In vitro P-glycoprotein activity does not completely explain in vivo efficacy of novel centrally effective oxime acetylcholinesterase reactivators

Abstract

Novel-substituted phenoxyalkyl pyridinium oxime acetylcholinesterase (AChE) reactivators (US patent 9,227,937) that showed convincing evidence of penetration into the brains of intact rats were developed by our laboratories. The oximes separated into three groups based on their levels of brain AChE reactivation following exposure of rats to the sarin surrogate nitrophenyl isopropyl methylphosphonate (NIMP). P-glycoprotein (P-gp) is a major blood–brain barrier (BBB) transporter and requires ATP for efflux. To determine if P-gp affinity screening could be used to reduce animal use, we measured in vitro oxime-stimulated ATPase activity to see if the in vivo reactivation efficacies related to the oximes’ functions as P-gp substrates. High efficacy oximes were expected to be poor P-gp substrates, thus remaining in the brain longer. The high efficacy oximes (24–35% brain AChE reactivation) were worse P-gp substrates than the low efficacy oximes (0–7% brain AChE reactivation). However, the oxime group with medium in vivo reactivation of 10–17% were even worse P-gp substrates than the high efficacy group so their reactivation ability was not reflected by P-gp export. The results suggest that in vitro P-gp ATPase activity can remove the low efficacy oximes from in vivo testing, but is not sufficient to differentiate between the top two tiers.

Keywords:

• P-glycoprotein
• oxime
• acetylcholinesterase
• reactivator
• BBB
• organophosphate

Disclosure statement

No potential conflict of interest was reported by M.B.D. and E.C.M.

J.E.C.: The novel oximes are under patent protection (US patent 9,227,937), but are not presently licensed or under commercial development. Therefore, I declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Additional information

Funding

This work was supported in part by the Defense Threat Reduction Agency [1.E0056–08-AHB-C] through the Henry Jackson Foundation for the Advancement of Military Medicine, INC. [000169320]; in part by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health [U01NS083430]; and in part by the Center for Environmental Health Sciences at Mississippi State University.