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Item: 6055 

GUIDANCE FOR IDENTIFYING PESTICIDE CHEMICALS AND OTHER  

SUBSTANCES THAT HAVE A COMMON MECHANISM OF TOXICITY

 

 ( January 29, 1999 )

EXECUTIVE SUMMARY

	The Food Quality Protection Act (FQPA) of 1996 requires the United
States Environmental Protection Agency (EPA) to assess the cumulative
risks to human health that can result from exposure to pesticides and
other substances that are toxic by a common mechanism.  The Agency is
currently developing a process for performing cumulative risk
assessments of this type.  Such assessments will play an increasingly
important role in the evaluation of risks posed by pesticides, and will
improve the Agency’s ability to make regulatory decisions that fully
protect public health and sensitive subpopulations, including infants
and children.  

	The identification of pesticides and other substances that cause a
common toxic effect by a common mechanism is the first step of the
cumulative risk assessment process. This document describes the approach
that EPA will use for identifying pesticides and other substances that
cause common toxic effects by common mechanisms of toxicity.  
Specifically, this document describes: EPA’s interpretation of common
mechanism of toxicity with respect to making a determination of safety;
the specific steps that will be taken for identifying mechanisms of
toxicity of pesticides and other substances that cause a common toxic
effect; the types of data and their sources that are needed; how these
data are to be used in reaching conclusions regarding commonality of
mechanisms of toxicity; and criteria the Agency will use for
categorizing pesticides and other substances for purposes of cumulative
risk assessments.  Details on the other aspects of the cumulative risk
assessment process will be discussed in a separate document.

	This document was developed from a draft version entitled Guidance for
Identifying Pesticide Chemicals that Have a Common Mechanism of
Toxicity, for Use in Assessing the Cumulative Toxic Effects of
Pesticides, that was released for public comment in August of 1998 (FR
63 42031, FRL-5797-7).  The Agency received comments from various
organizations.  Each of the commentors offered recommendations for
improving the science policy.  All comments were extensively evaluated
and considered by the Agency.  This revised version embodies many of the
sentiments and recommendations of the commentors. The public comments,
as well as a detailed summary of the Agency’s response to the comments
are being made available in the Federal Register.         

I.  INTRODUCTION

	The Food Quality Protection Act (FQPA) of 1996 stipulates, among other
things, that when determining the safety of a pesticide chemical EPA
shall base its assessment of the risk posed by the pesticide chemical
on: aggregate (i.e., total dietary, residential, and other
non-occupational) exposure to the pesticide and available information
concerning the cumulative effects to human health that may result from
dietary, residential, or other non-occupational exposure to other
substances that have a common mechanism of toxicity.  The Act
specifically mandates the Agency to consider the special susceptibility
of infants and children to the toxic effects caused by pesticides.  The
Agency must also base its risk assessment on available information
concerning the cumulative effects on infants and children to the
pesticide and other substances that have a common mechanism of toxicity.
 The reason for consideration of these factors is due to the possibility
that low-level exposures to multiple substances that cause a common
toxic effect by a common mechanism could lead to the same adverse health
effect as would a higher level of exposure to any of the chemicals
individually.  A person exposed to a pesticide at a level that is
considered safe may in fact experience harm if that person is also
exposed to other substances that cause a common toxic effect by a
mechanism common with that of the subject pesticide, even if the
individual exposure levels to the other substances are also considered
safe. 

  

	Hence, in assessing the risks posed by a given pesticide chemical, EPA
must assess the cumulative risks to human health that can result from
exposure to the pesticide, as well as from other pesticide chemicals and
other substances that are toxic by a common mechanism.   The goal of a
cumulative risk assessment, in regard to implementing FFDCA as amended
by FQPA, is to characterize the potential for a cumulative toxic effect
and the magnitude of the effect in individuals exposed to pesticides and
other substances that cause a common toxic effect by a common mechanism.
 In order to assess these cumulative toxic effects, the Agency needs to
first identify and categorize those pesticides and other substances that
cause a common toxic effect by a common mechanism.  The purpose of this
document is to describe the approach that EPA will use for identifying
and categorizing pesticides and other substances that cause common toxic
effects from common mechanisms of toxicity.  Specifically, this document
describes:

	

• 	EPA’s interpretation of common mechanism of toxicity 				with
respect to 	making a determination of safety under 				FFDCA as amended
by FQPA;

		

	• 	The specific steps that need to be taken for identifying 			
mechanisms of toxicity of pesticides and other substances 			that cause
a common toxic effect;

• 	The types of data (and their sources) that are needed for 				doing
so;

• 	How these data are to be used in reaching conclusions 				regarding
commonality of mechanisms of toxicity;

• 	Criteria the Agency will use for categorizing pesticides 				and
other substances for purposes of cumulative risk 				assessments. 

		

This document does not address how EPA will assess cumulative toxicity
when making determinations of safety.  This topic will be discussed in a
forthcoming Agency science policy document.

II.  DEFINITIONS OF TERMS 

	This document uses a number of terms that are necessary for discussions
of toxic effects, mechanism of toxicity, and the identification of
substances that cause a common toxic effect by a common mechanism. 
These terms are not defined (some are not mentioned) in FQPA.  The
definitions presented here represent EPA's interpretation of the terms
for purposes of implementing the requirements of FQPA.

	Analog(s).  Analog is a generic term used to describe substances that
are chemically closely related.  Structural analogs are substances that
have similar or nearly identical molecular structures.  Structural
analogs may or may not have similar or identical biological properties. 

  

	Toxic Effect.  A toxic effect is an effect known (or can reasonably be
expected) to occur in humans, that results from exposure to a chemical
substance and that will or can reasonably be expected to endanger or
adversely affect quality of life.  Some examples of toxic effects are
acute lethality, loss of hearing, renal tubule necrosis and
cardiomyopathy, to name just a few. 

	Site of a Toxic Effect.  The site of a toxic effect is the specific
anatomical or physiological site or locus (e.g., organ or tissue) at
which the effect occurs.

	Common Toxic Effect.  A pesticide and another substance that are known
to cause the same toxic effect in or at the same anatomical or
physiological site or locus (e.g., same organ or tissue) are said to
cause a common toxic effect.  Thus, a toxic effect observed in studies
involving animals or humans exposed to a pesticide chemical is
considered common with a toxic effect caused by another chemical if
there is concordance with both site and nature of the effect.

	Cumulative Toxic Effect.   A cumulative toxic effect is the net change
in magnitude of a common toxic effect resulting from exposure to two or
more substances that cause the common toxic effect by a common
mechanism, relative to the magnitude of the common toxic effect caused
by exposure to any of the substances individually.     

	Toxophore.  Substances that are capable of causing a toxic effect
contain a structural feature or moiety that bestows the toxic property. 
This structural feature or moiety is referred to generically as the
toxophore, or toxophoric moiety.  A toxic substance elicits its toxicity
through interaction of its toxophore with a biomolecular site (e.g.,
receptor) in cells of tissue or organs to cause changes or alterations
in normal cellular biochemistry. These biochemical changes or
alterations lead to  disruption of the physiological process(es) the
tissue or organs perform and, ultimately, the toxic effect.  The
toxicity of many substances, however, is not due to a direct interaction
with a biomolecular site.  Rather, the toxicity results from metabolism
of a structural substituent to a toxophore, which then causes the
toxicity.  Metabolic pathways that lead to toxicity are often called
bioactivation pathways.

	Mechanism of Toxicity.  Mechanism of toxicity is defined as the major
steps leading to a toxic effect following interaction of a pesticide
with biological targets.  All steps leading to an effect do not need to
be specifically understood.  Rather, it is the identification of the
crucial events following chemical interaction that are required in order
to describe a mechanism of toxicity.  Generally, the more that is
understood about the various steps in the pathway leading to an adverse
effect, the more confident one is about the mechanism of toxicity.  For
instance, a mechanism of toxicity may be described by knowing the
cascade of effects such as the following:  a chemical binds to a given
biological target in vitro, and causes the receptor-related molecular
response; in vivo it also leads to the molecular response and causes a 

number of intervening biological and morphological steps that result in
an adverse effect.  Other processes may describe a mechanism of toxicity
in other cases.

	Common Mechanism of Toxicity.  Common mechanism of toxicity pertains to
two or more pesticide chemicals or other substances that cause a common
toxic effect to human health by the same, or essentially the same,
sequence of major biochemical events.  Hence, the underlying basis of
the toxicity is the same, or essentially the same, for each chemical.

  

	Toxic Action. Toxic action of a given substance is its interaction with
biological targets, to lead to a toxic effect.

	

	Site of Toxic Action.  The site of toxic action of a given substance is
the anatomical or physiological site(s), locus, or loci  at which takes
place the interaction of the substance with its biological targets, to
lead to a toxic effect.

 	

	Structure-Activity Relationships.   Substances that contain or are
bioactivated to the same toxophore may cause a common toxic effect by a
common mechanism.  The relative toxic efficacy and potency among the
substances in their ability to cause the toxic effect may vary. 
Differences in potency or efficacy are directly related to: the specific
or incremental structural differences between the substances; the
influence these differences have on the ability of the toxophore to
reach and interact with its biomolecular site of action; and on the
intrinsic abilities of each of the substances to cause the effect.  The
ability of two or more structurally-related substances to cause a common
toxic effect and the influence that their structural differences have on
toxic efficacy and potency are referred to as structure-activity
relationships. 

	Weight-of-Evidence.  Weight-of-evidence refers to a qualitative
scientific evaluation of a chemical substance for a specific purpose.  
A weight-of-evidence evaluation involves a detailed analysis of several
or more data elements, such as data from different toxicity tests,
pharmacokinetic data, and chemistry data, followed by a conclusion in
which a hypothesis is developed, or selected from previous hypotheses.  
                     

	

III. PROCESS FOR IDENTIFYING PESTICIDE CHEMICALS AND OTHER SUBSTANCES
THAT  HAVE A COMMON MECHANISM OF TOXICITY. 

	To assess the cumulative toxicity of pesticides and other substances
that cause common toxic effects by common mechanisms, EPA will first
need to identify those pesticides and other substances that cause common
toxic effects by common mechanisms, and then group them in accordance
with commonality of toxic effect and toxic mechanism.  Once grouped,
combined risk assessments can be performed and the potential for
cumulative toxicity that may result from exposure to substances within a
group can be characterized.

  

	The conceptual framework of the process that EPA will use to identify
pesticide chemicals and other substances that cause a common toxic
effect by a common mechanism is illustrated in Figure 1.  This process
is designed to enable EPA to make accurate identification and
categorization of pesticides and other substances that are toxic from a
common mechanism, in both a timely and resource-effective manner. 
(Specific examples of the application of this process are in
preparation, and will be made available at a future date.)  To implement
the process, the Agency has convened a multidisciplinary team of EPA
scientists who are experts in chemistry, biology, pharmacology,
toxicology and pharmacokinetics.  It is the responsibility of this team
to identify and analyze data and information pertaining to toxic
mechanisms, and to make expert judgements regarding mechanisms of
toxicity of pesticides and other substances.  The following policies and
practices will be used by the Agency for identifying chemicals that have
a common mechanism of toxicity:

								

	A thorough identification and analysis of all relevant information will
be undertaken for each pesticide chemical and 	other substances under
consideration. This will provide the basis for identifying underlying
mechanisms of toxicity; 

	•	A “weight-of-evidence” approach will be used to support the
development of hypotheses pertaining to mechanisms of toxicity. 
Generally, no single piece of information will suffice to support the
characterization of a specific or common mechanism of toxicity; this
finding will be supported by the analysis and inter-relationships of
available pieces of information;

•	External review of EPA’s decisions concerning: utilization of
established toxic mechanisms; determination of toxic 	mechanisms for
specific substances; and grouping of substances by mechanism of toxicity
will be solicited as needed . 

		When identifying toxic effects, common toxic effects, mechanisms of
toxicity, and common mechanisms of toxicity for purposes of grouping
substances that cause a common toxic effect by a common mechanism of
toxicity, care must be taken not to confuse “mechanism of toxicity”
with “site of toxic action,” or “site of toxic action” with
“toxic effect” or “site of toxic effect.” (These terms are
defined near the beginning of this document.)  With many substances, the
site of a toxic effect is the same as the site of toxic action.  It is
also true, however, that with many other substances the site of a toxic
effect may be different than site of toxic action.  For example, a
substance inhibits the catalytic activity of the peroxidase enzyme
within the thyroid gland.  Inhibition of this enzyme prevents the
synthesis of thyroxine and triiodothyronine, and ultimately leads to
hypothyroidism, the toxic effect.  In this case, the site of the toxic
effect is the same as the site of toxic action: the thyroid gland. 
Another substance known 

to cause hypothyroidism does so by preventing the synthesis of
thyroid-stimulating hormone within the anterior pituitary gland.  Here
the site of the toxic effect is the thyroid gland, but the site of toxic
action is the anterior pituitary gland.  Although these two substances
cause a common toxic effect, they would not be considered for cumulative
risk assessment because they have different mechanisms of toxicity.

		 Many substances can cause more than one toxic effect, depending upon
level of exposure, and do so by different mechanisms of toxicity that
take place at different sites of toxic action.  However, a chemical may
also cause multiple toxic effects at multiple sites from a single
mechanism of toxicity taking place at a single site of toxic action,
provided that the function initially altered at the site of toxic action
normally controls other functions at distant sites.  For example, a
substance that prevents the conversion of cholesterol to corticosteroid
hormones in the adrenal cortex would ultimately cause many effects
throughout the body that would differ in site and nature.  The Agency
will group substances that cause multiple toxic effects by a common
mechanism from a common site of toxic action (e.g., the multiple effects
caused by certain endocrine disruptors) for purposes of cumulative risk
assessment, provided at least one of the toxic effects is common among
the substances.

	Step 1.  Identify a Candidate Set of Substances That Might Cause a
Common Toxic Effect by a Common Mechanism of Toxicity.  The process of
identifying pesticides and other substances that have a common mechanism
of toxicity begins with a preliminary grouping of chemicals that might
cause a common toxic effect by a common mechanism of toxicity (step 1,
Figure 1).  Substances that are related structurally, or have a similar
mechanism of pesticidal action, or share a general mechanism of
mammalian toxicity or cause what could be a common toxic effect in
humans or experimental animals are those that could cause a common toxic
effect by a common mechanism.  Hence, the initial, preliminary grouping
of substances will be based upon at least one of the following criteria:


			•	structural similarity;

			•	mechanism of pesticidal action;

			•	general mechanism of mammalian toxicity; 

			•	a particular toxic effect.

	Use of structural similarity as a starting point for grouping chemicals
relies on the assumption that substances that are structurally analogous
could contain a common toxophore (or may yield a common toxophore upon
metabolism) and may interact analogously with cellular biomolecular
sites to cause a common toxic effect.  To identify pesticides and other
substances that are structurally similar, the Agency will perform
substructure searches in databases containing: registered pesticides;
pesticides for which there are import tolerances; and other substances
(e.g., pharmaceuticals, industrial chemicals) that 

are used in commerce in the United States.  Search queries for
identification of structurally similar substances may include, for
example: toxophore (if known) or metabolic precursor of the toxophore;
base structure; and accompanying functional groups or other substituents
that may impact on the propensity of a substance to produce a
toxicological response common with those of structurally-related
chemicals.

	Preliminary grouping of pesticides based on mechanism of pesticidal
action is justifiable because the mechanisms by which a number of
pesticides are toxic to humans are fundamentally similar or, in some
cases, identical to their mechanisms of intended toxicity to pests. 
With such pesticides the portion of the molecule that is responsible for
pesticidal action is also responsible for human toxicity (i.e., the
portion of the molecule that bestows pesticidal activity is also the
human toxophore). The pesticidal action and human toxicity of these
pesticides are often due to analogous interactions of their toxophores
with specific biomolecular sites that are common to pests and humans,
respectively.

	Preliminary grouping of pesticides and other substances that share a
general mechanism of mammalian toxicity is based on the possibility that
such substances may cause a common toxic effect.  Examples of general
mechanism of toxicity include, for example, substances that uncouple
oxidative phosphorylation, or substances that are known to undergo the
same or similar bioactivation pathways, or that are metabolized to the
same or analogous metabolites that are toxic.  

	Preliminary grouping of pesticides and other substances that cause a
particular toxic effect known to occur in experimental animals or humans
is based on the possibilty that the effect could be common (i.e.,
concordant in both site and nature), and that commonality in toxicity
among two or more substances could be due to a common mechanism.  Since
this type of grouping is functionally-based, not structure-based, it
enables the identification of structurally unrelated substances that
cause a common toxic effect from a common mechanism that otherwise may
not be identifiable from groupings based on structural similarity or
mode of pesticidal action alone.   

	Not all toxic effects can be used as a preliminary basis for grouping
substances.  Toxic effects which have many possible unrelated causes, or
which could be defined as nonspecific in origin are not appropriate as
the primary basis for initial grouping of chemicals.  These effects,
such as body weight changes or death, can result from many unrelated
factors and are usually of limited value in understanding mechanism of
toxicity.  Therefore, such generalized effects, which could have many
different causes, ordinarily will not be used as a basis for initial
grouping of pesticides.  An exception, however, is genetic alterations. 
While genetic alterations can result from a variety of causes, knowledge
of the mechanism by which a chemical substance causes genetic
alterations can provide insight into the mechanism by which it causes
adverse human health effects. Therefore, data for chemicals with common
mutagenic effects may serve as a basis for initial grouping of such
chemicals.

	Following preliminary grouping of substances using any of the criteria
described above, other substances that are mammalian metabolic
precursors to the substances identified under step 1 will be added to
the initial grouping.  The basis for including a metabolic precursor to
a substance identified under step 1 is that since it is metabolized to
the substance, it may cause a common toxic effect by a mechanism common
with that of the substance. 

	It is important to emphasize and to make clear that the purpose of step
1 is for preliminary grouping only, and that substances (including any
metabolic precursors) identified under this step will not 

be included in a cumulative risk assessment if it is determined that
they do not cause a common toxic effect by a common mechanism.   For
example, while some substances that contain the same toxophore, or that
are otherwise structurally analogous, may cause a common toxic effect by
a common mechanism, others may not.    It is also possible for
substances to cause a particular toxic effect in which the nature of the
toxic effect caused by each substance is the same, but the organ or
specific location in the body where the effect occurs differs  among the
substances.  It is also possible for two substances, even those that are
structurally analogous, to cause entirely different toxic effects.  Such
differences between location or nature of a toxic effect can be ascribed
to the specific structural and physicochemical differences between the
substances, and the effect   these differences have on their respective
pharmacokinetics (i.e., absorption, distribution, metabolism, and
excretion of each substance) or pharmacodynamics (i.e., the interaction
of the toxophore with biomolecular  sites).  In these instances the
policy of the Agency is not to group such substances for cumulative risk
assessment purposes, because the toxic effects are not common, as
defined earlier.  It is also possible with substances that cause a
common toxic effect to cause the effect by different mechanisms of
toxicity.  Conversely, substances that share a general mechanism of
toxicity may not necessarily cause a common toxic effect(s).
Furthermore, substances that have a common mechanism of pesticidal
action may not necessarily have a common mechanism of toxicity in
mammals.  Again, in these instances the policy of the Agency is not to
group such substances for cumulative risk assessment purposes because
they are not consistent with the definition of common mechanism of
toxicity.

   

	It must also be stressed that step 1 (like step 2 discussed below) is a
very inclusive and necessary screening step to identify preliminary
groupings of substances for a rigorous assessment.  In particular, the
criteria used in step 1 are very broad, and thus there is a real
possibility that a substantial portion of the pesticide chemicals and
other substances which are included in a preliminary grouping may not
have a common mechanism of toxicity, and many will be dropped from a
group in subsequent steps.  Accordingly, EPA does not regard information
which shows substances meet the step 1 (and step 2) criteria for
grouping as reliable by itself to conclude that such substances have a
common mechanism of toxicity.  Nor does such information create a
sufficient presumption of the existence of a common mechanism of
toxicity that it compels EPA to complete the remaining steps described
below before making its safety determination.

	Hence, only those substances  that EPA determines, through the in-depth
review described below, cause a common toxic effect by a common
mechanism will be considered for cumulative risk assessment.  As shown
in the Figure, this examination will involve: a thorough evaluation of
toxicity data to determine which substances identified under step 1
cause a common toxic effect; determination of the mechanism of toxicity
by which each substance causes the common toxic effect; and subsequent
comparison of each mechanism to confirm or rule-out commonality.   It is
likely that EPA will conclude a substantial portion of the substances
identified in step 1 should not be included in a cumulative risk
assessment.  

  ADVANCE \u 654 

	Step 2.  Definitively Identify Those Substances from Step 1 That Cause
a Common Effect.   The primary purpose of step 2 is to further refine
the preliminary grouping created at step 1 by screening out substances
that obviously do not cause a common toxic effect.  Following the
preliminary grouping of substances (step 1, Figure 1) a detailed
evaluation of available toxicology data for each substance will be
undertaken to identify and characterize the toxic effects caused by each
substance, and to determine which of the substances cause toxic effects
that are common with other substances (i.e., toxic effects that are
concordant in both site and nature).  A primary data set to be used by
EPA will be toxicity data generated in support of regulatory activities
as outlined in 40 CFR 158.  The Agency may also use toxicity data
obtained from other studies, such as those described in government
reports, or the published literature. The evaluation of toxicology data
for purposes of identifying and characterizing toxic effects will be
conducted in a manner similar to that used by EPA in its pesticide
registration and re-registration programs.  

		Most substances, depending upon level of exposure, can elicit more
than one toxic effect (albeit one toxic effect is generally more readily
elicitable than the others).  All toxic effects caused by each
substance, regardless of the exposure levels required to induce the
effects, will be evaluated and compared to the toxic effects caused by
the other substances.  Only those substances that cause a common toxic
effect will remain grouped. Thus, for those substances initially grouped
(step 1) using the “particular toxic effect” criterion (step 1), a
determination as to which substances the toxic effect is in fact common
will need to be made.  The toxicity data for substances initially
grouped using any of the other criteria in step 1 will also be evaluated
to determine which of these substances cause a common toxic effect. 
Substances may be placed in more than one group in instances where
substances cause more than one common toxic effect.   Pesticide
chemicals that do not cause a toxic effect that is common with at least
one other substance identified under step 1 will be eliminated from the
group and, thus, will not undergo further cumulative risk consideration.

	Step 3.  Determine the Toxic Mechanism(s) by Which Each Substance
Causes a Common Toxic Effect.  The next phase of the review process
(step 3, Figure 1) is to determine the mechanisms by which the
substances cause the common toxic effect(s) identified under step 2
(Figure 1).  Generally, the more that is understood about the various
biochemical events that lead to a toxic effect, the more apparent and
scientifically acceptable is the mechanism of toxicity.  While
desirable, all of the specific biochemical events involving a substance
in the causation of its toxicity do not need to be known or completely
characterized in order to describe its mechanism of toxicity.  What is
needed, as a minimum, is an understanding of those biochemical events
that are most crucial in causing the toxicity.  Once the critical
biochemical events pertaining to toxicity are understood for each
substance, they can be compared and identification of those substances
that are toxic from a common mechanism can be made.  Hence, the goal of
step 3 is to determine, to the extent possible for each substance
identified under step 2 as causing a common toxic effect, those
biochemical events that are most critical in causing the effect. 

		The toxic mechanisms of some classes of substances in causing a given
toxic effect have been characterized, and are described in various
literature sources (e.g., textbooks, journals, etc.).   These mechanisms
were elucidated from the development and comprehensive analysis of data 
pertaining to the structure, pharmacokinetics and toxicity of the
substances and their analogs.  The toxophoric moieties and
structure-activity relationships of many of these chemical classes were
similarly characterized. The toxic mechanisms, toxophores and
structure-activity relationships of other pesticides, however, have not
been fully characterized, either because of insufficient data, or
because available data have not yet been fully analyzed.

		Rather than reexamining de novo all of the relevant data, EPA will
assume that a substance is toxic by the mechanism that has been
previously determined provided that the mechanism is consistent with
current toxicological theory and deemed scientifically plausible by the
Agency for these purposes.  Thus, identification of toxic mechanisms
will involve an initial search of Agency databases and the literature
(step 3a, Figure 1) for assessments or studies that describe mechanisms
of toxicity for any of the pesticides grouped in step 2.  The types of
literature sources that will be searched and used include standard
reference and text books,  peer-reviewed journals, government reports,
and study reports submitted to the Agency.  This will allow segregation
of the substances into two sub-groups: those for which mechanism(s) of
causing a common toxic effect have been determined; and those for which
their mechanism(s) have not been determined.  When deemed necessary,
more comprehensive literature or Agency database searches will need to
be conducted to identify data that support or invalidate previously
determined mechanisms of toxicity for which uncertainty exists.

 		EPA will attempt to determine the mechanisms of toxicity of those
substances whose toxic mechanisms are not known or not well understood,
or for which there is an absence of direct mechanistic data.  The
determination of a toxic mechanism will be based upon an evaluation of
various data elements.  The types of data and information that the
Agency will use to develop a scientifically defensible determination of
a given pesticide’s toxic mechanism are structural data,
pharmacokinetic data, and toxicity data.  In situations in which such
data are not available or are insufficient for a pesticide, the Agency
will review and may use mechanistic, structural, pharmacokinetic or
toxicity data pertaining to one or more analogs of the pesticide (or
other substance) as a basis for determining the toxic mechanism of the
pesticide.   Identifying and obtaining pesticide or analog data will
involve a comprehensive search of the literature and Agency databases. 
A primary source of these data and information will be studies that have
been submitted to the Agency in support of registration and
reregistration decisions.  Other sources of data will include
peer-reviewed journals, text books and government reports. 

		The Agency will analyze these data and, using a weight-of-evidence
approach, will attempt to determine the major biochemical events
involving a pesticide (or other substance) that are most critical in
causing its toxicity (step 3b).  From an analysis of a substance’s
structure, for example, the recognition of moieties that are known or
expected to react with biological macromolecules, or are known or
expected to be metabolized to reactive (e.g., radical, electrophilic)
intermediates, or are otherwise known or expected to bestow toxicity may
allow one to infer one or more biochemical events that are responsible
for the substance’s toxicity.  Data that define the metabolism,
distribution and excretion of a pesticide in the body are also very
useful for determining its mechanism of toxicity.  Metabolism data that
show the formation of toxic metabolites in vivo are especially useful
for characterizing metabolic pathways which may be operative in causing
toxic effects.  Distribution and excretion data show the partitioning
patterns of a substance in the body, and may in some cases be used to
infer the types of metabolic transformations that are most likely to
occur and where they are most likely to take place.  These data, in
conjunction with structural and toxicity data, may also provide
explanations for differences in toxicity of structurally similar
substances.  Toxicity data can be helpful in the determination of a
toxic mechanism in many ways.  Genetic alterations, for example, are
important in the causation of cancers and developmental effects. Tests
for genetic alterations that show that a substance (or a metabolite
thereof) forms a covalent adduct with DNA may be useful to infer or
support a mechanism by which a pesticide known to cause cancer or
developmental toxicity causes either of these effects.  

		Data pertaining to analogs of a pesticide or other substance will be
reviewed and may be used in situations in which mechanism-related data
are not available for the pesticide.  An established mechanism of
toxicity of a pesticide’s analog(s), for example, may serve as a basis
for determining the toxic mechanism of the pesticide.  Conclusions based
on the toxic mechanisms of an analog or analogs will only be made when:
there is evidence that shows that the toxicological effects caused by
the pesticide and the analogs are common; there is sufficient evidence
that supports the toxic mechanism of the analog(s); and there is
sufficient evidence for the Agency to conclude that the mechanism of
toxicity of the pesticide is common with the mechanism of toxicity of
the analog(s).  Pharmacokinetic, toxicity and structure-activity
relationship data that are available for analogs of a pesticide will
also be used as a basis for determining the toxic mechanism of the
pesticide (step 3b, Figure 1).  For example, it is stated above that
test data pertaining to genetic alterations may be useful to infer or
support a mechanism of a pesticide known to cause cancer or
developmental toxicity.  Genetic alterations data available for analogs
of a pesticide known to cause cancer or developmental toxicity may be
useful for inferring the mechanism by which the pesticide causes these
effects.   Genetic alterations that are similar among a pesticide and
its analogs are also useful in a weight-of-evidence confirmation of the
validity of such inferences, particularly when mechanistic data are
available for the analog but not for the subject pesticide.

		Relationships between structure and toxicity within a given series of
structurally-similar substances or of a single given substance are often
discernable from an analysis of: the general structure; the chemical
properties of the substance(s); information pertaining to the
pharmacokinetics and toxicity of the substance(s); and the structural
differences within the series and their corresponding affect on toxic
efficacy and potency.  While knowledge of the mechanism of toxicity is
usually not necessary in order to discern a causal relationship between
structure and activity (toxicity), the relationship becomes more
apparent and more useful when the mechanism of toxicity is known.   Once
deduced, the structure-activity relationship of the substance or the
series can be useful for inferring the likelihood of an analogous,
untested chemical to cause the same toxicological effect, and for
estimating its toxic potency.  In cases where the mechanism of toxicity
is known for a substance or a group of substances, structure-activity
relationships are useful for inferring the mechanism of toxicity of an
analogous, untested substance and for supporting or refuting proposed
mechanisms of toxicity of analogous untested substances. 

	Steps 4 and 5. Comparison of Mechanisms of Toxicity (Step 4) and
Refined Grouping of Substances  (Step 5).   Once the mechanism of
toxicity of each substance has been identified, comparisons of
mechanisms will be made to determine which substances identified under
step 2 as causing a given common toxic effect do so by a common
mechanism.  Determinations that two or more substances are toxic by a
common mechanism will be based on similarities in both the nature and
sequence of the major biochemical events that cause toxicity. 
Mechanistic similarities that would support a finding of a common toxic
mechanism include, for example, analogous interactions of the pesticides
or other substances with identical or similar biological targets, or the
occurence of similar metabolic transformations that yield common or
structurally analogous metabolites that interact with similar biological
targets, or that are otherwise involved in causing toxicity.  Substances
that cause a common toxic effect by different mechanisms will excluded
from the refined grouping (Step 5).  

		Peer review of EPA’s decisions concerning: utilization of
established toxic mechanisms; identification of toxic mechanisms for
specific substances; and grouping (or non-grouping) of substances for
purposes of cumulative risk assessment will be solicited in situations
in which the Agency believes additional evaluation is needed to ensure
that Agency decisions are consistent, well-reasoned and reflect current
scientific thinking. 

IV. ASSESSING THE CUMULATIVE TOXICITY POSED  BY TWO OR MORE SUBSTANCES
THAT ARE TOXIC BY A COMMON MECHANISM. 

		Cumulative toxicity represents the net change in toxicity resulting
from exposure to two or more chemical substances, relative to the
toxicity caused by each substance alone.  The evaluation of cumulative
toxicity will be conducted in accordance to a cumulative risk assessment
process being developed by the Agency.  The goal of the cumulative risk
assessment process, in regard to implementing FFDCA as amended by FQPA,
is to characterize the potential for a cumulative toxic effect and the
magnitude of the effect in individuals following known or anticipated
exposures to substances that cause the effect by a common mechanism. 
Pesticide chemicals and other substances within a refined common
mechanism grouping (step 5, Figure 1) will undergo cumulative risk
assessment to determine the potential cumulative toxicity posed by
exposures to such substances.  This will involve consideration of a
number of factors that pertain to: exposure; the pharmacokinetics of
each substance; the nature of the common toxic effect; the
pharmacodynamics of each substance in causing the effect;
pharmacokinetic or pharmacodynamic interactions that may take place
between the substances; subpopulations for which exposures are
anticipated; and susceptibility and sensitivity of exposed individuals
or subpopulations to the common toxic effect.  A discussion of how these
factors affect cumulative toxicity is beyond the scope of this document.
 In addition to the substances wihitn a refined grouping, the Agency
will also consider the potential contribution to cumulative toxicity
from other substances that undergo environmental degradation or
metabolism in plants to any of the substances within the refined group. 
Substances that degrade in the environment, or that are metabolized in
plants to substances in the refined grouping will be included in a
cumulative risk assessment because such precursor substances may
represent an additional source of exposure to the substances in the
refined grouping. The cumulative risk assessment process that the Agency
will use will be described in a forthcoming Agency science policy
guidance document.  The document will include detailed discussions of
the above factors, how these and other factors will be considered by the
Agency in assessing cumulative toxicity, and what the Agency will do
when there are data gaps with the above factors.  

		 

      

	

	

		

	For details see The Federal Insecticide, Fungicide, and Rodenticide Act
(FIFRA) and Federal Food, Drug, and Cosmetic Act (FFDCA) As Amended by
the Food Quality Protection Act (FQPA) of August 3, 1996; U.S.
Environmental Protection Agency, Office of Pesticide Programs, document
# 730L97001, March, 1997.  

	“Other substances” includes pesticide chemicals, pharmaceutical
substances (e.g., drug products), industrial chemicals, and other
substances to which the general population is exposed.

 	Toxic effect is not synonymous with toxic endpoint.  Toxic endpoint is
a quantitative expression of a toxic effect occuring at a given level of
exposure.  For example, acute lethality is a toxic effect, whereas an
LD50 value (median lethal dose) is the toxic endpoint that pertains to
the effect. 

4	The term “toxophore” with respect to toxic substances, is akin to
the term “pharmacophore” with respect to drug substances: the
pharmacophore is that structural moiety of a drug substance or
substances which imparts a desired pharmacological property. 	      

	A biomolecular site refers to a specific area on a particular type of
biomolecule (e.g, DNA, RNA, peptide, protein, lipoprotein, enzyme, etc.)
within a cell.  The toxophoric portion of a given pesticide may interact
reversibly or irreversibly with its biomolecular site, depending upon
the reactive nature of the toxophore and the biomolecular site.	

	In the context of this document, mechanism of toxicity refers to the
mechanism by which a pesticide substance is toxic to humans or
experimental animals, and not the mechanism by which it is toxic to
target or intended species (i.e., its mechanism of pesticidal action).
With some pesticides, however, the mechanism responsible for causing
toxicity to humans or experimental animals is similar to the mechanism
of pesticidal action.    

		

7	 Toxic efficacy is the intrinsic ability for a substance to produce a
given toxic effect. Maximal toxic efficacy is reached when an increase
in dose no longer causes an increase in the magnitude (intensity) of the
effect.  Toxic potency is the magnitude of the toxic effect that results
from a given exposure level (or dose), or the range in magnitude of the
toxic effect that corresponds to a range in levels of exposure. 
Relative toxic potency refers to a comparison of the exposure level or
dose required of an individual substance to the exposure levels or doses
required of other substances to cause a common toxic effect of an
equivalent magnitude (e.g, LD50, ED50) by a common mechanism of
toxicity.

	In the context of this document the term “activity” is synonymous
with toxicity. 

