The Effect of Route, Vehicle, and Divided Doses on the Pharmacokinetics of Chlorpyrifos and Its Metabolite Trichloropyridinol in Neonatal Sprague-Dawley Rats

Chlorpyrifos and Δ9 Tetrahydrocannabinol exposure and effects on parameters associated with the endocannabinoid system and risk factors for obesity

Marilyn Silva. Retired from a career in toxicology and risk assessment. Increased childhood and adult obesity are associated with chlorpyrifos (CPF), an organophosphate pesticide. Cannabis (Δ9Tetrahydrocannabinol: Δ9THC) use has increased globally with legalization. CPF applications on cannabis crops lacks federally regulated tolerances and may pose health risks through exposure during development and in adulthood. Both CPF and Δ9THC affect the endocannabinoid system (eCBS), a regulator of appetite, energy balance, and gut microbiota, which, if disrupted, increases risk for obesity and related diseases. CPF inhibits eCB metabolism and Δ9THC is a partial agonist/antagonist at the cannabinoid receptor (CB1R). Effects of each on obesogenic parameters were examined via literature search. Male rodents with CPF exposure showed increased body weights, dysbiosis, inflammation and oxidative stress, potentially associated with increased eCBs acting through the gut-microbiota-adipose-brain regulatory loop. Δ9THC generally decreased body weights via partial agonism at the CB1R, lowering levels of eCBs. Dysbiosis and/or oxidative stress associated inflammation occurred with CPF, but these parameters were not tested with Δ9THC. Database deficiencies included limited endpoints to compare between chemicals/age-groups, inter-study variables (dose ranges, dosing vehicle, rodent strain, treatment duration, etc.). CPF and Δ9THC were not tested together, but human co-chemical effects would depend on exposure ratio, subject age, exposure duration, and health status, among others. An overriding concern is that both chemicals are well-documented developmental neurotoxins in addition to their low dose effects on energy balance. A co-exposure risk assessment is warranted with increased use and lack of federal CPF regulation on cannabis.

Translational outcomes relevant to neurodevelopmental disorders following early life exposure of rats to chlorpyrifos

BACKGROUND

Neurodevelopmental disorders (NDDs), including intellectual disability, attention deficit hyperactivity disorder (ADHD), and autism spectrum disorder (ASD), are pervasive, lifelong disorders for which pharmacological interventions are not readily available. Substantial increases in the prevalence of NDDs over a relatively short period may not be attributed solely to genetic factors and/or improved diagnostic criteria. There is now a consensus that multiple genetic loci combined with environmental risk factors during critical periods of neurodevelopment influence NDD susceptibility and symptom severity. Organophosphorus (OP) pesticides have been identified as potential environmental risk factors. Epidemiological studies suggest that children exposed prenatally to the OP pesticide chlorpyrifos (CPF) have significant mental and motor delays and strong positive associations for the development of a clinical diagnosis of intellectual delay or disability, ADHD, or ASD.

METHODS

We tested the hypothesis that developmental CPF exposure impairs behavior relevant to NDD phenotypes (i.e., deficits in social communication and repetitive, restricted behavior). Male and female rat pups were exposed to CPF at 0.1, 0.3, or 1.0 mg/kg (s.c.) from postnatal days 1-4.

RESULTS

These CPF doses did not significantly inhibit acetylcholinesterase activity in the blood or brain but significantly impaired pup ultrasonic vocalizations (USV) in both sexes. Social communication in juveniles via positive affiliative 50-kHz USV playback was absent in females exposed to CPF at 0.3 mg/kg and 1.0 mg/kg. In contrast, this CPF exposure paradigm had no significant effect on gross locomotor abilities or contextual and cued fear memory. Ex vivo magnetic resonance imaging largely found no differences between the CPF-exposed rats and the corresponding vehicle controls using strict false discovery correction; however, there were interesting trends in females in the 0.3 mg/kg dose group.

CONCLUSIONS

This work generated and characterized a rat model of developmental CPF exposure that exhibits adverse behavioral phenotypes resulting from perinatal exposures at levels that did not significantly inhibit acetylcholinesterase activity in the brain or blood. These data suggest that current regulations regarding safe levels of CPF need to be reconsidered.

Effect of in utero and lactational exposure of triazophos on reproductive system functions in male offsprings, Rattus norvegicus

In the present study, Triazophos (TZ) was used at acceptable daily intake (ADI) to investigate the consequence of prenatal and postnatal exposure on reproductive functions in the male offsprings. Pregnant females were divided into three groups, the first group was orally gavaged with olive oil (control), the second group was administered with 0.01 mg kg^-1 bw of the ADI of TZ from gestation day (GD) 1 until parturition (designated as P group) and the third group was gavaged with the same dose from GD1 to postnatal day (PND) 21 of lactation (marked as P + L group). Non-significant reduction occurred in the body weight of pups except at (PND) 35 during which body weight of P + L group pups significantly decreased. Male offsprings born to TZ exposed females showed significant changes at maturity (PND 63) in weight of liver, thyroid and testis, alterations in the levels of protein, urea, creatinine in plasma and abnormal levels of cholesterol, phospholipids and lipid peroxidation in testicular homogenate. Gonadal inhibition in TZ exposed progeny was reflected from a significant fall in sperm count, sperm motility, plasma testosterone level and histopathological alterations in testis. Hence, in utero and lactational exposure to ADI level of TZ influences testis development and functions in the male offsprings. Further investigations are suggested with germline studies of offsprings to examine the transgenerational effect of TZ exposure.

Sub-lethal Effects of Chlorpyrifos on Big Brown Bats (Eptesicus fuscus)

We determined dose-response curves for sublethal effects of the organophosphorus (OP) insecticide, chlorpyrifos, on bats. Big brown bats (Eptesicus fuscus, n = 64) were given a single dose of chlorpyrifos (nominal concentrations) of 0, 5, 10, 15, 20, 25, 30, or 60 µg/g body weight and examined at 12 or 24 h after dosing. A second experiment dosed 32 bats with 0 or 60 µg/g body weight and examined 1, 3, 7, or 14 days after dosing. Skin temperature and behavioral changes were recorded, and brain and plasma cholinesterase (ChE) activity were measured. The benchmark dose (BMD10) of chlorpyrifos that altered brain and plasma ChE activity at 24 h was 3.7 and 10.1 µg/g, respectively. The 95 % lower confidence limit for the BMD10 (i.e., BMDL10) was 1.6 and 7.7 µg/g. The best of five models (as determined by AIC) for impaired flight, impaired movement, or presence of tremors provided a BMD10 of 6.2, 12.9, and 7.8 µg/g body weight of chlorpyrifos, respectively. BMDL10 for impaired flight, impaired movement, or presence of tremors was 3.5, 6.6, and 5.3 µg/g body weight, respectively. In the wild, impaired ability to fly or crawl could be life-threatening. Brain and plasma ChE activity remained low for 3 days after dosing. Gradual recovery of enzyme activity was observed by 7 days in survivors. Brain and plasma ChE activity were still significantly lower than that of the control group at 14 days after dosing.

Effects of maternal chlorpyrifos diet on social investigation and brain neuroendocrine markers in the offspring - a mouse study

BACKGROUND

Chlorpyrifos (CPF) is one of the most widely used organophosphate pesticides worldwide. Epidemiological studies on pregnant women and their children suggest a link between in utero CPF exposure and delay in psychomotor and cognitive maturation. A large number of studies in animal models have shown adverse effects of CPF on developing brain and more recently on endocrine targets. Our aim was to determine if developmental exposure to CPF affects social responsiveness and associated molecular neuroendocrine markers at adulthood.

METHOD

Pregnant CD1 outbred mice were fed from gestational day 15 to lactation day 14 with either a CPF-added (equivalent to 6 mg/kg/bw/day during pregnancy) or a standard diet. We then assessed in the offspring the long-term effects of CPF exposure on locomotion, social recognition performances and gene expression levels of selected neurondocrine markers in amygdala and hypothalamus.

RESULTS

No sign of CPF systemic toxicity was detected. CPF induced behavioral alterations in adult offspring of both sexes: CPF-exposed males displayed enhanced investigative response to unfamiliar social stimuli, whereas CPF-exposed females showed a delayed onset of social investigation and lack of reaction to social novelty. In parallel, molecular effects of CPF were sex dimorphic: in males CPF increased expression of estrogen receptor beta in hypothalamus and decreased oxytocin expression in amygdala; CPF increased vasopressin 1a receptor expression in amygdala in both sexes.

CONCLUSIONS

These data indicate that developmental CPF affects mouse social behavior and interferes with development of sex-dimorphic neuroendocrine pathways with potential disruptive effects on neuroendocrine axes homeostasis. The route of exposure selected in our study corresponds to relevant human exposure scenarios, our data thus supports the view that neuroendocrine effects, especially in susceptible time windows, should deserve more attention in risk assessment of OP insecticides.

Incorporating Low-Dose Epidemiology Data in a Chlorpyrifos Risk Assessment

USEPA assessed whether epidemiology data suggest that fetal or early-life chlorpyrifos exposure causes neurodevelopmental effects and, if so, whether they occur at exposures below those causing the current most sensitive endpoint, 10% inhibition of blood acetyl-cholinesterase (AChE). We previously conducted a hypothesis-based weight-of-evidence analysis and found that a proposed causal association between chlorpyrifos exposure and neurodevelopmental effects in the absence of AChE inhibition does not have a substantial basis in existing animal or in vitro studies, and there is no plausible basis for invoking such effects in humans at their far lower exposure levels. The epidemiology studies fail to show consistent patterns; the few associations are likely attributable to alternative explanations. Human data are inappropriate for a dose-response assessment because biomarkers were only measured at one time point, may reflect exposure to other pesticides, and many values are at or below limits of quantification. When considered with pharmacokinetic data, however, these biomarkers provide information on exposure levels relative to those in experimental studies and indicate a margin of exposure of at least 1,000. Because animal data take into account the most sensitive lifestages, the use of AChE inhibition as a regulatory endpoint is protective of adverse effects in sensitive populations.

Incorporating Low-dose Epidemiology Data in a Chlorpyrifos Risk Assessment

USEPA assessed whether epidemiology data suggest that fetal or early-life chlorpyrifos exposure causes neurodevelopmental effects and, if so, whether they occur at exposures below those causing the current most sensitive endpoint, 10% inhibition of blood acetylcholinesterase (AChE). We previously conducted a hypothesis-based weight-of-evidence analysis and found that a proposed causal association between chlorpyrifos exposure and neurodevelopmental effects in the absence of AChE inhibition does not have a substantial basis in existing animal or in vitro studies, and there is no plausible basis for invoking such effects in humans at their far lower exposure levels. The epidemiology studies fail to show consistent patterns; the few associations are likely attributable to alternative explanations. Human data are inappropriate for a dose-response assessment because biomarkers were only measured at one time point, may reflect exposure to other pesticides, and many values are at or below limits of quantification. When considered with pharmacokinetic data, however, these biomarkers provide information on exposure levels relative to those in experimental studies and indicate a margin of exposure of at least 1,000. Because animal data take into account the most sensitive lifestages, the use of AChE inhibition as a regulatory endpoint is protective of adverse effects in sensitive populations.

Cholinesterase inhibition and toxicokinetics in immature and adult rats after acute or repeated exposures to chlorpyrifos or chlorpyrifos-oxon

The effect of age or dose regimen on cholinesterase inhibition (ChEI) from chlorpyrifos (CPF) or CPF-oxon (CPFO) was studied in Crl:CD(SD) rats. Rats were exposed to CPF by gavage in corn oil, rat milk (pups), or in the diet (adults) or to CPFO by gavage in corn oil. Blood CPF/CPFO levels were measured. With acute exposure, ChEI NOELs were 2 mg/kg CPF for brain and 0.5 mg/kg CPF for red blood cells (RBCs) in both age groups. In pups, ChEI and blood CPF levels were similar using either milk or corn oil vehicles. Compared to gavage, adults given dietary CPF (12 h exposure) had greater RBC ChEI, but lower brain ChEI at corresponding CPF doses, indicating an effect of dose rate. With repeated CPF exposures, ChEI NOELs were the same across ages (0.5 and 0.1 mg/kg/day for brain and RBCs, respectively). With CPFO dosing, the ChEI NOELs were 0.1 mg/kg (acute) and 0.01 mg/kg/day (repeated doses) for RBCs with no ChEI in brain at CPFO doses up to 0.5 (pup) or 10 mg/kg (adult) for acute dosing or 0.5 mg/kg/day for both ages with repeat dosing. Thus, there were no age-dependent differences in CPF ChEI via acute or repeated exposures. Pups had less ChEI than adults at comparable blood CPF levels. Oral CPFO resulted in substantial RBC ChEI, but no brain ChEI, indicating no CPFO systemic bioavailability to peripheral tissues.

Integration of epidemiology and animal neurotoxicity data for risk assessment

Most human health risk assessments are based on animal studies that can be conducted under conditions where exposure to multiple doses of a single chemical can be controlled. Data from epidemiology studies also provide valuable information about human exposure and response to pesticides. Human studies have the potential of evaluating neurobehavioral and other outcomes that may be more difficult to evaluate in animals. The human data together with animal data can contribute to a weight-of-evidence analysis in the characterization of human health risks. Epidemiology data do, however, pose challenges with respect to characterizing human health risks. Similarly, animal data at high doses or routes of exposure not typical for humans also pose challenges to dose-response evaluations needed for risk assessments. This paper summarizes some of the presentations given at a symposium held at the Xi'an, China, International Neurotoxicology Conference held in June 2011. This symposium brought together scientists from government, industry and academia to discuss approaches to evaluating and conducting animal and human neurotoxicity studies for risk assessment purposes, using the pesticides paraquat and chlorpyrifos as case studies.

Delayed reduction of hippocampal synaptic transmission and spines following exposure to repeated subclinical doses of organophosphorus pesticide in adult mice

Agricultural and household organophosphorus (OP) pesticides inhibit acetylcholinesterase (AchE), resulting in increased acetylcholine (Ach) in the central nervous system. In adults, acute and prolonged exposure to high doses of AchE inhibitors causes severe, clinically apparent symptoms, followed by lasting memory impairments and cognitive dysfunction. The neurotoxicity of repeated environmental exposure to lower, subclinical doses of OP pesticides in adults is not as well studied. However, repeated exposure to acetylcholinesterase inhibitors, such as chlorpyrifos (CPF), pyridostigmine, and sarin nerve agent, has been epidemiologically linked to delayed onset symptoms in Gulf War Illness and may be relevant to environmental exposure in farm workers among others. We treated adult mice with a subclinical dose (5 mg/kg) of CPF for 5 consecutive days and investigated hippocampal synaptic transmission and spine density early (2-7 days) and late (3 months) after CPF administration. No signs of cholinergic toxicity were observed at any time during or after treatment. At 2-7 days after the last injection, we found increased synaptic transmission in the CA3-CA1 region of the hippocampus of CPF-treated mice compared with controls. In contrast, at 3 months after CPF administration, we observed a 50% reduction in synaptic transmission likely due to a corresponding 50% decrease in CA1 pyramidal neuron synaptic spine density. This study is the first to identify a biphasic progression of synaptic abnormalities following repeated OP exposure and suggests that even in the absence of acute cholinergic toxicity, repeated exposure to CPF causes delayed persistent damage to the adult brain in vivo.

Evaluation of epidemiology and animal data for risk assessment: chlorpyrifos developmental neurobehavioral outcomes

Developmental neurobehavioral outcomes attributed to exposure to chlorpyrifos (CPF) obtained from epidemiologic and animal studies published before June 2010 were reviewed for risk assessment purposes. For epidemiological studies, this review considered (1) overall strength of study design, (2) specificity of CPF exposure biomarkers, (3) potential for bias, and (4) Hill guidelines for causal inference. In the case of animal studies, this review focused on evaluating the consistency of outcomes for developmental neurobehavioral endpoints from in vivo mammalian studies that exposed dams and/or offspring to CPF prior to weaning. Developmental neuropharmacologic and neuropathologic outcomes were also evaluated. Experimental design and methods were examined as part of the weight of evidence. There was insufficient evidence that human developmental exposures to CPF produce adverse neurobehavioral effects in infants and children across different cohort studies that may be relevant to CPF exposure. In animals, few behavioral parameters were affected following gestational exposures to 1 mg/kg-d but were not consistently reported by different laboratories. For postnatal exposures, behavioral effects found in more than one study at 1 mg/kg-d were decreased errors on a radial arm maze in female rats and increased errors in males dosed subcutaneously from postnatal day (PND) 1 to 4. A similar finding was seen in rats exposed orally from PND 1 to 21 with incremental dose levels of 1, 2, and 4 mg/kg-d, but not in rats dosed with constant dose level of 1 mg/kg-d. Neurodevelopmental behavioral, pharmacological, and morphologic effects occurred at doses that produced significant brain or red blood cell acetylcholinesterase inhibition in dams or offspring.

Pharmacokinetics of bisphenol A in neonatal and adult CD-1 mice: inter-species comparisons with Sprague-Dawley rats and rhesus monkeys

Bisphenol A (BPA) is an important industrial chemical used in the manufacture of polycarbonate plastic products and epoxy resin-based food can liners. The presence of BPA metabolites in urine of >90% of Americans aged 6-60 suggests ubiquitous and frequent exposure at levels largely below 1 μg/kg bw/d. The current study used LC/MS/MS to measure serum pharmacokinetics of unconjugated (active) and conjugated (inactive) BPA in adult and neonatal CD-1 mice by oral and subcutaneous (SC) injection routes. Deuterated BPA was used to avoid issues of background contamination. Significant inverse relationships were observed between postnatal age and measures of internal exposures (C(max)) to unconjugated BPA after oral administration. Phase II conjugation, area under the time-concentration curve (AUC), and elimination half-time of unconjugated BPA were also inversely related to age. In postnatal day (PND) 3 mice, the combination of under-developed Phase II metabolism, rapid absorption, and slow elimination kinetics led to equivalent internal exposures for unconjugated BPA from oral and SC routes; however, maturing metabolic capabilities in PND 10 and older mice, led to large and significant route effects. The significant inverse age-related developmental profiles from PND 3 through adulthood for unconjugated BPA internal exposure metrics from oral administration to CD-1 mice and Sprague-Dawley rats were remarkably similar; however, the developmental profile was quite different for neonatal rhesus monkeys in which small insignificant age-related differences were observed. These results suggest that an adverse effect from BPA observed in rodent models, attributable to exposure during a discrete time period of neonatal development, would be less likely for comparable neonatal primate dosing based on internal dosimetry. On the other hand in all adults of all species studied, including humans, a low oral dose of BPA produced similarly small internal exposures for the unconjugated form, reflecting the dominant effect of presystemic Phase II metabolism.

Pharmacokinetics and pharmacodynamics of chlorpyrifos in adult male Long-Evans rats following repeated subcutaneous exposure to chlorpyrifos

Chlorpyrifos (CPF) is a commonly used organophosphorus pesticide. Several pharmacokinetic and pharmacodynamic studies have been conducted in rats in which CPF was administered as a single bolus dose. However, there is limited data regarding the pharmacokinetics and pharmacodynamics following daily exposure. Since occupational exposures often consist of repeated, daily exposures, there is a need to evaluate the pharmacokinetics and pharmacodynamics of CPF under exposure conditions which more accurately reflect real world human exposures. In this study, the pharmacokinetics and pharmacodynamics of CPF were assessed in male Long-Evans rats exposed daily to CPF (0, 3 or 10mg/kg/day, s.c. in peanut oil) over a 10 day study period. Throughout the study, multiple pharmacokinetic (urinary TCPy levels and tissue CPF and metabolite levels) and pharmacodynamic (blood and brain AChE activity) determinants were measured. Average blood AChE activity on day 10 was 54% and 33% of baseline among animals in the 3 and 10mg/kg/day CPF treatment groups, respectively, while average brain AChE activity was 67% and 28% of baseline. Comparable dose-response relationships between brain AChE inhibition and blood AChE inhibition, suggests that blood AChE activity is a valid biomarker of brain AChE activity. The pharmacokinetic and pharmacodynamic measures collected in this study were also used to optimize a rat physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model for multiple s.c. exposures to CPF based on a previously published rat PBPK/PD model for CPF following a single bolus injection. This optimized model will be useful for determining pharmacokinetic and pharmacodynamic responses over a wide range of doses and durations of exposure, which will improve extrapolation of results between rats and humans.

Cytochrome P450-specific human PBPK/PD models for the organophosphorus pesticides: chlorpyrifos and parathion

Organophosphorus pesticides (OPs) remain a potential concern to human health because of their continuing use worldwide. Phosphororthioate OPs like chlorpyrifos and parathion are directly activated and detoxified by various cytochrome P450s (CYPs), with the primary CYPs involved being CYP2B6 and CYP2C19. The goal of the current study was to convert a previously reported human pharmacokinetic and pharmacodynamic (PBPK/PD) model for chlorpyrifos, that used chlorpyrifos metabolism parameters from rat liver, into a human CYP based/age-specific model using recombinant human CYP kinetic parameters (V(max), K(m)), hepatic CYP content and plasma binding measurements to estimate new values for acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibition and to use the model as a template for the development of a comparable parathion PBPK/PD model. The human CYP based/age-specific PBPK/PD models were used to simulate single oral exposures of adults (19 year old) and infants (1 year) to chlorpyrifos (10,000, 1000 and 100 μg/kg) or parathion (100, 25 and 5 μg/kg). Model simulations showed that there is an age dependency in the amount of blood cholinesterase inhibition observed, however additional age-dependent data are needed to further optimize age-specific human PBPK/PD modeling for these OP compounds. PBPK/PD model simulations estimated that a 4-fold increase or decrease in relative CYP2B6 and CYP2C19 content would produce a 9-22% inhibition in blood AChE activity following exposure of an adult to chlorpyrifos (1000 μg/kg). Similar model simulation produced an 18-22% inhibition in blood AChE activity following exposure of an adult to parathion (25 μg/kg). Individuals with greater CYP2B6 content and lower CYP2C19 content were predicted to be most sensitive to both OPs. Changes in hepatic CYP2B6 and CYP2C19 content had more of an influence on cholinesterase inhibition for exposures to chlorpyrifos than parathion, which agrees with previously reported literature that these CYPs are more reaction biased for desulfurization (activation) and dearylation (detoxification) of chlorpyrifos compared to parathion. The data presented here illustrate how PBPK/PD models with human enzyme-specific parameters can assist ongoing risk assessment efforts and aid in the identification of sensitive individuals and populations.

Adverse effects of exposure to low doses of chlorpyrifos in lactating rats

This study was conducted to shed light on the effect of exposure of lactating rat to chlorpyrifos (CPF). CPF was orally administered to lactating rats at 0.01 mg kg-1 b.wt. (acceptable daily intake, ADI), 1.00 mg kg-1 b.wt. (no observed adverse effects level, NOAEL) and 1.35 mg kg-1 b.wt. (1/100 LD 50) from postnatal day 1 (PN1) until day 20 (PN20) after delivery. Results indicated decreases in body weight and increases in relative liver and kidney weights of exposed dams. Significant damage to liver was observed via increased plasma levels of aminotransferases (aspartate aminotransferase (AST) and alanine aminotransferase (ALT)) lactate dehydrogenase (LDH) and γ-glutamyle transferase (γ-GT) in a dose-dependent manner. At two high doses of CPF (1.00 and 1.35 mg kg-1 b.wt.), the lactating mothers showed significant decrease in the activity of cholinesterase (ChE). Lipid peroxidation was significantly increased, while glutathione s-transferase (GST) and superoxide dismutase (SOD) were significantly decreased compared to control. At high dose of CPF (1.35 mg kg-1 b.wt.), total protein and uric acid levels were significantly increased. CPF caused dose-related histopathological changes in liver and kidney of the CPF-treated dams.

Adverse effects of lactational exposure to chlorpyrifos in suckling rats

The present study was undertaken to evaluate the oxidative damage, biochemical and histopathological alterations in sucking rats whose mothers were exposed to the insecticide chlorpyrifos (CPF). Dams were administered CPF, via oral route. Doses equalled 0.01 mg kg(-1) body weight (b.wt.; acceptable daily intake, ADI), 1.00 mg kg(-1) b.wt. (no observed adverse effects level, NOAEL) and 1.35 mg kg(-1) b.wt. (1/100 lethal dose [LD(50)]) from postnatal day 1 until day 20 after delivery. At two high doses of CPF, the body weight gain and relative liver and kidney weight of suckling pups were significantly decreased. Exposure of the mothers to CPF caused increase in lipid peroxidation (LPO) and decrease in superoxide dismutase (SOD) and glutathione-s-transferase (GST) in lactating pups. CPF altered the level of the marker parameters related to the liver and kidneys. Consistent histological changes were found in the liver and kidneys of the subjected pups, especially at the higher doses. The results suggested that the transfer of CPF intoxication through the mother's milk has resulted in oxidative stress and biochemical and histopathological alterations in the suckling pups. The data of this study may be considered as a contribution to the problem of lactational transfer of the relatively less persistent OP pesticides, such as CPF.

Comparative chlorpyrifos pharmacokinetics via multiple routes of exposure and vehicles of administration in the adult rat

Chlorpyrifos (CPF) is a commonly used organophosphorus pesticide. A number of toxicity and mechanistic studies have been conducted in animals, where CPF has been administered via a variety of different exposure routes and dosing vehicles. This study compared chlorpyrifos (CPF) pharmacokinetics using oral, intravenous (IV), and subcutaneous (SC) exposure routes and corn oil, saline/Tween 20, and dimethyl sulfoxide (DMSO) as dosing vehicles. Two groups of rats were co-administered target doses (5 mg/kg) of CPF and isotopically labeled CPF (L-CPF). One group was exposed by both oral (CPF) and IV (L-CPF) routes using saline/Tween 20 vehicle; whereas, the second group was exposed by the SC route using two vehicles, corn oil (CPF) and DMSO (L-CPF). A third group was only administered CPF by the oral route in corn oil. For all treatments, blood and urine time course samples were collected and analyzed for 3,5,6-trichloro-2-pyridinol (TCPy), and isotopically labeled 3,5,6-trichloro-2-pyridinol (L-TCPy). Peak TCPy/L-TCPy concentrations in blood (20.2 micromol/l), TCPy/L-TCPy blood AUC (94.9 micromol/lh), and percent of dose excreted in urine (100%) were all highest in rats dosed orally with CPF in saline/Tween 20 and second highest in rats dosed orally with CPF in corn oil. Peak TCPy concentrations in blood were more rapidly obtained after oral administration of CPF in saline/Tween 20 compared to all other dosing scenarios (>1.5 h). These results indicate that orally administered CPF is more extensively metabolized than systemic exposures of CPF (SC and IV), and vehicle of administration also has an effect on absorption rates. Thus, equivalent doses via different routes and/or vehicles of administration could potentially lead to different body burdens of CPF, different rates of bioactivation to CPF-oxon, and different toxic responses. Simulations using a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model for CPF are consistent with these possibilities. These results suggest that exposure route and dosing vehicle can substantially impact target tissue dosimetry. This is of particular importance when comparing studies that use varying exposure paradigms, which are then used for extrapolation of risk to humans.

Effect of different administration paradigms on cholinesterase inhibition following repeated chlorpyrifos exposure in late preweanling rats

Chlorpyrifos (CPS) is widely used in agricultural settings and residue analysis has suggested that children in agricultural communities are at risk of exposure. This has resulted in a large amount of literature investigating the potential for CPS-induced developmental neurotoxic effects. Two developmental routes of administration of CPS are orally in corn oil at a rate of 0.5 ml/kg and subcutaneously in dimethyl sulfoxide (DMSO) at a rate of 1.0 ml/kg. For comparison between these methods, rat pups were exposed daily from days 10 to 16 to CPS (5 mg/kg) either orally dissolved in corn oil or subcutaneously dissolved in DMSO, both at rates of either 0.5 or 1.0 ml/kg. A representative vehicle/route group was present for each treatment. Both the low and high volume CPS in DMSO subcutaneous groups were lower than that of the low and high volume CPS in oil oral groups. At 4 h following the final administration, serum carboxylesterase was inhibited > 90% with all treatments. For cholinesterase activity in the cerebellum, medulla-pons, forebrain, and hindbrain, and serum, inhibition in the CPS-oil groups was similar and inhibition in the CPS-DMSO groups was similar. However, significantly greater inhibition was present in the high volume CPS-DMSO group as compared to the CPS-oil groups. Inhibition in the low volume CPS-DMSO group was generally between that in the CPS-oil groups and the high volume CPS-DMSO group. These data suggest that using DMSO as a vehicle for CPS may alter the level of brain ChE inhibition.