Toxicological Sciences 53, 100-108 (2000)
Copyright © 2000 by the Society of Toxicology
Developmental Toxicity Studies in Rats and Rabbits with 3,5,6-Trichloro-2-pyridinol, the Major Metabolite of Chlorpyrifos
* Global Health, Environmental Safety and Regulatory, Dow AgroSciences, L. L. C., 9330 Zionsville Road, Indianapolis, Indiana 46268;
Health and Environmental Research Laboratories, The Dow Chemical Company, Midland, Michigan 48674;
Jefferson Medical College, Philadelphia, Pennsylvania 19107
Received August 30, 1998; accepted April 28, 1999
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ABSTRACT |
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3,5,6-Trichloro-2-pyridinol (TCP), the primary metabolite of chlorpyrifos and chlorpyrifos-methyl, was evaluated for potential developmental toxicity. Groups of 32–34 bred female Fischer 344 rats were given 0, 50, 100, or 150 mg TCP/kg/day by gavage on gestation days 6–15; the fetuses were evaluated on gestation day 21. Similarly, groups of 16 inseminated female New Zealand White rabbits were given 0, 25, 100, or 250 mg TCP/kg/day by gavage on gestation days 7–19, and fetuses were evaluated on gestation day 28. No clinical signs of toxicity attributed to TCP were noted in either species. In rats, at 150 mg/kg/day, maternal effects included slight decreases in feed consumption, significantly depressed body weight gain (25% relative to controls) resulting in significantly lower maternal terminal body weights, and increased relative liver weight. At 100 mg/kg/day, maternal body weight gain in rats was depressed approximately 22%. Among rabbits, maternal effects were limited to the group given 250 mg/kg/day, which lost an average of approximately 70 g during the treatment period (vs. 140 g in the controls). There were no effects on fetal weight or viability, nor were there significant increases in any fetal alteration in either species. A slightly higher (not statistically significant) than usual incidence of central nervous system anomalies occurred in rabbits, but these anomalies were found in both treated and control groups in this study as well as contemporaneous studies of unrelated compounds. This, and the fact that these anomalies were not seen with the parent compound, chlorpyrifos, suggest that their origin was spontaneous. Thus, TCP was not considered fetotoxic or teratogenic in either rats or rabbits, even at dose levels that produced maternal toxicity.
Key Words: TCP; organophosphates; chlorpyrifos; rats; rabbits; developmental toxicity.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Organophosphates (OPs) are potent inhibitors of serine esterases, and the toxicity of these materials is attributed to the inhibition of acetylcholinesterase through the binding of the phosphate moiety. Chlorpyrifos (CPF) [O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl)phosphorothioate] is a broad-spectrum OP insecticide used extensively in agriculture and for residential pest control throughout the world under the registered trademarks LORSBAN® (Dow AgroSciences, LLC) Insecticide and DURSBAN® (Dow AgroSciences, LLC) Insecticide. Because of its widespread use and the potential for human exposure, chlorpyrifos has been extensively studied, with more than 250 studies conducted examining the uses and impact on human health and the environment. CPF is activated through enzyme-mediated oxidative desulfuration to form the oxon. 3,5,6-Trichloropyridinol (TCP) is the major degradation product of both chlorpyrifos and chlorpyrifos-oxon (Racke, 1993
), as well as chlorpyrifos-methyl [O,O-dimethyl-O-(3,5,6-trichloro-2-pyridyl)phosphorothioate], in which the phosphate moiety has been removed, and a minor metabolite of the herbicide triclopyr (Timchalk et al., 1990). Bakke and co-workers (Bakke and Price, 1976; Bakke et al., 1976) have reported that following oral administration of either 14C-chlorpyrifos or 14C-chlorpyrifos-methyl to rats, at least 70% of the administered radiolabel was excreted in the urine, with 83% or more of the excreted radiolabel identified as TCP and its glucuronide conjugate. Low levels of TCP have also been detected in the tissues of farm animals treated with chlorpyrifos, including cattle (Dishberger et al., 1977; Ivey, 1979; McKellar et al., 1976), sheep (Ivey and Palmer, 1981), hogs (Ivey and Palmer, 1979) and goats (Cheng et al., 1989), as well as in fish (Barron et al., 1991; Marshall and Roberts, 1978). TCP was also identified in the urine of human volunteers exposed to chlorpyrifos (Davis, 1977; Nolan et al., 1984), and can be used to biomonitor for exposure to chlorpyrifos (Nolan et al., 1984).
The acute oral toxicity of CPF is considered moderate, with acute oral LD50 values in rats ranging from 118 to 245 mg/kg (McCollister et al., 1974). Multigeneration reproduction studies have been conducted in rats using chlorpyrifos, and developmental toxicity studies have been conducted in mice, rats, and rabbits. In an oral developmental toxicity study in CD-1 mice, dose levels ranging from 0.1 to 25 mg/kg/day were administered on gestation days 6–15. Severe maternal toxicity, including mortality, clinical signs of cholinesterase inhibition, and decreased maternal body weight, were seen at 25 mg/kg/day. The only effects on fetuses from treated mice were decreased body measurements (weight and length) and an increased incidence of minor skeletal variants in this high-dose group (Deacon et al., 1980). In a developmental toxicity study in rats, dose levels ranging from 0.1 to 15 mg/kg/day of chlorpyrifos were used. Clinically, cholinergic signs (salivation, urination, and tremors) were seen, and body weights and weight gains were decreased in maternal animals given 15 mg/kg/day. However, there were no adverse fetal effects in rats (Breslin et al., 1996). In a developmental toxicity study in rabbits, gavage doses of 1–140 mg/kg/day were administered on gestation days 7–19. Plasma cholinesterase activity was reduced in all treatment groups, with reduced feed consumption and decreased body weight gain seen at 140 mg/kg/day. Developmental effects were limited to decreased fetal body weights and slight delays in skeletal ossification only in the highest dosage group (EPA, 1998). There was no indication of any teratogenic effect in mice, rats, or rabbits
A developmental neurotoxicity study has also been conducted using Sprague-Dawley rats in which dams were exposed by gavage from gestation day (GD) 6 through postnatal day 10 to dosages of 0, 0.3, 1, or 5 mg/kg/day (Maurissen et al., manuscript in preparation). At 5 mg/kg/day, dams showed clinical signs of toxicity (muscle fasciculations, hyperactivity), and decreased survival promptly after birth and delayed maturation were noted in the pups, but no effects were evident at lower dosages. Despite the delayed maturation, learning and memory were unimpaired and there were no signs of CNS or any other structural anomalies in the offspring. Using the same dosage regimen as the developmental neurotoxicity study, Mattsson et al. (1999) demonstrated that in dams given 5 mg CPF/kg/day, concentrations of TCP in the blood of fetuses on GD 20, 4 h postdosing, were similar to blood levels in the dams (1800–2000 ng/g), although blood CPF levels in dams were only 108 ng/g, with even lower CPF blood levels in fetuses (46 ng/g).
TCP has a low intrinsic toxicity and has no cholinesterase activity. The oral LD50 in rats is approximately 800 mg/kg, whereas the dermal LD50 in rabbits is greater than 2000 mg/kg. The liver and kidneys have been identified as the primary target organs in rats following 4-week dietary exposure to the sodium salt of TCP at dose levels of 120 mg/kg/day and greater (Unreported data, The Dow Chemical Company). Due to the rapid detoxification of CPF to TCP (Racke, 1993) resulting in exposure primarily to TCP, and because residues of TCP are found in humans as well as in a variety of animal species exposed to chlorpyrifos, the potential of TCP to produce developmental effects was evaluated as part of the commitment of Dow AgroSciences to product stewardship. Dose levels of TCP used in this study were as much as 3- to 18-fold higher on a molar basis than respective rat and rabbit developmental toxicity studies conducted with the parent compound, chlorpyrifos.
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MATERIALS AND METHODS |
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Test materials.
TCP (Lot # AGR143197) was obtained from the Agricultural Products Department of The Dow Chemical Company (Midland, MI). Analysis of the test material using differential scanning calorimetry indicated a purity of 99.7%. TCP was suspended in a 0.5% aqueous METHOCELTM (The Dow Chemical Company) A4M methylcellulose ethers solution for administration. Analysis of a suspension of 25 mg TCP/ml demonstrated that TCP was uniformly distributed and was stable for at least 76 days. The structure of TCP is depicted in Figure 1
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Rats.
Male and female Fischer 344 rats, approximately 10 weeks of age, were obtained from Charles River Breeding Laboratories (Kingston, NY) and allowed to acclimate to laboratory conditions for at least 2 weeks prior to breeding. Animals were housed individually, except during mating, in stainless steel suspended cages with wire mesh floors in climate-controlled rooms at 65–76°F, 21–70% relative humidity, and a 12-h light:12-h dark photocycle. Purina Certified Rodent Chow No. 5002 (Purina Mills, Inc., St. Louis, MO) and tap water were available ad libitum. Adult, virgin females at least 11 weeks old and weighing approximately 150–225 g were bred overnight with adult males of the same strain. Vaginal smears were taken early in the morning following cohabitation and females were considered to have mated if sperm and/or a vaginal plug was observed. The day on which evidence of mating was observed was defined as gestation day 0 (GD 0). Mated females were randomly assigned to test groups using a computer program designed to equalize the GD 0 mean group body weights. Animals were individually identified with a numbered metal ear tag upon assignment to study.
Groups of 32–34 bred female rats were given 0, 50, 100, or 150 mg TCP/kg/day by oral gavage at a dose volume of 4 ml/kg on GD 6–15. Dose volumes were adjusted daily based on individual body weights. All animals were observed twice daily for signs of treatment-related effects. Maternal body weights were recorded on GD 0, daily on GD 6–16, and on GD 21. Feed and water consumption were measured at 3-day intervals beginning on GD 6. At Cesarean section on GD 21, a complete gross postmortem examination was performed, and the maternal liver, kidneys, and gravid uterus were weighed. The number of corpora lutea, and number and position of implantations, resorptions, and live or dead fetuses were recorded. Uteri with no visible implantations were stained with a 10% solution of ammonium sulfide (Kopf et al., 1964) and examined for evidence of early resorptions. Each fetus was individually identified, weighed, sexed, and given a gross examination for external malformations/ variations including observation for palatal defects. All fetuses were euthanized by CO2 asphyxiation, and approximately one-half of the fetuses in each litter were evaluated for visceral malformations/variations (Staples, 1974). The heads of fetuses selected for visceral examination were removed, placed in Bouin's fixative, and subsequently sectioned and examined for craniofacial defects (Wilson, 1965). All fetuses were then eviscerated and processed; the ossified skeletal structures were stained with alizarin red S (Dawson, 1926) and examined for skeletal alterations.
Rabbits.
Stock supplies of female New Zealand White rabbits, approximately 5 months of age, were obtained (Hazleton-Dutchland, Inc., Denver, PA), examined upon receipt in the laboratory by a veterinarian for health status, and acclimated to laboratory conditions for at least 3 weeks. The animal rooms of the facility were regulated for rabbits to maintain temperature at 68–72°F, relative humidity at 40–60%, and a 12-h light:12-h dark photocycle. A priming dose of 50 IU of human chorionic gonadotropin (hCG; W. A. Butler) was administered by IV injection to all females on test 3 weeks prior to insemination. Adult females, approximately 6 months old, weighing 3471–4222 grams were artificially inseminated (Gibson et al., 1966) with fresh semen collected from bucks of the same strain, and the day of insemination was considered GD 0. Upon insemination, rabbits were given an intravenous injection of 100 IU of hCG and randomized by body weight into the 4 groups using a computer-generated procedure. Animals were housed individually in cages with wire floors and identified using uniquely coded alphanumeric metal ear tags. Animals were maintained on 8 oz/day of Certified Laboratory Rabbit Chow No. 5322 (Purina Mills, Inc., St. Louis, MO). Municipal tap water was available ad libitum.
Groups of 16 inseminated adult female New Zealand White rabbits were administered TCP at dose levels of 0, 25, 100, or 250 mg/kg/day by oral gavage once daily on GD 7–19 of gestation at a dose volume of 2 ml/kg body weight. All animals were observed daily for treatment-related alterations in behavior or demeanor. Body weights were recorded on GD 0, daily during the dosing period, and on GD 20 and 28. Dose volumes were adjusted daily based on individual body weights. On GD 28, rabbits were euthanized by an intravenous injection of T-61 Euthanasia Solution (American Hoechst Corporation, Somerville, NJ), and a gross necropsy was performed. The weights of the maternal liver with gallbladder, kidneys, and gravid uterus, and any obvious gross pathologic alterations were recorded. The number of corpora lutea, and number and position of implantations, resorptions, and live or dead fetuses were recorded. Uteri with no visible implantations were stained with a 10% solution of ammonium sulfide (Kopf et al., 1964), and examined for evidence of early resorptions. Each fetus was individually identified, weighed, sexed, and given a gross examination for external malformations/variations to include observation for palatal defects. All fetuses were euthanized and examined by dissection under a low power stereomicroscope for evidence of visceral alterations (Staples, 1974). This examination also included a fresh examination of the brain. All fetuses were then preserved in alcohol, eviscerated, cleared, stained with alizarin red-S (Dawson, 1926) and examined for skeletal alterations.
Statistical evaluation.
Maternal body weights and body weight gains, fetal body weights, and organ weights (absolute and relative) were evaluated by Bartlett's test for equality of variances (Winer, 1971). Based on the outcome of Bartlett's test, a parametric (Steel and Torrie, 1960) or nonparametric (Hollander and Wolfe, 1973) analysis of variance (ANOVA) was performed. If the ANOVA was significant, analysis by Dunnett's test (Winer, 1971) or the Wilcoxon Rank-Sum test (Hollander and Wolfe, 1973) with Bonferroni's correction (Miller, 1966), respectively, was performed. Descriptive statistics (means and standard deviations) were reported for feed and water consumption (rats only). Statistical evaluation of the frequency of preimplantation loss, resorptions, and fetal alterations among litters and the fetal population was performed using a censored Wilcoxon test (Haseman and Hoel, 1974) with Bonferroni's correction. The number of corpora lutea and implants, as well as litter size, were evaluated using a nonparametric ANOVA followed by the Wilcoxon Rank-Sum test with Bonferroni's correction. Pregnancy rates were analyzed using the Fisher exact probability test (Siegel, 1956). Fetal sex ratios were analyzed using a binomial distribution test. Nonpregnant animals were excluded from the appropriate analyses. Statistical outliers were identified using a sequential method (Grubbs, 1969), but except for feed and water consumption, were not excluded unless justified by sound scientific reasons unrelated to treatment. The level of statistical significance was set a priori at 
= 0.05.
Good Laboratory Practice.
These studies complied with Good Laboratory Practice Standards (EPA, 1983), and were conducted according to the procedures outlined in the Pesticide Assessment Guidelines, Subdivision F- Hazard Evaluation: Human and Domestic Animals (NTIS, 1984). The laboratory is fully accredited by the Association for Accreditation of Laboratory Animal Care, Internation (AALAC).
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RESULTS |
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Rats
Administration of TCP to pregnant rats produced no clinical signs of toxicity that were considered treatment related. However, dose-related significant decreases in maternal body weight gain were observed during the treatment period in rats given 100 or 150 mg TCP/kg/day (Table 1
). At 150 mg/kg/day, weight gain was significantly depressed over each of the intervals evaluated during treatment (GD 6–9, 9–12, 12–16), and weight gain during the overall treatment period (GD 6–16) was depressed approximately 25% relative to controls, which resulted in a significantly lower terminal body weight in this dose group on GD 21. At 100 mg/kg/day, weight gain was depressed on GD 6–9 and 9–12, with weight gain over the treatment period depressed approximately 22%. Feed consumption during the treatment period in these two groups was consistent with the lower body weight gain, but water consumption was unaffected (data not shown). Feed consumption was depressed approximately 10% during treatment in females given 150 mg TCP/kg/day and approximately 5% in females given 100 mg/kg/day. The only organ weight change noted at necropsy was an increase in the relative liver weight among female rats given 150 mg/kg/day, which was considered secondary to the decreased body weights in these animals (data not shown).
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Despite the effects on maternal body weights, weight gains, and feed consumption, there were no indications of any adverse effects on fetal development in rats at any dose level. Reproductive parameters (pregnancy rates, resorption rates, litter size, etc.) among pregnant rats were unaffected by treatment (Table 2
). Though the mean litter size among females given 150 mg/kg/day was lower (but not statistically significantly) than in the controls, this was considered a function of a lower number of implantations seen in this group, but, as is standard with studies of this type, treatment began after implantation. All values were, however, well within the range of historical control values for Fischer 344 rats used in this laboratory.
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Examination of the fetuses from rats treated with TCP revealed no evidence of any treatment-related adverse effects on embryonal or fetal development. A low incidence of malformations scattered throughout the control and treated groups reflected the normal background for this rat strain (Table 3
). Externally, micrognathia was seen in one fetus from the low dose (50 mg/kg/day), and anophthalmia was seen in one middle-dose (100 mg/kg/day) fetus. At the high dose (150 mg/kg/day), one fetus was seen with multiple facial anomalies that included agnathia, maxillary aplasia, microphthalmia, and misshapen skull bones. Visceral examination revealed two fetuses from a single litter with dilated cerebral ventricles at 100 mg/kg/day. Skeletal examination revealed one control fetus that was missing one-half of a vertebra and had fused ribs, and one high-dose fetus that had a pair of cervical (extra) ribs.
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The only statistically identified difference in the incidence of fetal alterations was an increase in the incidence of vertebral spurs in fetuses from the low-dose group (50 mg TCP/kg/day), but the lack of any effects at higher dose levels indicated this difference was a reflection of the variability normally seen in this strain.
Rabbits
As in rats, there were no clinical signs of toxicity noted in rabbits administered TCP during gestation (data not shown). Overall body weight gain during the treatment period in females given 250 mg/kg/day was significantly depressed relative to the control group (Table 4), though there were no statistically significant differences in mean body weights or body weight gains over discrete intervals (i.e., GD 7–10, 10–13, 13–16, or 16–20) among rabbits in any dose group. In fact, females at this dose level lost approximately 1.7% of their GD 7 body weight over the course of the treatment period; all other groups gained weight, with the controls gaining approximately 3.5%. There were no significant differences in absolute or relative organ weights among rabbits administered TCP (data not shown).
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Consistent with the results in rats, there were no indications of any adverse effects on reproductive parameters in rabbits administered TCP (Table 5
). The number of corpora lutea and implantation, litter sizes, resorption rates, and fetal body weights measured at Cesarean section in treated animals were comparable to the values seen in the controls, and were consistent with the historical values seen in this species and strain.
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Examination of the fetuses from this study revealed a number of malformations scattered among the various treatment groups (Table 6
). Among the controls, three fetuses from different litters were found with malformations; one had severely dilated cerebral ventricles, one had dilated renal pelves, and the third had calloused ribs. There were no malformations detected in the 25 mg/kg/day dose group. Among fetuses from the 100 mg/kg/day dose group, there were seven fetuses involving five litters with malformations. Three fetuses from the same litter had CNS anomalies (either hydrocephaly or dilated cerebral ventricles). Three fetuses from this dose group also had multiple malformations; one had scoliosis, cleft palate, micrognathia, and hydrocephaly, one had Tetralogy of Fallot, and the third exhibited multiple visceral malformations. One malformed fetus in this group had calloused ribs. In the 250 mg/kg/day dose group, there were a total of seven fetuses from six different litters with malformations, one of which appeared to be a recent death with no evidence of resorption at the time of Cesarean section. Including the dead fetus, there were a total of five fetuses with CNS anomalies found in the 250 mg/kg/day dose group. One of these fetuses with a CNS anomaly also had cranioschisis, forelimb flexure, and cleft palate. Single fetuses from separate litters also had fused lobes of the lungs and scoliosis.
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There were, however, no statistically significant increases in the incidence of any developmental effects in any of the treated groups when compared to the controls. Examination of those end points that are typically associated with delayed fetal development (i.e., ossification of sternebrae, vertebral centra, etc. which represent the last centers to ossify) did not indicate any evidence of a treatment-related effect. Low incidences (one or two in a single treatment group) of minor variations such as pale or bilobed spleen, irregular patterns of ossification, or extra sites of ossification normally seen in rabbits at a low incidence were not included in the table. There was no pattern in the incidence of malformations (either singly or combined) to suggest any association with TCP administration.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The present studies indicated that the potential of TCP to adversely affect fetal development is low. Dose levels of TCP that significantly decreased maternal weight gains in both rats and rabbits produced no evidence of treatment-related effects on the developing fetus. In rats, dose-related body weight effects were seen at the two higher dose levels (100 and 150 mg/kg/day), accompanied by decreases in feed consumption. However, there were no differences in any of the reproductive parameters or measures of embryonal or fetal development among rats in any dose group. Among rabbits, weight gain was affected only in the high-dose group (250 mg/kg/day), but the maternal animals in this group lost approximately 2% of their body weight. Despite this weight loss, there was no indication of any treatment-related effects on measures of fetal development in rabbits in any dose group. Fetal body weights were consistent across all dose groups, and there were no indications of any delays in skeletal ossification.
The incidence of CNS malformations (hydrocephalus and severely dilated cerebral ventricles) in rabbits in the 100 and 250 mg/kg/day dose groups initially suggested a possible relationship to treatment with TCP. However, there were no other indications of any adverse fetal effects in this study to support an association with treatment. An examination of historical control data revealed that in the 3 years prior to the conduct of this study, only one CNS anomaly had been noted among control rabbits in our laboratory; that being hydrocephaly. However, an increase in the spontaneous incidence of CNS malformations in the control rabbit population was observed in rabbit teratology studies conducted in our laboratory in the 5–7 years surrounding this study. In five studies conducted subsequent to this study, a total of 11 control fetuses with these same CNS anomalies were reported (5 having severely dilated lateral ventricles and 6 hydrocephalus), 4 of which occurred in a single study with a control size of 132 fetuses (3.0%), and another 3 in a second study with a control group of 138 fetuses. Figure 2 depicts the incidence, by year of reporting, of CNS malformations noted in control rabbit populations in our laboratory. As can be seen in this figure, a cluster of hydrocephalus/dilated cerebral ventricles appeared in control fetuses in 1987–1988, the years immediately surrounding the conduct of the study described herein. This historical database includes evaluation of 878 control fetuses from 198 litters in this 2- year period, with individual study incidences of 0%–2.2% of control fetuses with CNS malformations.
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The ranges in Figure 2
are in agreement with published incidence data for NZW rabbits. Fetal incidences of hydrocephalus ranging from 0.03% to 0.19% had been reported prior to 1992 (Clemens et al., 1994; Palmer, 1972; Palmer, 1978; Stadler et al., 1983). In a more recent compendium of historical control data compiled by the Middle Atlantic Reproduction and Teratology Association (MARTA) and the Midwest Teratology Association (MTA) from developmental studies conducted between 1992 and 1994 using the NZW rabbit from the same source used in the studies reported herein, the average fetal incidence of hydrocephalus was 0.14%. However, individual studies with control incidences as high as 5.2% were reported, while litter incidences of 1.70% (average) and 50% (upper limit) were seen. Dilated cerebral ventricles were reported separately in this compendium, with an average fetal incidence of 0.40%, and incidences as high as 14.1% in individual studies (MARTA/MTA, 1996). Kalter (1968) described epidemic-like waves of hydrocephalus in various strains of rabbits; one wave initially reported in 1940 lasted approximately 2 years, with an overall incidence of hydrocephalus of 143 in 810 fetuses examined, while a second wave reported in 1966 had a 13.4% incidence (148/1103) of hydrocephalus. According to Kalter, these outbreaks could not be explained on a hereditary, infectious, or nutritional basis. These reports support the interpretation that a transient spontaneous increase in the incidence of hydrocephalus/dilated cerebral ventricles in the rabbit population of unknown etiology is the explanation for the increased incidence of hydrocephalus seen in our laboratory. In any case, it is evident that CNS anomalies have been found in other laboratories at incidences within control populations similar to those seen in the high-dose groups in the present study.
The CNS malformations seen in this study also need to be viewed in a broader context. These CNS anomalies were very specific in nature, with no indication of any other CNS anomalies such as exencephalus or spinal bifida to suggest neural tube development as a unique target. Also, in the present study, there were no increases in malformations in any other organ system, and no evidence of any delayed maturation such as depressed fetal body weights or skeletal ossification to suggest any adverse effect. Even with classic teratogenic agents such as thalidomide and its association with limb defects, numerous other malformations were observed (Shepard, 1998). It should also be noted that there was no indication of any treatment-related CNS malformations in fetal rabbits exposed to a dosage as high as 140 mg CPF/kg/day, a molar equivalent dose level of ~80 mg TCP/kg/day roughly equivalent to the mid-dose level used in the current study with TCP (EPA, 1998). Thus, the lack of any additional effects in rabbit fetuses from this study, the lack of any evidence of CNS anomalies in rabbits following exposure to CPF, and the historical control incidence both within our laboratory as well as in others support the conclusion that the CNS anomalies observed were most likely unassociated with treatment.
The dose levels at which the current studies were conducted with TCP were, on a molar equivalent basis, ~18-fold higher than the dose levels used to evaluate the parent material, chlorpyrifos, in rodent studies, and greater than 3-fold higher than the CPF level used in the developmental toxicity study in rabbits. TCP lacks the phosphate moiety of chlorpyrifos and does not inhibit cholinesterase activity, and it is this functional group that is responsible for the higher toxicity associated with the parent compound. Nonetheless, the lack of any significant developmental toxicity with TCP is consistent with the results of studies with chlorpyrifos. As noted previously, chlorpyrifos is rapidly and extensively hydrolyzed to TCP in mammalian systems. The results of the studies reported herein indicate that the TCP metabolite of chlorpyrifos has no intrinsic developmental toxicity and that the metabolism of chlorpyrifos to TCP represents an effective detoxification pathway that protects against developmental effects.
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NOTES |
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Presented at the 37th Annual Meeting of the Society of Toxicology, Seattle, Washington.
1 To whom correspondence should be addressed. Fax: (317) 337-4557. E-mail: trhanley{at}dowagro.com. 
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REFERENCES |
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