Protection Against Soman or VX Poisoning by Human Butyrylcholinesterase in Guinea Pigs and Cynomolgus Monkeys

Human butyrylcholinesterase (HuBuChE), purified from outdated human plasma, is being evaluated for efficacy against nerve agents in guinea pigs and cynomolgus monkeys. Previous studies in rodents and nonhuman primates demonstrated that pretreatment of animals with enzymes that can scavenge nerve agents could provide significant protection against behavioral and lethal effects of nerve agent intoxication. In preparation for evaluation of efficacy of HuBuChE prior to initiating an investigational new drug (IND) application, the pharmacokinetics of HuBuChE were evaluated in guinea pigs and in cynomolgus monkeys. HuBuChE was injected intramuscularly (i.m.) at two doses, and blood samples were taken to follow the time-course of HuBuChE in blood for up to 168 h after administration. In guinea pigs, the two doses of HuBuChE, 19.9 and 32.5 mg/kg, produced similar times of maximal blood concentration (T(max) of 26.0 and 26.8 h, respectively) and similar elimination half-times (t(1/2) of 64.6 and 75.5 h, respectively). Enzyme levels were still 10-fold over baseline at 72 h. Based on these data, guinea pigs were administered 150 mg/kg of enzyme i.m. and challenged at T(max). Soman or VX doses were approximately 1.5, 2.0 and 2.0 x LD50 administered subcutaneously (s.c.) in sequence at 90-120 min apart. None of the animals displayed signs of organophosphorus (OP) anticholinesterase intoxication at any of the challenge levels, and all survived for the 14-day duration of the experiment. Similar experiments were carried out with cynomolgus monkeys to determine the pharmacokinetics of HuBuChE and its efficacy against soman. The complete survival of nearly all animals tested to date, coupled with the maximal blood concentration and half-life elimination profile obtained for HuBuChE after i.m. injection, provides strong support for the continued development of HuBuChE as a product to protect against nerve agents.