Development of Novel Host-Directed and Antibacterial Compounds for Tuberculosis Therapy

Abstract

Tuberculosis (TB) remains a top 10 cause of death globally, with its causative agent Mycobacterium tuberculosis (Mtb) being the single deadliest infectious agent. The immune response is often unable to effectively fight Mtb infection as the bacteria is adept at manipulating and exploiting the immune system for its survival and persistence. Treatment for TB has been relatively unchanged for decades, involving a 6-month course of up to 4 antibiotics under the best of conditions, which is rapidly losing efficacy due to the acceleration in antibiotic resistance emergence. It is therefore imperative to not only develop novel antibiotics, but to also look beyond antibiotic therapy and develop alternative treatment strategies. One such alternative strategy is host-directed therapy (HDT), an approach aimed at targeting the host to improve the immune response to infection, which is significantly impaired by Mtb. In this thesis, I demonstrate the viability of targeting the host phosphatase PPM1A for TB HDT, and developed a novel small molecule inhibitor (SMIP-30) that is able to boost clearance of Mtb by macrophages. Mechanistically, SMIP-30 enhances autophagy in macrophages, which is the primary mechanism by which the drug promotes Mtb clearance. SMIP-30 also induces selective apoptosis of infected macrophages, leading to increased efferocytosis of apoptotic cells and enhances bacterial clearance through this mechanism as well. SMIP-30 can be used in combination treatments with TB antibiotics to achieve improved bacterial clearance compared to individual drug treatments, both in vitro and in vivo in Mtb-infected mice. Furthermore, SMIP-30 was successfully optimized to generate a derivative compound SMIP-031 with greater PPM1A-inhibition activity and reduced cytotoxicity. Compared to SMIP-30, SMIP-031 is a stronger activator of autophagy and results in enhanced Mtb clearance. These findings establish PPM1A as a druggable target for TB HDT and provides a pipeline for candidate compound design and optimization. While HDT is an important strategy, antibiotic therapy is still currently the main avenue of treatment, and an approach that combines both HDT with antibiotics would have even better therapeutic potential. For that, novel antibiotic discovery remains a priority. I have also discovered 2 novel benzophenanthridine compounds, BPD-6 and BPD-9, with narrow-spectrum antimycobacterial antibiotic activity. The more therapeutically viable candidate BPD-9 has bacteriostatic activity against Mtb and is able to maintain the bacteria in a non-replicating state, suggesting it may be able to prevent the reactivation of latent TB into active disease. Importantly, BPD-9 remained active against a panel of clinical and drug-resistant Mtb strains and was effective at clearing M. bovis BCG in vivo in infected mice, demonstrating therapeutic potential as a valuable new candidate compound in the development of novel TB antibiotics. Overall, my findings contribute to both aspects of TB treatment research, demonstrating the importance of and providing drug candidates for an alternative approach like HDT and traditional antibiotic therapy. Further investigations into the mechanisms of action within the cell and molecular targets of these compounds will allow for optimizations that iteratively increase therapeutic potential in order to ultimately develop viable candidates for clinical use.