Combination therapy is a successful approach to treat tuberculosis in patients with susceptible strains of Mycobacterium tuberculosis. However, the emergence of resistant strains requires identification of new, effective therapies. Pretomanid (PA824) and moxifloxacin (MXF) are promising options currently under evaluation in clinical trials for the treatment of susceptible and resistant mycobacteria. We applied our recently described screening strategy to characterize the interaction between PA824 and MXF toward the killing of M. tuberculosis in logarithmic growth phase (log phase), acid phase, and nonreplicating-persister (NRP) phase. Respective in vitro data generated for the H37Rv and 18b strains were evaluated in a microdilution plate system containing both drugs in combination. The Universal Response Surface Approach model from Greco et al. (W. R. Greco, G. Bravo, and J. C. Parsons, Pharmacol Rev 47:331–385, 1995) was used to characterize the nature of the interaction between both drugs; synergistic or additive combinations would prompt additional evaluation in the hollow-fiber infection model (HFIM) and in animal studies. The interaction between MXF and PA824 was additive against M. tuberculosis organisms in acid phase (interaction parameter [α] = 5.56e−8 [95% confidence interval {CI} = −0.278 to 0.278] and α = 0.408 [95% CI = 0.105 to 0.711], respectively), NRP phase (α = 0.625 [95% CI = −0.556 to 1.81] and α = 2.92 [95% CI = 0.215 to 5.63], respectively), and log phase (α = 1.57e−6 [95% CI = −0.930 to 0.930] and α = 1.83e−6 [95% CI = −0.929 and 0.929], respectively), prompting further testing of this promising combination for the treatment of tuberculosis in the HFIM and in animal studies.

The therapy for treatment of infections is long and arduous. It has been hypothesized that the therapy duration is driven primarily by populations of organisms in different metabolic states that replicate slowly or not at all (acid-phase and nonreplicative-persister [NRP]-phase organisms). Linezolid is an oxazolidinone antimicrobial with substantial activity against Log-phase Here, we examined organisms in acid-phase growth and nonreplicative-persister-phenotype growth and determined the effect of differing clinically relevant exposures to linezolid in a hollow-fiber infection model (HFIM). The endpoints measured were bacterial kill over 29 days and whether organisms that were less susceptible to linezolid could be recovered during that period. In addition, we evaluated the effect of administration schedule on linezolid activity, contrasting daily administration with administration of twice the daily dose every other day. Linezolid demonstrated robust activity when administered daily against both acid-phase and NRP-phase organisms. We demonstrated a clear dose response, with 900 mg of linezolid daily generating ≥3 Log(CFU/ml) killing of acid-phase and NRP-phase over 29 days. Amplification of a population less susceptible to linezolid was not seen. Activity was reduced with every 48-h dosing, indicating that the minimum concentration ()/MIC ratio drove the microbiological effect. We conclude that once-daily linezolid dosing has substantial activity against in acid-phase and NRP-phase metabolic states. Other studies have shown activity against Log-phase Linezolid is a valuable addition to the therapeutic armamentarium for and has the potential for substantially shortening therapy duration.

A major goal for improving tuberculosis therapy is to identify drug regimens with improved efficacy and shorter treatment durations. Shorter therapies improve patient adherence to the antibiotic regimens, which, in turn, decreases resistance emergence. Mycobacterium tuberculosis exists in multiple metabolic states. At the initiation of therapy, the bulk of the population is in log-phase growth. Consequently, it is logical to focus initial therapy on those organisms. Moxifloxacin has good early bactericidal activity against log-phase bacteria and is a logical component of initial therapy. It would be optimal if this agent also possessed activity against acid-phase and nonreplicative-persister (NRP) phenotype organisms. In our hollow-fiber infection model, we studied multiple exposures to moxifloxacin (equivalent to 200 mg to 800 mg daily) against strain H37Rv in the acid phase and against strain 18b in streptomycin starvation, which is a model for NRP-phase organisms. Moxifloxacin possesses good activity against acid-phase organisms, generating cell killing of 3.75 log10(CFU/ml) (200 mg daily) to 5.16 log10(CFU/ml) (800 mg daily) over the 28 days of the experiment. Moxifloxacin also has activity against streptomycin-starved strain 18b. The 400- to 800-mg daily regimens achieved extinction at day 28, while the no-treatment control still had 1.96 log10(CFU/ml) culturable. The lowest dose (200 mg daily) still had 0.7 log10(CFU/ml) measurable at day 28, a net kill of 1.26 log10(CFU/ml). Moxifloxacin is an attractive agent for early therapy, because it possesses activity against three metabolic states of M. tuberculosis.

Tuberculosis is the ninth-leading cause of death worldwide. Treatment success is approximately 80% for susceptible strains and decreases to 30% for extensively resistant strains. Shortening the therapy duration for is a major goal, which can be attained with the use of combination therapy. However, the identification of the most promising combination is a challenge given the quantity of older and newer agents available. Our objective was to identify promising 2-drug combinations using an strategy to ultimately be tested in an hollow fiber infection model (HFIM) and in animal models. We studied the effect of the combination of linezolid (LZD) and bedaquiline (BDQ) on strain H37Rv in log- and acid-phase growth and strain 18b in log- and nonreplicating-persister-phase growth in a plate system containing a 9-by-8 matrix of concentrations of both drugs alone and in combinations. A characterization of the interaction as antagonistic, additive, or synergistic was performed using the Greco universal response surface approach (URSA) model. Our results indicate that the interaction between LZD and BDQ is additive for bacterial killing in both strains for both of the metabolic states tested. This prescreen strategy was suitable to identify LZD and BDQ as a promising combination to be further tested in the HFIM. The presence of nonoverlapping mechanisms of drug action suggests each drug in the combination will likely be effective in suppressing the emergence of resistance by to the companion drug, which holds promise in improving treatment outcomes for tuberculosis.