dNew regimens based on two or more novel agents are sought to shorten or simplify treatment of tuberculosis (TB). Pretomanid (PMD) is a nitroimidazole in phase 3 trials that has significant bactericidal activity alone and in combination with bedaquiline (BDQ) and/or pyrazinamide (PZA). We previously showed that the novel combination of BDQ؉PMD plus the oxazolidinone sutezolid (SZD) had sterilizing activity superior to that of the first-line regimen in a murine model of TB. The present experiments compared the activity of different oxazolidinones in combination with BDQ؉PMD with or without PZA in the same model. The 3-drug regimen of BDQ؉PMD plus linezolid (LZD) had sterilizing activity approaching that of BDQ؉PMD؉SZD and superior to that of the first-line regimen. The addition of PZA further enhanced activity. Reducing the duration of LZD to 1 month did not significantly affect the activity of the regimen. Halving the LZD dose or replacing LZD with RWJ-416457 modestly reduced activity over the first month but not after 2 months. AZD5847 and tedizolid also increased the bactericidal activity of BDQ؉PMD, but they were less effective than the other oxazolidinones. These results provide optimism for safe, short-course oral regimens for drug-resistant TB that may also be superior to the current first-line regimen for drug-susceptible TB.A pproximately half a million new cases of multidrug-resistant (MDR) tuberculosis (TB) occur annually (1). Current recommendations call for up to 2 years of treatment with second-line drugs that are poorly tolerated, toxic, more difficult to administer, and less effective than the 6-month, so-called short-course regimen for drug-susceptible TB. Regimens containing at least 6 drugs, including newer fluoroquinolones in high doses, an injectable agent, clofazimine, pyrazinamide (PZA), and high-dose isoniazid (INH), have shown potential as effective 9-month regimens in MDR-TB cases with minimal bacillary resistance to second-line drugs (2-4). However, these regimens remain quite cumbersome to administer and are not expected to be as effective in the setting of resistance to fluoroquinolones and/or injectable agents (3). Novel regimens based on 3 or more oral agents with little or no preexisting resistance would provide simpler, more universally active regimens. If such novel regimens are more effective than the current first-line regimen for drug-susceptible TB, they may shorten and simplify treatment for pulmonary TB irrespective of resistance to existing drugs.Agents from 2 novel classes recently received conditional regulatory approval for use in MDR-TB, the diarylquinoline bedaquiline (BDQ) and the nitroimidazole-derivative delamanid. Aside from some mutations known to confer cross-resistance between BDQ and clofazimine (5, 6), these agents are not known to exhibit cross-resistance with other TB drugs. We recently reported that the 3-drug regimen of BDQ plus pretomanid (PMD; formerly known as PA-824), the second nitroimidazole to enter phase 3 clinical trials, and the oxazolidinone su...

dNew drugs to treat drug-resistant tuberculosis are urgently needed. Extensively drug-resistant and probably the totally drugresistant tuberculosis strains are resistant to fluoroquinolones like moxifloxacin, which target gyrase A, and most people infected with these strains die within a year. In this study, we found that a novel aminobenzimidazole, VXc-486, which targets gyrase B, potently inhibits multiple drug-sensitive isolates and drug-resistant isolates of Mycobacterium tuberculosis in vitro (MICs of 0.03 to 0.30 g/ml and 0.08 to 5.48 g/ml, respectively) and reduces mycobacterial burdens in lungs of infected mice in vivo. VXc-486 is active against drug-resistant isolates, has bactericidal activity, and kills intracellular and dormant M. tuberculosis bacteria in a low-oxygen environment. Furthermore, we found that VXc-486 inhibits the growth of multiple strains of Mycobacterium abscessus, Mycobacterium avium complex, and Mycobacterium kansasii (MICs of 0.1 to 2.0 g/ml), as well as that of several strains of Nocardia spp. (MICs of 0.1 to 1.0 g/ml). We made a direct comparison of the parent compound VXc-486 and a phosphate prodrug of VXc-486 and showed that the prodrug of VXc-486 had more potent killing of M. tuberculosis than did VXc-486 in vivo. In combination with other antimycobacterial drugs, the prodrug of VXc-486 sterilized M. tuberculosis infection when combined with rifapentine-pyrazinamide and bedaquiline-pyrazinamide in a relapse infection study in mice. Furthermore, the prodrug of VXc-486 appeared to perform at least as well as the gyrase A inhibitor moxifloxacin. These findings warrant further development of the prodrug of VXc-486 for the treatment of tuberculosis and nontuberculosis mycobacterial infections.

The antileprosy drug clofazimine was recently repurposed as part of a newly endorsed short-course regimen for multidrug-resistant tuberculosis. It also enables significant treatment shortening when added to the first-line regimen for drug-susceptible tuberculosis in a mouse model. However, clofazimine causes dose- and duration-dependent skin discoloration in patients, and the optimal clofazimine dosing strategy in the context of the first-line regimen is unknown. We utilized a well-established mouse model to systematically address the impacts of duration, dose, and companion drugs on the treatment-shortening activity of clofazimine in the first-line regimen. In all studies, the primary outcome was relapse-free cure (culture-negative lungs) 6 months after stopping treatment, and the secondary outcome was bactericidal activity, i.e., the decline in the lung bacterial burden during treatment. Our findings indicate that clofazimine activity is most potent when coadministered with first-line drugs continuously throughout treatment and that equivalent treatment-shortening results are obtained with half the dose commonly used in mice. However, our studies also suggest that clofazimine at low exposures may have negative impacts on treatment outcomes, an effect that was evident only after the first 3 months of treatment. These data provide a sound evidence base to inform clofazimine dosing strategies to optimize the antituberculosis effect while minimizing skin discoloration. The results also underscore the importance of conducting long-term studies to allow the full evaluation of drugs administered in combination over long durations.

c Pyrazinamide (PZA) is a key sterilizing drug in first-line tuberculosis (TB) regimens and exerts its activity entirely during the first 2 months in human infections. We recently described the reduced activity of PZA in C3HeB/FeJ mice with large caseous tubercles due to neutral pH. Here, we aimed to determine the contribution of PZA to the sterilizing activity of the first-line TB regimen in C3HeB/FeJ and BALB/c mice. Three regimens were compared (in combinations: R, rifampin; H, isoniazid; E, ethambutol; Z, pyrazinamide; with numbers indicating the treatment duration, in months): 2RHEZ/4RH, 2RHE/4RH, and 2RHEZ/4RHZ. Lung CFU counts were assessed after 0 and 2 months of treatment, and relapse rates were assessed 3 months after 3, 4.5, and 6 months of treatment. The relapse rates after 3 months of treatment were 53% and 95% in C3HeB/FeJ mice receiving 2RHEZ/1RH and 2RHE/1RH, respectively, and 67%, 100%, and 80% in BALB/c receiving 2RHEZ/1RH, 2RHE/1RH, and 2RHEZ/1RHZ, respectively. The relapse rates after 4.5 months of treatment were 32%, 20%, and 0% in C3HeB/FeJ mice receiving 2RHEZ/2.5RH, 2RHE/2.5RH, and 2RHEZ/2.5RHZ, respectively, and 0% and 67% in BALB/c receiving 2RHEZ/2.5RH and 2RHE/2.5RH, respectively. The month-6 relapse rates were 0%, 13%, and 0% in C3HeB/FeJ mice given 2RHEZ/4RH, 2RHE/4RH, and 2RHEZ/4RHZ, respectively, and 7% in BALB/c mice receiving 2RHE/4RH. The addition of PZA shortens the duration of treatment needed to prevent relapse in both mouse strains. However, while its contribution is limited to the first 2 months of treatment in BALB/c mice, continuing PZA beyond the first 2 months is beneficial in C3HeB/FeJ mice by preventing relapse among those with the highest disease burden. P yrazinamide (PZA, Z in combinations) has a unique and important role in the first-line regimen used to treat tuberculosis (TB). Whether administered alone or in combination with other first-line drugs, PZA has little or no effect on viable bacterial counts in sputum over the first 2 weeks of treatment. However, when it is absent from the regimen, the subjects are significantly less likely to have their sputum cultures sterilized by 2 months of treatment (1), and the recommended treatment duration is routinely extended from 6 to 9 months in order to achieve comparable rates of cure without relapse (2). Interestingly, PZA contributes this treatment-shortening, or sterilizing, effect within the first 2 months of treatment (3). Indeed, when PZA was administered in combination with rifampin (RIF, R in combinations), isoniazid (INH, H in combinations), and streptomycin (STR, S in combinations), no difference in relapse was observed between the arms treated with PZA for 2 months (2RHSZ/RH) and the arms treated with PZA for up to 6 months (2RHSZ/RHZ) (3). The limited activity of PZA in the continuation phase of treatment has also been demonstrated in murine models of TB (4, 5). This unique time-limited contribution of PZA in the first-line regimen is attributed to its pH-dependent activity, which is diminished with time w...

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A regimen comprised of bedaquiline (BDQ, or B), pretomanid, and linezolid (BPaL) is the first oral 6-month regimen approved by the U.S. Food and Drug Administration and recommended by the World Health Organization for the treatment of extensively drug-resistant tuberculosis.

Siderophores produced in soil by plant growth-promoting rhizobacteria (PGPRs) play several roles, including nutrient mobilizers and can be useful as plants defense elicitors. We investigated the role of a synthetic mixed ligand bis-catechol-mono-hydroxamate siderophore (SID) that mimics the chemical structure of a natural siderophore, fimsbactin, produced by Acinetobacter spp. in the resistance against the phytopathogen Pseudomonas syringaepv tomato DC3000 (Pst DC3000), in Arabidopsis thaliana. We first tested the antibacterial activity of SID against Pst DC3000 in vitro. After confirming that SID had antibacterial activity against Pst DC3000, we tested whether the observed in vitro activity could translate into resistance of Arabidopsis to Pst DC3000, using bacterial loads as endpoints in a plant infection model. Furthermore, using quantitative polymerase chain reaction, we explored the molecular actors involved in the resistance of Arabidopsis induced by SID. Finally, to assure that SID would not interfere with PGPRs, we tested in vitro the influence of SID on the growth of a reference PGPR, Bacillus subtilis. We report here that SID is an antibacterial agent as well as an inducer of systemic priming of resistance in A. thaliana against Pst DC3000, and that SID can, at the same time, promote growth of a PGPR.

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