The kinetics of bedaquiline diffusion in tuberculous cavities open a window for emergence of resistance.

Background: Cavitary tuberculosis is difficult to cure and constitutes a site of relapse. Bedaquiline has been a wonder drug in the treatment of multidrug resistanttuberculosis, but emergence of resistance threatens the sustainability of its success. We designed site-of-disease pharmacokinetic studies in preclinical species to spatially resolve the penetration of bedaquiline, and two next generation diarylquinolines TBAJ587 and TBAJ876, in the necrotic center (caseum) of cavities.

Methods: Rabbits with cavitary tuberculosis received the study drugs at human equivalent doses. A laser-capture microdissections scheme was developed to measure drug concentrations as a function of distance from blood supply in caseum. To simulate drug coverage in patient cavities, the data were modeled, and pharmacokinetic parameter estimates were linked to clinical plasma pharmacokinetic models for bedaquiline and the new diarylquinolines.

Results: Pharmacokinetic-pharmacodynamic simulations in caseum revealed that bedaquiline reaches steady state and efficacious concentrations in deep caseum after several weeks to months and lingers at subtherapeutic concentrations up to 3 years after therapy ends. TBAJ876 and TBAJ587, currently in clinical development, achieve bactericidal concentrations in caseum layers more rapidly (within 2 hours and 1.8 days respectively) and shorten the window of suboptimal concentrations post treatmentcompared to bedaquiline. Simulations of clinically plausible dosing schemes can inform strategies to mitigate resistance development and guide the design of clinical trials for cavitary TB.

Conclusions: The slow kinetics of diffusion of bedaquiline into and out of caseum creates spatio-temporal windows of subtherapeutic concentrations. Site-of-disease simulations of TBAJ587 and TBAJ876 predict reduced opportunities for resistance development.