More Medicines for Tuberculosis: Fuelling Drug Discovery against Mycobacterium tuberculosis

Tuberculosis (TB), whose etiological agent is Mycobacterium tuberculosis (M. tuberculosis), has plagued humanity since antiquity. Even with chemotherapy available today, TB is the leading cause of death due to an infectious disease. Modern day factors, such as the HIV epidemic and the emergence of drug-resistant TB strains, have redefined the complexities and challenges of tackling the TB pandemic. Current treatments for TB are lengthy, and poor adherence to such prolonged treatment further exacerbates the issue of drug resistance. Moreover, therapies for drug-resistant TB have low cure rates. There is therefore a pressing need for improved therapies that are short and efficacious against all TB strains, which can be achieved through new antimicrobials that are more potent and have novel mechanisms of action, in addition to being affordable, orally bioavailable, and without drug-drug interactions. Such new anti-TB drugs need to be discovered and developed through a long, risky, and costly process, in which attrition rates are high. While there are promising compounds currently being developed, including the benzothiazinones (BTZs), it is necessary to further populate and enhance the Global TB drug pipeline to ensure the availability of new drugs. This thesis aims to address this need through the discovery work of two new, highly promising families, the AX and PB compounds, and of BTZs. The piperazine-based AX analogs are easily synthesised and demonstrate potent activity against M. tuberculosis in vitro and in vivo. Their target, identified in this work, is the QcrB subunit of the cytochrome bc1-aa3 complex, a terminal oxidase of the mycobacterial respiratory chain. Notably, AX compounds are bactericidal in the absence of the alternate terminal oxidase, cytochrome bd. As this family interacts differently in the same binding site of QcrB as Q203, a drug candidate in clinical trials, AX compounds could potentially serve as a backup series for QcrB inhibitors. The PB family, derived from the natural product lapachol, is also easily synthesised and shows substantially improved activity against M. tuberculosis in vitro compared to the parent compound. PB analogs also demonstrated activity against the non-replicating bacillus and in infected macrophages. The mechanism of action of PB compounds relies on the F420 cofactor, although not on the F420-dependent nitroreductase Ddn, therefore this family has a novel mechanism of action which is highly specific to M. tuberculosis. To support the clinical development of PBTZ169, the mechanism of resistance to BTZs was further elucidated in this thesis. Five mutations at cysteine 387 of the target enzyme, DprE1, were identified as mediating resistance to BTZs, which would serve as resistance markers for clinical screening. The impact of these mutations on M. tuberculosis and on DprE1 was further characterised, revealing a fitness cost imposed on the bacillus intracellularly and reduced catalytic efficiency of the enzyme. This thesis additionally contributed to the characterisation of a PBTZ169 backup series and the design of an optimal regimen with other TB drugs. The compounds presented herein merit further optimisation so their full antibiotic potential may be realised for TB, and possibly for other mycobacterial diseases as well. Altogether, this thesis has contributed to the fuelling of drug discovery against M. tuberculosis, and a step towards more medicines for TB.