Bisquaternary pyridinium oximes: Comparison of in vitro reactivation potency of compounds bearing aliphatic linkers and heteroaromatic linkers for paraoxon-inhibited electric eel and recombinant human acetylcholinesterase

Abstract

Oxime reactivators are the drugs of choice for the post-treatment of OP (organophosphorus) intoxication and used widely for mechanistic and kinetic studies of OP-inhibited cholinesterases. The purpose of the present study was to evaluate new oxime compounds to reactivate acetylcholinesterase (AChE) inhibited by the OP paraoxon. Several new bisquaternary pyridinium oximes with heterocyclic linkers along with some known bisquaternary pyridinium oximes bearing aliphatic linkers were synthesized and evaluated for their in vitro reactivation potency against paraoxon-inhibited electric eel acetylcholinesterase (EeAChE) and recombinant human acetylcholinesterase (rHuAChE). Results herein indicate that most of the compounds are better reactivators of EeAChE than of rHuAChE. The reactivation potency of two different classes of compounds with varying linker chains was compared and observed that the structure of the connecting chain is an important factor for the activity of the reactivators. At a higher concentration (10(-3)M), compounds bearing aliphatic linker showed better reactivation than compounds with heterocyclic linkers. Interestingly, oximes with a heterocyclic linker inhibited AChE at higher concentration (10(-3)M), whereas their ability to reactivate was increased at lower concentrations (10(-4)M and 10(-5)M). Compounds bearing either a thiophene linker 26, 46 or a furan linker 31 showed 59%, 49% and 52% reactivation of EeAChE, respectively, at 10(-5)M. These compounds showed 14%, 6% and 15% reactivation of rHuAChE at 10(-4)M. Amongst newly synthesized analogs with heterocyclic linkers (26-35 and 45-46), compound 31, bearing furan linker chain, was found to be the most effective reactivator with a k(r) 0.042min(-1), which is better than obidoxime (3) for paraoxon-inhibited EeAChE. Compound 31 showed a k(r) 0.0041min(-1) that is near equal to pralidoxime (1) for paraoxon-inhibited rHuAChE.

Figure 1

Examples of mono- and bisquaternary pyridinium oximes

Figure 2

Comparison of reactivation potency at three different concentrations (0.001 M – 0.00001 M) for selected compounds with aliphatic linker (20, 22, 24 and 42) and heterocyclic linkers (30, 26 and 31) at three different concentrations (0.001 M – 0.00001 M) for EeAChE. Three different trends [(i). 20 and 22; (ii). 24, 42 and 30; (iii). 26 and 31] of % reactivation with increasing concentration of oxime are also shown. Bars are represented as % reactivation ± SEM.

Figure 3

Comparison of reactivation potency of 4,4′-bisoximes differing by a linker chain (Aliphatic- 2, 3, 25; heterocyclic- 28, 32, 35) for EeAChE at 10⁻³ M and 10⁻⁵ M. Bars are represented as % reactivation ± SEM.

Figure 4

Comparison of reactivation potency for selected compounds at 10⁻⁵ M for EeAChE and rHuAChE. Bars are represented as % reactivation ± SEM.

Scheme 1

Reagents and conditions: (a). DMF, 80 °C, 2–24 h, 24–89%.

Scheme 2

Reagents and conditions: (a). MeCN, 70–80 °C, 2–3 h, 60–75%; (b). DMF, 80 °C, 2–24 h, 55–75%.