Document ID: EPA-HQ-OPP-2002-0302-0175
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2007-12-05T05:00Z

SEQ CHAPTER \h \r 1 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

  SEQ CHAPTER \h \r 1 OFFICE OF

PREVENTION, PESTICIDES AND TOXIC SUBSTANCES

		OFFICE OF

		PREVENTION, PESTICIDES AND

		TOXIC SUBSTANCES



TXR No.  0053990

MEMORANDUM

DATE:  March 20, 2006

SUBJECT: 	Human Studies Review Board:  Final Weight of Evidence
Comparison of Human and Animal Toxicology Studies and Endpoints for DDVP
Human Health Risk Assessment and Discussion of Interspecies
Extrapolation in the Organophosphate Cumulative Risk Assessment.

DP Barcode:  327499	Reregistration Case#:  0310

PC Code:  084001	                                                       
    MRID #:  several

FROM:	Ray Kent, Ph.D., Chief		

		Reregistration Branch 4

		Health Effects Division (7509C)

		William Dykstra, Ph.D., Toxicologist

		Reregistration Branch 4

		Health Effects Division (7509C)

TO: 		Tina E. Levine, Ph.D., Director	

		Health Effects Division (7509C)

This weight of evidence (WOE) document for dichlorvos (DDVP) describes
the Agency’s rationale for selecting a repeated dose human toxicity
study (MRID 44248801) to assess risks from short- and intermediate term
residential and occupational exposure to DDVP.  In addition, the WOE
document also discusses why an acute human toxicity study (MRID
44248802) was not considered appropriate to assess risks from a single
exposure to DDVP.  The WOE document also discusses the Agency’s
conclusions regarding the usefulness of this study in the cumulative
risk assessment for the OPs.  

Background.  Dichlorvos is an organophosphate insecticide that is toxic
to mammals, including humans, through inhibition of the
acetylcholinesterase(s) of the peripheral and/or central nervous system.
 The technical registrant for dichlorvos has submitted a number of
toxicity studies involving direct dosing of humans to support their
contention that humans are no more or less sensitive to the effects of
dichlorvos than rats, dogs or other experimental animals.  In this
document,  HED scientists compare the strengths and weaknesses of the
human and animal toxicity studies and present how the human studies
compare with the animal studies, i.e., are the human data consistent
with animal data in terms of types of effects and effect levels or are
there notable differences between animals and humans.

This document focuses on two human studies in which humans were
intentionally dosed with dichlorvos; a single dose oral study (MRID
44248802) and a repeated dose oral study (MRID 44248801).  The human
data are compared with animal data from similar studies, and
recommendations for endpoint selection are made based on the most
appropriate studies and uncertainty factors.  All of the studies are
discussed in summary form and then the weight of evidence discussion of
endpoint selection follows.

Chemical and Hazard Characterization of Dichlorvos

 tc "Chemical and Hazard Characterization of Dichlorvos

" 

Dichlorvos is a phosphate triester with a molecular formula of
C4H7O4PCl2 and a molecular weight of 221.  It is a liquid at room
temperature with a relatively high vapor pressure of 0.032 mm (30 C).  

The high vapor pressure of dichlorvos is the basis for its use as a
fumigant for processed food commodities, food warehouses and
food-handling establishments.  Because of its volatility, dichlorvos is
also incorporated into resin strips for use at many sites including
family homes.

Like most other cholinesterase-inhibiting phosphotriesters, dichlorvos
is asymmetrical with respect to the ester substituents, with two
relatively difficult to hydrolyze methyl groups and a dichlorovinyl-
“leaving group” which is more readily hydrolyzed and is the group
displaced when dichlorvos reacts at the active site of cholinesterases. 
Many sulfur-containing organophosphate cholinesterase inhibitors require
metabolic activation to convert an unsubstituted phosphorous-sulfur
(P=S) group to a phosphorus-oxygen oxon (P=O) group before inhibition of
cholinesterase can occur; however, dichlorvos already exists in the oxon
form and needs no activation to inhibit cholinesterases.  

Dichlorvos is well absorbed by all routes of exposure and extensively
metabolized with excretion of metabolic products occurring mostly in the
urine and through exhalation as CO2.  Absorbed dichlorvos is initially
inactivated by an esterase found in plasma and liver.  The esterase
catalyzes the hydrolysis of dichlorvos to dimethyl phosphate and
dichlorovinyl alcohol which spontaneously rearranges to
2,2,-dichloroacetaldehyde which is then metabolized further.  Dichlorvos
may also be inactivated by a glutathione-dependent reaction to form
desmethyl dichlorvos.  The half-life of dichlorvos in the blood is very
short, 15 minutes or less.

	

Dichlorvos inhibits plasma, erythrocyte, and brain cholinesterase in a
variety of species, but does not cause organophosphate-induced delayed
neurotoxicity (OPIDN) in the hen (MRID 43433501).  In acute and 90-day
neurotoxicity studies in rats (MRIDs 42497901, 41004101, there was no
neuropathology associated with changes in FOB and motor activity. 
Subchronic and chronic oral exposures in rats and dogs (MRIDs 41004701,
41593101) as well as chronic inhalation exposure in rats (MRIDs
00057695, 00532569) resulted in significant decreases in plasma, red
blood cell and/or brain cholinesterase activity.  Animal toxicity
studies do not show evidence of gender susceptibility. Repeated, oral
subchronic exposure in male humans was associated with statistically and
biologically significant decreases in red blood cell cholinesterase
activity (MRID 44248801). 

	There was no evidence of increased susceptibility following in utero
exposure to rats and rabbits as well as pre/post natal exposure to rats
in developmental and reproduction studies (MRIDs 41802401, 41951501,
42483901).  However, increased sensitivity to dichlorvos-induced
inhibition of brain cholinesterase activity was observed in repeated
gavage studies in preweaning rats in comparison to young adult rats
(MRID 46153304).  These findings necessitated that a special Food
Quality Protection Act (FQPA) 3x safety factor be retained for
assessment of risks (other than risks from acute exposure) where the
endpoint is based on RBC cholinesterase inhibition as a result of
repeated exposure.  A factor of 3x is considered appropriate since the
differences in brain cholinesterase inhibition were minimal.  The factor
of 3x is not needed for assessing acute risks, since there was no
increased sensitivity in brain cholinesterase activity in preweaning
rats in comparison to young adults following a single gavage dose of
dichlorvos.

Specific Toxicity Studies

Single Dose Oral Studies:

A. Human

In a single dose human study with DDVP (MRID 44248802), the NOAEL for
RBC cholinesterase depression is 1.0 mg/kg bw based on the absence of
statistically significant ChE depression in 6 fasted young healthy male
volunteers administered a 70 mg oral dose of DDVP.  In this study, the
first cholinesterase measurement was recorded 24 hours after dosing.  In
another study (MRID 46153303) on the measurement of RBC and brain ChE
activity in pre-weaning and adult female rats treated with a single dose
of 15 mg/kg dichlorvos, time-course data demonstrate that the time of
peak effect for both RBC and brain ChE measurements is 1-3 hours
post-dosing  and that by 24 hours post-dosing, RBC cholinesterase
activity has recovered to levels similar to the controls (MRID
46153303).  Therefore, the absence of biologically significant RBC ChE
depression in the human study may be due to the absence of blood
sampling at the time of peak effect (1-3 hours), since in the human
study, the first measurement did not occur until 24 hours after dosing.

B. Animals

Single dose comparative cholinesterase studies in preweaning and adult
rats which measured both RBC and brain ChE depression at 1 hour
following oral exposure were analyzed by a Benchmark Dose (BMD)
procedure (MRIDs 45805703, 45842301).  The BMDS (Benchmark Dose Software
version 1.3.2) model was used to derive the BMD10, the estimated dose
that results in 10% inhibition of cholinesterase, and the BMDL10, the
lower 95% confidence interval on the BMD10, for the cholinesterase data
evaluated.  For this analysis, the continuous polynomial model was used
with the default option of relative deviation for the benchmark response
(BMR) type.  A BMR factor of 0.1 was the basis for BMD10 and BMDL10
derivation.

A BMDL10 of 0.8 mg/kg (BMD = 1.6 mg/kg) based on female brain ChE
depression was selected as the lowest value of all the studies available
which were analyzed.  Consistent with EPA’s Draft Technical Benchmark
Dose Guidance (2000), the BMDL, not the BMD, is used to extrapolate
risk.  BMD analysis of studies with pup and adult ChE depression results
did not demonstrate any substantial age-related numerical differences in
BMDL values (all values were approximately 1 mg/kg) for either RBC or
brain cholinesterase. 

Repeated Dose Oral Studies

A. Human

In a single blind oral study (MRID 44248801), 6 fasted male volunteers
were administered 7 mg of DDVP in corn oil (equivalent to approximately
0.1 mg/kg/d) via capsule daily for 21 days. Three control subjects
received corn oil as a placebo.  Baseline values for RBC cholinesterase
activity for each study participant were determined.  After dosing
started, RBC cholinesterase activity was monitored on days 2, 4, 7, 9,
11, 14, 16, and 18, then on day 25 or 28 post dosing.  No toxicity
attributable to administration of DDVP was reported.  Mean RBC
cholinesterase activity was statistically significantly reduced in
treated subjects on days 7, 11, 14, 16, and 18. These values were 8, 10,
14, 14, and 16 percent below the pre-dose mean.  Although the percent
mean depression was less than 20%, the blood samples were not taken
until just before the next day’s dose, at the point of maximum
recovery.   Under the conditions of the study, a LOAEL for RBC
cholinesterase inhibition was established at 0.1 mg/kg/d based on the
consistent, statistically significant ChE depression over time, although
ChE depression during the study was less than 20%. A NOAEL was not
established.

The repeated dose human study has been criticized for a number of
reasons including:

1) Too few subjects.   There were six treated adult males and three
adult males served as placebo controls.  All of the treated males
responded to some extent to repeated dosing of dichlorvos with a mean
response of 16%.  If there had been no response, then the argument that
there were insufficient subjects might have more merit, however, the
Agency has determined that the administered dose is a LOAEL.

2) Use of males only.   All subjects were adult males.  Animal toxicity
studies do not show evidence of gender susceptibility.

3) Administration of only a single dose level.  A single dose level of 7
mg per person per day was administered for 21 days.  This dose resulted
in sufficient RBC cholinesterase inhibition that we consider the
response to be a LOAEL.  If there had been no response, interpretation
of the results would have been problematic.

4) Blood sampling did not occur until 24 hours after dosing.  This is
considered a critical deficiency for the single dose study; however,
after 21-days there is a clear response which the Agency considers a
LOAEL.  If blood had been sampled at 1-3 hours after dosing, RBC
cholinesterase inhibition may have been somewhat greater.

B.  Animal

Comparative cholinesterase (7-day rat).  In a comparative cholinesterase
inhibition study (MRID 46153304), dichlorvos was administered by gavage
in  seven daily doses of 0, 0.1, 7.5, or 15 mg/kg/day to groups of 5
rats/sex beginning on either PND 12 (pre-weaning rats) or 42 (young
adults) and animals were sacrificed one hour after the last dose.  RBC
and brain ChE activities were measured in all animals in each study.  In
pre-weaning rats, tremors were observed in 5/5 males and 5/5 females at
15 mg/kg/day on 3-5 days of the dosing interval.  In young adult rats at
15 mg/kg/day, tremors were observed in 3/5 males and 5/5 females on one
to four days of the dosing interval.  In addition, tremors were seen in
one adult male after the last dose of 7.5 mg/kg/day.  No clinical signs
of toxicity were observed in the remaining groups.  Dose-related
inhibition of RBC and brain ChE activities was apparent after repeated
dosing in both adult and pre-weaning rats.  Biologically significant
inhibition of RBC enzyme activity (>50%) occurred at doses of 7.5 and 15
mg/kg/day in both sexes of adults and pre-weaning and at the low dose
for adult animals (11-17%).  Brain enzyme activity was statistically and
biologically inhibited in both sexes at doses of 7.5 and 15 mg/kg/day
for adults (>50%) and at all doses for pups (>20%).  The LOAEL for
inhibition of RBC cholinesterase was 0.1 mg/kg/day and a NOAEL was not
identified based on findings in young adults.  The LOAEL for inhibition
of brain cholinesterase was 0.1 mg/kg/day and a NOAEL was not identified
based on findings in pre-weaning pups.  

Subchronic neurotoxicity (90-day rat).  In a subchronic oral
neurotoxicity study (MRID 42958101), dichlorvos was administered in
deionized water to 15 Sprague-Dawley rats/sex/group at gavage doses of
0, 0.1, 7.5, or 15.0 mg/kg/day for 90 days.  Within each dose group, 10
rats/sex were allocated for brain cholinesterase determination and 5
rats/sex were allocated for neuropathology evaluation. Additionally,
blood samples were collected for cholinesterase measurements prestudy
and on study weeks 3, 7, and 13 .  Five rats/sex/dose from the
cholinesterase group and 5/sex/dose from the neuropathology group were
evaluated with the Functional Observational Battery (FOB) and motor
activity tests pretest and on study weeks 3, 7, and 12.  Body weight and
food consumption were measured weekly.

There was no treatment-related mortality.  Mean body weight in high-dose
females was consistently lower than the control (11-21%) throughout the
study.  No body weight effects were observed in any other animals, and
there was no treatment-related effect on food consumption.  Tremors,
salivation, exophthalmos, lacrimation, and clear material on the
forelimbs were observed in high-dose males and females approximately 15
minutes post-dosing.  Rales, chromodacryorrhea, and red/yellow/orange
material around the nose and mouth were also seen in high-dose rats. 
Tremors were observed in three mid-dose males and nine mid-dose females.
 Generally, the clinical signs occurred during the third week of
treatment in the mid-dose animals and as early as the first week of
dosing and throughout the study in the high-dose rats.  Cholinesterase
activity was decreased in mid- and high-dose male and female rats as
follows: plasma 30-58%; erythrocyte 8-35%; brainstem and brain cortex
10-16%.  There were no treatment-related effects in the FOB or motor
activity tests.  No treatment-related neurohistopathological lesions and
no apparent changes in brain weight or size were observed. 

Based on decreased cholinesterase activity and clinical cholinergic
signs, the LOAEL for dichlorvos is 7.50 mg/kg and the NOAEL is 0.1
mg/kg. 

Chronic (One-year dog).  In a chronic feeding study (MRID 41593101),
groups of 4/sex/dose beagle dogs were administered dichlorvos by capsule
for 52 weeks at dose levels of 0, 0.1, 1.0 and 3.0 mg/kg/day.  The 0.1
mg/kg/day dose was lowered to 0.05 mg/kg/day on day 22 due to the
inhibition of plasma cholinesterase noted after 12 days (plasma
cholinesterase was decreased in males (21.1%) and females (25.7%) at
week 2 in the 0.1 mg/kg/day group).  At time points after week 2, plasma
cholinesterase activity was only significantly reduced in males (39.1 to
59.2%) and females (41.0 to 56.7%) in the mid-dose group and in males
(65.1 to 74.3%) and females (61.1 to 74.2%)in the high dose group. 
Although RBC cholinesterase activity was reduced in males (23.6%) and
females (50.1%) at week 6 in the low-dose group, this was believed to be
residual effect on RBC cholinesterase of the higher dose of 0.1
mg/kg/day.  RBC cholinesterase inhibition was not observed in this group
after week 6.  At time points after week 6, RBC cholinesterase activity
was only significantly decreased in males (43.0 to 53.9) and females
(38.0 to 51.9) in the mid-dose group and in males (81.2 to 86.9%) and
females (79.2 to 82.5%) in the high-dose groups.  Brain cholinesterase
activity was significantly reduced in males (22%) in the mid-dose group
and in males (47%) and females (29%) in the high dose group.  The NOAEL
was 0.05 mg/kg/day and the LOAEL was 0.1 mg/kg/day based on plasma and
RBC cholinesterase inhibition in males and females.

Other animal studies.  There are several other animal studies by the
oral route in which RBC and plasma cholinesterase were measured: 1)
90-day rat subchronic in rats (MRID 41004701), LOAEL = 1.5 mg/kg/day
based on plasma and RBC cholinesterase inhibition, NOAEL = 0.1
mg/kg/day; 2) Range-finding study for the rabbit developmental study
(MRID 41802401), LOAEL = 1 mg/kg/day based on RBC cholinesterase
inhibition, NOAEL = 0.1 mg/kg/day.  Repeated dose (28-day) delayed
neurotoxicity study in hens (MRID 43433501), LOAEL = 0.3 mg/kg/day based
on inhibition of brain cholinesterase, NOAEL = 0.1 mg/kg/day.

Endpoint Selection.

Acute RfD.  In the past, the acute dosing study in humans was considered
suitable for use in establishing an acute RfD, but recently received
time-course data in rats indicate that peak inhibition of RBC
cholinesterase occurs 1-3 hours after oral dosing and that by 24-hours
post-dosing, cholinesterase activity returns to near control levels. 
The risk assessment team concludes that the lack of cholinesterase
measurements prior to 24 hours post-treatment in the acute human study
is a deficiency so critical that it has opted not to rely on the acute
human study for either establishing an acute RfD or to decrease the
interspecies uncertainty factor.

The rat acute BMDL10, 0.8 mg/kg, was selected for assessment of acute
exposure scenarios.  An uncertainty factor of 100 (10x for interspecies
differences and 10x for intraspecies variation) was applied.  It was
concluded that an additional special FQPA factor is not needed, since
BMD analysis of studies with pup and adult ChE depression results for
either RBC or brain cholinesterase inhibition did not demonstrate any
substantial numerical differences in the acute BMDL values for either
RBC or brain cholinesterase inhibition (all values were approximately 1
mg/kg).  Based on this assessment, an acute RfD of 0.008 mg/kg/day for
the general population was derived for DDVP. 

Short term residential & occupational exposure.  There are a number of
repeated dose studies that are under consideration, either individually
or collectively, for providing appropriate endpoints for risk assessment
for short- term durations.  

 -  The 21-day repeated dose study in humans with a LOAEL for RBC
cholinesterase inhibition of 0.1 mg/kg/day is of an adequate duration
for selection of endpoints.

 -  In the 7-day repeated dose comparative cholinesterase study in
rats, the LOAEL for adult rats for RBC cholinesterase inhibition was 0.1
mg/kg/day, whereas in pre-weaning rats, 0.1 mg/kg was a NOAEL for RBC
cholinesterase inhibition.  

 -  In the 7-day repeated dose comparative cholinesterase study in rats,
the NOAEL for inhibition  of brain cholinesterase inhibition in adult
rats was 0.1 mg/kg/day, whereas 0.1 mg/kg/day was a LOAEL in preweaning
rats.

The DDVP risk assessment team concludes that the 21-day human study is
sufficiently robust and is more reflective of the short-term exposure
duration of 30 days or less, and has therefore selected that study for
assessment of short-term risks via all routes of exposure.

Intermediate term residential & occupational exposure.  There are a
number of repeated dose studies that are under consideration, either
individually or collectively, for providing appropriate endpoints for
risk assessment for intermediate-term durations.  

-  The 21-day repeated dose study in humans with a LOAEL for RBC
cholinesterase inhibition of 0.1 mg/kg/day is of an adequate duration
for selection of endpoints.

 - The NOAEL in a 90-day rat subchronic neurotoxicity study was 0.1
mg/kg/day and the LOAEL was 7.5 mg/kg/day based on clinical signs of
neurotoxicity and inhibition of plasma, RBC and brain cholinesterase.

 -  In a chronic feeding study in dogs, the LOAEL for plasma and RBC
cholinesterase inhibition was 0.1 mg/kg/day and the NOAEL was 0.05
mg/kg/day measured at 3 and 6 weeks.  The NOAEL for brain cholinesterase
inhibition measured at the end of the study was 0.05 mg/kg/day.

The findings in the repeated dose studies are consistent across species
and over study durations ranging from seven days to one year, with the
LOAEL or NOAEL fluctuating around 0.1 mg/kg/day for RBC or brain
cholinesterase inhibition.  The HED dichlorvos risk assessment team is
of the opinion that the endpoint of RBC cholinesterase inhibition in the
human repeated dose study is well supported by several animal studies
and should serve as the basis for comparison in assessing short- and
intermediate-term risks.  A MOE of 100 is recommended to account for
intraspecies variability (10x), the lack of NOAEL in this study (3x) and
a Special FQPA factor of 3x.  The Special FQPA factor is based on
residual concern for the sensitivity shown by young rats to brain
cholinesterase inhibition in the repeated dose comparative
cholinesterase study (MRID 46153304).  A Special Factor of 3x is
considered sufficient since the percent inhibition (25%) in brain
cholinesterase in preweaning rats at 0.1 mg/kg/day is not substantial. 
The factor should be applied in any situation where the endpoint is
based on RBC cholinesterase inhibition.  A factor of 3x rather than 10x
was used to account for a lack of a NOAEL since the RBC cholinesterase
inhibition in humans during the exposure period of 21 days was minimal. 
 

The dichlorvos risk assessment team acknowledges that there may be some
uncertainty associated using the 21-day study for exposures up to 6
months in duration, particularly given that steady state has not yet
been reached in the human study.  However, the Team notes that for
purposes of quantitative risk assessment, use of the steady state BMDL10
of 0.4 mg/kg/day from the rat 90 day subchronic study (see below) with
10X factors for intra- and inter-species extrapolation yields a
regulatory endpoint of 0.004 mg/kg/day.  Use of the human LOAEL of 0.1
mg/kg/day with a 3X for LOAEL to NOAEL with a 10X factor for
intra-species extrapolation yields a regulatory endpoint of 0.003
mg/kg/day.  These two approaches provide very similar regulatory
endpoints. Thus, the human LOAEL provides a reasonable endpoint for
extrapolating human health intermediate-term risk. 

Chronic RfD and long term residential & occupational.  The same
rationale described under short- and intermediate-term scenarios could
be used for long-term scenarios since study duration didn’t seem to
make any difference in the NOAELs and LOAELs; however, the risk
assessment team concluded that it would be difficult to justify using a
21-day human study for chronic or long-term scenarios.  The risk
assessment team recommends continuing to use the one-year dog study for
establishing a chronic oral RfD and to assess long-term occupational or
residential risk.

For the chronic dog study with a NOAEL of 0.05 mg/kg/day, an uncertainty
factor of 300x was selected (10x for interspecies variation, 10x for
intraspecies extrapolation, and 3x for a Special FQPA factor as
described above.  The risk assessment team considered reducing the
interspecies factor to 3x based on the similarity of the human data to
the dog data, but chose to continue with the standard 10x value based
again on the difficulty in justifying use of a 21-day study (with a
LOAEL) to decrease the interspecies factor for a chronic study.  Based
on this information, a chronic oral RfD of 0.00017 mg/kg/day was
established for DDVP based on the dog study. 

Note on Human Studies Cited in MacGregor et al (2005)

In its submissions to, and discussions with the Agency about the risk
assessment of dichlorvos, the registrant has consistently made the
argument that the body of experimental animal and human data indicate
that humans are no more sensitive to the cholinesterase-inhibiting
effects of dichlorvos than are laboratory animals, and therefore, the
interspecies factor reflecting uncertainty about sensitivity across
species should be 1x.  In a publication appearing last year (MacGregor
et al, 2005), the registrant and its consultants reiterated their
argument for removing the interspecies factor and cited key human and
experimental animal studies conducted over four decades in support of
their thesis.

EPA has looked at these studies and remain of the opinion that a weight
of evidence argument such as MacGregor et al. present is only as robust
as the individual studies that are used to build the argument.  The
publication lists 29 studies “available for assessment of DDVP
toxicity potential”, but only 11 of these are used for the
quantitative interspecies comparison (capsulized in Appendix A).  There
are four oral studies listed for quantitative comparison, two of which
are discussed in this weight of evidence document (Gledhill, 1997a,
1997c).  One of the remaining oral studies, (Slomka and Hine, 1981)
utilizes a slow-release formulation of dichlorvos entrained in plastic
beads.  The availability of dichlorvos in this formulation is unknown. 
The remaining study (Gledhill, 1997b) is deficient as well (Appendix A).

There are 7 human inhalation studies, 3 of which use children as
subjects and are therefore are excluded in accord with the Human Studies
Rule.  Two of the remaining 4 studies (Smith et al., 1972 and Witter et
al., 1961) limit exposure to 2 hours or less which is of little use in
assessing current use patterns, or for comparing with animal data. One
study (Stein et al., 1966) monitors exposure to workers over a few
weeks, but it isn’t possible from the study to determine what the
actual exposures to the subjects were. One study (Ueda and Nishimura,
1967) in which four subjects were exposed to relatively high levels for
2 days may have limited use for comparing with animal data.

The Agency has concluded that these studies are insufficient to support
a weight of evidence argument that the interspecies factor should be
reduced to 1X.  

OP Cumulative Risk Assessment

The Food Quality Protection Act (FQPA) was passed by Congress in 1996. 
The FQPA made key changes to the approaches used by EPA to assess
pesticide chemicals.  One of these changes was the requirement to
consider cumulative risk to those pesticides which act by a common
mechanism of toxicity.  Pesticides are determined to have a "common
mechanism of toxicity" if they act the same way in the body--that is,
the same toxic effect occurs in the same organ or tissue by essentially
the same sequence of major biochemical events.   OPP established the
organophosphate pesticides (OPs) as a common mechanism group and in
accordance with FQPA has developed a cumulative risk assessment for this
group of pesticides (USEPA, 2002a).   DDVP is a member of the OP common
mechanism group.  

OPP has developed a guidance document for developing cumulative risks
assessments under FQPA (USEPA, 2002b).  This guidance indicates that
when developing multi-chemical hazard assessments, comparison of toxic
potency should be made using a uniform basis of comparison, by using to
the extent possible a common response derived from a comparable
measurement methodology, species, and sex for all the exposure routes of
interest.  In the OP cumulative risk assessment, brain ChE data from rat
toxicity studies in duration of 28 days and longer have been used by EPA
to estimate relative potency and to develop the points of departure for
extrapolating cumulative risk.  ChE inhibition of OPs typically reaches
steady state in rats at or near 28 days of exposure.  Thus, potency
estimates for exposures in duration from approximately one month or
longer are consistent and show less variation than potency estimates for
shorter exposure durations.  Brain data have been selected over RBC data
as brain ChE inhibition represents a direct measure of the target tissue
(as opposed to blood data which is considered a surrogate measure) and
brain ChE tend to have less variation and thus confer less uncertainty
on cumulative risk estimates.  

In the DDVP human multi-dosing study, the subjects exhibited 8, 10, 14,
14, and 16 percent below the pre-dose mean on days 7, 11, 14, 16, and 18
respectively.  These data indicate that there is an increase in
inhibition from days 7 to 18 of exposure.  As such, steady state in
humans may not have yet been reached during the DDVP study.  The Agency
notes given the relatively small increase (2%) in inhibition observed
from days 14 to 18, it is unlikely that RBC ChE inhibition would
increase substantially with prolonged exposure.  

Incorporation of the results from the DDVP human multi-dosing study may
create a mis-match with the relative potency factors and points of
departure being used in the cumulative risk assessment which are based
on ChE inhibition at steady state.  The Agency has determined that for
the cumulative risk assessment, the inter-species factor for oral
exposure to DDVP will be 10X.  This determination is based, in part, on
uncertainties associated with lack of steady state ChE inhibition in the
human study and potential mis-matching with the data being used for
relative potency and points of departure.   The 10X inter-species
extrapolation factor is further supported by a comparison of the results
from the human study with RBC BMD10s estimated by EPA previously (USEPA,
2001) from the DDVP rat subchronic study where steady state was reached
(MRID no. 41004701; see table below).  Dividing rat BMD10 of
approximately 0.6 mg/kg/day by the human endpoint (0.1 mg/kg/day) yields
a rat to human extrapolation factor of approximately 6X.  It is notable
that rat benchmark response is based on 10% RBC ChE inhibition whereas
the human endpoint is 16%.  It is reasonable to expect that the dose to
result in 10% RBC ChE inhibition in humans would be lower than that
resulting in 16% RBC ChE inhibition.  As such, the 6X ratio is likely to
be higher (ie, closer to 10X).

Sex and Age	RBC

Human (male)	16%  at  0.1 mg/kg/day on Day 18

Rat 	BMD10	BMDL10

Male	0.57- 0.60	0.41 – 0.49

Female	0.65	0.45 – 0.54

Bibliography

Gledhill, A. (1997) Dichlorvos: A Single Blind, Placebo Controlled,
Randomised Study to Investigate the Effects of Multiple Oral Dosing on
Erythrocyte Cholinesterase Inhibition in Healthy Male Volunteers: Lab
Project Number: CTL/P/5392: XH6063. Unpublished study prepared by Zeneca
Central Toxicology Lab. 52 p. (MRID 44248801)

Gledhill, A. (1997) Dichlorvos: A Study to Investigate the Effect of a
Single Oral Dose on Erythrocyte Cholinesterase Inhibition in Healthy
Male Volunteers: Lab Project Number: CTL/P/5393: XH6064. Unpublished
study prepared by Zeneca Central Toxicology Lab. 44 p. (MRID 44248802)

USEPA, 2000. “Benchmark Dose Technical Guidance Document” Draft
report. Risk Assessment Forum, Office of Research and Development, U.S.
Environmental Protection Agency. Washington, DC. EPA/630/R-00/001

USEPA (2001).  Preliminary Organophosphorus Pesticide Cumulative Risk
Assessment.  Office of Pesticide Programs, U.S. Environmental Protection
Agency.  Washington,
DC.http://www.epa.gov/pesticides/cumulative/pra_op/.

USEPA  (2002a).  “Guidance on Cumulative Risk Assessment of Pesticide
Chemicals That Have a Common Mechanism of Toxicity.”  January 14,
2002.  (67 FR 2210; January 16, 2002) 
http://www.epa.gov/oppfead1/trac/science/#common

USEPA  (2002b).  Revised Organophosphorus Pesticide Cumulative Risk
Assessment.  Office of Pesticide Programs, U.S. Environmental Protection
Agency.  Washington, DC. June 10, 2002. 
http://www.epa.gov/pesticides/cumulative/rra_op

Table I. Guideline Toxicology Studies for Dichlorvos in Experimental
Animals and Humans

Study Type/Guideline No.	MRID No.	Results

Acute Oral Cholinesterase Inhibition Study (1st) in Adult Rats/870.1100
(modified)	45805701

Acceptable	ChEI NOAEL (RBC and Brain) = not established

ChEI LOAEL (RBC and Brain) = 2.1 mg/kg

Acute Oral Cholinesterase Inhibition Study (2nd) in Adult Rats/870.1100
(modified)	45805702

Acceptable	ChEI NOAEL (RBC and Brain)  = 1 mg/kg

ChEI LOAEL (RBC and Brain) = not established

Acute Oral Cholinesterase Inhibition Study (3rd) in Adult Rats/870.1100
(modified)	45805703

Acceptable	ChEI NOAEL (RBC and Brain)  = 1 mg/kg

ChEI LOAEL (RBC and Brain)  = 5 mg/kg

RBC ChE (F/M) BMD10 = 1.4/1.7 mg/kg; 

RBC ChE (F/M) BMDL10 = 1.2/1.3 mg/kg; 

Brain ChE (F/M) BMD10 = 1.3/1.6 mg/kg 

Brain ChE (F/M) BMDL10 = 0.8/1.0 mg/kg 

Acute Oral Cholinesterase Inhibition Study in Preweaning Rat
Pups/870.1100 (modified)	45842301

Acceptable	ChEI NOAEL (RBC)  = not established

ChEI LOAEL (RBC)  = 1 mg/kg

ChEI NOAEL (Brain)  = 1 mg/kg

ChEI LOAEL (Brain)  = 5 mg/kg

PND8 RBC ChE (F/M)BMD10 = 1.5/1.8 mg/kg; 

PND8 RBC ChE (F/M)BMDL10 = 1.0/1.3 mg/kg; 

PND8 Brain ChE (F/M)BMD10 = 2.2/1.8 mg/kg 

PND8 Brain ChE (F/M)BMDL10 = 1.6./1.5 mg/kg 

PND15 RBC ChE (F/M)BMD10 = 1.3/1.5 mg/kg; 

PND15 RBC ChE (F/M)BMDL10 = 1.1/1.2 mg/kg; 

PND15 Brain ChE (F/M)BMD10 = 1.4/1.6 mg/kg 

PND15 Brain ChE (F/M)BMDL10 = 1.0./1.3 mg/kg 

Single Dose Cholinesterase Inhibition Study-Humans (Non-Guideline)
44248802

Un-Acceptable

	NOAEL = 1.0 mg/kg/day

LOAEL = not established

70 mg/person, single oral (capsule) dose to 6 male volunteers with no
placebos - missed time of peak effect

Time Course of Cholinesterase Inhibition in Preweaning and Adult
Rats/870.8223 (Non-Guideline)	46153303

Acceptable	Brain and RBC enzyme activities were maximally inhibited one
hour after single dosing in both adult and preweaning female rats.
Thereafter, ChE inhibition in both compartments decreased to
approximately control levels by 8 hours post dosing.

7-Day, Repeat Dose Cholinesterase Inhibition Study in Preweaning and
Adult Rats/(Non-Guideline)	46153304

Acceptable	PND 18 (M/F) ChEI NOAEL (Brain)  = not established

PND 18 (M/F) ChEI LOAEL (Brain)  = 0.1 mg/kg/d PND 48 (M/F) ChEI NOAEL
(Brain)  = 0.1 mg/kg/d

PND 48 (M/F) ChEI LOAEL (Brain)  = 7.5 mg/kg/d 

PND 18 (M/F) ChEI NOAEL (RBC)  = 0.1 mg/kg/d

PND 18 (M/F) ChEI LOAEL (RBC)  = 7.5 mg/kg/d

PND 48 (M/F) ChEI NOAEL (RBC)  = not established

PND 48 (M/F) ChEI LOAEL (RBC)  = 0.1 mg/kg/d

21-Day Oral (capsule) Cholinesterase Inhibition Study-Humans
(Non-Guideline)	44248801

Acceptable

	NOAEL = not established

LOAEL = 0.1 mg/kg/day (RBC ChE)

7 mg/day for 21 Days in 6 male volunteers plus 3 male volunteers as
placebos 

Single Dose and Repeated Dose Cholinesterase Inhibition Studies-Humans
(Non-Guideline)	44317901

Acceptable	Phase I (Single Dose of 35 mg)

NOAEL = 0.5 mg/kg/day (RBC ChE)

Phase II (Repeated Dose of 21 mg/day for 12 or 15 Days)

NOAEL = not established

LOAEL = 0.3 mg/kg/day (RBC ChE)

28-Day Delayed Neurotoxicity- Hen/870.6100	43433501

Acceptable	Cholinesterase inhibition (brain ChEI) 

NOAEL = 0.1 mg/kg/day

LOAEL = 0.3 mg/kg/day

No neuropathology.

90-Day Subchronic Oral  Toxicity - Rat/870.3100	41004701

Acceptable	NOAEL = 0.1 mg/kg/day

LOAEL = 1.5 mg/kg/day (plasma and RBC ChE)

90-Day Neurotoxicity - Rat/870.6200	42958101

Acceptable	NOAEL = 0.1 mg/day

LOAEL = 7.5 mg/kg/day (plasma, red blood cell (RBC) and brain ChEI).

Chronic-Feeding-Dog/870.4100	41593101

Acceptable	NOAEL = 0.05 mg/kg/day

LOAEL = 0.1 mg/kg/day (plasma and RBC ChEI in both sexes).

Chronic-Inhalation-Rats/

Guideline	00057695, 00632569

Acceptable	NOAEL = 0.00005 mg/L 

LOAEL =  0.0005 mg/L based on plasma, RBC and brain cholinesterase
inhibition.  

aChronic-Inhalation-Human/870.4100	45812001,

00060486

g/L (0.000125 mg/L)

LOAEL = 0.180 g/L (0.000180 mg/L) (Headaches and RBC ChEI)

Developmental Toxicity-Rat/870.3700	41951501

Acceptable	Maternal toxicity	NOAEL = 3 mg/kg/day

LOAEL = 21 mg/kg/day

(clinical signs, decreased body weight gain and reductions in food
consumption and efficiency)

Developmental toxicity NOAEL = > 21 mg/kg/day (HDT)

Developmental Toxicity-Rabbit/870.3700	41802401

Acceptable	Maternal toxicity  	NOAEL = 0.1 mg/kg/day

LOAEL = 2.5 mg/kg/day 

(mortality, decreased body weight gain at LOAEL)

Developmental toxicity NOAEL= > 7 mg/kg/day (HDT)

ChEI was not measured.in main study

Range-Finding: Doses were 0, 0.1, 1.0, 2.5, 5.0, 10 mg/kg/day

Maternal toxicity         ChE NOAEL = 0.1 mg/kg/day

                                     ChE LOAEL = 1.0 mg/kg/day

Reproductive Toxicity - Rat/870.3800	42483901

Acceptable	Parental/Systemic	NOAEL = 2.3 mg/kg/day

LOAEL = 8.3 mg/kg/day

(decreased % of females with estrous cycle  and increased % of females
with abnormal cycling)

 Offspring	NOAEL= 2.3 mg/kg/day

 LOAEL = 8.3 mg/kg/day 

(reduced # dams bearing litter, fertility index, pregnancy index and pup
weight).

Preliminary Developmental

Neurotoxicity - Rat/(Non-Guideline)	46153301

Acceptable	Systemic NOAEL = 7.5 mg/kg/day Maternal

Systemic LOAEL = not identified Maternal

RBC ChEI NOAEL = 0.1 mg/kg/day Maternal

RBC ChEI LOAEL = 1.0 mg/kg/day Maternal

Brain ChEI NOAEL = 1.0 mg/kg/day Maternal

Brain ChEI LOAEL = 7.5 mg/kg/day Maternal

Systemic NOAEL = 7.5 mg/kg/day Offspring

Systemic LOAEL = not identified Offspring

RBC ChEI NOAEL = 1.0 mg/kg/day Fetuses (GD 22)

RBC ChEI LOAEL = 7.5 mg/kg/day Fetuses (GD 22)

Brain ChEI NOAEL = 1.0 mg/kg/day Fetuses (GD 22)

Brain ChEI LOAEL = 7.5 mg/kg/day Fetuses (GD22)

Offspring (Pups) did not demonstrate ChEI during PND 2-22

Developmental Neurotoxicity - Rat/870.6300	46153302

Not-acceptable	Maternal LOAEL/NOAEL could not be identified due to low
viability indices

Offspring LOAEL/NOAEL could not be identified due to low viability
indices

Offspring Effects at 7.5 mg/kg/day (HDT) included increased startle
response in males on PND 23, impaired memory in males on PND 27 and 62,
and alterations in brain morphometry on PND 62.

Study is unacceptable/not upgradable

Developmental Neurotoxicity - Rat/870.6300	46239801

Not-acceptable	LOAEL/NOAEL could not be identified due to low viability
index in controls.

No maternal or offspring effects in FOB, motor activity, auditory
startle reflex habituation, learning and memory tests, brain weight,
neuropathology and morphometry at 7.5 mg/kg/day (HDT).

Study is unacceptable/not upgradable



Appendix A.

 MacGregor et al, 2005 Human Studies

Usable?	Citation

Oral studies used in the interspecies comparison

No _ Missed peak effect	Gledhill, A. (1997a) Dichlorvos: A Study to
Investigate the Effect of a Single Oral Dose on Erythrocyte
Cholinesterase Inhibition in Healthy Male Volunteers: Lab Project
Number:  CTL/P/5393: XH6064.  Unpublished study prepared by Zeneca
Central Toxicology Lab.  44 p.

No _ Missed peak effect (acute) & Gledhill 1997b dose is lower
(multidose) 	Gledhill, A. (1997b) Dichlorvos: A Study to Investigate
Erythrocyte Cholinesterase Inhibition Following Oral Administration to
Healthy Male Volunteers: Lab Project Number: XH5170: CTL/P/5251. 
Unpublished study prepared by Central Toxicology Lab.  66 p.

Yes _ Multidose study used in the RA	Gledhill, A. (1997c) Dichlorvos: A
Single Blind, Placebo Controlled, Randomised Study to Investigate the
Effects of Multiple Oral Dosing on Erythrocyte Cholinesterase Inhibition
in Healthy Male Volunteers: Lab Project Number: CTL/P/5392: XH6063. 
Unpublished study prepared by Zeneca Central Toxicology Lab.  52 p.

No _ Plastic bead formulation _ limited bioavailability	Slomka, M.B. and
C.H. Hine.  1981.  Clinical pharmacology of dichlorvos.    Acta.
Pharmacol. Toxicol.  49(Suppl. 5): 105_108.

Inhalation studies used in the interspecies comparison

No _ children exposed	Funckes, A.J., Miller, S., Hayes, W., 1963.
Initial field studies in upper volta with dichlorvos residual fumigant
as a malaria eradication technique. Bull. World Health Org. 29, 243_246.

No _ children exposed _ study had been used in RA	Johnston, J.E.,
Barraj, L., Petersen, B., Youngren, S.H., 2002. A reanalysis of
observations on occupants of Arizona homes containing 20% vapona
insecticide resin strips, Arizona II home study, Exponent Inc., Project
Identification Number DDVP_ 02_01, December 4, 2002. 

No _ children exposed	Shell Chemical Com pany, 1970. The third Arizona
home study: quantitation of DDVP residues in foods consumed by human
volunteers exposed to No_Pest Strip insecticide. May 1970, unpublished
report from Shell Chemical Company.

No _ 1 hr exposure _ not relevant  to use	Smith, P.W., Mertens, H.,
Lewis, M.F., Funkhouser, G.E., Higgins, E.A., Crane, C.R., Sanders,
D.C., Endecott, B.R., Flux, M., 1972. Toxicology of dichlorvos at
operational aircraft cabin altitudes. Aerosp. Med. 43, 473_478. 

No _ Exposure levels to individuals uncertain	Stein, W.J., Miller, S.,
Fetzer, L.E., 1966. Studies with dichlorvos residual fumigant as a
malaria eradication technique in Haiti. III. Toxicological studies. Am.
J. Trop. Med. Hyg. 15, 672_675. 

Limited _ 4 subjects exposed 2 days to high conc.	Ueda, K.; Nishimura,
M. (1967) Effect of Vapona/Strips to Human Beings.  (Unpublished study
prepared by Tokyo Dental College, Japan)

No _ 1-2 hr exposure _ not relevant  to current use	Witter, R.F.;
Gaines, T.B.; Short, J.G.; et al. (1961) Studies on the safety of DDVP
for the disinsection of commercial aircraft.  Bulletin of the World
Health Organization 24(?): 635_642.

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