Evaluating the attractiveness of the terminal oxidases as drug targets in mycobacterium abscessus

• Main Author: Sorayah, Ria
• Other Authors: Kevin Pethe
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Biological sciences::Microbiology
• Online Access: https://hdl.handle.net/10356/155733
• Institution: Nanyang Technological University

The global incidence and prevalence of rapidly-growing mycobacteria (RGM) have increased at a startling rate, particularly in the developed countries. Among them is the Mycobacterium abscessus (Mabs) complex which accounts for the majority of RGM-related infections. Mabs primarily causes pulmonary infections among immunocompromised individuals and individuals with underlying lung pathology, such as cystic fibrosis, bronchiectasis, chronic obstructive pulmonary diseases (COPD) and tuberculosis sequelae, although infections in healthy individuals have also been reported in one-third of cases. Mabs infections are notoriously difficult to treat and they often develop into a chronic incurable disease as the bacterium is intrinsically resistant to numerous antibiotics, including those used to treat tuberculosis (TB). Therefore, there is an urgent need to discover and develop effective pharmacological options to treat Mabs infections. The oxidative phosphorylation (OxPhos) pathway has been identified as a promising drug target due to its essentially for growth and survival in mycobacteria. Over the years, several compounds have been discovered to target the components of the OxPhos pathway in Mycobacterium tuberculosis (Mtb). While most anti-TB drugs are ineffective against Mabs, those affecting the OxPhos pathway were reported to retain some potency thus suggesting that targeting this pathway could be a useful strategy in drug development against Mabs. Mabs possesses a branched electron transport chain consisting of two terminal oxidases, namely the cytochrome bcc:aa3 (cyt-bcc:aa3) supercomplex and the cytochrome bd (cyt-bd) terminal oxidase. In this study, the potency of the anti-tuberculosis drug Q203 (Telacebec) and TB47 were evaluated against Mabs. Q203 is a clinical-stage drug candidate, while TB47 is a close analogue of Q203. Both drugs target the QcrB subunit of the cyt-bcc:aa3. Our findings revealed that naturally-occurring polymorphisms in the Mabs QcrB are responsible for the high resistance of the bacterium to Q203. Considering that all of the cyt-bcc:aa3 inhibitors discovered to date share the same binding pocket as Q203, this finding has important implications for the repurposing and development of drugs targeting the Mabs cyt-bcc:aa3. Despite the limitation of a direct drug repurposing approach for Q203 and other related cyt-bcc:aa3 inhibitors, the cyt-bcc:aa3 remains an attractive target for drug development in Mabs due to its essentiality for optimum growth under aerobic conditions. Cyt-bd is generally regarded as an attractive drug target due to its exclusivity to prokaryotes. In this study, genetic deletion of cyt-bd was observed to sensitise Mabs to the type II NADH dehydrogenase (NDH-2) effector clofazimine, thus suggesting a role for cyt-bd in the bacteria defence against clofazimine. Whole-cell screening assay of small-compound libraries led to the identification of two putative Mabs cyt-bd inhibitors, named compound-6 (cpd-6) and compound-12 (cpd-12). Although cpd-6 and cpd-12 are ineffective on their own, they synergised clofazimine to effectively kill Mabs. Overall, this study supports the attractiveness of the cyt-bd as a drug target in Mabs, especially as part of a rational drug combination targeting the OxPhos pathway.