Introduction

The design of oxime reactivators for both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibited by organophosphorus compounds (OPs), including nerve agents (NAs), remains challenging, with limited success [1]. The main problem in designing effective cholinesterase (ChE) reactivators is the poor structure-activity relationship of newly developed oximes, which very rarely achieve the reactivation efficacy of standard oximes such as pralidoxime (2-PAM), asoxime (HI-6), obidoxime and dipyroxime (TMB-4) [2]. The structure of the OP that inhibits ChE is another critical factor in the effectiveness of reactivation. For example, oxime HI-6 is a potent reactivator of cyclosarin-inhibited AChE, a moderately effective reactivator of sarin or VX-inhibited AChE, but an ineffective reactivator of tabun-inhibited AChE [3].

Worek and co-workers commented on the structure-activity relationship in the development of oxime reactivators: “Since the invention of the first clinically used oxime, pralidoxime (2-PAM) in the 1950s, ongoing research attempted to identify more effective oximes. In fact, several thousand oximes were synthesized in the past six decades. These include charged and non-charged compounds, mono- and bispyridinium oximes, asymmetric oximes, oximes with different substitutes and more recently non-oxime reactivators. Multiple in vitro and in vivo studies investigated the potential of oximes to reactivate OP-inhibited AChE. The inconsistent effectiveness of oximes in the treatment of OP-pesticide-poisoned patients led to a continuous discussion on the value of oximes. In order to provide a forward-looking evaluation of the significance of oximes in OP poisoning, multiple aspects, including intrinsic toxicity, in vitro reactivation potency, efficacy and pharmacokinetics, as well as the impact of the causative OP, have to be considered.” [1].

Oximes are primarily designed to reactivate AChE, given its vital role in neurotransmission, thereby attenuating toxic OP effects. Yet, the related enzyme BChE, which, unlike AChE, doesn’t have an essential physiological role, has been recognised as a pseudo-catalytic scavenger of OP compounds if paired with an effective BChE oxime reactivator [4-8]. An efficient pseudo-catalytic scavenger would degrade the OP compound before AChE inhibition, thus protecting the victim from the poisoning [4,9]. However, effective AChE reactivators show poor efficacy in BChE reactivation due to differences in the amino acid composition lining the active-site gorge of AChE. The design of effective BChE reactivators needs to be guided by specific structural properties of the BChE active site that arise from differences in the composition of aromatic residues. Instead of the six aromatic residues present in the AChE active site, matching aliphatic residues are found in the BChE active site, thus changing the aromatic properties and enlarging the volume of the BChE active site ~0.200 nm3 (200 Å3) [10]. As mentioned, reactivation effectiveness greatly depends on the structure of the OP compound that inhibited ChE. Upon inhibition, the volume of the active site is reduced depending on the type of inhibiting OP compound, which is one reason oxime reactivation efficacy varies significantly among different OPs [11,12]. Finding a universal reactivator, the oxime that would be effective in the reactivation of ChE inhibited by various OPs, would be considered the ultimate goal, and in silico studies can be a helpful tool.

In this study, we used our previously published results on the in vitro evaluation of the kinetic properties of a library of 115 oximes for the reactivation of BChE inhibited by OPs: sarin (GB), cyclosarin (GF), tabun (GA), and VX [9]. The library contained triazole oximes synthesised using the click-chemistry method and their pyridinium or miscellaneous building blocks (Figure 1). A set of standard oximes known for their efficacy in AChE reactivation inhibited by OPs: 2-PAM, HI-6, obidoxime, and TMB-4, was used for comparison. The aim of the study was to verify if the principal component analysis (PCA) could be used as a statistical method for the evaluation of the oxime’s reactivation efficacy. PCA is a technique for analysing relatively large datasets, and it was chosen for its ability to increase interpretability while minimizing information loss [13-16]. We analysed the reactivation efficacy of the library of oximes using kinetic parameters together with pharmacological parameters such as ADME (absorption, distribution, metabolism, and excretion) and drug design parameters. The rationale for this type of analysis is also the prediction of CNS (central nervous system) reactivation activity of the studied oximes. Due to their permanent positive charge, which limits passive diffusion across the blood-brain barrier (BBB), these oximes may have limited reactivation activity toward ChEs in the CNS [17,18].

Figure 1.: Chemical structures of effective oxime reactivators of BChE inhibited by organophosphorus compounds: tabun (GA), sarin (GB), cyclosarin (GF), and VX, screened from a library of 115 oximes [9].