A novel pyrazolopyridone class of inhibitors was identified from whole cell screening against Mycobacterium tuberculosis (Mtb). The series exhibits excellent bactericidality in vitro, resulting in a 4 log reduction in colony forming units following compound exposure. The significant modulation of minimum inhibitory concentration (MIC) against a Mtb strain overexpressing the Rv3790 gene suggested the target of pyrazolopyridones to be decaprenylphosphoryl-β-D-ribose-2'-epimerase (DprE1). Genetic mapping of resistance mutation coupled with potent enzyme inhibition activity confirmed the molecular target. Detailed biochemical characterization revealed the series to be a noncovalent inhibitor of DprE1. Docking studies at the active site suggest that the series can be further diversified to improve the physicochemical properties without compromising the antimycobacterial activity. The pyrazolopyridone class of inhibitors offers an attractive non-nitro lead series targeting the essential and vulnerable DprE1 enzyme for the discovery of novel antimycobacterial agents to treat both drug susceptible and drug resistant strains of Mtb.

From the phenotypic screening of the AstraZeneca corporate compound collection, N-aryl-2-aminobenzimidazoles have emerged as novel hits against the asexual blood stage of Plasmodium falciparum (Pf). Medicinal chemistry optimization of the potency against Pf and ADME properties resulted in the identification of 12 as a lead molecule. Compound 12 was efficacious in the P. berghei (Pb) model of malaria. This compound displayed an excellent pharmacokinetic profile with a long half-life (19 h) in rat blood. This profile led to an extended survival of animals for over 30 days following a dose of 50 mg/kg in the Pb malaria model. Compound 12 retains its potency against a panel of Pf isolates with known mechanisms of resistance. The fast killing observed in the in vitro parasite reduction ratio (PRR) assay coupled with the extended survival highlights the promise of this novel chemical class for the treatment of malaria.

Whole-cell high-throughput screening of the AstraZeneca compound library against the asexual blood stage of Plasmodium falciparum (Pf) led to the identification of amino imidazoles, a robust starting point for initiating a hit-to-lead medicinal chemistry effort. Structure-activity relationship studies followed by pharmacokinetics optimization resulted in the identification of 23 as an attractive lead with good oral bioavailability. Compound 23 was found to be efficacious (ED90 of 28.6 mg·kg(-1)) in the humanized P. falciparum mouse model of malaria (Pf/SCID model). Representative compounds displayed a moderate to fast killing profile that is comparable to that of chloroquine. This series demonstrates no cross-resistance against a panel of Pf strains with mutations to known antimalarial drugs, thereby suggesting a novel mechanism of action for this chemical class.

Reactive metabolites (RMs) have been implicated as causal factors in many drug-associated idiosyncratic toxicities. This study aims at identification and mitigation of an RM liability associated with aminoimidazole and amino(aza)benzimidazole structural motifs from an antimalarial project. Nineteen compounds with different structural modifications were studied in rat and human liver microsomes using glutathione (GSH) and N-acetyl cysteine (NAC) as trapping agents for RM. Metabolite profiling of aminoimidazole compounds in initial studies revealed the presence of dihydrodiol metabolites suggestive of reactive epoxide precursors, confirmed by the identification of a dihydrohydroxy GSH conjugate in GSH supplemented incubations. Substitution of methyl group at a potential site of metabolism blocked the epoxidation; however, formation of an imine-methide RM was suspected. Masking the site of metabolism via benzimidazole and 4/7-azabenzimidazole resulted in the possible formation of quinone-imine intermediates as a product of bioactivation. Further, substitutions with electron withdrawing groups and steric crowding did not address this liability. Mitigation of bioactivation was achieved with 5/6-azabenzimidazole and with CF3 substitution at the 6-position of the 7-azabenzimidazole ring. Moreover, compounds devoid of imidazole -NH2 do not undergo bioactivation. This study, therefore, establishes aminoimidazole and amino(aza)benzimidazoles as potential toxicophores and describes ways to mitigate this bioactivation liability by chemical modification.

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