NCEA-C-1743
November 2006
Toxicological Reviews of Cyanobacterial
Toxins: Anatoxin-A
National Center for Environmental Assessment
U.S. Environmental Protection Agency
26 West Martin Luther King, Jr. Drive
Cincinnati, OH 45268
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DISCLAIMER
This report is an external draft for review purposes only and does not constitute Agency
policy. Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
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TABLE OF CONTENTS
Page
LIST OF TABLES vi
LIST OF FIGURES vi
LIST OF ACRONYMS vii
PREFACE viii
AUTHORS, CONTRIBUTORS AND REVIEWERS ix
ACKNOWLEDGMENTS x
1. INTRODUCTION 1
2. CHEMICAL AND PHYSICAL INFORMATION 3
3. TOXICOKINETICS 4
3.1. ABSORPTION 4
3.2. DISTRIBUTION 4
3.3. METABOLISM 4
3.4. ELIMINATION 4
3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS 4
4. HAZARD IDENTIFICATION 5
4.1. STUDIES IN HUMANS - EPIDEMIOLOGY, CASE REPORTS,
CLINICAL CONTROLS 5
4.2. ACUTE, SHORT-TERM, SUBCHRONIC AND CHRONIC
STUDIES AND CANCER BIO AS SAYS IN ANIMALS - ORAL
AND INHALATION 6
4.2.1. Oral Exposure 6
4.2.1.1. Acute Studies 6
4.2.1.2. Short-Term Studies 6
4.2.1.3. Subchronic Studies 8
4.2.1.4. Chronic Studies 8
4.2.2. Inhalation Exposure 8
4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES - ORAL AND
INHALATION 8
4.3.1. Oral Exposure 8
4.3.2. Inhalation Exposure 9
4.4. OTHER STUDIES 9
4.4.1. Neurotoxicity by Parenteral Exposure 9
4.4.2. Other Effects by Parenteral Exposure 12
4.4.3. Effects by Intranasal Instillation 12
4.4.4. In Vitro Studies 12
4.4.5. Interactions with Other Cyanotoxins 13
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TABLE OF CONTENTS cont.
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4.5. MECHANISTIC DATA AND OTHER STUDIES IN SUPPORT OF THE
MODE OF ACTION 13
4.5.1. Mechanisms of Neurotoxicity 13
4.5.2. Structure-Activity Relationships 14
4.5.3. Other Studies 14
4.6. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER
EFFECTS 15
4.6.1. Oral 15
4.6.2. Inhalation 20
4.6.3. Mode of Action Information 20
4.7. WEIGHT-OF-EVIDENCE EVALUATION AND CANCER
CHARACTERIZATION 20
4.7.1. Summary of Overall Weight-of-Evidence 20
4.7.2. Synthesis of Human, Animal, and Other Supporting Evidence 20
4.8. SUSCEPTIBLE POPULATIONS AND LIFE STAGES 20
4.8.1. Possible Childhood Susceptibility 20
4.8.2. Possible Gender Differences 20
4.8.3. Other Possible Susceptible Populations 21
5. DOSE-RESPONSE ASSESSMENTS 22
5.1. NARRATIVE DESCRIPTION OF THE EXTENT OF THE DATABASE 22
5.2. ORAL REFERENCE DOSE (RfD) 23
5.2.1. Data Considered in Deriving Reference Values 23
5.2.2. Acute Duration 23
5.2.2.1. Choice of Principal Study and Critical Effect - with
Rationale and Justification 23
5.2.3. Short-Term Duration 23
5.2.3.1. Choice of Principal Study and Critical Effect - with
Rationale and Justification 23
5.2.3.2. Methods of Analysis - Including Models 24
5.2.3.3. RfD Derivation - Including Application of Uncertainty Factors 24
5.2.4. Subchronic Duration 25
5.2.4.1. Choice of Principal Study and Critical Effect - with
Rationale and Justification 25
5.2.4.2. Methods of Analysis - Including Models 25
5.2.4.3. RfD Derivation - Including Application of Uncertainty Factors 25
5.2.5. Chronic Duration 26
5.2.5.1. Choice of Principal Study and Critical Effect - with
Rationale and Justification 26
5.2.6. Route-to-Route Extrapolation 26
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TABLE OF CONTENTS cont.
age
5.3. INHALATION REFERENCE CONCENTRATION (RfC) 27
5.4. CANCER ASSESSMENT 27
6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD
AND DOSE RESPONSE 28
6.1. HUMAN HAZARD POTENTIAL 28
6.2. DOSE RESPONSE 28
7. REFERENCES 29
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LIST OF TABLES
No. Title Page
2-1 Experimental Property Data for Racemic (+/-)-Anatoxin-a Hydrochloride
and Anatoxin-a Fumarate 3
4-1 Summary Results of Major Oral Toxicity Studies of Anatoxin-a in
Experimental Animals 16
LIST OF FIGURES
No. Title Page
2-1 Chemical Structure of Anatoxin-a (protonated state) 3
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LIST OF ACRONYMS
ALT Alanine aminotransferase
ASP Aspartate aminotransferase
BMD Benchmark dose
CCL Contaminant Candidate List
CI Confidence interval
ED50 Effective dose in 50% of subjects
EPA Environmental Protection Agency
FEL Frank effect level
GD Gestation day
GGT Glutamyl transpeptidase
LD50 Dose lethal to 50% of population
LOAEL Lowest-observed-adverse-effect level
NOAEL No-observed-adverse-effect level
PND Postnatal day
RfC Reference concentration
RfD Reference dose
SDWA Safe Drinking Water Act
UF Uncertainty factor
VFDF Very Fast Death Factor
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PREFACE
The Safe Drinking Water Act (SDWA), as amended in 1996, requires the Environmental
Protection Agency (EPA) to publish a list of contaminants that, at the time of publication, are not
subject to any proposed or promulgated national primary drinking water regulations, are known
or anticipated to occur in public water systems, and may require regulations under SDWA. This
list, known as the Contaminant Candidate List (CCL), was first published in 1998 and then again
in 2005. The 1998 and 2005 CCLs include "cyanobacteria (blue-green algae), other freshwater
algae, and their toxins" as microbial contaminants.
In 2001, a meeting was held among EPA, researchers from the drinking water industry,
academia and government agencies with expertise in the area of fresh water algae and their
toxins. The goal of this meeting was to convene a panel of scientists to assist in identifying a
target list of algal toxins that are likely to pose a health risk in source and finished waters of the
drinking water utilities in the U.S. Toxin selection was based on four criteria: health effects,
occurrence in the United States, susceptibility to drinking water treatment and toxin stability.
Anatoxin-a was identified at this meeting as being a toxin of high priority based on those criteria.
The National Center for Environmental Assessment has prepared this Toxicological
Review of Cyanobacterial Toxins: Anatoxin-a as one in a series of dose-response assessments to
support the health assessment of unregulated contaminants on the CCL. The purpose of this
document is to compile and evaluate the available data regarding anatoxin-a toxicity to aid the
Office of Water in regulatory decision making. It is not intended to be a comprehensive treatise
on the chemical or toxicological nature of anatoxin-a.
In Section 6, Major Conclusions in the Characterization of Hazard and Dose Response,
EPA has characterized its overall confidence in the quantitative and qualitative aspects of the
hazard and dose response by addressing knowledge gaps, uncertainties, quality of data and
scientific controversies. The discussion is intended to convey the limitations of the assessment
and to aid and guide the Office of Water in the ensuing steps of the human health risk assessment
of anatoxin-a.
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AUTHORS, CONTRIBUTORS AND REVIEWERS
AUTHORS
Belinda Hawkins, Ph.D. (Chemical Manager)
National Center for Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH
Stephen Bosch
Syracuse Research Corporation
Syracuse, NY
Marc Odin
Syracuse Research Corporation
Syracuse, NY
David Wohlers
Syracuse Research Corporation
Syracuse, NY
REVIEWERS
INTERNAL EPA REVIEWERS
Joyce Donohue, Ph.D.
Office of Water
Washington, DC
Elizabeth Hilborn, D.V.M., M.P.H.
National Health and Environmental Effects Research Laboratory
Office of Research and Development
Research Triangle Park, NC
James Sinclair, Ph.D.
Office of Water
Cincinnati, OH
Jeff Swartout
National Center for Environmental Assessment
Office of Research and Development
Cincinnati, OH
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AUTHORS, CONTRIBUTORS AND REVIEWERS cont.
EXTERNAL REVIEWERS
ACKNOWLEDGMENTS
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1 1. INTRODUCTION
2
3
4 This toxicological review presents background and justification for hazard and dose-
5 response assessments of anatoxin-a. U.S. Environmental Protection Agency (EPA) toxicological
6 reviews may include oral reference doses (RfD) and inhalation reference concentrations (RfC)
7 for chronic and less-than-lifetime exposure durations and a carcinogenicity assessment.
8
9 The RfD and RfC provide quantitative information for use in risk assessments for health
10 effects known or assumed to be produced through a nonlinear (possibly threshold) mode of
11 action. The RfD (expressed in units of mg/kg-day) is defined as an estimate (with uncertainty
12 spanning perhaps an order of magnitude) of a daily exposure to the human population (including
13 sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a
14 lifetime. The inhalation RfC (expressed in units of mg/m3) is analogous to the oral RfD, but
15 provides a continuous inhalation exposure estimate. The inhalation RfC considers toxic effects
16 for both the respiratory system (portal-of-entry) and for effects peripheral to the respiratory
17 system (extrarespiratory or systemic effects). Reference values are generally derived for chronic
18 exposures (up to a lifetime), but may also be derived for acute (24 hours), short-term (up to 30
19 days), and subchronic (up to 10% of average lifetime) exposure durations, all considered to be
20 daily exposures, continuously or intermittently, throughout the duration specified.
21
22 The carcinogenicity assessment provides information on the carcinogenic hazard
23 potential of the substance in question and quantitative estimates of risk from oral exposure and
24 inhalation exposure. The information includes a weight-of-evidence judgment of the likelihood
25 that the agent is a human carcinogen and the conditions under which the carcinogenic effects
26 may be expressed. Quantitative risk estimates are presented in three ways. The slope factor is
27 the result of application of a low-dose extrapolation procedure and is presented as the risk per
28 mg/kg-day. The unit risk is the quantitative estimate in terms of either risk per |J,g/L drinking
29 water or risk per |J,g/m3 air breathed. Another form in which risk is presented is a drinking water
30 or air concentration providing cancer risks of 1 in 10,000; 1 in 100,000; or 1 in 1,000,000.
31
32 Development of these hazard identification and dose-response assessments for anatoxin-a
33 has followed the general guidelines for risk assessment as set forth by the National Research
34 Council (NRC, 1983). EPA guidelines that were used in the development of this assessment
35 include the following: Guidelines for the Health Risk Assessment of Chemical Mixtures (U.S.
36 EPA, 1986a), Guidelines for Mutagenicity Risk Assessment (U.S. EPA, 1986b), Guidelines for
37 Developmental Toxicity Risk Assessment (U.S. EPA, 1991), Guidelines for Reproductive Toxicity
38 Risk Assessment (U.S. EPA, 1996), Guidelines for Neurotoxicity Risk Assessment (U.S. EPA,
39 1998a), Guidelines for Carcinogen Assessment (U.S. EPA, 2005a), Supplemental Guidance for
40 Assessing Susceptibility from Early-Life Exposure to Carcinogens (U.S. EPA, 2005b),
41 Recommendations for and Documentation of Biological Values for Use in Risk Assessment (U.S.
42 EPA, 1988), (proposed) Interim Policy for Particle Size and Limit Concentration Issues in
43 Inhalation Toxicity Studies (U.S. EPA, 1994a), Methods for Derivation of Inhalation Reference
44 Concentrations and Application of Inhalation Dosimetry (U.S. EPA, 1994b), Use of the
45 Benchmark Dose Approach in Health Risk Assessment (U.S. EPA, 1995), Science Policy Council
46 Handbook: Peer Review (U. S. EPA, 1998b, 2000a), Science Policy Council Handbook: Risk
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1 Characterization (U.S. EPA, 2000b), Benchmark Dose Technical Guidance Document (U.S.
2 EPA, 2000c) and Supplementary Guidance for Conducting Health Risk Assessment of Chemical
3 Mixtures (U.S. EPA, 2000d) and A Review of the Reference Dose and Reference Concentration
4 Processes (U. S. EPA, 2002).
5
6 Literature searches were conducted for studies relevant to the derivation of toxicity and
7 carcinogenicity values for anatoxin-a. The following databases were searched: MEDLINE
8 (PubMed), TOXLINE, BIOSIS, CANCERLIT, TSCATS, CCRIS, DART/ETIC, EMIC,
9 GENETOX, HSDB and RTECS. The relevant literature was thoroughly reviewed through May
10 2006.
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2. CHEMICAL AND PHYSICAL INFORMATION
Anatoxin-a is a naturally occurring toxin produced by some strains ofAnabaena
(particularly Anabaena flos-aquae} and at least four other genera of freshwater cyanobacteria
(commonly referred to as blue-green algae), including Aphanizomenon, Microcystis, Planktothrix
and Oscillatoria (Fawell et al., 1999; Viaggiu et al., 2004). A structural analog, homoanatoxin-a
(methylene-anatoxin-a), has been isolated from a strain of Oscillatoria formosa (Skulberg et al.,
1992). Anatoxin-a has a semi-rigid bicyclic secondary amine structure, 2-acetyl-
9-azbicyclo[4:2: l]non-2-ene, a molecular formula of CioHi5NO and a molecular weight of
165.26 (Lewis, 2000). Of the two enantiomeric forms of anatoxin-a, only (+)-anatoxin-a is
produced in nature. The pKa of (+)-anatoxin-a is 9.4 (Valentine et al., 1991), indicating that the
molecule is almost completely protonated at acidic and neutral pH (e.g., in natural water). The
structure of the protonated molecule is shown below.
Figure 2-1. Chemical Structure of Anatoxin-a (protonated state)
Chemical and physical property data are not available in the open literature for anatoxin-a
as the free base. Anatoxin-a is commercially produced as both hydrochloride and fumarate salts
on a small scale for research purposes (A.G. Scientific Inc., 2006; BIOMOL International LP,
2006; MacPhail et al., 2005). Both of these salts have been used in toxicity studies of anatoxin-a
because they readily ionize in water. Available experimental property data for racemic (+/-)
anatoxin-a hydrochloride and anatoxin-a fumarate are provided in Table 2-1 (Sigma, 1995); data
were not located for the (+)-enantiomer.
Table 2-1. Experimental Property Data for Racemic (+/-)-Anatoxin-a Hydrochloride and
Anatoxin-a Fumarate
Name
Molecular Formula
Molecular Weight
Appearance
Melting Point
Solubility in Water
(+/-)-Anatoxin-a Hydrochloride
CioHi5NO-HCl
187.67
Information not available
Information not available
Information not available
(+/-)- Anatoxin-a Fumarate
Ci0Hi5NO-C4H4O4
281.31
Light brown hydroscopic solid;
[a]20D=+28°(c=0.29, MeOH)
124-126°C (decomposes)
1.51xl04mg/mL
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1 3. TOXICOKINETICS
2
3
4 3.1. ABSORPTION
5
6 No quantitative data were located regarding the rate or extent of absorption of anatoxin-a
7 in humans or animals. Acute oral toxicity studies in animals indicate that anatoxin-a is rapidly
8 absorbed from the gastrointestinal tract, as shown by the occurrence of clinical signs of
9 neurotoxicity including loss of coordination, muscular twitching, and death from respiratory
10 paralysis within several minutes of exposure (Fitzgeorge et al., 1994; Stevens and Krieger,
11 1991).
12
13 3.2. DISTRIBUTION
14
15 No information regarding the tissue distribution of anatoxin-a was identified in the
16 materials reviewed for this assessment.
17
18 3.3. METABOLISM
19
20 No information regarding the metabolism of anatoxin-a was identified in the materials
21 reviewed for this assessment.
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23 3.4. ELIMINATION
24
25 No information regarding the elimination of anatoxin-a was identified in the materials
26 reviewed for this assessment.
27
28 3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS
29
30 No physiologically based toxicokinetic models have been developed for anatoxin-a.
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1 4. HAZARD IDENTIFICATION
2
3
4 Investigations into the characterization of anatoxin-a effectively began in 1961 following
5 the deaths of cows that had ingested water from a lake containing an algal bloom in
6 Saskatchewan, Canada (Carmichael and Gorham, 1978; Carmichael et al., 1975; Devlin et al.,
7 1977; Moore, 1977). Toxin-producing and non-toxin-producing unialgal colony isolates of A.
8 flos-aquae were isolated from specimens of the bloom in 1964, and toxicity testing showed that
9 the cultured toxin-producing isolates elicited the same physiological effects as the parent bloom.
10 The toxin was termed Very Fast Death Factor (VFDF) because intraperitoneal (i.p.) injection of
11 toxin-producing cells or cell culture filtrates into mice induced paralysis, tremors, mild
12 convulsions and death within 2-7 minutes. VFDF was chemically isolated and purified from
13 lyophilized cells of one of the colony isolates (NRC-44h) in 1966, the structure of VFDF was
14 determined and re-named anatoxin-a in 1977, and methods were subsequently developed to
15 synthesize both racemic anatoxin-a and optically pure (+)-anatoxin-a. Experimental studies on
16 the health effects of anatoxin-a began in the 1970s in response to poisoning outbreaks in animals
17 that drank water from lakes, ponds and reservoirs containing blooms of A. flos-aquae. Although
18 some preliminary studies were performed using suspensions of cultured cells, essentially all of
19 the pertinent toxicity studies, which are summarized below, used anatoxin-a that was purified
20 from cell extracts or synthesized de novo. Interest in anatoxin-a has continued due to concern for
21 possible negative impacts on human drinking and recreational water quality and because it has
22 been found to be a particularly useful molecule for characterizing the properties of acetylcholine
23 receptors in the nervous system.
24
25 4.1. STUDIES IN HUMANS - EPIDEMIOLOGY, CASE REPORTS, CLINICAL
26 CONTROLS
27
28 Cases of non-lethal human poisonings, predominantly manifested as acute
29 gastrointestinal disorders (e.g., nausea, vomiting and diarrhea), have been attributed to the
30 ingestion of lake water containing unspecified species ofAnabaena and Microcystis
31 (Schwimmer and Schwimmer, 1968). A number of these cases were documented by the
32 detection ofAnabaena., either alone or with Microcystis., in the feces. An allergy to Anabaena
33 was demonstrated in a young woman who developed skin papulo-vesicular eruptions whenever
34 she swam in a lake containing a bloom of the algae (Schwimmer and Schwimmer, 1968).
35
36 Anatoxin-a was implicated in the death of a 17-year-old boy who died 2 days after
37 swallowing water while swimming in a pond containing an algal bloom (Behm, 2003). The boy
38 went into shock and suffered a seizure before dying from heart failure. A companion teenage
39 boy who also swallowed some of the pond water while swimming later became sick with severe
40 diarrhea and abdominal pain but survived. Three other teenage boys who swam in the pond at
41 the same time as other two, but had not been fully submerged in the water, developed only
42 unspecified minor symptoms. Tests of stool samples from the two affected boys revealed the
43 presence of A. flos-aquae cells (Behm, 2003). Results of initial analyses of liver, blood and
44 ocular (vitreous) fluid samples from the boy who died indicated the presence of anatoxin-a but
45 were negative for other cyanobacterial toxins (microcystins, cylindrospermopsins and saxitoxins)
46 (Carmichael et al., 2004). The coroner concluded that anatoxin-a was the most reasonable cause
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1 of the death based on the available information, but a definitive diagnosis was confounded by the
2 delay between exposure and overt toxicity and the lack of other anatoxin-a -related human
3 fatalities for a temporal comparison (Behm, 2003). In particular, the time of death is inconsistent
4 with what is known about anatoxin-a toxicity as determined from laboratory animal studies (i.e.,
5 that signs of neurotoxicity and death typically occur within minutes to several hours of
6 exposure). More recent (unpublished) analyses determined that the compound detected in the
7 body fluids and liver tissue samples was not anatoxin-a but the amino acid phenylalanine
8 (Carmichael et al., 2004), further confounding the diagnosis.
9
10 4.2. ACUTE, SHORT-TERM, SUBCHRONIC AND CHRONIC STUDIES AND
11 CANCER BIOASSAYS IN ANIMALS - ORAL AND INHALATION
12
13 4.2.1. Oral Exposure
14
15 4.2.1.1. Acute Studies
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17 An acute oral (single dose gavage) LD50 value of 16.2 mg/kg (95% confidence interval
18 [CI]: 15.4-17.0) was determined for synthetic (+)-anatoxin-a hydrochloride (commercial product,
19 >98% pure) in male Swiss Webster ND-4 mice (Stevens and Krieger, 1991). This LD50 is
20 equivalent to 13.3 mg anatoxin-a/kg (95% CI: 12.8-14.1). A single dose gavage LD50 of >5
21 mg/kg was determined for anatoxin-a in newly weaned CBA/BalbC mice of unspecified sex
22 (Fitzgeorge et al., 1994); the anatoxin-a in this study was a commercial product in a "suitably
23 purified" but unspecified form. Deaths occurred within 2 minutes of gavage administration and
24 were due to neurotoxicity, with manifestations that included loss of coordination, muscular
25 twitching and death by respiratory paralysis (Fitzgeorge et al., 1994). A single dose gavage LD50
26 value of 6.7 mg anatoxin-a/kg was determined for male Swiss Webster ND-4 mice administered
27 the toxin as a lysate solution of \yophi\izedA.flos-aquae (NRC-44-1) cells (Stevens and Krieger,
28 1991).
29
30 Anatoxin-a has been implicated in case reports of poisonings and deaths in dogs,
31 livestock and waterfowl that consumed water containing blooms of toxin-producing
32 cyanobacteria (Carmichael and Gorham, 1978; Edwards et al., 1992; Gunn et al., 1992; Pybus et
33 al., 1986). Signs of toxicity were predominantly neurologic, with deaths due to respiratory
34 paralysis. Quantitative exposure data were not reported.
35
36 4.2.1.2. Short-Term Studies
37
38 A 5-day oral toxicity study was performed in which groups of two male and two female
39 Crl:CD-l(ICR)BR mice were administered aqueous (+)-anatoxin-a hydrochloride (commercial
40 product, purity not reported) by gavage in daily doses of 1.5, 3, 7.5 or 15 mg/kg (equivalent to
41 1.2, 2.5, 6.2 or 12.3 mg anatoxin-a/kg) (Fawell and James, 1994; Fawell et al., 1999). This is a
42 range-finding study that was conducted to determine the maximum tolerated dose to be used in
43 the 28-day study summarized below. The dosing of the 6.2 and 12.3 mg/kg-day groups
44 commenced approximately 24 hours after the dosing of the 1.2 mg/kg-day group. The 2.5
45 mg/kg-day group was established 5 days after dosing of the 6.2 and 12.3 mg/kg-day groups as an
46 intermediate level due to toxicity at these dosages (discussed below). No control group was
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1 included. Clinical signs, body weight and food consumption were assessed, and surviving
2 animals were necropsied. All high-dose mice and one female mouse in the 6.2 mg/kg-day group
3 died within 5 minutes of dosing during the first 4 days. Males in the 6.2 mg/kg-day dose group
4 were hyperactive following the third dose; no other signs of neurotoxicity were reported and
5 none of the other surviving animals had any abnormal clinical signs. No changes in body weight
6 or food consumption or unusual necropsy findings were observed in any animals. The highest
7 no-observed-adverse-effect level (NOAEL), 2.5 mg/kg-day (3 mg anatoxin-a
8 hydrochloride/kg-day), was selected as the maximum tolerated dose for the 28-day main study.
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10 In the main study, groups of 10 male and 10 female Crl:CD-l(ICR)BR mice were
11 administered aqueous (+)-anatoxin-a hydrochloride (commercial product, purity not reported) by
12 gavage in daily doses of 0 (vehicle control), 0.12, 0.6 or 3 mg/kg (0, 0.1, 0.5 or 2.5 mg
13 anatoxin-a/kg) for 28 days (Fawell and James, 1994; Fawell et al., 1999). Endpoints that were
14 examined included general condition and behavior (daily), body weight (weekly), food
15 consumption (weekly), ophthalmoscopic condition (final week), hematology (final week;
16 erythrocyte count, packed and mean cell volumes, hemoglobin concentration, mean cell
17 hemoglobin, mean cell hemoglobin concentration, total and differential leukocyte counts and
18 platelet counts) and blood chemistry (final week; blood urea nitrogen, glucose, alkaline
19 phosphatase, alanine and aspartate aminotransferases [ALT and ASP, respectively], total protein,
20 albumin, albumin/globulin ratio, sodium, chloride, potassium, calcium, inorganic phosphorus,
21 total bilirubin, creatinine and cholesterol). Gross pathology and organ weights (liver, kidneys,
22 adrenals and testes) were evaluated in all animals at the end of the study. Comprehensive
23 histological examinations were performed on the control and high dose groups, on animals that
24 died or were sacrificed during the study and on gross lesions from all animals. Histology was
25 evaluated in the following tissues: adrenals, aorta, brain, cecum, colon, duodenum,
26 epididymides, eyes (including optic nerves), femur (including marrow), heart, jejunum, kidneys,
27 liver (including gall bladder), lungs (including mainstem bronchi), mammary gland, mesenteric
28 lymph node, esophagus, ovaries, pancreas, pituitary, prostate, rectum, salivary gland, sciatic
29 nerve, seminal vesicles, skeletal muscle, skin, spinal cord, spleen, stomach, submandibular
30 lymph node, testes, thymus, thyroid, parathyroid, trachea, urinary bladder and uterus.
31
32 There were three deaths during the course of the study. One death was not treatment-
33 related: a male in the 0.1 mg/kg-day group was humanely sacrificed after being attacked by its
34 cage mates. One 0.5 mg/kg-day male and one 2.5 mg/kg-day female died within 2.5 hours of
35 dosing on days 10 and 14 of treatment, respectively. Both of these animals were clinically
36 unremarkable prior to death, and the postmortem examinations were unable to establish the cause
37 of death, leading the authors to conclude that a possible relationship to treatment could not be
38 ruled out. The only other effects reported in treated animals were several minor hematology and
39 blood chemistry changes that were not considered to be lexicologically significant. These
40 alterations included statistically significant (p<0.05) increases in mean cell hemoglobin in males
41 at >0.1 mg/kg-day, mean cell hemoglobin concentration in females at >0.5 mg/kg-day and serum
42 sodium in females at >0.5 mg/kg-day. The alterations also included a nonsignificant increase in
43 mean serum ASP in males at >0.5 mg/kg-day and sporadic changes in a few other blood
44 chemistry indices. The study authors concluded that the NOAEL was 0.1 mg/kg-day based on
45 the two deaths that occurred at the higher dose levels. This conclusion was based on their
46 inability to determine the cause(s) of death (i.e., to completely rule out a relationship with
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1 treatment), and they indicated that the true NOAEL may actually be 2.5 mg/kg-day. Due to the
2 low incidences of mortality that showed no dose-response or gender consistency (1/10 males at
3 0.5 mg/kg-day and 1/10 females and 2.5 mg/kg-day), the lack of characteristic clinical signs of
4 acute neurotoxicity in the two animals that died, and the absence of lexicologically significant
5 effects in the surviving mice, as well as the lack of effects at 2.5 mg/kg-day in mice as reported
6 in the 5-day study discussed above and a developmental toxicity study (discussed below) (Fawell
7 and James, 1994; Fawell et al., 1999), EPA concludes that the deaths are likely to be incidental
8 and that the actual NOAEL is 2.5 mg/kg-day.
9
10 4.2.1.3. Subchronic Studies
11
12 Groups of 20 female Sprague-Dawley rats were administered anatoxin-a in the drinking
13 water in concentrations of 0, 0.51, or 5.1 ppm for 7 weeks (Astrachan and Archer, 1981;
14 Astrachan et al., 1980). The anatoxin-a used in this study was extracted from the culture media
15 of A. flos-aquae (NRC-44-1) cells and partially purified; purity was not quantified, but the toxin
16 had a UV absorbance spectrum that qualitatively indicated that anatoxin-a was the principal UV-
17 absorbing component. The authors assumed that the test rats consumed 0.1 mL/g body weight-
18 day (based on a preliminary water consumption study), indicating that the estimated daily intakes
19 of anatoxin-a in the low and high dose rats were 0.05 and 0.5 mg/kg-day, respectively.
20 Endpoints evaluated throughout the study included clinical signs, food consumption, body
21 weight (weekly), red and total white blood cell counts (weekly) and serum enzyme activities
22 (alkaline phosphatase, ALT, gamma glutamyl transpeptidase [GGT] and cholinesterase)
23 (weekly). Endpoints assessed at the end of the exposure period included hepatic mixed function
24 oxidase activity (aldrin epoxidation in vitro), organ weights (liver, kidneys, spleen), gross
25 pathology and histology (liver, kidneys, spleen, adrenals, heart, lungs and brain). Additional
26 information regarding the design of this study was not reported. No treatment-related effects
27 were observed, indicating a free-standing NOAEL of 0.5 mg/kg-day.
28
29 4.2.1.4. Chronic Studies
30
31 No information regarding the chronic effects of oral exposure to anatoxin-a in animals
32 was identified in the materials reviewed for this assessment.
33
34 4.2.2. Inhalation Exposure
35
36 No information regarding the inhalation toxicity of anatoxin-a was identified in the
37 materials reviewed for this assessment.
38
39 4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES - ORAL AND INHALATION
40
41 4.3.1. Oral Exposure
42
43 A developmental toxicity screening study was conducted in which groups of 10 and 12
44 pregnant Crl:CD-l(ICR)BR mice were administered aqueous (+)-anatoxin-a hydrochloride
45 (commercial product, purity not reported) by gavage in doses of 0 (vehicle control) or 3
46 mg/kg-day (0 or 2.5 mg anatoxin-a/kg), respectively, on gestation days (GD) 6-15 (Fawell and
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1 James, 1994; Fawell et al., 1999). Clinical signs and body weight were recorded until day 18 of
2 gestation, at which time the maternal animals were sacrificed and necropsied. Developmental
3 endpoints appear to have been limited to numbers of implantations (live and dead) and live
4 fetuses, post implantation loss and fetal body weight, sex ratio and external abnormalities. No
5 treatment-related maternal or fetal effects were observed although it was noted that mean fetal
6 weight (male, female and total) in the treated group was marginally lower than in controls (data
7 not reported). The lack of adverse effects in dams and fetuses identifies 2.5 mg/kg-day as a free-
8 standing NOAEL for maternal and developmental toxicity.
9
10 4.3.2. Inhalation Exposure
11
12 No information regarding the reproductive or developmental effects of inhalation
13 exposure to anatoxin-a was identified in the materials reviewed for this assessment.
14
15 4.4. OTHER STUDIES
16
17 4.4.1. Neurotoxicity by Parenteral Exposure
18
19 Acute (single dose) i.p. LDso values have been determined in mice; values include 0.25
20 mg/kg (95% CI: 0.24-0.28) for (+)-anatoxin-a hydrochloride (commercial product, >98% pure)
21 (0.21 mg anatoxin-a/kg) (Stevens and Krieger, 1991) and 0.375 mg/kg for commercial
22 anatoxin-a (form and purity not reported) (Fitzgeorge et al., 1994). Lethal i.p. doses were
23 characterized by neurotoxic effects that included loss of coordination, muscular twitching and
24 death by respiratory failure within 2 minutes (Fitzgeorge et al., 1994). Another study compared
25 acute lethality in male BalbC mice that were administered single i.p. injections of (+)-, racemic
26 or (-)-anatoxin-a hydrochloride (all >95% pure) and observed for 30 minutes following dosing
27 (Valentine et al., 1991). LD50 values were determined to be 386 ng/kg (95% CI: 365-408) for
28 (+)-anatoxin-a hydrochloride (0.32 mg anatoxin-a/kg) and 913 ng/kg (95% CI: 846-985) for
29 racemic anatoxin-a hydrochloride (0.76 mg anatoxin-a/kg). No deaths or clinical signs occurred
30 in mice treated with doses of (-)-anatoxin-a hydrochloride as high as 73 mg/kg (i.e., doses 189
31 times higher than (+)-anatoxin-a hydrochloride). The approximately 2-fold potency difference
32 between (+)-anatoxin-a and the racemic mixture and the lack of toxicity with (-)-anatoxin-a is
33 consistent with mechanistic data indicating that (+)-anatoxin-a is the biologically active
34 enantiomer (see Section 4.5.2).
35
36 An incompletely reported 2-day study in mice was performed to determine the doses for a
37 neurodevelopmental study (Rogers et al., 2005). Doses of anatoxin-a (commercial product,
38 >90% purity) in distilled water ranging from 10 to 400 ng/kg were administered by i.p. injection
39 to female CD-I mice for 2 consecutive days. Another study by the same laboratory identified
40 the same commercial product and lot number as racemic (+/-)-anatoxin-a hydrochloride
41 (MacPhail et al., 2005). Individual dose levels of anatoxin-a hydrochloride included 10, 100,
42 200, 250, 300 and 400 ng/kg (0.008, 0.08, 0.17, 0.21, 0.25 and 0.33 mg anatoxin-a/kg); however,
43 it was not reported if these were the only levels tested. The authors noted that the study was
44 conducted with 18 mice, but it is unclear if this refers to total number of animals or group size.
45 Endpoints other than survival and clinical signs of toxicity were not evaluated. After one dose,
46 mortality in the 0.08, 0.17, 0.21, 0.25 and 0.33 mg/kg groups was 0, 0, 50, 100 and 100%,
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1 respectively; all rats that received a second dose survived. Observations in mice administered
2 lethal doses (>0.21 mg/kg-day) included decreased motor activity, altered gait, difficulty
3 breathing and convulsions. The onset of clinical signs was noted after 5-6 minutes and death
4 occurred within 10 minutes. Similar clinical signs (decreased motor activity level, altered gait
5 and breathing difficulties) occurred at 0.17 mg/kg and in the 0.21 mg/kg mice that survived, but
6 the convulsion stage was never reached and recovery occurred by 15-20 minutes. Additional
7 information on this study, including results for doses lower than 0.08 mg/kg, was not reported.
8
9 In the neurodevelopmental study, groups of 8-11 time-pregnant CD-I mice were
10 administered (+/-)-anatoxin-a hydrochloride (commercial product, >90% purity) via i.p. injection
11 in distilled water in doses of 0, 125 or 200 |ig/kg-day (0, 0.10 or 0.17 mg anatoxin-a/kg-day) on
12 GD 8-12 or 13-17 (Rogers et al., 2005). All mice were allowed to give birth and body weight
13 and viability of the pups were determined on postnatal days (PND) 1 and 6.
14 Neurodevelopmental maturation was assessed by testing righting reflex, negative geotaxis and
15 hanging grip time on PND 6, 12 and/or 20 in pups from dams exposed on GD 13-17. These
16 behavioral tests were only conducted in the pups exposed on GD 13-17 because this gestational
17 interval follows the onset of neurogenesis in the mouse brain (Rice and Barone, 2000). The
18 litters from the dams exposed on GD 13-17 were normalized to eight pups (four males and four
19 females) on PND 6, and a randomly selected male and female pup from each litter was evaluated
20 on each test day. Righting reflex was tested on PND 6 and 12 by gently turning the pup over and
21 holding it on its back and then quickly releasing it and measuring the time for the pup to return to
22 an upright position with all feet on the horizontal surface. Negative geotaxis was tested on PND
23 6, 12 and 20 by placing the pup on an inclined screen facing downhill and determining the time
24 to rotate to facing up the incline. Hanging grip time was tested on PND 12 and 20 by holding the
25 pups by the base of the tail above the work surface and allowing them to grasp a bar with their
26 front feet, releasing them to hang and measuring the time until each pup let go.
27
28 Maternal toxicity was observed at 0.17 mg/kg-day, as shown by decreased motor activity
29 immediately after treatment (additional details not reported) (Rogers et al., 2005). There were no
30 effects on pup viability (number of live pups) on PND 1 or 6 in mice treated on GD 8-12 or
31 13-17 or on pup body weight on PND 1 or 6 in mice treated on GD 8-12. Pups treated on GD
32 13-17 showed a statistically significant dose-related trend for reduced body weight on PND 1
33 (p<0.05) but not on PND 6 (p<0.07). Body weight on PND 1 in the pups exposed on GD 13-17
34 was 7.1 and 8.7% less than controls at 0.10 and 0.17 mg/kg-day, respectively, and the differences
35 between the treated and control groups were not significant. Although the trend data could have
36 been interpreted as a treatment-related effect during the latter part of gestation, the investigators
37 believed that the marginal effect on pup weight was due to random variability in litter size: the
38 litter size of the GD 13-17 controls was noticeably smaller than the treated groups (p=0.09), and
39 a difference in litter size would impact both birth weight and growth on PND 1-6 because pups
40 in smaller litters are larger at birth (McCarthy, 1967) and grow more rapidly postnatally (Rogers
41 et al., 2003). Almost all of the results of the righting reflex, negative geotaxis and hanging grip
42 time tests showed no statistically significant differences between exposed and control groups or
43 dose-related trends, indicating a lack of postnatal neurotoxicity. Findings in the righting reflex
44 test included a non-significant (p<0.086) dose-related trend towards slower righting in males on
45 PND 6 and a significantly (p value not reported) slower reflex in females than males in all
46 treatment groups on PND 6 but no treatment or gender differences in righting reflex were
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1 observed on PND 12. The negative geotaxis test was complicated by control values (turning
2 times) that did not decrease from PND 6 to 20 as expected and by many control and dose groups
3 in which 1-4 pups fell off the screen before turning. Although the latter finding was a
4 confounding factor because only data from mice that stayed on the inclined screen could be
5 evaluated, there were no significant differences across treatment groups in either the number of
6 fallen mice or the average turning times. There also were no treatment-related differences in
7 hanging grip time on either test day. Hang time increased significantly from PND 12 to 20 in
8 females, as expected, although males did not show the expected increase in hang time. The
9 investigators observed that gender differences are usually not evident at this age, indicating that
10 random variability in the tested population may account for the finding in the male pups.
11
12 The mouse pups that were exposed to anatoxin-a on GD 13-17 in the Rogers et al. (2005)
13 study were subsequently tested as adults to determine the effect of prenatal exposure to
14 anatoxin-a on the motor activity of adult mice and their responses to nicotine challenge
15 (MacPhail et al., 2005). Motor activity was measured in 30-minute sessions using a photocell
16 device when the offspring were approximately 8 months old. Preliminary testing was performed
17 in which groups of 12 male and 12 female mice were subcutaneously administered a single 0,
18 0.1, 0.3, 1.0 or 3.0 mg/kg dose of nicotine in saline approximately 5 minutes before testing motor
19 activity. These mice were taken from litters that received saline vehicle on GD 8-12 or 13-17
20 (Rogers et al., 2005) and were assigned to the nicotine dose groups regardless of gestational
21 period. Dose-related decreases in both horizontal and vertical activity were observed and 0.65
22 mg/kg was estimated to be the effective dose in 50% of subjects (ED50) for nicotine in both
23 sexes. Mice exposed to 0, 0.10 or 0.17 mg anatoxin-a/kg-day on GD 13-17 were then given the
24 nicotine ED50 or saline vehicle approximately 5 minutes before testing motor activity. The
25 nicotine ED50 and saline vehicle treatments were separated by 1 week. Group sizes were 10 per
26 gender, except for the high-dose anatoxin-a female group, which contained nine mice. There
27 were no differences in horizontal or vertical motor activity between the anatoxin-a-treated mice
28 and the controls. The report presents the results of the activity tests in bar graphs but provides no
29 indication that the comparisons were based on a statistical evaluation of the data.
30
31 Additional information on neurobehavioral effects of anatoxin-a is available from
32 intravenous and subcutaneous injection studies. Mice that were administered a single dose of
33 (+)-anatoxin-a hydrochloride (commercial product, purity not reported) by intravenous injection
34 were evaluated using the Irwin Screen and rota-rod tests at levels of 10-100 ng/kg (8-83 |j,g
35 anatoxin-a/kg) and 30-60 ng/kg (25-50 g anatoxin-a/kg), respectively (Fawell and James, 1994;
36 Fawell et al., 1999). The Irwin Screen is a standard functional observational battery used to
37 characterize CNS effects, including motor activity, behavioral changes, coordination and
38 sensory/motor reflex responses. The rota-rod test assesses sensorimotor coordination by
39 evaluating the animal's ability to remain on a rotating rod. There were no exposure-related
40 effects in either of these tests although the highest doses caused clinical signs of neurotoxicity
41 and death within 1 minute of exposure. The clinical signs of neurotoxicity included increased
42 respiration, salivation, micturition, hyperactivity and Straub tail (contraction of the
43 sacrococcygeus muscle, resulting in vertical erection of the tail). Testing in rats that were
44 administered aqueous (+)-anatoxin-a fumarate by subcutaneous injection showed that a single
45 dose of 0.1 mg/kg (0.06 mg anatoxin-a/kg) caused decreased locomotor activity as well as a
46 partial nicotine-like discriminative stimulus effect in animals trained to discriminate nicotine
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1 from saline in an operant conditioning procedure (Stolerman et al., 1992). As reported in an
2 abstract, anatoxin-a also decreased response and reinforcement rates in multiple-schedule
3 operant performance tests in rats treated by subcutaneous injection, although substantial
4 tolerance developed upon repeated administration (Jarema and MacPhail, 2003).
5
6 4.4.2. Other Effects by Parenteral Exposure
7
8 The short-term parenteral toxicity of anatoxin-a was evaluated in groups of 18 female
9 Sprague-Dawley rats that were administered daily doses of 0 or 0.062 mg/kg via i.p. injection for
10 21 days (Astrachan and Archer, 1981; Astrachan et al., 1980). The anatoxin-a used in this study
11 was extracted from the culture media of A. flos-aquae (NRC-44-1) cells and partially purified;
12 purity was not quantified, but the toxin had a UV absorbance spectrum that qualitatively
13 indicated that anatoxin-a was the principal UV-absorbing component. Endpoints included body
14 weight, food consumption, serum enzyme activities (alkaline phosphatase and ALT),
15 cholinesterase activity (whole blood and brain) and gross pathology and histology (scope not
16 specified). No exposure-related effects were observed.
17
18 The developmental toxicity of i.p. injected anatoxin-a was evaluated in groups of five or
19 six golden hamsters that received doses of 0.2 mg/kg once a day on gestation days 8-14 or 0.125
20 or 0.2 mg/kg 3 times a day on gestation days 8-11 or 12-14 (Astrachan et al., 1980). The
21 anatoxin-a used in this study was extracted from the culture media of A. flos-aquae (NRC-44-1)
22 cells and partially purified; purity was not quantified, but the toxin had a UV absorbance
23 spectrum that qualitatively indicated that anatoxin-a was the principal UV-absorbing component.
24 Vehicle (not reported) and untreated control groups were included in the study. Maternal
25 animals were sacrificed on gestation day 15 for assessment of endpoints that were limited to
26 numbers of implantations and resorptions, fetal body weight and numbers of fetuses with
27 malformations (external, visceral and skeletal). No maternal toxicity was observed in any of the
28 treated animals (scope of examinations not reported). Fetal weights were significantly reduced
29 in the groups exposed to 0.2 mg/kg-day on gestation days 8-14 (18% less than controls), 0.125 or
30 0.2 mg/kg 3 times a day on gestation days 8-11 (9% less than controls at both dose levels) and
31 0.125 mg/kg 3 times a day on gestation days 12-14 (24% less than controls). The only other
32 reported effect was hydrocephaly that was not likely to be chemical related; it occurred in all 10
33 fetuses in one out of six total litters in the 0.125 mg/kg group treated 3 times a day on gestation
34 days 12-14.
35
36 4.4.3. Effects by Intranasal Instillation
37
38 An acute LDso value of 2 mg/kg was determined for intranasal instillation of anatoxin-a
39 (commercial product, form and purity not reported) in mice (Fitzgeorge et al., 1994). Additional
40 information regarding this study was not reported.
41
42 4.4.4. In Vitro Studies
43
44 In vitro developmental toxicity was evaluated in groups of 9-13 mouse whole embryos
45 (GD 8, mouse strain not reported) that were exposed to 0, 0.1, 1.0, 10 or 25 |jM of (+/-)-
46 anatoxin-a hydrochloride (0, 0.08, 0.8, 8.3 and 20.8 |jM anatoxin-a) (commercial product, >90%
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1 purity) in culture medium for 26-28 hours (Rogers et al., 2005). The range of concentrations
2 included some with no adverse effects. At the end of the culture period the embryos were
3 evaluated for dysmorphogenesis; endpoints included somite number, normal morphology and
4 yolk sac dysmorphology. Perturbations in yolk sac vasculature, i.e., a decrease in large caliber
5 vessels and a reduction in arborization, occurred in 0, 0, 44.4, 100 and 100% of the conceptuses
6 at 0, 0.08, 0.8, 8.3 and 20.8 |iM, respectively.
7
8 An in vitro amphibian toxicity test of (+/-)-anatoxin-a hydrochloride (commercial
9 product, >90% purity) was conducted using toad embryos (Bufo arenarum) beginning at life
10 cycle Stage 18 (muscular response) or Stage 25 (complete operculum) (Rogers et al., 2005).
11 Stage 18 embryos were exposed to 0.03, 0.3, 3.0 or 30 mg/L concentrations of anatoxin-a
12 hydrochloride (0.025, 0.25, 2.5 or 25 mg anatoxin-a/L) for 10 days, and Stage 25 embryos were
13 exposed to 30 mg/L of anatoxin-a hydrochloride (25 mg anatoxin-a/L) for 10 days. Embryos
14 were monitored for viability and functional impairments during the 10-day exposures and for 3
15 days post-exposure. Main effects included narcosis and mortality; loss of equilibrium and edema
16 were also noted. The narcosis was transient (times of occurrence not reported), dose-dependent
17 and affected >70% of the embryos at 25 mg/L in both embryonic stages. The mortality was
18 delayed in both embryonic stages, occurring after 6-10 days of exposure. Mortality in Stage 18
19 embryos occurred at 0.25-25 mg/L on days 10-13 (i.e., up to 3 days after cessation of exposure).
20 At 25 mg/L, mortality was 20% on day 8 and reached 100% between days 10 and 13. In Stage
21 25 embryos, mortality was initially observed at day 6 and reached 100% by day 9. Additional
22 information regarding the results of this study was not reported.
23
24 4.4.5. Interactions With Other Cyanotoxins
25
26 Possible interactions between anatoxin-a and other cyanotoxins are of interest because
27 algal blooms can contain multiple cyanotoxin-producing algal species and many toxin-producing
28 species produce more than one toxin. Potential synergism was tested in CD-I mice (number and
29 sex not reported) that were administered a single gavage dose of 0, 500 or 1000 ng/kg (0, 0.5 or
30 1 mg/kg) of microcystin-LR (commercial product, >98% purity) in distilled water, followed 50
31 minutes later by similar administration of 0, 500, 1000 or 2500 |J,g/kg (+/-)-anatoxin-a
32 hydrochloride (0, 0.4, 0.9 or 2.5 mg anatoxin-a/kg) (commercial product, >95% purity) (Rogers
33 et al., 2005). Mice were monitored for clinical signs of acute intoxication and changes in body
34 weight (mice were weighed prior to treatment and 3 hours later). No deaths or definitive signs of
35 intoxication (decreased motor activity, rough hair coat, altered gait, convulsions or failure to eat)
36 occurred in any group, and there were no differences in pre- to post-treatment changes in body
37 weight.
38
39 4.5. MECHANISTIC DATA AND OTHER STUDIES IN SUPPORT OF THE MODE
40 OF ACTION
41
42 4.5.1. Mechanisms of Neurotoxicity
43
44 In vitro studies have clearly demonstrated that (+)-anatoxin-a mimics the action of
45 acetylcholine at neuromuscular nicotinic receptors (Aronstam and Witkop, 1981; Biggs and
46 Dryden, 1977; Carmichael et al., 1975, 1979; Swanson et al., 1986) and is significantly more
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1 potent than acetylcholine and nicotine as an agonist (see Section 4.5.2). Anatoxin-a has become
2 a very useful agent for investigating nicotinic acetylcholine receptors because it is resistant to
3 enzymatic hydrolysis by acetylcholinesterase and because it is 100-fold more selective for
4 nicotinic acetylcholine receptors than for muscarinic acetylcholine receptors (Aronstam and
5 Witkop, 1981). When acetylcholine is released at the neuromuscular junction of motor neurons,
6 it binds to muscle cell receptor molecules consisting of a neuromuscular binding site and an ion
7 channel, which triggers ionic currents that induce muscle cell contraction. Extracellular
8 acetylcholinesterase acts on acetylcholine by degrading the neurotransmitter to prevent
9 overstimulation of the muscle cells. Because anatoxin-a is not degraded by cholinesterase or any
10 other known cellular enzymes, muscle cells continue to be stimulated, causing muscular
11 twitching, fatigue and paralysis. Severe overstimulation of respiratory muscles may result in
12 respiratory arrest and rapid death, as observed in acute lethality studies in animals (Carmichael et
13 al., 1975, 1977; Devlin et al., 1977; Stevens and Krieger, 1991).
14
15 Anatoxin-a also acts as a nicotinic cholinergic agonist at receptors in the cardiovascular
16 system of rats, resulting in increased blood pressure and heart rate (Adeyemo and Siren, 1992;
17 Dube et al., 1996; Siren and Feuerstein, 1990), as well as in rat and human brain neurons
18 (Durany et al., 1999; Thomas et al., 1993; Zhang et al., 1987). Anatoxin-a is a potent agonist of
19 the secretory response of bovine adrenal chromaffm cells, presumably via neuronal-type
20 nicotinic receptor activation (Molloy et al., 1995).
21
22 Anatoxin-a is capable of eliciting the release of neurotransmitters from presynaptic
23 neuromuscular and brain cell terminals. Incubation of guinea pig ileum longitudinal muscle-
24 myenteric plexus preparations with anatoxin-a resulted in dose-dependent release of
25 acetylcholine (Gordon et al., 1992). Anatoxin-a stimulated the release of dopamine from rat
26 striatal synaptosomes in a dose-dependent manner (Clarke and Reuben, 1996; Rowell and
27 Wonnacott, 1990; Soliakov et al., 1995; Wonnacott et al., 2000). These findings indicate that
28 anatoxin-a can bind to presynaptic nicotinic receptors to trigger neurotransmitter release.
29 Increased neurotransmitter release could contribute to increased stimulation of postsynaptic
30 receptors.
31
32 4.5.2. Structure-Activity Relationships
33
34 Anatoxin-a is produced as the natural stereoisomer, (+)-anatoxin-a, by some strains of
35 Anabaena (particularly A. flos-aquae) and at least four other genera of freshwater cyanobacteria,
36 including Aphanizomenon, Microcystis, Planktothrix and Oscillatoria (Devlin et al., 1977;
37 Fawell et al., 1999; Huber, 1972; Viaggiu et al., 2004). As discussed in Section 4.5.1,
38 (+)-anatoxin-a is a nicotinic acetylcholine receptor agonist that exerts its effects at both
39 peripheral and central cites in the nervous system. It is generally believed that nicotinic agonists
40 form hydrogen bonds in the planar region and contain a bulky cationic group approximately 5.9
41 A from the hydrogen bond (Beers and Reich, 1970; Chothia and Pauling, 1970; Spivak and
42 Albuquerque, 1982); anatoxin-a exhibits these characteristics. The importance of
43 stereospecificity was demonstrated in assays of contracture potency in frog rectus abdominis
44 muscle preparations; natural (+)-anatoxin-a exhibited at least a 2.5- and 150-fold greater potency
45 than racemic and (-)-anatoxin-a, respectively (Spivak et al., 1983; Swanson et al., 1986). Similar
46 potency differences were demonstrated in in vivo lethality assays in mice (Valentine et al., 1991).
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1 As discussed in Section 4.4.1, acute i.p. LD50 values of 386 and 913 ng/kg were determined for
2 (+)-anatoxin-a hydrochloride and racemic anatoxin-a hydrochloride, respectively. No clinical
3 signs or deaths occurred in mice that were similarly treated with doses of (-)-anatoxin-a
4 hydrochloride as high as 73 mg/kg. These findings indicate that (+)-anatoxin-a was 2.4 and at
5 least 189 times as potent as racemic and (-)-anatoxin-a, respectively.
6
7 (+)-Anatoxin-a is significantly more potent than acetylcholine and nicotine as an agonist
8 at neuromuscular nicotinic acetylcholine receptors. Anatoxin-a has been shown to bind to the
9 nicotinic acetylcholine receptor with an affinity approximately 3.6 times greater than
10 acetylcholine (Swanson et al., 1986). Following complete inhibition of acetylcholinesterase
11 activity in frog rectus abdominis muscle preparations, anatoxin-a exhibited an 8-fold greater
12 potency by contracture than acetylcholine (Swanson et al., 1986). Anatoxin-a was 7-136 times
13 more potent than nicotine in a series of in vitro acetylcholine receptor (guinea pig ileum, rat
14 phrenic nerve, chick biventer cervicis muscle) and mouse intravenous (neuropharmacological
15 signs) screening studies (Fawell and James, 1994; Fawell et al., 1999). Additionally, the agonist
16 potency of (+)-anatoxin-a was 3-50 times more than nicotine and approximately 20 times more
17 than acetylcholine at neuronal nicotinic acetylcholine receptors from rat hippocampal
18 synaptosomes, fetal rat hippocampal neurons, mouse M10 cells and frog (Xenopus) oocytes
19 (Thomas et al., 1993).
20
21 Neuromuscular and neuronal assays of structure activity relationships indicate that
22 N-methylation of anatoxin-a greatly reduces the acetylcholine-mimicking effect at nicotinic
23 cholinergic receptors (Aracava et al., 1987; Costa et al., 1990; Kofuji et al., 1990; Stevens and
24 Krieger, 1990; Swanson et al., 1989, 1991; Wonnacott et al., 1991).
25
26 4.5.3. Other Studies
27
28 Anatoxin-a caused apoptosis in rat thymocytes and monkey kidney (Vero) cells that was
29 characterized by DNA fragmentation and apparently mediated by generation of reactive oxygen
30 species and caspase activation (Rao et al., 2002). Anatoxin-a also caused cytotoxicity in cultured
31 mouse B- and T-lymphocytes, but apoptosis was not induced; the cytotoxic action appeared to be
32 non-selective and non-specific, and the mechanism remains to be elucidated (Teneva et al.,
33 2005).
34
35 4.6. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER EFFECTS
36
37 4.6.1. Oral
38
39 The main oral toxicity studies of anatoxin-a are summarized in Table 4-1. A limited
40 amount of information is available on the health effects of anatoxin-a in humans. One report
41 surveyed several cases of nonlethal human poisonings caused by ingestion of lake water
42 containing Anabaena sp. (Schwimmer and Schwimmer, 1968). The most prominent and best
43 documented effects were acute gastrointestinal disorders. In a more recent report, anatoxin-a
44 was implicated in the poisonings of two teenage boys who swallowed water from a pond
45 containing an algal bloom (Behm, 2003). One of the boys suffered a seizure and died from heart
46 failure 2 days after swallowing the water, and the other boy became sick with severe diarrhea
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Table 4-1. Summary Results of Major Oral Toxicity Studies of Anatoxin-a in Experimental Animals
Species
Sex
Average Daily
Dose
(mg/kg-day)
Exposure
Duration
NOAEL
(mg/kg-day)
LOAEL
(mg/kg-day)
Responses
Comments
Reference
Acute Exposure
No suitable acute studies are available
Short-term Exposure
Mouse
Mouse
Mouse
M,F
M,F
F
1.2,2.5,6.2,
12.3
0,0.1,0.5,
2.5
0,2.5
5 days
28 days
9 days;
gestation
days 6-15
2.5
2.5
2.5
6.2*
ND
ND
PEL = 6.2 mg/kg-day due to
mortality in the two highest
dose groups.
Two deaths at>0.5 mg/kg-
day; it was unclear whether
these deaths could be
attributed to compound
administration. Minor
hematology and blood
chemistry changes at >0. 1
mg/kg-day were not clearly
exposure-related and/or
lexicologically significant.
No exposure-related adverse
maternal or fetal effects.
Mean fetal weight was
marginally reduced at 2.5
mg/kg-day (data not
reported) but not considered
to be lexicologically
significant.
No clinical signs or effects on
body weight, food consumption,
survival or necropsy at <2.5
mg/kg-day. Other endpoints not
examined. No control group and
small group sizes (2/sex/dose).
Well-designed study that
investigated clinical signs, body
weight, food consumption,
ophthalmic condition,
hematology, blood chemistry,
gross pathology, organ weights
and histology (comprehensive).
10 mice/sex/dose.
Maternal endpoints included
clinical signs, body weight and
necropsy. Developmental
endpoints included numbers of
implantations and live fetuses,
post implantation loss, body
weight, sex ratio and external
abnormalities. No fetal internal
examinations. 10-12 mice/dose.
Fawell and
James, 1994;
Fawell et al.,
1999
Fawell and
James, 1994;
Fawell et al.,
1999
Fawell and
James, 1994;
Fawell et al.,
1999
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Table 4-1 cont.
Species
Sex
Average Daily
Dose
(mg/kg-day)
Exposure
Duration
NOAEL
(mg/kg-day)
LOAEL
(mg/kg-day)
Responses
Comments
Reference
Subchronic Exposure
Rat
F
0, 0.05, 0.5
7 weeks
0.5
ND
None
Limited number of endpoints:
clinical signs, food consumption,
body weight, red and total white
blood cell counts, serum enzyme
activities (alkaline phosphatase,
ALT, GOT, cholinesterase),
hepatic MFO activity (aldrin
epoxidation in vitro), organ
weights (liver, kidneys, spleen),
gross pathology and histology
(liver, kidneys, spleen, adrenals,
heart, lungs and brain). 20
rats/dose.
Astrachan and
Archer, 1981;
Astrachan et
al., 1980
Chronic Exposure
No suitable chronic studies are available
*FEL
PEL = Frank effect level
LOAEL = Lowest-observed-adverse-effect level
ND = not determined
MFO = mixed function oxidase
17
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1 and abdominal pain but survived. Testing of stool samples from both boys revealed the presence
2 of A. flos-aquae cells, and analyses of blood, liver tissue and ocular fluid from the boy who died
3 found a compound initially identified as anatoxin-a. A definitive diagnosis of anatoxin-a as the
4 cause of death was confounded by the apparent delay between exposure and overt toxicity
5 (Behm, 2003), and subsequent analysis determined that the detected compound was not
6 anatoxin-a (Carmichael et al., 2004). Relevant dose-response information, including estimated
7 amounts of water or toxin ingested, was not provided in either of the above reports.
8
9 Anatoxin-a has also been implicated in cases of animal poisonings following
10 consumption of water containing blooms of A. flos-aquae (Carmichael and Gorham, 1978;
11 Edwards et al., 1992; Gunn et al., 1992; Pybus et al., 1986) although no quantitative exposure
12 data are available. The preponderance of experimental studies on anatoxin-a are in vitro and
13 pertain to its mode of neurotoxic action (see Section 4.5.1).
14
15 Information on the in vivo effects of anatoxin-a in orally-exposed laboratory animals is
16 available from several acute and short-term studies and one subchronic study that provide a
17 limited amount of dose-response data on systemic toxicity and developmental toxicity, as
18 summarized below.
19
20 Acute toxicity data for anatoxin-a are essentially limited to the results of lethality assays
21 in mice that determined a single-dose LD50 value of 13.3 mg anatoxin-a/kg and identified
22 neurotoxicity as the cause of death (Fitzgeorge et al., 1994; Stevens and Krieger, 1991).
23
24 Information on the short-term oral toxicity of anatoxin-a is available from 5- and 28-day
25 systemic toxicity studies in mice (Fawell and James, 1994; Fawell et al., 1999) and a
26 developmental toxicity study in mice (Fawell and James, 1994; Fawell et al., 1999). The 5-day
27 mouse study used four dose levels (1.2, 2.5, 6.2 and 12.3 mg/kg-day by gavage) but is limited by
28 small numbers of animals (2/sex/dose), lack of controls and few endpoints (clinical signs, body
29 weight, food consumption, and necropsy). Based on dose-related mortality at 6.2 mg/kg-day
30 (1/4 mice) and 12.3 mg/kg-day (4/4 mice), the NOAEL and FEL are 2.5 and 6.2 mg/kg-day,
31 respectively.
32
33 The 28-day mouse study used three dose levels (0.1, 0.5 and 2.5 mg/kg-day by gavage)
34 and was generally comprehensive in that hematology, blood chemistry, gross pathology and
35 histology were included in the evaluations (Fawell and James, 1994; Fawell et al., 1999). The
36 only remarkable findings were one death (1/10 males) at 0.5 mg/kg-day and one death (1/10
37 females) at 2.5 mg/kg-day that occurred within 2.5 hours of dosing on days 10 and 14 of
38 treatment. The deaths were not preceded by any clinical signs of distress and postmortem
39 examination of the animals did not identify a cause of death. Accordingly, the authors could not
40 completely rule out a possible relationship to anatoxin-a exposure and concluded that the
41 NOAEL was 0.1 mg/kg-day, although they indicated that the actual NOAEL may be 2.5 mg/kg-
42 day. Considering the lack of clinical signs (anatoxin-a is an acute neurotoxin with characteristic
43 clinical signs before death), very low incidences of mortality that showed no dose-response or
44 gender consistency, absence of lexicologically significant effects in surviving mice and lack of
45 effects at 2.5 mg/kg-day in the 5-day and developmental toxicity studies, EPA concludes that the
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1 deaths in the 28-day study are likely to be incidental and that the actual NOAEL is 2.5
2 mg/kg-day.
3
4 The developmental toxicity study in mice was limited by use of only one dose level (2.5
5 mg/kg-day by gavage) and fetal assessments that lacked internal soft tissue and skeletal
6 examinations (Fawell and James, 1994; Fawell et al., 1999). No maternal effects (clinical signs,
7 body weight, necropsy) or developmental effects (numbers of implantations and fetuses, fetal
8 body weight and sex ratio, external abnormalities) were observed. Although this study provides
9 insufficient information on possible fetal abnormalities, a developmental toxicity study of
10 intraperitoneally-injected anatoxin-a in hamsters found no external, soft tissue or skeletal
11 changes in fetuses at doses high enough to cause reduced fetal weight (Astrachan et al., 1980).
12 Additionally, there were no effects on postnatal neurodevelopmental maturation, as shown by
13 righting reflex, negative geotaxis and hanging grip time tests on PND 6-20, in offspring of mice
14 that were gestationally administered anatoxin-a at intraperitoneal doses high enough to cause
15 decreased maternal motor activity (Rogers et al., 2005). The lack of effects on fetal weight and
16 other endpoints in the oral mouse study indicates that 2.5 mg/kg-day is a free-standing NOAEL
17 for maternal toxicity and developmental toxicity by oral exposure.
18
19 As indicated above, the 28-day toxicity study in mice (Fawell and James, 1994; Fawell et
20 al., 1999) is the best-designed short-term oral study of anatoxin-a. The 2.5 mg/kg-day NOAEL
21 for systemic toxicity in this study is supported by the 2.5 mg/kg-day NOAEL in the 5-day mouse
22 study (Fawell and James, 1994; Fawell et al., 1999) and the 2.5 mg/kg-day NOAEL for maternal
23 and developmental toxicity in mice (Fawell and James, 1994; Fawell et al., 1999). No adverse
24 effect levels were identified in the 28-day and developmental toxicity studies although 6.2
25 mg/kg-day was a FEL in the 5-day study. Considering the evidence for the 2.5 mg/kg-day
26 NOAEL from two studies and its apparent proximity to the threshold for adverse effects (as
27 indicated by the 6.2 mg/kg-day FEL), the NOAEL is an adequate basis for quantitative
28 assessment of short-term noncancer risks of anatoxin-a (see Section 5.2.3). The lack of adverse
29 effects at doses below those causing death is consistent with the acute neurotoxic nature of the
30 chemical.
31
32 Information on the subchronic oral toxicity of anatoxin-a is available from a 7-week
33 drinking water study in rats (Astrachan and Archer, 1981; Astrachan et al., 1980). This study is
34 limited by the use of only two dose levels (0.05 and 0.5 mg/kg-day) and non-comprehensive
35 examinations, particularly for hematology (two indices), blood chemistry (four serum enzymes)
36 and histology (seven tissues). No treatment-related effects were found, indicating that 0.5
37 mg/kg-day is a subchronic NOAEL. The small number of endpoints and lack of an adverse
38 effect level to provide information on proximity of the NOAEL to the toxicity threshold are
39 problematic, but data from the short-term studies are supportive of the subchronic NOAEL. In
40 particular, the NOAELs of 2.5 mg/kg-day in the 5-day, 28-day and developmental toxicity
41 studies and FEL of 6.2 mg/kg-day in the 5-day study (Fawell and James, 1994; Fawell et al.,
42 1999) provide indications that the 0.5 mg/kg-day NOAEL in the 7-week study is a reliable value
43 and a sufficient basis for subchronic risk assessment.
44
45 No information is available on the chronic oral toxicity of anatoxin-a.
46
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1 4.6.2. Inhalation
2
3 No information is available on the inhalation toxicity of anatoxin-a.
4 4.6.3. Mode of Action Information
5
6 The main known toxic effect of anatoxin-a is acute neurotoxicity that is manifested as
7 progressive clinical signs that include loss of coordination, muscular fasciculations, convulsions
8 and death by respiratory paralysis. It is well documented that anatoxin-a acts by mimicking the
9 action of acetylcholine at neuromuscular nicotinic receptors (Aronstam and Witkop, 1981; Biggs
10 and Dryden, 1977; Carmichael et al., 1975, 1979; Swanson et al., 1986). As an agonist that is
11 significantly more potent than acetylcholine and is not degraded by cholinesterase (Swanson et
12 al., 1986; Thomas et al., 1993), (+)-anatoxin-a interacts with the nicotinic acetylcholine receptors
13 to cause persistent stimulation of the muscle cells.
14
15 4.7. WEIGHT-OF-EVIDENCE EVALUATION AND CANCER
16 CHARACTERIZATION
17
18 4.7.1. Summary of Overall Weight-of-Evidence
19
20 No cancer or genotoxicity studies, no information on potential modes of carcinogenic
21 action nor other carcinogenicity data are available for anatoxin-a. In accordance with the
22 Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005a), the weight of evidence
23 descriptor for the carcinogenic hazard potential of anatoxin-a is "Inadequate Information to
24 Assess Carcinogenic Potential."
25
26 4.7.2. Synthesis of Human, Animal and Other Supporting Evidence
27
28 No information regarding carcinogenicity in humans or animals or on possible
29 carcinogenic processes and mode(s) of action of anatoxin-a was identified in the materials
30 reviewed for this assessment.
31
32 4.8. SUSCEPTIBLE POPULATIONS AND LIFE STAGES
33
34 4.8.1. Possible Childhood Susceptibility
35
36 There is no information on the degree to which children might differ from adults in the
37 disposition of, or response to, anatoxin-a.
38
39 4.8.2. Possible Gender Differences
40
41 There is no information on possible gender differences in the disposition of, or response
42 to, anatoxin-a.
43
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1 4.8.3. Other Possible Susceptible Populations
2
3 Anticholinergic agents have been recommended for the treatment of various medical
4 conditions, but therapeutic uses are mainly in four areas: atony of the smooth muscle of the
5 intestinal tract and urinary bladder, glaucoma, myasthenia gravis and termination of the effects
6 of competitive neuromuscular blocking agents (Taylor, 1996). It is conceivable that people
7 using anticholinergic agents for therapeutic purposes could be at risk of experiencing an increase
8 in unwanted side effects if exposed to anatoxin-a due to the potential for additivity of adverse
9 effects.
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1 5. DOSE-RESPONSE ASSESSMENTS
2
3
4 5.1. NARRATIVE DESCRIPTION OF THE EXTENT OF THE DATABASE
5
6 Studies on the absorption, tissue distribution, metabolism and elimination of anatoxin-a
7 have not been performed. Acute oral toxicity studies in animals indicate that anatoxin-a is
8 rapidly absorbed from the gastrointestinal tract as shown by clinical signs of neurotoxicity and
9 death within several minutes of exposure.
10
11 The only information on the toxicity of anatoxin-a in humans consists of two reports
12 implicating ingestion of lake or pond water containing Anabaena sp. in several cases of non-
13 lethal gastrointestinal poisonings and in one death. Anatoxin-a has also been implicated in cases
14 of domestic and wild animal neurotoxicity and death following consumption of water containing
15 blooms of A. flos-aquae. Details regarding most of these human and non-laboratory animal
16 exposures and effects were not reported and doses were not estimated.
17
18 The acute in vivo neurotoxicity of anatoxin-a in animals is well-documented and
19 characterized by tremors, altered gait, convulsions and death by respiratory paralysis. Little
20 information is available on in vivo neurotoxicity at sublethal doses; findings include no effects of
21 gestational intraperitoneal exposure on postnatal neurodevelopmental maturation in mice and no
22 effects of acute intravenous exposure on motor activity, coordination, sensory/motor reflexes and
23 other central nervous system responses in mice. The preponderance of experimental studies of
24 anatoxin-a are in vitro, pertain to its mode of neurotoxic action and have established that it is a
25 nicotinic acetylcholine receptor agonist that exerts its effects at both peripheral and central sites
26 in the nervous system. In vitro studies also indicate that anatoxin-a can affect non-neuronal
27 cells, causing effects that include apoptosis via production of reactive oxygen species and
28 caspase activation in rat thymocytes and monkey kidney cells, and cytotoxicity without apoptosis
29 in mouse lymphocytes.
30
31 Information on the in vivo effects of anatoxin-a in orally exposed laboratory animals is
32 available from single-dose lethality assays in mice, 5- and 28-day studies in mice, a 7-week
33 study in rats and a developmental toxicity study in mice. These studies only provide a limited
34 amount of dose-response data on systemic toxicity and developmental toxicity due to limitations
35 in experimental design and reporting, including insufficient numbers of dose levels and study
36 endpoints. In particular, the oral database is limited by a preponderance of NOAELs and no
37 LOAELs; information on adverse effects essentially consists of FELs for neurotoxicity-induced
38 mortality in the single-dose and 5-day studies. The 28-day and 7-week studies provide bases for
39 derivation of short-term and subchronic oral RfDs, but the remaining oral data are insufficient
40 for deriving acute and chronic RfDs.
41
42 No information is available on the inhalation toxicity of anatoxin-a.
43
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1 5.2. ORAL REFERENCE DOSE (RfD)
2
3 5.2.1. Data Considered in Deriving Reference Values
4
5 Data considered in deriving oral reference values for each duration of exposure are
6 summarized in Table 4-1 (see Section 4.6.1).
7
8 5.2.2. Acute Duration
9
10 5.2.2.1. Choice of Principal Study and Critical Effect - with Rationale and
11 Justification
12
13 The available acute duration oral toxicity data for anatoxin-a are inadequate to support
14 derivation of an acute RfD. Cases of non-lethal human poisonings, manifested mainly as acute
15 gastrointestinal disturbances, have been attributed to ingestion of lake or pond water containing
16 anatoxin-a-producing Anabaena sp. (Behm, 2003; Schwimmer and Schwimmer, 1968).
17 Anatoxin-a was implicated in the death of a person who suffered a seizure and heart failure 2
18 days after swallowing pond water containing A. flos-aquae in an algal bloom (Behm, 2003;
19 Carmichael et al., 2004). None of these case reports provide dose information or unequivocally
20 establish anatoxin-a as the causal agent. Acute oral experimental data for anatoxin-a in animals
21 are essentially limited to the results of two lethality assays in mice that determined a single-dose
22 LD50 value of 13.3 mg anatoxin-a/kg and identified neurotoxicity as the cause of death
23 (Fitzgeorge et al., 1994; Stevens and Krieger, 1991). Derivation of an acute oral RfD based on
24 the human or animal data is precluded by insufficient information on sensitive endpoints and
25 associated dose-response relationships.
26
27 5.2.3. Short-Term Duration
28
29 5.2.3.1. Choice of Principal Study and Critical Effect - with Rationale and
30 Justification
31
32 Information on the short-term oral toxicity of anatoxin-a is available from 5- and 28-day
33 systemic toxicity studies in mice (Fawell and James, 1994; Fawell et al., 1999) and a
34 developmental toxicity study in mice (Fawell and James, 1994; Fawell et al., 1999), as discussed
35 in Sections 4.2.1.2 and 4.6.1. The best designed of these studies is the 28-day study in mice,
36 which tested groups of 10 mice/sex at dose levels of 0, 0.1, 0.5 and 2.5 mg/kg-day and identified
37 an apparent NOAEL of 2.5 mg/kg-day. The authors concluded that the NOAEL was 0.1 mg/kg-
38 day due to deaths in 1/10 males at 0.5 mg/kg-day and 1/10 females at 2.5 mg/kg-day. This
39 conclusion was based on an inability to determine the cause(s) of death and completely rule out a
40 relationship with treatment, but the study authors indicated that the true NOAEL may actually be
41 2.5 mg/kg-day. EPA concludes that the actual NOAEL is 2.5 mg/kg-day due to the low
42 mortality incidences that showed no dose-response or gender consistency, a lack of clinical signs
43 of acute neurotoxicity prior to death (the animals died within 2.5 hours of dosing on days 10 and
44 14), a lack of lexicologically significant effects in the surviving animals (comprehensive
45 evaluations were performed that included hematology, clinical chemistry and histology) and
46 supporting NOAELs of 2.5 mg/kg-day in the 5-day and developmental toxicity studies. The
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1 5-day study identified an FEL of 6.2 mg/kg-day for mortality but no LOAEL, and no LOAEL or
2 FEL was identified in the developmental toxicity studies. The proximity of the 6.2 mg/kg-day
3 FEL to the 2.5 mg/kg-day NOAEL indicates that the NOAEL is close to the toxicity threshold
4 region, and therefore, is an appropriate basis for RfD assessment.
5
6 5.2.3.2. Methods of Analysis - Including Models (Physiologically Based
7 Pharmacokinetic [PBPK], Benchmark Dose [BMD], etc.)
8
9 The NOAEL/LOAEL approach is used to derive the RfD due to limitations in the
10 available studies. BMD analysis is precluded by lack of appropriate dose-response data (adverse
11 effects other than mortality were not observed) and by predominantly qualitative reporting of
12 results.
13
14 5.2.3.3. RfD Derivation - Including Application of Uncertainty Factors (UFs)
15
16 To derive the short-term RfD, the 2.5 mg/kg-day NOAEL for systemic toxicity was used
17 as the point of departure (POD). Dividing the POD of 2.5 mg/kg-day by a composite uncertainty
18 factor (UF) of 1000 results in a short-term RfD for anatoxin-a of 3xlO"3 mg/kg-day.
19
20 Short-Term RfD = NOAEL-UF
21 =2.5 mg/kg-day - 1000
22 = 0.0025 mg/kg-day or 3x103 mg/kg-day
23
24 The composite UF of 1000 includes a factor of 10 for interspecies extrapolation, a factor
25 of 10 to account for interindividual variability in the human population and a factor of 10 for
26 database limitations, as follows.
27
28 • A default 10-fold UF is used to account for the interspecies variability in extrapolating
29 from laboratory animals (mice) to humans. No relevant information is available on the
30 toxicity of anatoxin-a in humans, and data on toxicokinetic differences between animals
31 and humans in the disposition of ingested anatoxin-a are not available.
32
33 • An intraspecies UF of 10 is used to account for variations in sensitivity within human
34 populations because there is no information on the degree to which humans of varying
35 gender, age, health status or genetic makeup might vary in the disposition of, or response
36 to, ingested anatoxin-a.
37
38 • A 10-fold UF is used to account for deficiencies in the database. There is no information
39 on the short-term toxicity of anatoxin-a in orally-exposed humans. The 2.5 mg/kg-day
40 NOAEL is based on a generally well-designed 28-day study, but no adverse effect level
41 was identified and some uncertainty in the NOAEL exists due to the low incidence of
42 deaths that the study authors could not rule out as being possibly exposure-related. The
43 2.5 mg/kg-day NOAEL in the 28-day study is supported by NOAELs of 2.5 mg/kg-day in
44 the 5-day and developmental toxicity studies, but neither of these studies was
45 comprehensive or identified a LOAEL. Although the 6.2 mg/kg-day FEL for mortality in
46 the 5-day study suggests that the 2.5 mg/kg-day NOAEL is close to the toxicity threshold
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1 region, the 5-day study is limited by very small numbers of animals, indicating that the
2 lack of a LOAEL is an important limitation of the database. The developmental toxicity
3 study is limited by use of only one dose level and fetal assessments that lacked internal
4 soft tissue and skeletal examinations. The database for short-term oral exposure is
5 additionally limited by lack of reproductive toxicity data as well as toxicity testing in a
6 second species.
7
8 5.2.4. Subchronic Duration
9
10 5.2.4.1. Choice of Principal Study and Critical Effect - with Rationale and
11 Justification
12
13 Information on the subchronic oral toxicity of anatoxin-a is available from a 7-week
14 drinking water study in rats (Astrachan and Archer, 1981; Astrachan et al., 1980). As discussed
15 in Sections 4.2.1.3 and 4.6.1, this study identified a NOAEL of 0.5 mg/kg-day but is limited by
16 insufficiencies that include two dose levels, a minimal number of endpoints and the lack of an
17 adverse effect level. Although the study provides no information on proximity of the 0.5
18 mg/kg-day NOAEL to the toxicity threshold, due to the lack of a LOAEL or FEL, results of the
19 short-term studies discussed in Section 5.2.3.1 are supportive of the subchronic NOAEL. In
20 particular, the NOAELs of 2.5 mg/kg-day in the 5-day, 28-day and developmental toxicity
21 studies and FEL of 6.2 mg/kg-day in the 5-day study (Fawell and James, 1994; Fawell et al.,
22 1999) indicate that the 0.5 mg/kg-day NOAEL in the 7-week study is a reliable value and a
23 sufficient basis for RfD derivation.
24
25 5.2.4.2. Methods of Analysis - Including Models (Physiologically Based
26 Pharmacokinetic [PBPK], Benchmark Dose [BMD], etc.)
27
28 The NOAEL/LOAEL approach is used to derive the RfD due to limitations in the
29 available study. BMD analysis is precluded by lack of appropriate dose-response data (adverse
30 effects were not observed) and predominantly qualitative reporting of results.
31
32 5.2.4.3. RfD Derivation - Including Application of Uncertainty Factors (UFs)
33
34 To derive the subchronic RfD, the 0.5 mg/kg-day NOAEL for systemic toxicity was used
35 as the point of departure (POD). Dividing the POD of 0.5 mg/kg-day by a composite uncertainty
36 factor (UF) of 1000 results in a subchronic RfD for anatoxin-a of 5xlO"4mg/kg-day.
37
38 Subchronic RfD = NOAEL-UF
39 = 0.5 mg/kg-day-1000
40 = 0.0005 mg/kg-day or 5x104 mg/kg-day
41
42 The composite UF of 1000 includes a factor of 10 for interspecies extrapolation, a factor
43 of 10 to account for interindividual variability in the human population and a factor of 10 for
44 database limitations, as follows.
45
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1 • A default 10-fold UF is used to account for the interspecies variability in extrapolating
2 from laboratory animals (rats) to humans. No relevant information is available on the
3 toxicity of anatoxin-a in humans, and data on toxicokinetic differences between animals
4 and humans in the disposition of ingested anatoxin-a are not available.
5
6 • An intraspecies UF of 10 is used to account for variations in sensitivity within human
7 populations because there is no information on the degree to which humans of varying
8 gender, age, health status or genetic makeup might vary in the disposition of, or response
9 to, ingested anatoxin-a.
10
11 • A 10-fold UF is used to account for deficiencies in the database. Only one subchronic
12 study was conducted and it is limited by deficiencies that include two dose levels, a
13 minimal number of endpoints (e.g., two hematology indices, four clinical chemistry
14 indices, seven tissues for histological examination) and lack of an adverse effect level.
15 Some supporting data are provided by short-term systemic and developmental toxicity
16 studies, but these studies have limitations, as discussed in Section 5.2.3.3. Additionally,
17 the database for subchronic oral exposure is limited by the lack of reproductive toxicity
18 data as well as toxicity testing in a second species.
19
20 5.2.5. Chronic Duration
21
22 5.2.5.1. Choice of Principal Study and Critical Effect - with Rationale and
23 Justification
24
25 Insufficient data are available to support derivation of a chronic duration oral RfD for
26 anatoxin-a. No chronic oral studies have been performed and use of the subchronic study for
27 chronic RfD estimation by extrapolation across exposure durations is precluded by the study
28 limitations discussed in Section 5.2.4.
29
30 5.2.6. Route-to-Route Extrapolation
31
32 Derivation of RfD values for anatoxin-a by route-to-route extrapolation could not be
33 considered due to a lack of inhalation data.
34
35 5.3. INHALATION REFERENCE CONCENTRATION (RfC)
36
37 No information is available on the inhalation toxicity of anatoxin-a.
38
39 5.4. CANCER ASSESSMENT
40
41 There is no information on carcinogenicity in humans or animals or on possible
42 carcinogenic processes and mode(s) of action for anatoxin-a. Under the Guidelines for
43 Carcinogen Risk Assessment (U.S. EPA, 2005a), the database is inadequate for an assessment of
44 human carcinogenic potential.
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1 6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD
2 AND DOSE RESPONSE
3
4
5 6.1. HUMAN HAZARD POTENTIAL
6
7 Anatoxin-a is a naturally occurring chemical produced by Anabaena (particularly A. flos-
8 aquae) and at least four other genera of freshwater cyanobacteria. Toxicokinetic studies of
9 anatoxin-a have not been performed, although the rapid onset of clinical signs in toxicity studies
10 indicate that it is quickly absorbed from the gastrointestinal tract. Anatoxin-a is a well
11 documented acute neurotoxin based on observations in domestic and wild animals that consumed
12 water containing blooms of Anabena flos-aquae and results of oral and parenteral toxicity studies
13 in laboratory animals. Mode of action studies indicate that anatoxin-a is a nicotinic acetylcholine
14 receptor agonist that exerts effects at both peripheral and central cites in the nervous system,
15 presumably via action of the parent compound. Acute gastrointestinal effects in humans (e.g.,
16 nausea, vomiting and diarrhea) and potentially one death have been attributed to the ingestion of
17 lake water containing Anabaena and Microcystis.
18
19 The acute clinical effects of high doses of anatoxin-a are mainly neuromuscular in nature
20 and characteristically include signs such as muscular twitching, gasping respiration, convulsions
21 and death from paralysis of respiratory muscle. The database is limited in the number and
22 quality of studies on effects of anatoxin-a following oral exposure to sublethal levels. No
23 adverse effects other than frank neurotoxicity and death have been observed in acute, short-term
24 and subchronic oral studies in animals. These studies identified a preponderance of NOAELs
25 and no LOAELs for systemic and developmental toxicity. No information is available on the
26 reproductive toxicity, chronic toxicity or carcinogen!city of anatoxin-a by any route of exposure.
27 Testing following inhalation has not been performed.
28
29 There is inadequate evidence to evaluate the carcinogenicity of anatoxin-a.
30
31 6.2. DOSE RESPONSE
32
33 The available data were sufficient for derivation of short-term and subchronic oral RfDs.
34 Based on a NOAEL of 2.5 mg/kg-day for systemic toxicity in mice exposed to anatoxin-a for 28
35 days (Fawell and James, 1994; Fawell et al., 1999), a short-term RfD of 3xlO"3 mg/kg-day was
36 derived by dividing the NOAEL by an uncertainty factor of 1000. The uncertainly factor
37 comprises component factors of 10 for interspecies extrapolation, 10 for interindividual
38 variability and 10 for database deficiencies. Based on a NOAEL of 0.5 mg/kg-day for systemic
39 toxicity in rats exposed to anatoxin-a for 7 weeks (Astrachan and Archer, 1981; Astrachan et al.,
40 1980), a subchronic RfD of 5xlO"4 mg/kg-day was derived by dividing the NOAEL by an
41 uncertainty factor of 1000. The uncertainly factor comprises component factors of 10 for
42 interspecies extrapolation, 10 for interindividual variability and 10 for database deficiencies.
43
44 Acute and chronic oral RfDs could not be derived due to inadequate data.
45
46 Inhalation RfC derivation is precluded by the lack of data for this route of exposure.
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1 7. REFERENCES
2
3 Adeyemo, O.M. and A.-L. Siren. 1992. Cardio-respiratory changes and mortality in the
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26 U.S. EPA. 1986b. Guidelines for Mutagenicity Risk Assessment. Fed. Reg.
27 51(185):34006-34012.
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33 DRAFT: DO NOT CITE OR QUOTE
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15
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