June 5, 1992
Part V


Protection  Agency

Draft Report: A Cross-Species Scaling
Factor for Carcinogen Risk Assessment
Based on Equivalence of mg/kg3/VDay;

Federal  Register / Vol. 57, No. 109 / Friday. June  5. 1992  / Notices


Draft Report: A Cross-Species Scaling
Factor for Carcinogen Risk
Assessment Based on Equivalence of

AGENCY: U.S. Environmental Protection
ACTION: Request for comments on the
draft report: A Cross-Species Scaling
Factor for Carcinogen Risk Assessment
Based on Equivalence of mg/kg3'4/day.

SUMMARY: Three Federal regulatory
agencies, the Environmental Protection
Agency, the Food and Drug
Administration, and the Consumer
Product Safety Commission, are today
asking for public comments on the draft
report: A Cross-Species Scaling Factor
for. Carcinogen Risk Assessment Based
on Equivalence of mg/kg 3'4/day.
  The report is intended to serve as the
basis for a common and unified science
policy among these three agencies on a
default methodology for determining
equivalence of doses—to be used when
existing agent-specific data are
insufficient for a case-by-case
determination—when extrapolating
results of rodent carcinogen bioassays
to humans.
  The public is invited to comment, and
public  comments will be considered in
final revision of the report and in the
final adoption of science policies by the
participating agencies on cross-species
extrapolation of equivalent doses in
assessing potential human risks from
putative chemical carcinogens.
  Commenters are  asked to focus on the
report's discussion of several issues: (1)
The bearing of empirical data on
carcinogenic potencies in experimental
animals and in humans to the
appropriate choice of a dose-scaling
methodology; (2) the use of allometric
scaling as a means for suggesting
appropriate dose scaling methods; (3)
the appropriate use of pharmacokinetic
and other data in defining a default
methodology and particularly in
supplanting such default assumptions
with case-specific, data-based analysis
of dose equivalence; (4) distinguishing
the contributions of pharmacokinetic
and pharmacodynamic factors to
species differences in a carcinogen's
potency; and (5) the advisability of
adopting the proposed dose-scaling
methodology as a common default
methodology for the participating
  The complete text of the draft report
is published as the last section of this
                 DATES: The draft document is being
                 made available for public review and
                 comment until August 4,1992.
                 Comments must be in writing and must
                 be postmarked by August 4,1992.
                 INSPECTION AND COPYING: This notice,
                 references, supporting documents, and
                 other relevant materials are available
                 for inspection and copying from the
                 ORD Public Information Shelf at the
                 EPA Headquarters Library, 401 M Street,
                 SW., Washington, DC, Telephone: (202)
                 260-5926 or FTS: 260-5926. The Library
                 is open daily between the hours of 8
                 a.m. and 5:30 p.m., except weekends  and
                 ADDRESSES: Comments may be mailed
                 or delivered to: Project Officer for Cross-
                 Species Scaling Factor Report, c/o
                 Technical Information Staff, Office of
                 Health and Environmental Assessment,
                 U.S. EPA (RD-689), 401 M Street, SW.
                 (room 3703), Washington, DC 20460.
                 Dr. Lorenz Rhomberg, Human Health
                 Assessment Group, Office of Health  and
                 Environmental Assessment, U.S. EPA
                 (RD-689), Washington, DC 20460,
                 Telephone: (202) 260-5723 or FTS: 260-
                 document reports a consensus reached
                 by representatives of the U.S.
                 Environmental Protection Agency (EPA),
                 the Food and Drug Administration
                 (FDA), and the Consumer Product Safety
                 Commission (CPSC) in discussions
                 conducted under the auspices of the
                 Interagency Pharmacokinetics Group, a
                 workgroup of Federal scientists dealing
                 with issues of common interest arising
                 in the application of pharmacokinetics
                 to chemical health risk assessment. The
                 report is a product of the Interagency
                 Pharmacokinetics Group. It comprises
                 an analysis of empirical and theoretical
                 aspects of the cross-species dose-scaling
                 question, together with an argument for
                 adopting the method of scaling daily
                 administered doses by body mass raised
                 to the % power to achieve presumed
                 equivalence in lifetime carcinogenic risk
                 in different mammalian species. These
                 recommendations have been reviewed
                  and endorsed by the EPA, the FDA,  and
                  the CPSC.
                    If such a policy is adopted, it would
                  replace the current practices in
                  carcinogenic risk assessment of scaling
                  daily administered amounts by body
                  mass (as at FDA) or by body surface
                  area (as at EPA and CPSC). The
                  consensus recognizes that there is
                  considerable scientific uncertainty
                  around any scaling method; it does not
                  claim to have overturned these previous
                  methods with one of superior scientific
                  validity or reduced uncertainty. Rather,
in view of the benefits of having the
major practitioners of carcinogen risk
assessment in the Federal government
adhere to a single, consistent
methodology, the proposal provides a
common default procedure to encourage
consistent analyses in cases where
agent-specific information is insufficient
to suggest appropriate dose-
equivalencies on a case-by-case basis.
Such case-specific information is always
to be preferred to the default
methodology proposed herein, and its
development and appropriate use are
encouraged. Since the scaling
methodologies in current use by the
agencies participating in this proposal
are within the span of scientific
uncertainty surrounding the cross-
species scaling question, it is not
proposed to retroactively change or
adjust any risk assessments completed
under current policies.
  This document has undergone a
preliminary interagency review under
the auspices of the Ad Hoc Working
Group on Risk Assessment of the
Federal Coordinating Council for
Science, Engineering, and Technology
(FCCSET). This request for public
comment and a concurrent external
scientific peer review will contribute to
the development of a final report on this
topic. This final report of the
Interagency Pharmacokinetics Group
will provide the basis for a
recommendation of a uniform, default
science policy on interspecies scaling for
carcinogen risk assessment, to be
endorsed by the FCCSET Working
Group and used by a broad segment of
Federal agencies.
  Dated: May 22,1992.
F. Henry Habicht II,
Deputy Administrator.
I. Introduction
II. Approaches to Choosing a Cross-Species
    Scaling Factor
A. Empirical Approach
B. Allometric Approach
  1. Species Differences in Pharmacokinetics
  2. Species Differences in
  3. Toxicological Equivalence
  4. A Physiological Time Approach to
    Toxicological Equivalence
III.  Discussion
IV. Conclusions
V. References

A Cross-Species Scaling Factor for
Carcinogen Risk Assessment Based on
Equivalence of mg/kga/4/Day

I. Introduction
  As a matter of necessity, the potential
for a chemical agent to cause  toxic
reactions in humans is often

                      Federal  Register / Vol. 57. No.  109 / Friday, June 5.  1992 / Notices
 investigated by exposing and observing
 the reactions of experimental animals,
 usually rats and mice. This practice
 rests on the high degree of physiological,
 biochemical,  and anatomical similarity
 among mammalian species; the
 biological reactions in the experimental
 animals may be taken as evidence that
 humans might show similar responses to
 the same chemical exposures. When the
 objective is to use the animal data to
 predict the degree or probability of
 response in humans—that is, when the
 aim is quantitative extrapolation—one
 must define the dose levels for humans
 and animals that are expected to
 produce the same degree of effect. For
 this, it is necessary to take into account
 the pronounced difference in scale
 between the tested model organisms
 and humans.  That is, even if
 fundamental  similarity is presumed, one
 must allow for the fact that humans are
 much larger than experimental rodents
 and will experience chronic exposure to
 a toxicant for a longer lifetime.
  Defining such "lexicologically
 equivalent" doses has been problematic.
 Alternatives that have found use include
 scaling daily administered amounts by
 body weight or by body surface area;
 scaling cumulative lifetime intake by
 body weight;  equating exposures to
 contaminated air, food, or water
 according to the concentration of toxic
 agent; and others. Despite considerable
 study and debate (Pinkel, 1958; Freireich
 et al., 1966; Mantel and Schneiderman,
 1975; Rail, 1977; Hoel, 1977; Hogan and
 Hoel, 1982; Calabrese, 1983,1987; Crump
 et al., 1985; Davidson et al., 1986;
 Gillette, 1987; Vocci and Farber, 1988;
 Hill et al.,  1986), no alternative has
 emerged as clearly preferable, either on
 empirical or theoretical grounds. The
 various Federal agencies conducting
 chemical risk assessments have
 developed their own preferences and
 precedents for cross-species scaling
 methodology. This variation stands
 among the chief causes of variation
 among estimates of a chemical's
 potential human risk, even when
 assessments are based on the same
  The variety of cross-species scaling
methods in use correctly reflects the
uncertainty about the best procedure,
but the resulting disagreement in risk
 estimates results in some awkwardness
in the regulatory arena. Increasingly,
regulatory procedures are being
mandated  that establish decision points
contingent on whether a certain human
risk level is to be expected according to
"generally accepted" risk assessment
procedures. Variation in methodology
frequently leads to ambiguity as to
 whether regulatory action should take
 place. It has therefore become important
 to resolve differences in cross-species
 scaling assumptions.
  A second impetus for reexamining the
 scaling question comes from the
 increasing availability of comparative
 pharmacokinetic information on toxic
 agents. Pharmacokinetic analysis uses
 data on absorption of agents into the
 body, distribution among the tissues,
 metabolic activation or detoxification,
 and elimination to develop a picture of
 the disposition of a dose by the body
 and consequent exposure of the actual
 target tissues of toxic action.
 Pharmacokinetic differences among
 species clearly contribute to the
 magnitude of equipotent doses.
 However, the appropriate use of such
 information for the dose equivalency
 question hinges on resolving the role of
 pharmacokinetics compared to that of
 species differences in the magnitude of
 toxic reaction to a given degree of
 target-tissue exposure (i.e.,
 "pharmacodynamics"). Distinguishing
 the roles of these two aspects of potency
 scaling  has been hampered by
 imprecisely articulated rationales for  the
 various methods.
  In view of the above considerations,
 the Federal agencies with primary
 responsibility for conducting chemical
 risk assessments have endeavored to
 define a uniform cross-species scaling
 methodology and rationale for use when
 extrapolating results of rodent
 carcinogen bioassays to humans.
 Discussions and debate on the issues
 have been held under the auspices of  the
 Interagency Pharmacokinetics Group
 (IPG), an ongoing workgroup of Federal
 scientists that deals with issues of
 common interest arising in the
 application of pharmacokinetics to risk
 assessment. The present report is a
 product of the Interagency
 Pharmacokinetics Group, and represents
 a statement of the consensus
 recommendation resulting from these
  The consensus is that, in the absence
 of adequate information on
 pharmacokinetic and sensitivity
 differences among species, doses of
 carcinogens should be expressed in
 terms of daily amount administered per
unit of body mass raised to the %
power. Equal doses in these units (i.e., in
mg/kg3' 4/day),  when experienced daily
for a full lifetime, are presumed to
produce equal lifetime cancer risks
across mammalian species. This
proposed scaling method has the
advantage of being intermediate
between the two currently used methods
(scaling daily amount by body mass or
 by body surface area). It is not merely a
 compromise; it is as well supported by
 the empirical data on carcinogen
 potencies in animals and humans as the
 methods it would replace.  It also has an
 explicit rationale (the concept of
 species-independent "physiological
 time") that may be derived from
 principles of interspecific allometric
 variation in anatomy, physiology, and
 pharmacokinetics. That  is, it can be
 interpreted as a correction for readily
 observable scale differen ;es among
 species as their essentially similar
 biology varies in a regular quantitative
 way as a function of size.
   The consensus does not pretend to
 have solved the underlying scientific
 issues. Former methodologies have not
 been shown to be in error; the
 consensus should not be construed as
 overturning previous assumptions and
 replacing them with one of superior
 scientific validity. Rather, the consensus
 achieves the benefits of  having all
 Federal risk assessments adhere  to a
 single, consistent methodology that is in
 accord with current scientific knowledge
 on the scaling question. Moreover, the
 method corresponds to a fully
 articulated rationale with explicitly
 stated assumptions about the roles and
 interactions of various underlying
 determinants of carcinogenic potency.
 This aids in consistent and scientifically
 appropriate application.  Furthermore, as
 information is gained on how the
 biology of carcinogenesis varies among
 species, it will be clearer how the
 arguments and previous  presumptions
 should be modified to accommodate
 these new insights.
   The balance of this document reviews
 the evidence and arguments that may be
 adduced to address the question of
 cross-species scaling of equally
 carcinogenic doses, and  outlines the
 support for the recommended position of
 equipotent doses in terms of mg/kg3'4/
 II. Approaches to Choosing a Cross-
 Species Scaling Factor
  There are two broad and
 complementary approaches to choosing
 a cross-species scaling factor. The first
 is empirical; one may seek  cases in
 which human epidemiologic data  allow
 a direct estimate of an agent's potency,
 and then investigate the  success of
 various scaling methods  in predicting
 that potency from animal data. The
 second approach is theoretical, and is
grounded in the principles of allometry,
which is the study of the regular
variation in features of anatomy and
physiology as a function  of overall body
size.  The strategy for  this second

Federal  Register / Vol. 57, No.  109 / Friday, June 5,  1992 / Notices
approach is to develop a scientific
rationale for a particular scaling factor
by investigating the allometric variation
of the biological features and processes
that influence and underlie carcinogenic
  Clearly, in many cases there will be
agent-specific ways in which humans
and experimental animals differ in a
nonsystematic fashion. These may
include metabolic activation or
detoxification, interaction with key
receptors or target molecules, and
others. Such factors create
unpredictable deviation from the general
pattern of scaling, and must be
discovered and accounted for on a case-
by-case basis. The factor proposed here
is a default scaling factor, by which is
meant one that is to be applied in the
absence of adequate case-specific
information. Lacking such information,
one provisionally assumes that the agent
in question is an example of a "typical"
or "average" chemical that follows a
general pattern of cross-species potency
differences. This presumption may be
modified as information becomes
available, but the default assumptions
still serve as the benchmark against
which the new information is evaluated.

A. Empirical Approach
  This approach  attempts to find a
factor value that is empirically
successful in producing good estimates
of potency in humans from data on
potencies in other species. The
underlying reason why such a factor
works is a secondary consideration. The
 advantage of an empirical approach is
 that, by directly examining carcinogenic
potencies (rather than influences on
 potency, such as pharmacokinetics), all
 relevant factors are included. The
 disadvantage is that the data are few
 and of low resolution. One must hope
 that the agent-specific factors,
 mentioned above, average out to give a
 good estimate of the general
   A number of studies have sought
 general scaling factors empirically.
 Freireich et al. (1966), testing and
 extending the suggestion of Pinkel
 (1958), examined maximum tolerated
 doses (MTDs) of 18 antineoplastic drugs
 in mice, rats, hamsters, dogs, monkeys,
 and humans. LDios were used for
 rodents, and were presumed to be an
 equivalent level of toxicity to an MTD.
 Doses from experiments of different
 length were reexpressed in terms of an
 exposure regimen of 5 consecutive days,
 on the assumption that cumulative dose
 is proportional to effect. The authors
 concluded that, when doses were
 expressed as mg/m2 body surface area/
 day, good predictions of human MTDs
                  were obtained from all animal species,
                  but that body weight scaling of doses
                  overpredicted human MTDs (i.e.,
                  underpredicted potency in humans) by a
                  margin that increased as one
                  extrapolates from smaller and smaller
                  species. Since an MTD is intended to be
                  a dose causing no lethality, while an
                  LDio causes 10% lethality, the
                  equivalence of these two end points can
                  be questioned. Antineoplastic drugs
                  typically have very steep dose-response
                  curves, however, and survival near the
                  MTD is maintained by close monitoring
                  and intervention, which the rodent LDio
                  determination lack.
                    Collins et al. (1986,1990) have found
                  that the human MTD for 16
                  antineoplastic drugs is well predicted on
                  average by the mouse LDio when doses
                  are expressed as mg/m2 of body surface
                  area. (If the MTD is considered to be a
                  less severe end point, in such
                  comparisons potencies in the larger
                  species are overestimated vis-a-vis
                  those in rodents; a bias would then be
                  created that would increase the
                  apparent success of surface area scaling
                  compared to scaling by body weight.)
                  That is, if these endpoints of acute
                  toxicity are taken as equivalent, scaling
                  doses in proportion to surface area
                  tends to equalize toxicity across species.
                  Moreover, Collins et al. (1990) compared
                  the blood levels (in terms of the areas-
                  under-the-curve of concentration in
                  plasma as it declines over time, or "C x
                  T") that correspond to equally toxic
                  administered doses and found that these
                  were an even better predictor, in that
                  they displayed less case-by-case
                  variation. These results illustrate three
                  points that are returned to in Section B,
                  below: (1) Scaling administered doses in
                  this way tends to equalize blood levels
                  across species; (2)  areas-under-the-curve
                  of blood concentration can serve as a
                  predictive measure of the toxic response
                  to a dose, even across species; and (3)
                  obtaining pharmacokinetic data on
                  internal dose measures can increase the
                  precision of the cross-species prediction
                   of equivalently toxic doses by
                   accounting for case-by-case variation.
                    Travis and White (1988) reanalyzed
                   the Freireich et al. (1966) data set and
                   nearly doubled the number of drugs by
                   adding a similar data set of Schein et al.
                   (1979). Instead of simply examining the
                   success of prevously proposed scaling
                   methods, they used regression
                   techniques empirically to determine the
                   optimal power of body weight to
                   achieve  the best fitting allometric
                   relationship of MTDs across species. For
                   both data  sets individually and for the
                   combined data set, a power of 0.72 to
                   0.74 led to the best cross-species
predictions. In the analysis of the
combined data, a power of unity (body
weight scaling) was clearly rejected at
the 95% level of significance, and a
power of 2/3 (surface area scaling) was
barely rejected. The authors discuss the
history of empirical studies of allometric
variation in a number of physiological
features, primarily basal metabolism,
and arque that their result is part of a
general empirical support for scaling by
the 3/4 power of body weight.
  The difficulty with applying these
studies to the present question is that
they address acute systemic toxicity of a
rather narrowly defined type rather than
carcinogenesis. Although dose-scaling
for different toxic end points should
have some features in common (notably
pharmacokinetics), it is not altogether
clear how lifelong risks that accumulate
over time (such as cancer risk) should
relate to short-term toxicity dependent
only on immediate insults to target
   Some empirical  studies of
comparative potencies of carcinogens in
different species have been done.  Such
studies face the difficulty of precisely
determining potencies in humans based
on epidemiologic data. There is also
some ambiguity in defining potencies in
animals, owing to the variations in rout
of exposure, sex and strain differences,
varying experimental designs, and so on.
Nonetheless, such studies represent the
direct investigation of the question at
   The National Academy of Sciences
(NAS, 1975) examined the potencies of
six carcinogenic agents in bioassays
using mice and rats and from human
epidemiologic studies. They
recommended as a dose measure
cumulative lifetime amount of agent
 administered (in mg) per kg body
weight. Such scaling is more
 "conservative" (i.e., predictive of higher
 human risk from animal results) than
 either surface area scaling or body
 weight scaling (from which it differs by
 a factor of 35, owing to the lack of
 adjustment for differences in length of
 lifetime). The NAS conclusion was not
 based on formal quantitative
 comparison with surface area scaling
 (mg/kg2/3/day) or body weight scaling.
   The paucity of carcinogen potencies in
 humans known directly from
 epidemiologic data limits the precision
 of such comparisons. Crouch and
 Wilson (1979) instead investigated dose
 scaling between rats and mice in about
 70 ingestion cancer bioassays from the
 National Cancer Institute testing
 program. They measured potency by the
 parameter of a fitted one-hit dose-
 response model (in units of risk per mg/

                      Federal Register  /  Vol. 57.  No. 109 / Friday, June 5. 1992 / Notices
kg/day), focusing on the tumor site/type
producing the greatest potency
(excluding testicular tumors in Fisher
344 rats, and skipping cases in which
potency was less than twice sensitivity
in either species). A geometric mean of
potencies in each sex (which were
highly correlated) was used.
Interspecies comparisons were based on
the best-fitting line of unit slope on a
plot of the logarithm of potency in rats
against  the logarithm of potency in mice.
The intercept of such a line gives the
geometric mean of the factor by which
the rat potency must be divided to give
the mouse potency. Body weight scaling
predicts a factor of one (i.e., equal risk
per mg/kg/day in both species) while
surface  area scaling predicts a factor of
about 2.1 to 2.3, depending on the exact
body weights. (For comparison, the
scaling by mg/kg3' 4/day, as advocated
herein, predicts a ratio of about 1.8 or
1.9.) The results depend on the strain of
rat used. In the 17 cases of comparison
between Osborne-Mendel rats and
B6C3F1  mice the mean ratio of potencies
was 0.40; these rats were somewhat less
sensitive than mice, contrary to the
expectations of both scaling
methodologies. When Fischer 344 rats
were compared to the same mouse
strain (18 cases) a mean ratio  of 4.5 was
obtained, indicating that rats were even
more sensitive than  surface area scaling
would expect. (A geometric mean of
these  two ratios is 1.3. To attempt
definition of a general mammalian
cross-species allometric relationship
using  only two species is fraught with
pitfalls,  especially when  they are as
close in size as are rats and mice.
Nonetheless, for the purposes of this
discussion one may note that, using
typical body weights—70 kg for a
human, 40 g for a mouse, 467 g for a rat
of unspecified strain, 500 g for an
Osborne-Mendel rat, and 360 g for a
Fischer rat—the ratio of 1.3 implies
scaling by body weight to the 0.89
  Crouch and Wilson (1979) also
examined ratios of rodent potency to
epidemiologically derived human
potency, comparing "insofar as
possible" studies with the same route of
exposure and duration in fraction of a
lifetime. Owing to imprecision in the
epidemiologically based human
estimates, no precise curve fitting was
attempted, but the authors state that
humans appear to be more sensitive to a
mg/kg/day dose by about a factor of 5
compared to either rats or mice. (Using
the typical body weights  listed
previously, a factor of 5 corresponds to
scaling doses by a power of body weight
 of 0.7 and 0.8 based on the rat and
 mouse results, respectively.)
  A similar comparison of rats and
 mice, based on an expanded base of 187
 NCI bioassays, was conducted by
 Crouch (1983). (Despite the larger
 original database, there were only a few
 more chemicals in the final analysis,
 apparently owing to more stringent
 requirements for significance of
 portency estimates.) Again, the rat
 strain influenced the results: for
 Osborne-Mendel rats the mean ratio
 was 0.63 while for Fischer 344 rats it
 was 2.29. (A geometric mean of these
 two ratios is 1.20.) Separate analysis of
 males and females changed these ratios
 only slightly. An analysis irrespective of
 rat strain yielded a ratio of 1.62. (Using
 the typical body weights listed
 previously, rations of 1.20 and 1.62 imply
 scaling by body weight to the 0.92 and
 0.80 power, respectively.)
  Gaylor and Chen (1986) examined
 data on rats, mice, and hamsters in the
 extensive database of Cold et al. (1984)
 on TDsoS, the dose (in mg/kg/day)
 leading to a halving of the actuarially
 adusted percentage of tumor-free
 animals at the end of a standard
 lifespan. The tumor site/type showing
 highest potency (i.e., lowest TD5o) was
 chosen to represent the species, and
 only agents with responses in both
 species were included. For 190
 compounds administered in the diet, the
 geometric mean ratio of TD5os in rats
 and mice was 0.455=1/2.20. That is, rats
 were on average about 2.2-fold more
 sensitive. (Using the typical body
 weights listed previously, this
 corresponds to scaling by body weight
 to the 0.68 power.) Ratios for other
 routes of exposure varied somewhat,
 although based on much lower sample
 sizes than the ingestion results cited
 above. By gavage, 32 compounds had a
 mean ratio 1/1.32, in drinking water 10
 compounds had a mean ratio of 1.45 (i.e.,
 rats were less sensitive), and by
 inhalation 7 compounds had a mean
 ratio of 1/11.2 (i.e., rats were much more
  Chen and Gaylor (1987) investigated
 NCI/NTP cancer bioassays of
 compounds administered orally to rats
 and mice. They compared "virtually safe
 doses" (VSDs), defined as doses
 associated with a lifetime cancer risk of
 one in a million. These were determined
 by the method of Gaylor and Kodell
 (1980), i.e., a linear extrapolation was
 conducted from an upper bound on a
fitted multistage model dose-response
curve. Thus, both the rat and mouse
VSDs are in some sense "upper
bounds." Chemicals were included if
judged by the NTP to be positive in at
 least one species, and when in only one,
 if there was at least a positive trend in
 the other species for the same tumor
 site/type. Unlike the studies mentioned
 above, Chen and Gaylor (1987) focused
 on Correspondence of VSDs at the same
 site and sex across species. VSDs were
 expressed in terms of concentration
 (parts per million [ppm]); as discussed
 further in the following section on
 allometry, since intakes of contaminated
 media (air, food, water) tend to be
 proportional to body surface area, the
 expectation from surface area scaling is
 that VSDs expressed in ppm would be
 about equal across species, while body
 weight scaling would expect a ratio of
 rat to mouse VSDs to be slightly greater
 than 2. Again, the results depend on the
 strain of rat used: For Fischer 344 rats
 the mean ratio is 1.15, for Osborne-
 Mendel rats it is 1.68, and for  Sprague-
 Dawley rats it is 1.78. Ignoring rat strain
 gives a mean ratio of 1.27. These  results
 are intermediate between the
 expectations of surface area and  body
 weight scaling. For ease of comparison
 with other studies, one may convert
 these ratios from a ppm basis to a mg/
 kg/day basis using empirically based
 daily food and water consumption
 patterns in rats and mice (for  food, 5%
 and 13% of body weight for rats and
 mice, respectively, and for water, 7.8%
 and 17% [U.S. EPA, 1984]). On a mg/kg/
 day basis, the ratimouse VSD ratios are
 0.44-0.53 for Fischer rats, 0.647-0.771 for
 Osborne-Mendel rats, and 0.69-0.82 for
 Sprague-Dawley rats. (The range
 reflects using rat:mouse ratios of  water
 and food consumption, respectively,
 which differ slightly.) Using the typical
 body weights listed previously, and
 assuming a weight of 540g for  Sprague-
 Dawley rats, these ratios correspond to
 scaling doses by body weight  to the
 0.63-0.71 power (when based on Fischer
 rats, which constituted most of the
 cases), 0.83-0.90 power (when based on
 Osborn-Mendel rats), and 0.86-0.92
 (when based on Sprague-Dawley  rats).
  Metzger et al. (1989) expanded
 Crouch's (1983) earlier data set by
 including all 264 cases from the Gold et
 al. (1984) database in which a  significant
TD5o was obtained in an oral study of
rats and mice (of any strain), i.e.,
including studies that were not in the
NCI/NTP database. A best-fitting line of
unit slope showed a TD50 ratio of  1.46
between mice and rats. This is
intermediate between the ratio of 1.0
expected from body weight scaling and
2.5 from suface area scaling (using the
authors' assumptions about body
weights—this implies a power of body
weight of 0.86).

Federal  Register / Vol. 57, No. 109 / Friday, June  5, 1992 / Notices
  A major study of animal-to-human
extrapolation of cancer potencies was
carried out by Allen et al. (1987), and
reported on by Crump et al. (1987,1989}
and Allen et at. (1988). Twenty-three
chemicals were identified that permitted
quantitative evaluation of potency in
humans and in animals. "Risk-Related
Doses" (RRDs) were calculated, defined
as the average daily dose per kg of body
weight that would be expected to result
in an extra cancer risk of 25% over a
lifetime. Chemicals were included even
if RRD estimates were "infinite" for one
species, as happens when no
carcinogenic effect is observed. Unlike
the studies reviewed above, the Allen et
al. (1987) study considered a large
number of alternative ways of
representing the potency in animals as
well as various methods for
extrapolating the resulting RRDs to
humans. Alternative sets of "risk
assessment assumptions" restricted the
animal database according to various
criteria of experimental design, route of
exposure, and tumor type. Different
levels of averaging results over
experiments, sex, and species were
tried. Finally, different methods for
combining the multiple animal results on
a given chemical into a single measure
of its "potency in animals" were
examined. This complexity allows an
admirably comprehensive look at
animal-to-human extrapolation, but it
also makes manifest a problem that is
latent in the other extrapolation studies:
The performance of a scaling factor
depends on how the animal potency is
characterized. A factor that tends to
overpredict human risk can be
"rescued" by a method for characterzing
animal potency that tends to produce a
low estimate, and vice versa.
   When the objective is to examine
alternative dose-scaling factors, it would
seem that the best approach is to
examine analyses that aim at broadly
based and unbiased estimates of the
potency in animals. Risk assessment
practices such as using upper bounds on
dose-response curves and extrapolating
from the most sensitive sex and species
of animal are explicitly conservative;
they may be appropriate science policies
for regulatory purposes,  but when the
issue is empirically to choose a best-
performing scaling factor, they introduce
a bias, favoring a less conservative
factor to compensate for their
conservatism and restore a good
prediction of the known human potency.
   To compare potencies, Allen et al.
(1987) fit a line of unit slope to the data
of epidemiologically observed log RRD
in humans plotted against the predicted
human log RRD based on the animal
                 data and the chosen scaling
                 methodology. The intercept of this line
                 gives an average ratio of the observed to
                 predicted potency, with a ratio of unity
                 indicating unbiased prediction. The
                 analyses discussed prominently in the
                 Allen et al. (1987,1988) and Crump et al.
                 (1987,1989) reports show that body
                 weight scaling leads to a ratio of
                 approximately one to somewhat less
                 than one depending on the particular
                 suite of risk assessment assumptions
                 chosen  (i.e. slightly underpredicting
                 human risk), while surface area scaling
                 overpredicts human risk several-fold.
                   These results are sometimes cited as
                 tending to support mg/kg/day scaling,
                 but such a conclusion should be
                 tempered. The particular choice of risk
                 assessment  assumptions (among many
                 examined) in the widely cited analysis
                 is the one with results least favorable to
                 surface area scaling; most of the
                 alternatives discussed by Allen et al.
                 (1987) show that body weight scaling
                 underestimates human risks by about
                 the degree to which surface area
                 overestimates it. Moreover, these
                 analyses contain a bias of the sort
                 outlined above—the animal potency for
                 a chemical is characterized by the
                 median of the lower bounds on the
                 RRDs for the various animal data sets
                 rather than on best estimates. At present
                 it is unresolved how much the use of
                 central  estimates of animal risk to
                 predict  central estimates of human
                 risk—a  more appropriate analysis for
                 resolving the scaling factor—would shift
                 the results toward favoring surface area
                   Two  additional studies of
                 comparative cancer potencies should
                 briefly be mentioned,  both favoring a
                 somewhat more conservative scaling
                 factor. Raabe et al. (1983) compared
                 bone cancer risks from radium in watch
                 dial painters (who ingested radium by
                 tipping  brushes on their tongues) and in
                 beagle dogs exposed to radium by
                 injection. Doses were measured as dose
                 to bone of deposited radium, so this
                 camparison can be seen as lacking the
                 pharmacokinetic component of cross-
                 species differences. Potency was
                 measured by the relative mean degree of
                 life-shortening as a function  of does. The
                 authors argued that a cumulative
                 lifetime radiation dose per unit of bone
                  seemed to give good correspondence
                 between human and dog. This result
                  could be related to mg/kg/lifetime
                  scaling for chemical agents.
                    Kaldor et al. (1988)  examined
                  carcinogenic potency of five
                  antineoplastic drugs, using potencies
                  derived from bioassays in rodents and
                  from the secondary tumors the drugs
caused in human cancer patients. They
argued that potency seemed to be
related to total cumulative lifetime
exposure per kg of body weight.
  The empirical evidence on cross-
species scaling of carcinogen potencies
can be summed up as follows. The
correlation of agents' potencies across
species is clearly and strongly
demonstrated. This correlation extends
to humans, so far as is ascertainable
from the limited number of agents for
which potencies can be estimated
epidemiologically. There is a remarkable
agreement among studies that the dose-
scaling methods in current use span a
range that appears approximately
correct. The resolution of the data
available at present, however, does not
permit a clear choice between surface
area and body weight scaling.
Empirically chosen scaling factors tend
to fall in between these two choices in
most cases, but the specific results
depend on the laboratory strains used,
route of administration, details of the
methods for characterizing the
carcinogenic potency in animals, and
the statistical methods used in curve
fitting. The data seem consistent in
indicating that body weight scaling
somewhat underestimates risks in larger
species. The exception is  when
Osborne-Mendel or Sprague-Dawley
rats are compared to B6C3F1 mice, in
which comparison the rats are seen to
be less affected even by doses scaled to
body weight. The preponderance of data
are from Fischer 344 rats, however, and
this is the strain used in most modern
  Several points should be borne in
mind while interpreting the empirical
scaling data. First, although several
studies are reviewed, they overlap
considerably in their databases; the
individual studies are not independent
tests. Second, the specific results of a
study depend on details of the
methodology. The Allen et al. (1987)
study showed that whether potencies
were averaged over sexes, whether both
benign and malignant tumors were
counted, whether projections were made
for specific tumor sites or for the most
potent site, and other such factors could
swing the analysis toward favoring one
scaling method or another. It is hard
confidently to identify and isolate the
specific contribution of dose scaling
among the many factors that contribute
to the final predictions of human risk.
Third, the epidemiologically based
human potencies  that serve as "targets"
for the animal-based extrapolations are
themselves very uncertain and, as in the
animal data, dependent on the specifics
of the methodology used  in their

                      Federal  Register / Vol. 57, No.  109 / Friday, June 5,  1992 / Notices
 estimation. As a result of this and of the
 previous point, the comparability of
 animal- and human-based potencies
 may be problematic. (For example,
 potencies calculated from human data
 are usually based on cancers that were
 the cause of death following partial
 lifetime exposure, while animal-based
 estimates usually reflect incidental as
 well as fatal tumors arising after full
 lifetime exposure.) A final point to be
 borne in mind is that the report
 empirically derived factors represent
 averages over large numbers of cases.
 Although the means vary over a narrow
 range, the individual chemicals show
 ratios of potencies in different species
 that span orders of magnitude. Most of
 the rat-to-mouse comparisons were
 within an order of magnitude of the
 average scaling relationship, but  several
 agents showed a 100-fold difference.
 Variances of rodent-to-human potency
 ratios were higher, reflecting the
 uncertain determination in humans and
 the lack of standardized experimental
 design. The existence of this scatter of
 cases around the mean helps to define
 the limits to the resolution of any scaling
 method and emphasizes the importance
 of case-to-case variation. Moreover, it
 provides some insight into the
 distribution of uncertainty in the cross-
 species dose extrapolation step of risk
  Despite these shortcomings, the
 empirical data support the general
 practice of scaling rodent potencies to
 humans, and show that, on average, the
 current methods perform satisfactorily.
 Certainly, any method that produces
 average results an order of magnitude
 higher or lower than the range
represented by body weight and surface
 area scaling would be in contradiction
to the empirical data. The data suggest
that a scaling factor in between the
surface area and body weight scaling
 can be considered to have empirical

 B. Allometric Approach
   The complement to the empirical
 investigation of potency scaling is a
 more theoretical approach that seeks to
 identify the biological factors whose
 variation underlies the variation in a
 carcinogen's potency across species,
 and then attempts to adjust for their
 effect. Clearly, these factors are
 numerous and, for the most part, poorly
 understood. Fortunately, there are some
 rather simple and general quantitative
 patterns in the variation of many
 features of anatomy and physiology
 across differently sized mammalian
 species, representing broad trends in the
 way the essentially similar mammalian
 system operates in large and small
 editions. Although specific processes
 acting on specific chemicals can (and
 do) deviate from these broad trends, it is
 argued below that the general patterns
 can provide a benchmark that expresses
 the expectation about a chemical's
 carcinogenic potency in small mammals
 such as experimental rodents and larger
 ones such as humans.  This expectation
.can be refined (or refuted) by case-
 specific biological and mechanistic data,
 when available, showing how the actual
 processes of metabolism and
 carcinogenesis differ from the
 presumptions of the broad trend
 analysis that serves as the default.
  The aim of a dose-scaling
 methodology is to estimate administered
 daily doses to experimental rodents and
 humans that result in equal lifetime
 cancer risks. That is, the scaled doses
 are intended to be "toxicologically
equivalent." It is useful to recognize two
components to this equivalence. The
first, which might be termed
"pharmacokinetic equivalence,"
concerns adjustment of the administered
 dose to a rodent or human so that the
 corresponding tissues that constitute the
 targets of the agent's toxicity receive
 similar exposures to the toxin. The
 second, or "pharmacodynamic
 equivalence," relates to the relative
 tissue doses that, when experienced
 daily for a lifetime, yield equal lifetime
 cancer risks. This latter aspect includes,
 but goes beyond the question of
 "sensitivity" to address species
 differences in the operation of the
 carcinogenic processes as they relate to
 tissue does. For both the
 pharmacokinetic and the
 pharmacodynamic component, scaling
 questions arise and the problem of
 defining "equivalence" must be faced.
  By way of illustration, consider a
 hypothetical agent with rather simple
 pharmacokinetics (first order
 elimination from a single compartment)
 given by intravenous injection to a
 mouse and a human. As shown in Figure
 1, such a compound will demonstrate an
 almost instantaneous peak in its blood
 concentration, followed by exponential
 decline. If the administered doses are
 equal in terms of mg/kg body weight,
 the peak concentrations are the same in
 the mouse and the human, but the
 mouse rids itself of this body burden
 faster, owing to its more rapid
 metabolism and elimination compared
 to the human. As a result, the area under
 the curve (AUC) of blood concentration
 as it declines with time is much less in
 the mouse. If the amount injected is
properly adjusted, as illustrated in
Figure 2, a concentration profile can be
achieved in which the initial peak blood
Concentration is much less in the human,
and yet is balanced by the compound's
longer persistence to generate an AUC
equal to that of the mouse.

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Federal  Register / Vol. 57, No.  109 / Friday, June 5,  1992 / Notices
  This example illustrates two points:
that knowledge of a compound's
pharmacokinetics can suggest scaling of
administered doses so as to equalize the
exposure of internal targets of toxicity,
and that "equal" internal exposure
requires further definition. The area
under the concentration curve
encompasses both the amount of a
compound that is present and the
duration of its  presence, providing a
measure of the compound's  opportunity
to interact with the targets of toxicity.
Moreover, since the AUC is the integral
of concentration X time—that is, the
"sum" of many momentary
concentration levels—dividing the AUC
by the time interval over which it is
measured gives the average
concentration  during  that interval. As
such, the AUC is more representative of
the target organ's total exposure to the
agent than is the peak concentration.
The AUC provides a measure of the
agent's opportunity to participate in
critical reactions at the target site. For
example, for DNA-reactive compounds,
the AUC is predictive of the rate of
generation of DNA adducts (Hattis,
1990), while for moderate levels of
receptor mediated carcinogens it tends
to be proportional to  average receptor
occupancy. For such reasons,
pharmacokinetic equivalence is usually
defined in terms of equality of AUCs.
  If this hypothetical chemical is
assumed to be a carcinogen, an added
difficulty in defining pharmacodynamic
equivalence is also readily apparent. It
should be remembered that equally
carcinogenic doses are defined in terms
of exposures repeated every day over a
full lifetime. An adjusted daily dose that
yields pharmacokinetic equivalence for
one day's exposure of the target organ
(as illustrated  in Figure 2) is repeated for
2 years in the lifetime of a mouse, but 70
years in a human's. Furthermore, if the
agent's stress on the physiological
system at any  given moment is not
proportional to its concentration, the
fact that the pharmacokinetically
"equivalent" equal AUCs are achieved
from different  time-patterns of target
organ exposure (as seen in Figure 2)
could affect the carcinogenic
consequences. These and other issues
will be discussed at greater length
further on in this document; they are
raised here to  emphasize that
pharmacokinetic equivalence need not
lead to carcinogenic  equivalence
without first employing further scaling
  Clearly, actual pharmacokinetic and
pharmacodynamic processes will be
more complex than the simple
considerations mentioned above would
                  indicate. Nevertheless, there are some
                  well recognized general trends in
                  species differences (e.g., the higher
                  metabolic rate in small mammals, the
                  longer tumor latency in humans via-a-
                  vis experimental rodents) that clearly
                  influence the appropriate scaling of
                  doses of carcinogens, and for which we
                  should attempt to account in our scaling
                  rationale (Boxenbaum, 1982,1983;
                  Schmidt-Nielsen, 1970,1975,1984; Travis
                  et al., 1990; Ings, 1990). An analysis of
                  the effects of major general trends in
                  cross-species physiological differences
                  not only helps guide our choice of
                  appropriate scaling factors, but it
                  provides the benchmark against which
                  increasingly available case-specific data
                  on the complex details of
                  pharmacokinetics  and carcinogenesis
                  may be compared. Without such a
                  framework, the impact of data on a
                  single component—metabolic activation
                  of a carcinogen in  a target tissue in mice
                  and humans, for example—is difficult to
                  guage (U.S.  EPA, 1987a,b). The analysis
                  presented below is not a definitive
                  solution to the cross-species scaling
                  problem. Rather, it is presented as an
                  attempt to accommodate present
                  knowledge about the major quantitative
                  trends in comparative anatomy and
                  physiology into a scaling rationale with
                  explicity stated assumptions.
                    The scaling of the myriad
                  physiological processes that underlie the
                  processing of carcinogens and their
                  toxic effects can be drawn  together into
                  a single scheme by referring to the
                  concept of physiological time. This
                  concept proposes that quantitative
                  differences  across mammalian species
                  in physiological processes can be seen
                  largely as the consequence of
                  fundamentally similar anatomical and
                  biochemical machinery operating at
                  different rates in differently sized
                  species, smaller species having faster
                  physiological "clocks." By correcting for
                  these differences in size and  time one
                  can express dose-response problems in
                  terms of a single scale-free mammalian
                  system in which scaled doses should
                  yield equal responses. (It is this very
                  similarity, after all, that leads us to  use
                  experimental animals as surrogates for
                  humans in risk assessment.) In the
                  sections that follow, the issues of
                  pharmacokinetic and pharmacodynamic
                  equivalence are considered in turn.
                  1. Species Differences in
                    The physiological time concept
                  emerges from the  study of the allometry
                  of key physiological and anatomical
                  variables that affect pharmacokinetics.
                  Allometry studies the variation in
                  features (and the consequences of that
variation) as a function of body size and
some other parameters. Most
quantitative features that vary among
mammals are well described by the so-
called allometric equation,
Y = a PV,

where b is the power of body weight
[W] to which attribute Ymaintains a
constant proportionality, a. A review of
the large literature on this subject is
beyond the scope of the present paper.
The reader is referred to a number of
excellent reviews (Adolph, 1949;
Kleiber, 1932,1961; Lindstedt and
Calder, 1976,1981; Schmidt-Nielsen,
1970, 1975,1984).
  The key point for the present
argument is that there is great regularity
in the value of b for certain classes of
attributes relevant to pharmacokinetics
(Travis et al., 1990). Volumes and
capacities (blood  volume, volumes of
distribution, organ sizes, lung capacity,
etc.) tend to remain in approximately
constant proportion to body size (i.e.,
b~1.0) in large and small mammals.
  Rates, in contrast, tend to maintain
proportionality with body weight to the
3/4 power (i.e., bxO.75). Such rates
include cardiac output, minute volume,
basal metabolic rate and oxygen
consumption, glomerular filtration rate,
and  many others. Consumption rates
also tend to scale this way, including
daily intakes of food, air, and water. A
rate that scales in this way becomes
smaller per unit weight (or volume) in
larger animals. For example, a human
has a total cardiac output (mL/min)
about 300 times greater than a mouse,
but in proportion  to the human's 2000-
times more massive body, the rate of
blood delivery per gram of tissue is
approximately seven-fold smaller (in
terms of mL/min/g).
   Several authors have suggested that
this  consistent scaling of rates of
physiological processes leads to a useful
concept of physiological time (Dedrick
et al., 1970; Dedrick, 1973; Boxenbaum,
1982,1983,1984,1986; Lindstedt and
Calder,  1981; Mordenti, 1986; Lindstedt,
1987; Travis et al., 1990). A mouse is
carrying out the same set of
physiological processes as a human, but
each process proceeds at a rate some 7-
times faster. The  various processes stay
in proportion to one another, but all of
them are relatively sped up in smaller
species.  If one scales the units of time
by dividing them by  the fourth root of
body mass (i.e., min»W~1/4, correcting
the physiological time scale) then the
time-course of physiological processes
becomes congruent across species. If
time were measured according to some
internal, physiological  standard (such as

                     Federal Register / Vol. 57, No.  109 / Friday, June 5,  1992 / Notices
heartbeats, breaths, blood circuit times,
clearance half-lives, etc.), rather than in
minutes, then the rates of
pharmacokinetic processes, the time
course of disposition of a dose, and even
life milestones and lifespan would all be
about equal across species. (As
discussed more fully below, humans
tend to be an outlier in the  relationship
of lifespan to W l/ 4, living longer than
expected. Some authors have addressed
this by including brain weight as a
second factor in the allometric equation
[Boxenbaum, 1986].)
  This concept is illustrated by the
simple example introduced in  the
previous section (shown graphically in
Figure 1)—a single intravenous dose of a
compound to a mouse and a human, and
its subsequent blood concentration as it
is removed  from a single body
compartment. (The simplicity is for
illustration; the argument can be shown
to hold for more complex
pharmacokinetic models as well, e.g.,
Travis et al., 1990.) If doses are scaled to
body weight (mg/kg) then initial
concentrations are equal, but the blood
level takes much longer to decline in the
human, owing to slower processing of
the compound. The human has a bood
volume (which is proportional to body
weight) some 2000-fold higher than the
mouse, but the compound must be
cleared from this volume by processes
(metabolism and/or excretion) that
operate only 300-fold faster (or seven-
fold slower per unit blood volume). As a
result, the human has an area under the
blood concentration curve (or AUC) that
is 7-fold higher. The AUC has units of
[cone.]'[time], e.g., (mg/L)»min.-
  There are two kinds of scaling one
could imagine to accommodate the
species difference in pharmacokinetic
behavior. The first has already been
illustrated in Figure 2; one could give a
smaller initial dose to the human—one
that is seven-fold smaller in terms of
mg/kg but equal in terms of mg/kg31 *.
The initial concentration is lower, but
this is balanced by the slower removal
to give the same AUC as seen in the
  Alternatively, one could give the same
initial mg/kg dose, but scale the time
axis, expressing time in "physiological
time units" (i.e., minutes divided by
W ll"). This is illustrated in Figure 3.
Such graphs are sometimes called
"Dedrick plots," following the
demonstration of Dedrick et al. (1970)
that scaling time in this way leads to
congruity of methotrexate
pharmacokinetics among several
species. The mouse and human curves
are identical on such a graph, falling to
the same concentration after the same
amount of physiological time has
elapsed. (Of course, it still takes 7-times
more minutes in a human for a given
interval of physiological time to elapse.
The AUC in the usual chronological time
units is still bigger in the human, but in
units of [conc.]»[physiological time] it is

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                     Federal Register / Vol.  57, No. 109 / Friday.  June 5, 1992  /  Notices
  It can be shown that these two scaling
approaches—shrinking doses or
stretching the time scale—give
equivalent ways of dealing with scale
differences as long as saturable
pharmacokinetic processes do not figure
prominently (O'Flaherty, 1989). For
example, consider the slightly more
complex case of repeated dosing.
Figures 4 and 5 show blood
concentration versus time curves for
bolus dosing repeated at regular
intervals. If dosing is daily (i.e., inter-
dose intervals are equal for animal and
human in clock time, as in Fig. 4) then
scaling the bolus amount by W31 *
achieves an equal area under the curve
after a given number of days, as well as
an equal average steady-state blood
concentration. Alternatively (Fig. 5), one
can give equal mg/kg doses spaced
according to equal intervals of
physiological time (e.g., daily in the
mouse and every seven days  in the
human)  to achieve the same end.



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Federal  Register / Vol. 57, No. 109 / Friday,  June 5, 1992  /  Notices
  The foregoing examples are of course
simplified and hypothetical, designed to
illustrate the principles of allometric
variation in physiological rates and
volumes and their impact on the relation
of administered dose to the degree of
"internal" exposure. The same
principles, however, can be shown to
apply to much more complex
pharmacokinetic systems as well,
including multicompartment models,
multiple routes of uptake and
elimination, and multiple metabolic
pathways causing carcinogenic
activation and/or detoxification. The
arguments have been most extensively
developed by Mordenti (1986),
O'Flaherty (1989), and Travis  et al.
(1990). The complete  elaboration of the
allometry of pharmacokinetics is too
complex to detail here, but a few
important points should be made.
  First, the ability to predict the
pharmacokinetic consequences of
variation in the dozens of parameters
that affect a chemical's uptake,
distribution, processing, and elimination
rests on the regularity in their cross-
species variation and the congruence of
these patterns for certain classes of
parameters (rates, volumes, etc.). If
physiological features varied
haphazardly across species, or if all
features had independent allometric
patterns unrelated to one another, then
no dose scaling method could be defined
(W31 * or any other) to approximate
pharmacokinetic equivalence without
first knowing the compound's
pharmacokinetics in detail.
  Owing to their importance,  it is well
briefly to examine the  starting
assumptions that form the basis of the
allometric, "physiological time" concept
and its predictions. They are: (a)
Volumes and capacities (organ sizes,
blood volumes) retain  proportionality to
 W; (b) the absolute rates of
physiological processes are proportional
to W31"; these rates include cardiac
output, minute volume, glomerular
filtration, and the rates of specific
metabolic steps; (c) physicochemical
 and thermodynamic properties of
 compounds (solubilities in various
 tissues) are equal in all species; and (d)
 for metabolic pathways with saturable
 metabolism, the Michaelis constant (the
 substrate concentration at which half
 the maximum reaction velocity is
 achieved) is invariant, while  the
 maximum velocity scales as  W314. A
 corollary to points (a)  and (b) is that
 when rates are figured relative to body
 size (or to a volume, or in terms of
 concentration rather than absolute
 amount), they scale as W3"/W = W^
 as illustrated by the cardiac output
 example shown earlier.
   Most of the above assumptions are
 well supported by data on comparative
 anatomy and physiology, as detailed in
 the allometry references cited
 previously. Collectively, they embody
 the concept of a basically similar
 mammalian physiological and
 anatomical plan that varies primarily in
 scale from one species to another. The
 most problematic issue is the scaling of
 rates of individual metabolic
 transformation reactions as W3/ 4. Not
 only are there few data on such scaling,
 but some individual metabolic enzyme
 activities are shown to vary rather
 haphazardly across species (e.g.,
 Gillette, 1987; Calabrese 1986a,b).
 Several points should be made,
 however. First, there are data that
 support the proposition of H/3'4 scaling
 in specific cases (e.g., Reitz et al., 1988).
 Second, overall metabolic rate  (O2
 consumption, resting metabolic rate)
 clearly scales as W31 *-, indeed,  this is
 the issue around which physiological
 allometry was developed. Scaling an
 individual metabolic step in this way
 corresponds to keeping it in proportion
 to general metabolism, which seems the
 best default. Third, daily intake of
 natural toxins (the usual targets of
 carcinogen-metabolizing enzymes)
 depends on intake of air, water, and
 food (which all scale as W314). That is,
 scaling detoxification  processes in
 proportion to their anticipated load also
 predicts W3' * scalirtg.
   Consideration of these points leads to
 the view that W314 scaling of the rates of
 individual  metabolic transformation
 reactions can be viewed as a benchmark
 around which different species (and
 individuals within a species) vary from
 instance to instance. Such variation
 does not invalidate the general scaling
 argument,  nor does it provide evidence
 for any different scaling factor. Rather,
 the variation simply illustrates that any
 single conception of cross-species
 scaling can accommodate only the
 general trends, not the diversity  of
 particular  instances. Clearly, when data
 on metabolic conversion are available in
 a particular case, they should be used in
 preference to the W314 default. In fact,
 instances of chemical-, dose-, and
 species-specific variation in metabolic
 transformation of a chemical may
 constitute  the principal reason for
 deviation from the allotmetric  default
 assumptions herein laid out.
 Accordingly, empirical determination of
  such metabolic variation constitutes the
  most important pharmacokinetic data
  that can be brought to bear on the
,  estimation of target tissue exposures.
                                                           A second major point to bear in mind
                                                         about the allometric analysis of
                                                         pharmacokinetics is that the cross-
                                                         species consequences of variation in the
                                                         many physiological parameters depend
                                                         not on the individual parameters, but on
                                                         their interrelation. It is misleading
                                                         simply to examine the scaling of one
                                                         component  (say, metabolic activation)  in
                                                         isolation. One must remember that the
                                                         many quantitative differences across
                                                         species are  having their influences
                                                         simultaneously; it is their interactions
                                                         and net results that determine the
                                                         consequences for doses to the tissues.
                                                         For example, metabolic rates alone are a
                                                         less important determinant of the
                                                         fraction of. a dose that is metabolically
                                                         activated than is the ratio of metabolic
                                                         activation rates to rates of other
                                                         competing processes (such as renal
                                                         clearance) that remove a compound
                                                         from the body.
                                                           The third major point is that, despite
                                                         the variety  and  diversity of underlying
                                                         pharmacokinetic processes that may
                                                         obtain from one case to another, the
                                                         allometric analysis of pharmacokinetics
                                                         makes rather general and simple
                                                         predictions about how administered
                                                         doses should relate  to target tissue
                                                         exposures in experimental rodents and
                                                         humans. These  predictions are:
                                                           For a  given dosing pattern in which
                                                         amounts are scaled to body weight,  fhe
                                                         tissue exposures (as measured by areas
                                                         under the concentration curve) tend to
                                                         be bigger in larger species by the ratio of
                                                         human to animal body weight to the 1/4
                                                         power (which amounts to almost seven-
                                                         fold for mouse-to-human scaling and not
                                                         quite four-fold for rat-to-human scaling).
                                                         If the administered amounts are kept in
                                                         proportion  to W31 * (rather than to W]
                                                         the doses tend to be
                                                         "pharmacokinetically equivalent" in the
                                                         sense of yielding similar areas under the
                                                         curve of concentration over time. Since
                                                         daily intakes of air, food, and water tend
                                                         to be in proportion to W31 * across
                                                         species, calling exposures to
                                                         environmental media equivalent on a
                                                         ppm basis (i.e., when they are equally
                                                         contaminated) produces essentially the
                                                         same expectation of pharmacokinetic
                                                         equivalence as scaling by W314 (Hattis,
                                                            In fact, all variables containing [time]
                                                         in their units will scale in a way that
                                                         leads to the human value being bigger
                                                         by the ratio of body weights to the 1/4
                                                         power.  If these variables are
                                                         reexpressed in terms of "physiological
                                                         time units," i.e., [time] 'H/'1'4, then their
                                                         values are equal across species.
                                                            The above conclusions apply to
                                                         parent compound and to metabolites,
                                                         since (in this generalized scheme)

                      Federal  Register / Vol. 57, No. 109 / Friday,  June 5. 1992  /  Notices
 metabolites are also subject to scale-
 affected clearance processes. In humans
 a metabolite may be formed more
 slowly, but the amount that is formed
 persists longer, resulting in similar
 AUCs as seen in rodents. The
 pharmacokinetic equivalence applies
 not only to an agent's concentration in
 blood, but also to concentrations in any
 specified organ or tissue. Thus, the
 scaling applies to the AUC of the
 ultimate carcinogenic species (be it
 parent compound or metabolite) at the
 particular site in the body that
 constitutes the target of carcinogenesis
 (presuming the target site to be the same
 across species).
  The proportion of the administered
 dose that ends up having any particular
 ultimate fate (e.g., being excreted
 unchanged, being metabolized by a
 particular biochemical pathway at a
 particular site, being excreted as a
 conjugate in the urine, etc.) is predicted
 to be the same independent of species.
 That is, if a mouse given 10 mg/kg of an
 agency ends up metabolizing 4 mg/kg
 into a form that has an AUC in the
 spleen of 100 (mg/L)«min, then the
 allometric prediction for a human given
 10 mg/kg is that 4 mg/kg will be-
 metabolized, but the AUC in the spleen
 will be 700 (mg/L)»min, owing to the
 metabolite's slower clearance.
  A difficult situation arises when the
 active carcinogen is neither the parent
 compound nor a stable metabolite, but
 rather a very reactive metabolite,
 perhaps an intermediate formed
 ephemerally during the course of
 metabolic transformation. If this
 reactive compound is removed by
 spontaneous reaction (rather than
 further enzymatic processing) and if
 such spontaneous reaction is so rapid
 that the moiety never leaves the tissue
 in which it is formed, then the removal
 rate may no longer be species-
 dependent; instead, it may hinge only on
 physicochemical properties of the
 reactant and its milieu. In such a case,
 without species differences in
 persistence,  the AUC of the reactive
 moiety in its tissue of formation may be
 proportional to the amount formed. Such
 AUCs would tend to be equalized when
 doses are scaled to body weight, rather
 than to W 3/ 4 (Travis, 1990).
  It may be well to reiterate at this point
that the reason for constructing these
general allometric arguments is to
predict the AUC of the proximate
carcinogenic agency at its site of action
in those cases (which constitute the
majority of cases at present) for which
no better means exists to determine
relative target tissue doses in rodents
and humans. Clearly, if better  means
 exist to characterize target tissue
 exposures, they should take precedence.
 Pharmacokinetic modeling of a
 particular compound may demonstrate
 that the allometric presumptions are in
 error. Two possible causes of such error
 are: (a) species differences in metabolic
 processing that do not adhere to the rule
 of proportionality to W3/ 4, and (b)
 saturation of metabolism in one but not
 the other species as a result of
 comparing markedly different dose
 levels or dosing regimens. The
 importance of the "reactive metabolite"
 scenario outlined in the previous
 paragraph is best determined by case-
 specific characterization  of metabolic
 activation and its effects.
 Macromolecular adducts may be
 particularly useful in this regard since,
 under certain circumstances (including
 negligible repair), their accumulation in
 a tissue would be expected to be
 proportional to the AUC of the adduct-
 forming moiety in that tissue.
  It must be conceded that, in actuality,
 mice and rats are not simply scale-
 model humans; certain particular
 characteristcs (metabolism among them)
 do not necessarily vary in a simple way
 with body size. However, the long-
 standing toxicological practice of using
 rodent exposures to toxic agents as
 surrogates for the human experience
 rests on the belief that, to a first
 approximation, the similarities that stem
 from a shared mammalian anatomy and
 physiology outweigh the differences.
 The species differences in size, uptake
 rates, basal metabolism, blood flows,
 organ sizes, and so on are clearly
 important to acknowledge in any
 dosimetric scheme. The allometric
 arguments adduced here attempt to
 construct a logical and consistent
 framework for investigating cross-
 species dosimetry. This framework
 provides a basis for articulating the
 expected consequence of those broad
 general patterns of cross-species
 difference in size scale and time scale
 that we understand, while providing
 rebuttable default positions for those
 aspects, such as chemical-specific
 metabolism, that are less  well
 2. Species Differences in
  The overall aim of dose scaling is to
 achieve toxicological equivalence across
 species. The foregoing section discussed
pharmacokinetic equivalence. For such
results to be useful for carcinogen risk
 assessment—that is, to complete the
equation  of exposure and tumorigenic
response—it remains to determine what
toxicological consequences to expect
from given target tissue exposures in
 humans and animals. As argued earlier,
 the principles of pharmacodynamic
 equivalence are far from self-evident.
   The issues about pharmacodynamic
 equivalence fall into three categories.
 First, the appropriate measures of
 "delivered dose" would seem to depend
 on details of the mechanism of toxic
 action, details that are frequently poorly
 understood. In the foregoing section,
 scaling of administered doses was
 discussed in terms of tendency to
 equalize the AUC, an integrated
 measure of target tissue concentration.
 Although this is a frequent and widely
 accepted measure of a  target organ's
 exposure to a toxin (Voisin, et al., 1990),
 its use as a measure of carcinogenic
 equivalence of .doses rests on the
 presumed proportionality of the rates of
 toxicological reactions to the AUC. If
 the underlying reactions that comprise
 the process of carcinogenicity are
 markedly nonlinear with target-tissue
 concentration, if they include capacity-
 limited steps or magnitudes below
 which significant stress on the system is
 absent, then proportionality of toxic
 response to the AUC (or to any other
 easily characterized summary measure
 of target-tissue exposure) becomes
 problematic. Thus, use of the AUC as an
 "equivalent" tissue dose should be
 regarded as a default that corresponds
 to the presumption that the processes
 constituting carcinogenicity operate in
 proportion to the concentration of the
 carcinogen at the target. In particular
 applications, this assumption should be
 critically examined, and relevant data
 brought to bear, if possible.
  The second issue returns to the
 question of scale. For corresponding
 organs bathed in an equal concentration
 of carcinogen, a human will have many
 more target cells exposed than a rodent,
 only one of which need be transformed
 to found a tumorigenic clone. Moreover,
 during the course of a full lifetime under
 this dosing regime, a human's cells will
 be exposed for much longer and undergo
 many more cell divisions (NAS, 1975;
 U.S. EPA, 1987a). Although this would
 seem to suggest a much larger
 sensitivity to carcinogens in larger
 species, the empirical evidence shows
 instead a rough lifetime-to-lifetime
 equivalence across species of both the
 magnitude of spontaneous cancer risk
 and the age pattern of its appearance.
 When arguments from first principles
 lead to answers that are clearly off
 track, it indicates that key factors have
not been brought into consideration. In
 this case, the role of species differences
in repair processes may enter. Also, the
number of cells (or cell divisions) at risk
may be less different among species

Federal  Register / Vol. 57, No.  109 / Friday, June 5,  1992 / Notices
than presumed, owing to slower
turnover, stem cell populations that are
not proportional to tissue volume, or
other factors. The point is raised here
simply to emphasize that size and
timespan differences across species may
have key roles in comparative
pharmacodynamics just as they do in
comparative pharmacokinetics, although
the particulars are not clear at present.
In the face of this  difficulty, it has been
the ususal practice to assume lifetime
equivalence when projecting
carcinogenesis patterns across species,
an assumption that has held up well in
experience. This point will be returned
to below.
  The third issue in pharmacodynamic
equivalence also parallels one in
pharmacokinetics—that of the
uniqueness and species-specificity of
carcinogenic responses that tends to
obscure overall trends and patterns. The
pharmacodynamic reasons for
differences in sensitivity of potential
target organs among species are perhaps
more obscure than the pharmacokinetic
reasons, but they  surely exist. As with
the case-by-case particulars of
pharmacokinetic processes, the
idiosyncratic and species-specific
variations in  responsiveness to
carcinogenic  stimuli create an
unavoidable envelope of uncertainty
around the predictions of a scaling
methodology that can only characterize
the average behavior of carcinogens
overall. When data are available that
enable the investigator to incorporate
knowledge of species differences in the
carcinogenic  reactions to a given level
of target-tissue dose, they should be
considered in the analysis and
incorporated when appropriate.
   Although certain pieces of the puzzle
of cellular and molecular biology that
underlie carcinogenesis are known, and
despite rapid progress,  it not yet
possible to undertake a detailed
analysis of the magnitudes and causes
of species differences in the
carcinogenic process. At present, there
can be no empirical and allometric
characterizations of general cross-
species trends, as has been done in this
report for the pharmacokinetic part of
the equation. One can,  however, make
use of the observation of general
lifetime-equivalence, noted above, to
suggest how  the insights of cross-species
patterns in pharmacokinetics  might be
applied to the question of toxicological
3. Toxicological Equivalence
   When experimental animals and
humans are exposed to a chemical in
such a way that they experience equal
areas-under-the-curve of the proximate
                  carcinogenic agent (be it the parent
                  compound, a metabolite, or a reactive
                  intermediate of metabolism) at the
                  target of toxic action, then they will
                  have their susceptible tissues exposed
                  to equal average concentrations of the
                  carcinogen over the exposure period.
                  Over the course of a full lifetime of
                  exposure, the lifetime average target-
                  tissue concentrations are equal
                  (although the total accumulated AUC is
                  larger in humans, by virtue of their
                  longer lives). The earlier discussion of
                  pharmacokinetics argued that,  if daily
                  administered doses are scaled  in
                  proportion to W3/ * (or if exposures of
                  equal duration are equated on  a ppm
                  basis), such equality of resulting AUCs
                  tends to result across mammalian
                    If the empirical principle of lifetime-
                  to-lifetime equivalence is applied, then a
                  possible presumption is that such
                  pharmacokinetically equivalent lifetime
                  exposures (in terms of equal average
                  concentrations of the carcinogen at its
                  target) should be equivalent in the
                  degree of lifetime cancer risk they
                  engender (although other interpretations
                  of the consequences of pharmacokinetic
                  equivalence are possible). That is, it
                  may be assumed that equal carcinogen
                  concentrations at the target lead to
                  equal degrees of impact at the  cellular
                  level which, if continued for a lifetime,
                  yield equal lifetime probabilities that a
                  tumor will be caused in that target
                    The reasons for approximate lifetime
                  equivalence in the carcinogenic process
                  among species of different body size and
                  lifespan are not clear. One can,
                  however, rationalize this observation by
                  extending the concept of physiological
                  time from pharmacokinetic processes to
                  cover pharmacodynamic processes as
                  well. The following section explores this
                  4. A Physiological Time Approach to
                  Toxicological Equivalence
                    It is helpful to begin by considering
                  the case of "zero" dose, i.e., by
                  examining background or spontaneous
                  carcinogenesis. Although  the common
                  cancer types differ somewhat, humans
                  and experimental animals have roughly
                  similar lifetime cancer rates. Moreover,
                  the latency periods are greatly different
                  in animals and humans, but in a  way
                  that is roughly proportional to lifetime.
                  Age-specific incidences are also roughly
                  parallel when time is measured not in
                  years, but on a lifetime scale (Cutler and
                  Semsei, 1989). If these equivalencies
                  were not so, we would either never see
                  tumors in experimental animals (since
                  they would die of other causes before
                  the 20-to-40 year latency was
completed), or we would find humans to
be overwhelmed with spontaneously
arising tumors during childhood. These
results from spontaneous carcinogenesis
appear to be paralleled by chemically
induced cancers, in that such cancers
also arise and progress on a "lifetime"
time scale in experimental animals and
  The above results suggest that
carcinogenesis proceeds more slowly in
larger animals, in a way that makes its
progress roughly constant per lifetime,
rather than per unit of clock time. This is
in accord with the current risk
assessment practice of equating lifetime
cancer incidences in humans and
rodents. It would seem that the concept
of physiological time—that large
animals carry on their life processes at
an overall slower pace than smaller
ones—proves as useful in examining
pharmacodynamics as it does for
pharmacokinetics. As argued in the
previous section, the rates of the
underlying pharmacokinetic processes
tend to operate in proportion to a size-
dependent physiological time "clock,"
which allows appropriate scaling to
explain and correct for species
differences in pharmacokinetic end
points." In the case of carcinogenesis,
the component physiological features
and processes are less easily observed,
but the  "pharmacodynamic end point"
can be seen in the above-mentioned
cross-species patterns of spontaneous
carcinogenesis. In sum, not only may
"pharmacokinetic time" vary among
species in a regular way,
"pharmacodynamic time" may do so as
well. Total lifespans  of different species
generally scale in rough proportion to
W ll 4 (Sacher, 1959; Lindstedt and
Calder, 1976,1981). (In terms of the
physiological time concept, the
"processes of living" that proceed at a
rate proportional to W 3l *—or on a per
kg basis, to W~lli—go slower in a
larger animal, and so take chronological
time in proportion to W114 to go "to
completion.") Hence, the two
physiological time scales are quite
similar. However, humans live longer
than their allometric prediction by about
a factor of five.
   The above discussion of
pharmacodynamics suggests that
carcinogenesis (in common with other
physiological processes) proceeds more
slowly in humans than in rodents, in a
way that tends to be equivalent on a
lifetime basis. Together with the
pharmacokinetic results outlined
earlier—namely, that scaling daily
administered doses in proportion  to
 W3l 4 tends to result in
"pharmacokinetically equivalent"

                      Federal  Register / Vol. 57, No. 109 / Friday,  June 5. 1992 /  Notices
 exposures to corresponding organs and
 equal steady-state concentrations of
 agents and their metabolites—this
 suggests that administered doses of
 carcinogens be considered equal in
 lifetime risk when expressed in units of
 mg/kg 3/ 4/day. One possible
 interpretation of this line of reasoning is
 that tissues experiencing equal average
 concentrations of the carcinogenic
 moiety over a full lifetime should be
 presumed to have equal lifetime cancer
 risk. Under the arguments on
 pharmacokinetic allometry set out
 earlier, such equality of average
 concentrations would tend to be
 produced by daily administered doses
 scaled in proportion to W314. However,
 if the pharmacokinetically equivalent
 doses can be obtained by experimental
 means, under this line of reasoning, such
 results could replace the allometric
 presumptions, and equal risks would be
 expected when average daily AUCs are
 equal (or equivalently, when average
 concentrations are equal). If the default
 allometrically based  assumptions about
 pharmacokinetics are adhered to by a
 particular compound, the introduction of
 data in place of assumptions will leave
 the answer unchanged. Other
 interpretations of the question of the
 cross-species toxicological equivalence
 of delivered doses  are possible, and the
 issue remains one on which further
 insight would be helpful.
  If we use a scale of pharmacodynamic
 time based on the equivalence of
 lifetimes, then the 35-times larger
 exposure of human tissues to
 carcinogens that results from a lifetime
 of doses scaled by mg/jy3/ 4/day
 results in an equal lifetime cancer risk
 because the affected  physiological
 processes of carcinogenesis themselves
 are operating more slowly (by
 assumption, 35-times more slowly). A
 given span of clock time that a tissue
 spends under a given concentration
 regime yields less risk in a human (since
 the tissue has spent less
 "pharmacodynamic time" exposed).
  It should be clear that not every
 empirical measure  of "internal dose" is
 equally informative about species
 differences. As noted earlier, the amount
 of a dose metabolically activated, for
 example, may be equal in a mouse and a
human, but the human's AUC of
metabolite at the target may be much
larger. If an empirical measurement or
modeled result is to be used as a
surrogate for "internal dose" in a  cross-
species extrapolation, its value in
animals and humans should be
compared to the predictions of the
default assumptions of allometrically
scaled pharmacokinetics (which should
 be aided by a full analysis of the
 uncertainties in the available data and
 of reasonably likely alternative
 pharmacokinetic modeling approaches).
 With this kind of analysis, it is possible
 to judge whether those default
 assumptions have actually been
 contradicted by data for the case at
   Once again it should be stressed that
 the arguments set out here are intended
 as defaults. They attempt to gauge the
 expected effect of known major cross-
 species trends in the rates and
 magnitudes of the underlying
 physiological processes, both in the
 internal disposition of a dose and its
 subsequent carcinogenic effect. Just as
 the pharmacokinetic presumptions may
 be able to be replaced with sufficiently
 validated case-specific modeling, the
 pharmacodynamic presumptions may be
 replaced with suitable biologically
 based dose-response models. The true
 pharmacodynamic situation is clearly
 more complex than represented here. In
 particular, there may be dose-rate
 effects,  in which higher concentrations
 have more-than-proportionally stronger
 effect (Hattis, 1990). The effect of one
 moment's exposure may also depend on
 age or on the degree of exposure earlier
 in life. Such effects have no
 generalizable patterns, however, and
 cannot serve as a basis for default
 scaling of effects. Again, we seek a
 simple default principle to guide our
 expectations, while allowing for the use
 of case-specific experimental or
 epidemiologic insights (when available)
 to improve the estimate based on the
 simplifying assumptions.
   It should also be pointed out that this
 scheme, with its explicit treatment of
 time, pharmacokinetics, and
 pharmacodynamics, provides a
 conceptual framework for examining
 such crucial emerging issues as risks
 from partial lifetime exposures,
 potencies in children vis-a-vis adults,
 and other similar questions. Failing to
 provide  such an explicit argument from
 stated assumptions dooms a scaling
 factor to be inapplicable to such
 questions and provides no means for
 incorporating biological insights, such as
 data on  pharmacokinetics and
mechanism of action, when they are

III. Discussion
  This proposal aims at arriving at a
very broad generalization about
carcinogen exposures that can be
considered of equal risk in experimental
animals and humans—one that can be
applied to potentially carcinogenic
chemicals lacking adequate information
on pharmacokinetics and mechanisms of
 action. It attempts to provide a rational
 basis for a prima facie characterization
 of potential risks in humans, consistent
 with our empirical knowledge of
 carcinogen potencies in animals and
 humans and with the known general
 consequences of species variation in
 body size and the rates of physiological
   To achieve this wide applicability and
 generality, it is necessary to rely on
 simplified, broad patterns and trends of
 biological variation, while bypassing
 many details and causes of case-by-case
 variation. This is not to deny the
 importance of these details, nor to
 denigrate the value of case-specific data
 that show species- or dose-related
 differences in uptake, metabolism, or
 physiological actions of putative
 carcinogenic agents. To the contrary, the
 intention is to provide a framework for
 the use of such data, allowing (and
 indeed, encouraging) one to go beyond
 the prima facie case based on overall
 trends to address the impact of specific
 knowledge about the chemical and its
   The empirical data on carcinogen
 potencies estimated in various animal
 species and in humans demonstrate the
 large variability involved. Although
 scaling doses by W3l 4, as proposed
 herein, characterizes the trend fairly
 well, individual chemicals may deviate
 from this overall pattern by two orders
 of magnitude or more in either direction.
 In the case of the allometric arguments,
 there are dozens of points in the chain of
 inference where one could raise
 counterexamples to simplifying
 assumptions, arguing that the
 generalized W 3l 4 scaling method
 thereby would over- or underestimate
 human risks for that case. For example,
 Gillette (1985) lists  a number of
 physiological factors with high
 variability that would influence the
 accuracy of extrapolation of a dose's
 toxicity to an exposed human, not the
 least of which is the 20-to-50-fold
 variation among individual humans in
 their ability to take up and metabolize
 an agent and to repair any resulting
  The existence of such underlying
variation means that the extrapolation
of chemically induced risks observed in
one circumstance (say, in a mouse
lifetime cancer bioassay) to another
(say, to people exposed to
environmental pollutants) needs to be
carefully and properly interpreted.
Clearly, the projection of an equivalent
dose is not merely a conversion of units,
with the resulting human dose achieving
an equal factual standing to the original
animal observation. The projection is an

Federal  Register / Vol. 57, No. 109 / Friday,  June 5, 1992  /  Notices
hypothesis, formulated in the face of
uncertainty. In the most basic case—
when there is little additional
information that may be brought to
bear—this hypothesis is framed in terms
of the general features of anatomical
and physiological differences among
species that should affect all chemicals.
It represents a best guess  based on
general principles and the recognition of
overall trends. This best guess is
surrounded by an envelope of
considerable uncertainty, owing to the
dozens of particulars that make each
chemical's  disposition and toxic effects
in various species unique, despite the
overall trends. When applicable
pharmacokinetic and mechanistic
insights into the particular chemical and
its actions are available, they can (and
should) be  used to refine the projections
by identifying and accounting for these
chemical-specific factors.
  Every projection of human equivalent
dose, no matter how sophisticated,  will
have associated with it both uncertainty
and variability. The uncertainty
concerns whether the scaling method
employed has correctly embodied and
utilized the information at hand (be it
general cross-species trends over all
chemicals or case-specific insights from
pharmacokinetics and mechanistic
studies). The variability arises because
even a sophisticated projection, when
applied to a population of cases,  will at
best predict the mean of an array of
actual values that reflect  the myriad
individual  factors that no analysis can
completely take into account. The "true"
dose of equivalent risk will vary  among
exposed humans according to how  each
individual  deviates from the overall
human norm, owing to genetic factors,
environmental influences, age, sex,
lifestyle, and countless details of
personal history.
  The goal of a cross-species scaling
methodology, then, is not to arrive at
"true" values of equivalent does under
all circumstances (for this is impossible,
even in principle). Rather, it is to
embody correctly and without bias the
impact of the information at hand,
providing rational estimates that take
into account what is known, recognizing
that true values will vary around this
estimate as a result of case-by-case
particulars, many of which are either
unknown to vary among  the individuals
 for whom  the projections are being
   The proposed scaling of daily
 administered doses of putative
 carcinogens by W 3l 4 is intended to be
 such an unbiased projection; i.e., it is to
 be thought of as a "best" estimate  rather
 than one with some conservatism built
                  in to assure that any error is on the side
                  of being overly protective. It should not
                  be interpreted as a "safety factor" or
                  other intentional bias designed to "err
                  on the side of safety." Thus, it is to be
                  expected that some individual
                  compounds will have their human
                  potencies overestimated by this
                  procedure, while others will have them
                    This having been said, it must be said,
                  it must be acknowledged that there is
                  considerable uncertainty about the best
                  scaling method to achieve this unbiased
                  projection. In particular, the empirical
                  data on comparative carcinogen
                  potencies are also compatible with both
                  body weight and surface area scaling,
                  the methodologies that we propose to
                  abandon in favor of W3l * scaling. The
                  W3/ 4 scaling is chosen  both to achieve
                  unity of default methods and because it
                  can be related to an explicit rationale
                  based on allometric variation of the
                  underlying anatomy and physiology.
                  Former methodologies have not been
                  shown to be false, however, and it is
                  considered that risk assessments
                  conducted under these methodologies
                  are  not in need of revision on account of
                  any agreement to utilize a common
                  methodology in the future.
                    The utility of the "physiological time"
                  concept for understanding the patterns
                  of cross-species differences in a
                  carcinogen's action lies in its simplicity
                  and generality. Because organ volumes
                  tend to share a common pattern of
                  allometric variation, while rates of
                  physiological processes share another,
                  the general predictions  of cross-species
                  differences is independent of specific
                  hypotheses about target organs or
                  mechanisms of action. One could, for
                  instance, envisage an alternative
                  allometric formulation that, rather than
                  relying on overall patterns for
                  unspecified organs in all mammals,
                  focuses instead on the details of specific
                  organs (common target organs or sites of
                  metabolic transformation, say) in
                  specific laboratory animal strains and in
                  humans. For example, instead of relying
                  on the approximation that breathing
                  rates vary as W3l 4, one could make
                  precise measurements of rates in
                  B6C3F1 mice and in the humans whose
                  risks are being evaluated. The utility of
                  such an approach for a default scaling
                  factor is doubtful, however, since the
                  generality of the argument is lost, and
                  the analysis becomes contingent on the
                  details of the specific physiological
                  hypothesis being elaborated. If such
                  specificity is possible in an individual
                  instance, it should become part of the
                  case-specific pharmacokinetic and
pharmacodynamic analysis that
overrides the default methodology.
  It is sometimes suggested that there
should be more than one "default"
scaling methodology, with different
generalized procedures to be applied to
different classes of chemical
carcinogens. At present, it is not clear
how such division  of cases would be
made, however, nor what the
consequences on a generalized method
should be. For example, tissue area-
under-the-curve of the toxic moiety
would seem to be the best prima facie
dosimeter for the effects of both
genotoxic and non-genotoxic
carcinogens on their target organs.
Similarly,  the general allometric
arguments for how AUCs are  expected
to vary across species apply both to
agents active as the parent compound
and to those requiring metabolic
   A possible exception to this pattern
has been mentioned earlier. The
generalized allometric pattern assumes
that the rate of clearance of a metabolite
from the target site of toxic action, like
other rates, scales in proportion to W3l 4.
If a  compound acts through a very
reactive metabolite that is
spontaneously and fully deactivated by
purely physical-chemical processes
within the target tissue itself,  then the
rate of detoxification may be  species-
independent, and the AUC may be more
related to the amount metabolized,
which by  default is expected  to retain
proportionality to  body mass  (Travis,
1990). Such a situation is not only
plausible, it may be frequent. There is
no particular indication from  the
empirical data, however, that different
rules apply to metabolically activated
compounds. Moreover, since  the
reactive intermediate scenario breaks
the symmetry of the physiological time
argument, it is difficult to know exactly
what the carcinogenic consequences
should be. This remains an important
problematical area that requires future
attention. For the present, however,
there do not seem to be grounds for
specifying when and how one should
alter the default proposal.
   The analysis presented herein is
oriented around scaling doses so as to
yield equal areas under the carcinogen's
concentration curve at the target site.
This definition of  equivalence of target
 "doses" is in line with common practice.
The AUC provides a measure of the
 agent's opportunity to interact with the
 target. Equal AUCs  over a fixed time
 interval correspond to equal  average
 concentrations of the agent during that
 interval. It should be borne in mind,
 however, that other measures of target

                      Federal Register / Vol. 57. No. 109 /  Friday. June 5. 1992 / Notices
tissue dose might be more appropriate
for specific mechanisms of
carcinogenicity. For example, if a critical
concentration must be reached or if
there is a nonlinear dependence of toxic
stress on concentration of the agent.
Such alternative have no generalizable
consequences or patterns, however, and
there is no evident way to bring them
into  a default methodology. When case-
specific pharmacokinetic analysis is
undertaken, careful attention should
also  be paid to the measure of target
tissue dose that is being considered to
yield equivalent lifetime carcinogenic
effect,  and alternatives should be
  When AUCs from daily exposures are
equal,  then average concentrations of
the agent at the target sites are equal.
And when dosing producing equal daily
average concentrations is continued for
a lifetime, then average lifetime
concentrations are equal. If one
presumes that such average lifetime
concentrations yield equal cancer risk,
then the argument follows common
practice and is in accord with the
general finding that age-specific tumor
incidence patterns tend to be congruent
across species when expressed on a
lifetime scale. (Other presumptions
about the impact of such equal
concentrations can be held, however.)
The  underlying biological basis for
lifetime equivalence, and the conditions
under which it might be violated, are not
clear at present. This is an area in need
of further investigation, and increased
understanding will be key to
determining how to scale the results of
cell-kinetically based models of
carcinogenesis from animal models to
  It  should be borne in mind that the
arguments for scaling doses by W3/ 4
have been cast in very general terms to
reflect constant, low-level, lifetime
dosing and consequent lifetime cancer
risks. Care should be taken when
applying the methodology to specific
exposure scenarios that deviate from
this  pattern. For example, the allometric
arguments are adduced for variation
among mammals. Other groups of
animals have their own characteristic
allometric patterns, but they are
different than the mammalian ones. To
extrapolate across classes of
vertebrates with the proposed
methodology, for example, would violate
the basic presumption of the variation in
a basically similar anatomical and
physiological plan among differently
sized mammals.
  The allometric patterns relied on by
the present argument represent variation
among species for adult organisms.
Allometric patterns among variously
sized individuals of the same species
can (and generally do) differ from the
pattern seen from one species to
another. The metabolic and lifespan
patterns across species do not really
describe  variation among differently
sized humans, for example. In other
words, the scaling arguments presented
here do not necessarily apply for the
adjustment of doses to larger and
smaller humans. In such cases, it is
probably preferable to use mg/kg
scaling (although the difference between
this and W 3l * scaling is minor).
Similarly, the allometric patterns
describing the changes within an
individual as he or she grows and
matures from child to adult generally
differ from both the cross-species
pattern and from the variation among
differently sized adults. Compared to
adults, children do have faster
metabolic rates and greater intakes of
food, water and air per unit of body
weight, but these relations are not well
described by proportionality W3l 4, as
they are across species. Moreover,
children also have proportionally faster
rates of cell division  (i.e., both
pharmacokinetic and pharmacodynamic
time are accelerated compared to
adults). This a complex and problematic
issue that is beyond the scope of the
present document. It is deserving of
further study. At present, it seems most
reasonable to follow current practice,
i.e., to  scale doses for adults and
children  (and for differently sized
adults) on a mg/kg basis. For similar
reasons,  the present scaling arguments
provide no special insight into the
problem  of partial lifetime exposures.
  Finally, it should be borne in mind
that the scaling arguments are made for
similar levels and patterns of exposure
in animals and humans. When
experimental animals are exposed to
much higher levels than humans (as is
common in carcinogenicity bioassays)
there is the possibility of saturation of
metabolism in animals that is not shared
with human exposures. Such effects will
obscure the usual pattern of equivalence
of internal doses projected on the
assumption of similar exposure regimes.
In other words, dose scaling cannot
solve the high-to-low-dose extrapolation
problem, which must be addressed by
other means. Case-specific
pharmacokinetic analysis can, however,
provide very valuable insight into
differences in target tissue doses
between rodents at high bioassay
exposures and humans at much lower
IV. Conclusions

  This notice is an announcement of a
consensus reached by the
Environmental Protection Agency, the
Food and Drug Administration, and the
Consumer Product Safety Commission
to consider that lifetime cancer risks
will be presumed to be equal when daily
amounts administered are in proportion
to body weight raised to the 3/4 power.
It should be reiterated that former
methodologies have not been shown to
be in error, and this agreement should
not be construed as overturning those
practices with one of superior scientific
  The empirical data on comparative
carcinogenic potencies in different
species support the general practice of
scaling rodent potencies to humans,  and
show that, on average,  current methods
perform rather well. The data are not of
sufficient resolution, however, to
distinguish between surface area and
body weight dose scaling. The data are
fully consistent with the proposal
contained herein for scaling by body
weight to the 3/4 power.
  Theoretical support for scaling
carcinogen doses by the 3/4 power of
body weight is available from analysis
of the allometric variation of key
physiological parameters across
mammalian species. Such an analysis
has the benefit of providing an
articulated rationale for the scaling
methodology and of setting out the
underlying assumptions explicitly.

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[FR Doc. 92-13207 Filed 6-4-92; 8:45 am]
*U.S. Government Printing Office  : 1992 - 312-014/40149