Development of a small molecule probe targeting non-replicating M. tuberculosis

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DESCRIPTION (provided by applicant): Tuberculosis (TB) remains a global public health challenge, infecting almost nine million people worldwide each year and killing over a million. Two major obstacles to TB control efforts include (1) the protracted drug regimens required to cure TB and (2) the inability of human immunity to effectively clear M. tuberculosis (Mtb) infection, resulting in latent infection. It is estimated that 1/3 of the world's population has laent TB, providing an enormous reservoir from which reactivation to acute disease can occur. New drugs that can simplify or shorten the existing treatment courses for both acute and latent TB infection are badly needed to help improve patient compliance and thus limit the emergence of drug resistant TB. Currently, successful treatment of active, symptomatic TB infection requires a complex drug regimen lasting at least six months, while latent infection requires multi-drug therapy for 3 months or mono-therapy for as long as 9 months. It is believed that this slow clearance of TB by our current antibiotics is due to subpopulations of bacteria within the host tissues that have adopted a drug-tolerant, non-replicating physiological state. Drug regimens that are better at eliminating these persistent, non-replicating bacilli could shorten the required duration of therapy for both active and latent infection. Thus, the identification of small molecules that kill non-replicating Mtb would be invaluable, first to our understanding of the biology of the non-replicating physiological state, and then to the development of more effective, potentially shorter course TB therapy. To find such molecules, we adapted a nutrient deprivation model to high-throughput chemical screening in order to identify compounds that kill carbon-starved, non-replicating Mtb. From an initial oxadiazole hit compound, we have developed ML338 as a small molecule probe that selectively targets non-replicating Mtb bacilli. Because the starvation response has been shown to be necessary for TB persistence in animal models, ML338 represents a valuable tool for not only identifying essential functions and vulnerabilities of the Mtb bacilli in the non-replicating state, but also for testing the relevance of the non-replicating state in vivo during infection. We will first work to identify the molecular target and mechanism of action of ML338. Target identification will entail a series of parallel approaches, including both genetic approaches to isolate resistant mutants as well as affinity-based assays to determine binding partners. Understanding the mechanism of action of ML338 is critical, as it will reveal a vulnerable pathway or pathways in non-replicating Mtb. In addition, we will leverage ML338 as a tool to explore the relevance of the non-replicating state to TB chemotherapy using a mouse infection model.