v-xEPA
                                  United States
                                  Environmental Protection
                                  Agency
                                  Environmental Research
                                  Laboratory
                                  Duluth MN 55804
                                  Research and Development
                                  EPA-600/S3-81 -029 June 1981
Project  Summary
                                  State-of-the-Art of
                                  Structure  Activity  Methods
                                  Development
                                 Oilman D. Veith
                                   The overall objective of this project
                                  is to provide the Agency with the tech-
                                  nical basis for estimating the toxicity
                                  and environmental  behavior  of
                                  organic  chemicals  from  molecular
                                  structure.  The .project  is directed
                                  toward the evaluation of quantitative
                                  structure-activity relationships
                                  (QSAR)  for use by EPA Program
                                  Offices and toward the development
                                  of new data and QSAR methods to
                                  extend the technique to meet Agency
                                  needs.
                                   Specifically,  the objective of the
                                  Structure-Activity Project is  to
                                  develop methods to predict the toxic-
                                  ity, persistence,  and treatability of
                                  large numbers of untested chemicals
                                  using  QSAR  based on  structural,
                                  chemical, and biological properties of
                                  representative  reference data bases.
                                  Development of QSAR is being tai-
                                  lored of use in the (1) initial screening
                                 of chemicals under the Toxic Sub-
                                 stances Control Act (TSCA), (2) devel-
                                 opment of risk assessment strategies,
                                 (3)  prioritization of  chemicals  for
                                 Water Quality Criteria  development,
                                 and (4) the optimization  of national
                                 monitoring programs  for  toxic
                                 chemicals.
                                   This report summarizes the prog-
                                 ress during the first six months of the
                                 project. The report provides a litera-
                                 ture review and  perspective for apply-
                                 ing  structure-activity  methods  to
                                 aquatic  toxicity  of industrial
                                 chemicals.  Experimental work
                                 centered  on developing methods for
                                 estimating  molecular  descriptors
                                 such as log P and connectivity indexes
                                 and on the development of a system-
                                 atic structure-activity data base for
                                 aquatic toxicity. A new program for
                                 entering structures into a computer
                                 and calculating connectivity indexes
                                 is discussed. A general model for pre-
                                 dicting 96-hour LC50 for  narcotic
                                 chemicals is presented.
                                   This Project Summary was develop-
                                 ed by EPA's Environmental Research
                                 Laboratory, Duluth, MN, to announce
                                 key findings of the research project
                                 that is fully documented in a separate
                                 report of the same title (see Project
                                 Report ordering information at back).
                                 Introduction
                                   Developing technology to predict the
                                 toxicity or reactivity of new and existing
                                 industrial chemicals from structure re-
                                 quires the  simultaneous generation
                                 and  compilation  of  endpoints  to
                                 measure toxicity, etc. and of molecular
                                 descriptors  for representative struc-
                                 tures as well as the development of
                                 statistical methods of relating the two to
                                 form the predictive model. This report
                                 includes a literature review of statistical
                                 methods which indicates that methods
                                 are  available for most  models for
                                 QSAR deemed necessary for EPA pur-
                                 poses. A small effort by this project is
                                 aimed at compiling computer programs
                                 for the most useful statistical methods

-------
into  a QSAR  modeling library  which
uses common  input/output formats.
  The limiting factor for use of QSAR in
screening chemicals, therefore,  is the
availability of  data  on  endpoints  and
molecular  descriptors  for  industrial
chemicals. Consequently,  this project
placed major emphasis on generating a
systematic aquatic  toxicity data base
using the 96-hour  LC50  for fathead
minnows, and on generating  a data
base of important molecular descriptors
such as the n-octanol water partition
coefficients and molecular connectivity
indexes.
Experimental Procedures
  The chemicals  initially selected for
this  study  include a  wide variety of
alcohols, ketones, aldehydes,  ethers,
phenols, and chlorinated aliphatic and
aromatic hydrocarbons.  Toxicity  tests
were conducted in proportional diluters
(Mount and Brungs, 1967) each with a
dilution  factor of 0.6. Test chambers
were glass aquaria measuring 20 x 35 x
25 cm with a 9 cm standpipe, providing
a 6.3 L volume. A 16-hr light, 8-hr dark
photoperiod  with  no transition  was
used.
  All tests were conducted with fathead
minnows (Pimephales promelas)  from
the  Environmental Research Labora-
tory—Duluth  culture  units. Fish  were
hatched and  reared in Lake Superior
water and fed live brine shrimp  at least
twice daily. Typical fish tested were 30
days old and  weighed 0.12 g. Twenty-
five  fish were  randomly assigned to
each of 12 tanks in lots of five. Fish  were
not fed during the 96-hour tests. Deaths
were recorded at hours 1, 3, 6, 12, and
24,  and every  24 hours thereafter.
Concentrations for toxicity in each test
tank were  determined by  chemical
measurements throughout the test.
can  be  grouped  together  in  QSAR
models and that the chemical activity
needed to  produce narcosis in  fish is
similar to that which causes narcosis in
mammals.  The data showed that the
96-hour LC50 for industrial alcohols,
ethers,   alkyl  halides,  ketones,  and
benzene derivatives can be estimated
by the equation:

     log  1   - 1.17 + 0.94 log  P
LC50

where log P is the logarithm of the n-
octanol/water  partition   coefficient.
However, this model is limited to chem-
icals with a log P less than 4.0 until
more data can be generated. The data
also showed that chemicals which do
not behave as narcotics at lethal con-
centrations require additional QSAR for
their specific mode of action.
  An extensive computer program was
developed  which  permits  entry  of
chemical  structure through either a
graphics terminal or conventional TTY
and  calculations  of  134  connectivity
indexes for  even  polycyclic chemicals
were developed.   Preliminary   results
indicate  that the toxicity  of narcotics
can be estimated from structure by the
QSAR:

        log LC50 = 0.28 - 1XV
where 1XV is the first order valence con-
nectivity index of the chemical.
  This approach has the advantage that
the connectivity indexes are computed
rather than measured values,  which
eliminates  experimental error and the
need for data other than structure. It has
the potential for discriminating chemi-
cals in terms of expected mode of action
of lethality, other biological effects and
chemical   reactivity  through  cluster
analysis in a multi-dimensional struc-
ture space.
Results
  During the initial six months of this
project, toxicity tests with more than 50
organic  chemicals indicated  that the
majority of the chemicals caused death
by non-specific physical toxicity-termed
narcosis.  The  literature  review  on
narcosis   presented   in   the  report
showed that the toxicity data for fish are
consistent with literature data in mam-
malian tests in that homologous series
Conclusion
  QSAR  can  provide  cost-effective
methods of screening new and existing
chemicals for potential hazards in the
aquatic environment. The development
of a comprehensive QSAR system for
evaluating the 45,000 chemicals in the
TSCA inventory is limited largely by the
time needed  to generate the training
sets of toxicity data for the numerous
classes of chemicals.

-------
This Project Summary was authored by GilmanD. Veith who was also the EPA
  Project Officer (see below).
The complete report, entitled "State-of-the-Art of Structure Activity Methods
  Development," /Order No. PB 81-187 239; Cost: $11.00, subject to change)
  will be available only from:
        National Technical Information Service
        5285 Port Royal Road
        Springfield. VA 22161
        Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
        Environmental Research Laboratory
        U.S. Environmental Protection Agency
        Duluth, MN 55804
1 US GOVERNMENT PRINTING OFFICE: 1W1-757-OU/7149

-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
      PS   0000329
      U S ENVIR PROTECTION  AGENCY
      REGION 5  LIBRA**
      230 S  DEARBORN  STREET
      CHICAGO  IL  60604

-------