Optimizing inhaled delivery of antimycobacterial MmpL3 inhibitors

This study reports the discovery and preclinical characterization of MSU-43085, a new small-molecule inhibitor of MmpL3, a membrane transporter essential for Mycobacterium tuberculosis (Mtb) and nontuberculous mycobacteria (NTM). MmpL3 mediates mycolic acid transport and is a validated target for antimycobacterial drug development. MSU-43085 is an analog in a chemical series derived from a parent compound (HC2099) and is among several HC2099 analogs evaluated for improved potency and pharmacological properties. Structure-activity relationship studies described in the work enhanced potency across analogs, including MSU-43165 and MSU-43170, with dose response characterization providing comparative EC50 and EC90 metrics that positioned MSU-43085 as a prioritized compound for in vivo evaluation.

In vitro, MSU-43085 demonstrated potent inhibition of M. tuberculosis and M. abscessus survival. Importantly for translational development, MSU-43085 was orally bioavailable and showed efficacy in an acute murine model of Mtb infection when administered at studied doses (for example, 200 mg/kg was used in comparative in vivo activity experiments). In acute (2-week) infection studies in C57Bl/6 mice infected with Mtb Erdman, treatment with MSU-43085 reduced lung bacterial burden compared with vehicle controls, with statistical comparisons indicating significant effects relative to untreated groups.

However, the compound's performance diverged in chronic infection models: MSU-43085 was inactive against Mtb in chronically infected mice. Follow-up pharmacokinetic and metabolite identification studies linked this lack of efficacy in chronic infection to in vivo metabolism that results in a short half-life in treated mice. These pharmacokinetic limitations rapid metabolic clearance and short systemic exposure were identified as key barriers to sustained efficacy in longer-term infection settings. The authors frame these findings as proof-of-concept that, while MSU-43085 validates MmpL3 inhibition as a viable mechanism and shows promise in acute infection, further medicinal chemistry and formulation optimization are required to overcome metabolic instability and achieve durable in vivo exposures appropriate for treatment of chronic TB or NTM disease.

The manuscript situates MSU-43085 development within broader efforts to target MmpL3, referencing structural and mechanistic studies that support MmpL3's role as a mycolic acid flippase and the value of two-way regulation approaches to identify and validate inhibitors. The study also discloses relevant translational interests: two coauthors are owners of Tarn Biosciences, a company pursuing new antimycobacterial drugs, and several authors are named inventors on a patent application covering matter described in the study. Funded in part through U.S. NIH mechanisms (identified grants include 75N90019D00005/CL/CLC and R21 AI148909/AI/NIAID), the work provides a clear path for next steps improving metabolic stability, optimizing pharmacokinetics and advancing analogs from the MSU-43085 series toward potential therapies for tuberculosis and nontuberculous mycobacterial infections.