Estimation of the upper limit of human butyrylcholinesterase dose required for protection against organophosphates toxicity: a mathematically based toxicokinetic model.

Human butyrylcholinesterase (HuBChE) is a drug candidate for protection against organophosphates (OP) intoxication. A mathematically based model was validated and employed to better understand the role of the endogenous HuBChE in detoxification of OPs and to estimate the dose of exogenous HuBChE required for enhancing protection of humans from lethal exposure to OPs. The model addresses the relationship between the HuBChE dose needed to maintain a certain residual activity of human acetylcholinesterase (HuAChE) and the following parameters: (1) level and duration of exposure, (2) bimolecular rate constants of inhibition of HuAChE (kA) and HuBChE (kB) by OPs, and (3) time elapsed from enzyme load. The equation derived for the calculation of HuBChE dose requires the knowledge of kA/kB in human blood and the rate constant of HuBChE elimination. Predictions of HuBChE doses were validated by in vitro experiments and data of published human studies. These predictions highlight two parameters that are likely to decrease the calculated dose: (1) the rapid consumption of the less toxic isomers of OPs in human plasma, and (2) the volume of distribution of HuBChE that appears significantly greater than the volume of plasma. The first part of the analysis of the proposed model was focused on acute bolus exposures and suggests that upper limit doses of 134, 115, and 249 mg/70 kg are sufficient to protect RBC AChE above 30% of baseline activity following a challenge with 1 LD(50) VX, soman, and sarin, respectively. The principles of the validated model should be applicable for advanced predictions of HuBChE dose for protection against continuous exposures to OPs.