Inhibition of hen brain acetylcholinesterase and neurotoxic esterase by chloropyrifos in vivo and kinetics of inhibition by chlorpyrifos oxon in vitro: application to assessment of neuropathic risk

Chlorpyrifos (CPS, O,O-diethyl 3,5, 6-trichloro-2-pyridyl phosphorothionate; Dursban) is a widely used broad-spectrum organophosphorus (OP) insecticide. Because some OP compounds can cause a sensory-motor distal axonopathy called OP compound-induced delayed neurotoxicity (OPIDN), CPS has been evaluated for this paralytic effect. Early studies of the neurotoxicity of CPS in young and adult hens reported reversible leg weakness but failed to detect OPIDN. More recently, a human case of mild OPIDN was reported to result from ingestion of a massive dose (about 300 mg/kg) in a suicide attempt. Subsequent experiments in adult hens (the currently accepted animal model of choice for studies of OPIDN) showed that doses of CPS in excess of the LD50 in atropine-treated animals inhibited brain neurotoxic esterase (NTE) and produced mild to moderate ataxia. Considering the extensive use of CPS and its demonstrated potential for causing OPIDN at supralethal doses, additional data are needed to enable quantitative estimates to be made of the neuropathic risk of this compound. Previous work has shown that the ability of OP insecticides to cause acute cholinergic toxicity versus OPIDN can be predicted from their relative tendency to inhibit the intended target, acetylcholines (AChE), versus the putative neuropathic target, NTE, in brain tissue. The present study was designed to clarify the magnitude of neuropathic risk associated with CPS exposures by measuring hen brain AChE and NTE inhibition following dosing in vivo and determining the bimolecular rate constant of inhibition (ki) for each enzyme by the active metabolite, CPS oxon (CPO), in vitro. CPS administered to atropine-treated adult hens at 0, 75, 150, and 300 mg/kg po in corn oil produced mean values for brain AChE inhibition 4 days after dosing of 0, 58, 75, and 86%. respectively, and mean values for brain NTE inhibition of 0, 21. 40, and 77%, respectively. Only the high dose (six times the unprotected LD50 in hens) produced NTE inhibition above the presumed threshold of 70%, and these animals were in extremis from cholinergic toxicity at the time of euthanization despite continual treatment with atropine. When 150 mg/kg CPS po in corn oil was given to atropine-treated hens on Day 0, inhibition on Days 1, 2,4,8, and 16 for brain AChE was 86, 82, 72, 44, and 29%, respectively, and for brain NTE was 30, 28, 38, 29, and 6%, respectively. No signs of OPIDN were observed in any of the animals during the 16-day study period. Kinetic studies of the inhibition of hen brain AChE and NTE by CPO in vitro demonstrated that CPO exhibits high potency and extraordinary selectivity for its intended target, AChE. The ki values were 15.5 micromolar-1 min-1 for AChE and 0.145 micromolar-1 min-1 for NTE. The calculated fixed-time (20-min) I50 values were 2.24 nM for AChE and 239 nM for NTE, yielding an I50 ratio for NTE/AChE of 107. These results may be compared with data compiled for other OP compounds with respect to NTE/AChE I50 ratios and the corresponding doses required to produce OPIDN relative to the LD50. In general, NTE/AChE I50 ratios greater than 1 indicate that the dose required to produce OPIDN is greater than the LD50. Taken together, the results of this study indicate that acute exposures to CPS would not be expected to cause OPIDN except under extreme conditions such as attempted suicides involving medically assisted survival of doses considerably in excess of the LD50.