Abstract Rationale Mycobacterium avium complex (MAC) is a difficult-to-eradicate nontuberculous mycobacteria (NTM). Many drugs are discounted due to presumed lack of efficacy based on minimum inhibitory concentration (MIC), but it is unknown if certain combinations of these drugs have synergy to create individualized treatment regimens. Methods We randomly selected MAC clinical isolates from five patients with refractory MAC lung disease, first diagnosis going back more than a decade. We tested the most recent isolate from each patient to (i) determine doubling time and growth rates of individual isolates, (ii) test MIC of 19 drugs, including aminoglycosides, rifamycins, tetracyclines, oxazolidinones, fluoroquinolones, epetraborol, bedaquiline, clofazimine, and beta-lactams, and (iii) test nine different drug combinations and to calculate the kill below stasis (B0 in log10 CFU/mL after seven days of drug and bbacteria co-incubation). Each combination was tested in at least two clinical isolates. All experiments were performed in triplicate. Drug degradation was factored in for antibiotics known to degrade in solution at incubation temperature by supplementation to keep the concentration constant throughout the experiments. Results The doubling time differed between the five MAC clinical isolates, ranging from 26.87h to 96.44h, and so was the growth rate (log10 CFU/mL/day) from 0.07 to 0.25. Two-to eight-fold difference in the MIC was observed for several drugs between cation-adjusted Mueller-Hinton broth and Middlebrook 7H9 broth. All isolates were macrolide, amikacin, and ethambutol-resistant. Rifabutin had the lowest MIC among the three rifamycins. The lowest MIC of 0.06 mg/L was for bedaquiline, however, it differed between two culture media for some isolates. The kill below stasis (B0) with standard therapy ranged between 1.09 to 1.90 log10 CFU/mL in two clinical strains, showing individual drugs’ MICs as follows: clarithromycin 128 mg/L, ethambutol 0.015 and 128 mg/L, and rifabutin 4 and 1 mg/L. In comparison, B0 with minocycline (MICs range 32 to 128 mg/L)-based different azithromycin, ethambutol, and epetraborole combinations ranged between no effect in one strain to the highest kill of 3.676 + 0.728 log10 CFU/mL. The double beta-lactam combination of ceftriaxone plus ceftazidime-avibactam killed 0.928 + 0.118 log10 CFU/mL MAC. However, when ceftriaxone was combined with omadacycline (MIC 128 mg/L) and tedizolid (128 mg/L), the B0 was impressively 2.626 + 0.126 log10 CFU/mL. Conclusion While individual drug MIC is informative, we should be careful in discounting drugs presuming a lack of efficacy against MAC. Drugs behave differently in combination and, despite high MICs, may still have potential when tested for individualized regimens, as shown here. This abstract is funded by: NIAID, CFF, UT Tyler NTM Research & Education
Srivastava et al. (Fri,) studied this question.