Physicochemical, Biological And -Haematin Inhibiting Activity Of Pyrido-Dibemequines, Pyrido[1,2-A]Benzimidazoles And Their Derivatives

ABSTRACT There is an urgent need for new antimalarials following the emergence of Plasmodium falciparum strains with reduced sensitivity to the currently used artemisinin combination therapies. Classical aminoquinoline-based drugs inhibit the formation of haemozoin (HZ) thereby causing parasite death from the cellular accumulation of toxic ‘free’ haem. Coincidentally, this immutable pathway also exists in Schistosoma mansoni, and presents a vulnerable target for drug design in these haematophagus organisms. Therefore, it would be of interest to explore novel scaffolds that can inhibit HZ formation as well as exploit the merits of established drugs via structural modifications that would harness their pharmacological and pharmacokinetic advantages while circumventing their therapeutic shortcomings. This project investigated the physicochemical, biological and mechanistic profiles of pyridodibemequine (pDBQ) and pyrido[1,2-a]benzimidazole (PBI) derivatives whose structural motifs were informed by previously synthesised prototype molecules. Specifically, the aqueous solubility, membrane permeability, lipophilicity, metabolic stability and potential for cardiotoxicity of seven pDBQs, their metabolites and ten PBIs were tested through computational and experimental methods. In addition, their antiplasmodial and antischistosomal activities were determined and correlated with their respective physicochemical properties. As regards mechanistic evaluation, their ability to inhibit formation of abiotic HZ, β-haematin (βH), was assessed and intracellular inhibition of HZ formation probed. The pDBQs constitute reversed chloroquines with a 4-aminoquinoline nucleus hybridised to a dibenzylmethylamine side group that serves as a chemosensitising moiety. The pDBQ derivatives showed moderate to high solubility (52 – 197 µM) and permeability (LogPapp: -4.6 – -3.6) at pH 6.5. Their lipophilicity, indexed by cLogP, ranged between 3.7 and 5.6 while the mean LogD at both cytosolic (7.4) and vacuolar (5.0) pH was 3.15 and 0.93, respectively. The compounds also showed low-nanomolar range antiplasmodial activity against both chloroquine (CQ)-sensitive (CQS) and resistant (CQR) strains (IC50 range CQS: 14.4 – 126.6 nM, CQRDd2: 44.5 – 162 nM and CQR7G8: 69.6 – 307.1 nM), with no discernible crossresistance with CQ and the antiplasmodial activity directly correlated with lipophilicity. Mechanistically, all the pDBQs inhibited βH formation (IC50: 13 – 25 µM) and haem-pyridine fractionation profiles revealed they produced a CQ-like dose-dependent increase in toxic iii ‘free’ haem with corresponding decrease in HZ levels. Predicted human-Ether-a-Go-GoRelated Gene (hERG) channel inhibition pIC50 ranged between 6.2 and 6.6, and correlated strongly with the cLogP and molecular weight. The derivatives were also highly susceptibility to microsomal metabolism, with N-dealkylation identified as the main biotransformation route. The pDBQ metabolites exhibited solubility and membrane permeability profiles similar to the parent compounds at pH 6.5, albeit with reduced lipophilicity (cLogP range: 2.3 – 3.5). Their βH inhibition activity (IC50: 15 – 24 µM) was also comparable to the parent compounds as were the haem-pyridine fractionation profiles. However, they showed greater antiplasmodial activity, with 4/7 derivatives exhibiting IC50 < 80 nM against PfDd2 (CQR strain). The metabolites had reduced hERG channel inhibition potential (pIC50: 5.0 – 5.7) and significantly improved metabolic stability upon incubation with mouse and human liver microsomes. The PBIs comprise molecules with structural likeness to CQ, including a planar heterocyclic moiety and a basic amine side group. PBI analogues showed low to moderate solubility (