Analysis and integration of developmental neurotoxicity and ancillary data into risk assessment: a case study of dimethoate.

Dimethoate is an organophosphate (OP) pesticide used to control a wide variety of insects on agricultural crops and ornamentals. To ensure that dimethoate is used safely, it is important to determine exposure levels that protect against adverse effects at all life stages, including the developing fetus, infant, and child. Based on an analysis of a developmental neurotoxicity (DNT) study, a cholinesterase (ChE) sensitivity study, a cross-fostering study, and several single- and multigenerational reproductive toxicity studies, two potential critical endpoints for dimethoate were identified: brain ChE inhibition (ChEI) in adult females, and pup mortality. An initial evaluation concluded that pup mortality was a preferable endpoint, based on an increased number of pup deaths born to dams dosed with > or =3 mg/kg dimethoate via oral gavage. Closer examination, however, revealed that the pup deaths were clustered in a small number of litters in which the dams providing postnatal care exhibited maternal care deficits. When the data were analyzed using the dam as the unit of statistical significance, a significant increase in the average litter proportion of pup deaths was observed only when the dams were dosed postnatally with 6 mg/kg dimethoate while they were raising the pups. Gestational exposure (i.e., during pregnancy only) to 6 mg/kg dimethoate exerted no effect on pup survival. This leads to the conclusion that it is postnatal exposure of the nursing dams that is associated with pup mortality. Furthermore, a previous benchmark dose (BMD) meta-analysis approach revealed that BMDL(10) for adult females (the lower 95% bound of the dose resulting in a 10% reduction in the parameter of interest) for ChEI was > 3-fold lower than the BMDL(10) for pup mortality (0.19 and 0.68 mg/kg, respectively). Overall, this study underscores the importance of using the dam as the unit of statistical significance when assessing data collected in the perinatal period, and it is concluded that adult brain ChEI is the correct critical endpoint for assessing risk of dimethoate toxicity.

Use of benchmark dose and meta-analysis to determine the most sensitive endpoint for risk assessment for dimethoate.

A meta-analysis of several rat toxicity studies for dimethoate was conducted to determine the most sensitive endpoint for use in risk assessment. The analysis was motivated by a recent developmental neurotoxicity (DNT) study, which identified the same no observed adverse effect level (NOAEL) for pup mortality and cholinesterase inhibition. The pup mortality NOAEL was lower than that determined in a range-finding study for the DNT and other reproduction studies, and was highly influenced by a single total litter loss in the middle dose group, which made interpretation difficult. First, a meta-analysis was conducted of four recent studies by gavage dosing with very similar designs, including the DNT. Benchmark dose (BMD) modeling was used to determine the appropriate point of departure for regulatory purposes, the lower limit of the BMD for a 5% incidence for pup mortality (BMDL(5)) and the lower limit of a 10% inhibition of brain cholinesterase (BMDL(10)*), the asterisk denotes that the BMD is based on continuous response variable as opposed to an incidence level. For pup mortality, the BMDL(5) for post-natal days (PND) 1-4 was 0.64 mg/kg/day. For cholinesterase inhibition, the lowest BMDL(10)* was 0.19 mg/kg/day for the dams at gestation day 20. These results show that the regulatory point-of-departure for cholinesterase inhibition is more than threefold lower than pup mortality. Thus, risk assessments protecting against cholinesterase inhibition are likely to also be protective of pup mortality. In addition, cholinesterase inhibition and pup mortality were evaluated in two 2-generation reproduction studies by dietary exposure. Also, cholinesterase inhibition was evaluated in a 28-day dietary study. Dietary exposure is more relevant than gavage exposures for many human risk assessment scenarios. There was no consistent pup mortality at the highest doses of the two 2-generation dietary studies (6.0 and 6.5mg/kg/day). The average BMD(10)s for brain cholinesterase inhibition for the 2-generation studies was 0.65 mg/kg/day, with a range of 0.49--0.96 mg/kg/day. This suggests that cholinesterase inhibition is at least a 10-fold more sensitive endpoint than pup mortality for dietary exposures. For the 28-day dietary study, the BMD(10) for brain cholinesterase inhibition was 1.1mg/kg/day for males and 0.70 mg/kg/day for females. The exposure duration in the 28-day dietary study is closest to the durations in the gavage studies. Compared to the dams in the gavage studies, which had a BMDL(10) of 0.19 mg/kg/day, the animals were more than threefold more sensitive to cholinesterase inhibition by gavage compared to dietary exposure.