Bioscavengers for the protection of humans against organophosphate toxicity

Abstract

Current antidotes for organophosphorus compounds (OP) poisoning consist of a combination of pretreatment with carbamates (pyridostigmine bromide), to protect acetylcholinesterase (AChE) from irreversible inhibition by OP compounds, and post-exposure therapy with anti-cholinergic drugs (atropine sulfate) to counteract the effects of excess acetylcholine and oximes (e.g., 2-PAM chloride) to reactivate OP-inhibited AChE. These antidotes are effective in preventing lethality from OP poisoning, but they do not prevent post-exposure incapacitation, convulsions, seizures, performance decrements, or in many cases permanent brain damage. These symptoms are commonly observed in experimental animals and are likely to occur in humans. The problems intrinsic to these antidotes stimulated attempts to develop a single protective drug, itself devoid of pharmacological effects, which would provide protection against the lethality of OP compounds and prevent post-exposure incapacitation. One approach is the use of enzymes such as cholinesterases (ChEs), β-esterases in general, as single pretreatment drugs to sequester highly toxic OP anti-ChEs before they reach their physiological targets. This approach turns the irreversible nature of the OP: ChE interaction from disadvantage to an advantage; instead of focusing on OP as an anti-ChE, one can use ChE as an anti-OP. Using this approach, it was shown that administration of fetal bovine serum AChE (FBSAChE) or equine serum butyrylcholinesterase (EqBChE) or human serum BChE (HuBChE) protected the animals from multiple LD 50s of a variety of highly toxic OPs without any toxic effects or performance decrements. The bioscavengers that have been explored to date for the detoxification of OPs fall into three categories: (A) those that can catalytically hydrolyze OPs and thus render them non-toxic, such as OP hydrolase and OP anhydrase; (B) those that stoichiometrically bind to OPs, that is, 1 mol of enzyme neutralizes one or 2 mol of OP inactivating both, such as ChEs and related enzymes; and (C) and those generally termed as “pseudo catalytic”, e.g., a combination of ChE and an oxime pre-treatment such that the catalytic activity of OP-inhibited ChE can rapidly and continuously be restored in the presence of an oxime. Since the biochemical mechanism underlying prophylaxis by exogenous esterases such as ChEs is established and tested in several animal species, including non-human primates, this concept should allow a reliable extrapolation of results from animal experiments to human application. Having being extensively investigated by several groups, plasma derived HuBChE is judged to be the most suitable bioscavenger for its advancement for human use. The program is being developed at the present time for conducting a safety clinical trial in human volunteers. Several other candidate bioscavengers will follow; e.g., recombinant HuBChE expressed in the milk of transgenic goats, pseudo catalytic scavenger(s), e.g., a combination of ChE and oxime, and possibly PON 1 as a catalytic scavenger in the future.