Prevention of tolerance to the organophosphorus anticholinesterase paraoxon with carboxylesterase inhibitors

The contribution of carboxylesterase (CarbE) to the development of tolerance to the organophosphorus anticholinesterase (OP-ANTIChE) paraoxon (diethyl p-nitrophenyl phosphate) was investigated in rats. Daily injections (20 days) of paraoxon (0.09 mg/kg) led to a cumulative dose that was 9.0-fold higher than the acute ED50 of 0.20 mg/kg, s.c. During this period, the rats did not demonstrate visible signs of cholinergic hyperactivity nor did they die, despite the persistence of critically reduced brain acetylcholinesterase (AChE) activity (20-30% of control). In addition, none of these rats died following the administration of a dose of carbachol (3.1 mg/kg, i.p.) that was an LD90 in untreated rats. Daily treatment with the CarbE inhibitors CBDP [2-(o-cresyl)-4H-1,3,2-benzodioxaphosphorin-2-oxide] (2 mg/kg, s.c.) or iso-OMPA (tetraisopropylpyrophosphoramide) (3 mg/kg, i.p.) followed by paraoxon (0.09 mg/kg, s.c.) 60 min later prevented the development of tolerance to paraoxon, since signs of cholinergic hyperactivity were observed and rats died on day 4 of the combined treatment. In tolerant rats, one-time CBDP or iso-OMPA pretreatment increased toxicity to paraoxon, causing the death of all rats within 60 min. The increase in paraoxon toxicity was correlated with inhibition of a plasma CarbE, with high affinity toward alpha-naphthyl acetate (alpha-NA) and to the inhibitors CBDP, iso-OMPA, and paraoxon. Inhibition of a plasma CarbE with high affinity toward p-nitrophenyl acetate (p-NPA) and low affinity to the above inhibitors did not potentiate paraoxon toxicity significantly. Neither the liver CarbEs, which showed high affinity to iso OMPA, nor the inhibition of butyrylcholinesterase (BuChE) by iso-OMPA in plasma and liver potentiated paraoxon toxicity. By eliminating plasma CarbE (alpha-NA) as potential binding sites for paraoxon with either CBDP or iso-OMPA, paraoxon can exert its toxicity to a greater extent at its specific target site, the functionally important ACHE at cholinergic synapses. It is concluded that plasma CarbE (alpha-NA) provided a significant protection against paraoxon intoxication and that the inhibition of this enzyme prevented the tolerance development seen with repeated paraoxon treatments. (C) 1998 Elsevier Science Inc.