Acetylcholinesterase inhibition and anti-soman efficacy of homologs of physostigmine.

Inhibition of acetylcholinesterase (AChE) activity by physostigmine (PHY) is reversible due to spontaneous decarbamylation. Physostigmine has been shown to be effective as a pretreatment against potent anticholinesterase poisons (e.g., soman) in experimental animals, yet it is short acting and causes undesirable side effects in mammals. The two-fold purpose of this study was 1) to determine whether extension of the N-substituted alkyl chain (N-SAC) of PHY from N-methyl to N-ethyl (I), N-propyl (II), N-isopropyl (III), N-butyl (IV) or N-heptyl (V) affects anti-AChE potency and spontaneous decarbamylation of inhibited AChE of guinea pig blood in vitro and in vivo, and 2) to see whether chain extension affects efficacy as pretreatment in poisoning by soman. The in vitro AChE inhibition studies were done using whole blood incubated at 37 degrees C for 30 min. All 5 homologs possessed anti-AChE activity with I50s ranging from 1.1 to 27.6 x 10(-7)M; compound III was the least potent in vitro and in vivo. Lengthening of the N-SAC of PHY markedly extended the duration of anti-AChE activity when compared to PHY, but rendered the modified compounds ineffective as pretreatments against soman. These data support the premise that the decrease in decarbamylation rates observed upon extending the N-SAC of PHY is responsible for the loss of effectiveness of pretreatment regimens against soman. Perhaps, these homologs of PHY may have potential use in instances where sustained action of acetylcholine is required at cholinergic junctions because of disease conditions or drug overdosage.

Evaluation of several oximes as reactivators of unaged soman-inhibited whole blood acetylcholinesterase in rabbits.

The antidotal benefit of oximes against organophosphorus (OP) anticholinesterase intoxication is thought to be due to reactivation of the OP-inhibited acetylcholinesterase (AChE). This study was conducted to determine whether the antidotal efficacy against soman by the oximes 2-hydroxyiminomethyl-3-methyl-1-[2-(3-methyl-3-nitrobutyl oxymethyl)]-imidazolium Cl (ICD 467) and 1,1'-methylenebis[4-(hydroxyiminomethyl) pyridinium] di-Cl (MMB-4) resulted, in part, from reactivation of the inhibited AChE. These oximes were tested in parallel with pralidoxime Cl (2-PAM) and 1-(2-hydroxyiminomethyl-1-pyridinio-3-(4-carbamoyl-1-pyridinio+ ++)-2-oxapropane di-Cl (HI-6). Rabbits were atropinized (8 mg/kg, i.m.) and intoxicated with soman (13 micrograms/kg, i.v.; 1.2 x LD50) 5 min later. Three minutes after soman, animals were treated with oxime (50, 100 or 150 mumol/kg, i.m.). Whole blood was collected from a catheter in the central artery of the ear just before soman, at 2 min after soman and at 2, 5, 10, 15, 30, and 60 min after oxime or vehicle for determination of AChE activity. Shortly thereafter, animals were anesthetized and exsanguinated with immediate flushing using heparinized saline. AChE activity was also determined on the cortex, medulla-pons and diaphragm to assess central and peripheral reactivation. Treatment with HI-6 or MMB-4 (50 mumol/kg, i.m.) resulted in significant (P less than 0.05) reactivation of soman-inhibited whole blood AChE and diaphragm cholinesterase (ChE), but not brain AChE. In contrast, 2-PAM was completely ineffective in reactivating soman-inhibited AChE. HI-6 was significantly better than MMB-4 in reactivating blood AChE; they were essentially equal against soman-inhibited diaphragm ChE. Three animals exposed to soman and treated with ICD 467 died within 15 min. When animals not exposed to soman were treated with ICD 467 (25 mumol/kg, i.m.), whole blood AChE activity was depressed by 60% within 5-10 min after treatment. Furthermore, ICD 467 failed to reactivate significantly unaged soman-inhibited erythrocyte AChE, in vitro. These observations indicate that ICD 467 would be contraindicated as a therapy for anti-ChE intoxication and that the efficacy of HI-6 or MMB-4 can be explained, in part, by reactivation of soman-inhibited AChE.