Shyong_2021_J.Pharmacol.Exp.Ther__

Inhibition of acetylcholinesterase (AChE) by certain organophosphates (OP) can be life-threatening and requires reactivating antidote accessibility to the peripheral and central nervous systems to reverse symptoms and enhance survival parameters. In considering dosing requirements for oxime antidotes in OP exposures that inactivate AChE, clearance of proton ionizable, zwitterionic antidotes is rapid and proceeds with largely the parent antidotal compound being cleared by renal transporters. Such transporters may also control disposition between target tissues and plasma as well as overall elimination from the body. An ideal, small molecule antidote should access and be retained in primary target tissues: CNS (brain), skeletal muscle, and peripheral autonomic sites, for sufficient periods to reactivate AChE and prevent acute toxicity. We show here that we can markedly prolong the antidotal activity of zwitterionic antidotes by inhibiting P-glycoprotein (P-gp) transporters in the brain capillary and renal systems. We employ the P-gp inhibitor, tariquidar (TQD), as a reference compound and show tissue and plasma levels of RS194B, a hydroxyl-imino acetamido alkylamine reactivator, are elevated and plasma clearances are reduced. To examine mechanism, identify the transporter and establish the actions of a transport inhibitor, we compare the pharmacokinetic parameters in a P-glycoprotein knock-out mouse strain and see dramatic enhancements of short-term plasma and tissue levels. Hence, repurposed transport inhibitors, that are candidate or FDA approved drugs, should enhance target tissue concentrations of the zwitterionic antidote through inhibition of both renal elimination and brain capillary extrusion. Significance Statement We examine renal and brain capillary transporter inhibition for lowering dose and frequency of dosing of a blood-brain barrier (BBB) permanent, reactivating antidote, RS194B, an ionizable zwitterion. Through a small molecule, tariqudar (TQD), and gene knock-out mice, CNS antidote concentrations are enhanced and total body clearances are diminished. RS194B with repurposed transport inhibitors should enhance reactivation of central and peripheral OP-inhibited AChE. Activity at both disposition sites are desired features for replacing 2-PAM as an antidote for acute OP exposure.