Pharmacology, Pharmacokinetics, and Tissue Disposition of Zwitterionic Hydroxyiminoacetamido Alkylamines as Reactivating Antidotes for Organophosphate Exposure

Abstract

In the development of antidotal therapy for treatment of organophosphate exposure from pesticides used in agriculture and nerve agents insidiously employed in terrorism, the alkylpyridinium aldoximes have received primary attention since their early development by I. B. Wilson in the 1950s. Yet these agents, by virtue of their quaternary structure, are limited in rates of crossing the blood-brain barrier, and they require administration parenterally to achieve full distribution in the body. Oximes lacking cationic charges or presenting a tertiary amine have been considered as alternatives. Herein, we examine the pharmacokinetic properties of a lead ionizable, zwitterionic hydroxyiminoacetamido alkylamine in mice to develop a framework for studying these agents in vivo and generate sufficient data for their consideration as appropriate antidotes for humans. Consequently, in vitro and in vivo efficacies of immediate structural congeners were explored as leads or backups for animal studies. We compared oral and parenteral dosing, and we developed an intramuscular loading and oral maintenance dosing scheme in mice. Steady-state plasma and brain levels of the antidote were achieved with sequential administrations out to 10 hours, with brain levels exceeding plasma levels shortly after administration. Moreover, the zwitterionic oxime showed substantial protection after gavage, whereas the classic methylpyridinium aldoxime (2-pyridinealdoxime methiodide) was without evident protection. Although further studies in other animal species are necessary, ionizing zwitterionic aldoximes present viable alternatives to existing antidotes for prophylaxis and treatment of large numbers of individuals in terrorist-led events with nerve agent organophosphates, such as sarin, and in organophosphate pesticide exposure.

Organophosphorus compounds and oximes: a critical review

“…Among these, RS194B (Fig. 2 , compound 4) was considered as new lead compound but turned out to be a weak reactivator of paraoxon-, sarin-, cyclosarin-, VX- and tabun-inhibited human AChE and in most cases substantially less potent compared to 2-PAM (Radic et al 2012 ; Kovarik et al 2013 ; Sit et al 2018 ). Post-exposure therapy of OP poisoned mice by RS194B (125 mg/kg) and atropine (10 mg/kg) provided good protective ratios for paraoxon, sarin and VX (unfortunately, the protective ratios of atropine alone therapy were not provided for comparison) but had no effect with soman and tabun (Radic et al 2012 ).…”

Central neuroprotection demonstrated by novel oxime countermeasures to nerve agent surrogates

“…[25][26][27][28] A zwitterionic hydroxyiminoacetamido alkylamine, RS194b, has shown efficacy in reducing AChE inhibition and attenuating the signs of toxicity elicited by sarin and paraoxon in macaques, in addition to the remediation of AChE inhibition elicited by a sarin analogue in mice. [29][30][31][32][33]…”

Rational design, synthesis, and evaluation of uncharged, “smart” bis-oxime antidotes of organophosphate-inhibited human acetylcholinesterase

“…Based on the structure of the pyridinium monoxime 2PAM (1), structures and in vitro reactivation efficacies of pyridinium aldoxime-based antidotes have evolved during seventy years of research to yield more efficient bis-pyridinium bis-oximes, such as ortho7, obidoxime or MMB4 (2)(3)(4)(5). Nevertheless, those cationic antidotes are ineffective in vivo, for reactivation of brain AChE or for reactivation of AChE in any tissue when administered orally (6), due to inability of pyridinium cations to cross biological membranes (7). Within the past decade it has become increasingly clear that for effective and complete recovery from an OP intoxication, antidotal action is needed in both peripheral and central nervous systems (8,9).…”

“…One mechanism for antidotes to traverse biological membranes and reach the central nervous system (CNS) is by diffusion, which is most effective for uncharged molecular species when they are formed by fast equilibration with ionized species. An acetamido oxime RS194B (7) (Figure 1), is an example of an uncharged but ionizable antidote with demonstrated capacity to cross biological membranes including the blood-brain barrier (BBB) and effectively recover activity of OP inhibited AChE, both in vitro (6,7) and in vivo (7,12). Protonation states of its two ionizable groups (oxime and heterocyclic amine; both with pKa values of ~9 (7)) will equilibrate in an aqueous environment between four ionization species, cationic, anionic, zwitterionic and uncharged ( Figure 1A).…”