Pharmacodynamic Correlates of Linezolid Activity and Toxicity in Murine Models of Tuberculosis.

BACKGROUND: Linezolid (LZD) is bactericidal against Mycobacterium tuberculosis, but it has treatment-limiting toxicities. A better understanding of exposure-response relationships governing LZD efficacy and toxicity will inform dosing strategies. Because in vitro monotherapy studies yielded conflicting results, we explored LZD pharmacokinetic/pharmacodynamic (PK/PD) relationships in vivo against actively and nonactively multiplying bacteria, including in combination with pretomanid. METHODS: Linezolid multidose pharmacokinetics were modeled in mice. Dose-fractionation studies were performed in acute (net bacterial growth) and chronic (no net growth) infection models. In acute models, LZD was administered alone or with bacteriostatic or bactericidal pretomanid doses. Correlations between PK/PD parameters and lung colony-forming units (CFUs) and complete blood counts were assessed. RESULTS: Overall, time above minimum inhibitory concentration (T>MIC) correlated best with CFU decline. However, in growth-constrained models (ie, chronic infection, coadministration with pretomanid 50 mg/kg per day), area under the concentration-time curve over MIC (AUC/MIC) had similar explanatory power. Red blood cell counts correlated strongly with LZD minimum concentration (Cmin). CONCLUSIONS: Although T>MIC was the most consistent correlate of efficacy, AUC/MIC was equally predictive when bacterial multiplication was constrained by host immunity or pretomanid. In effective combination regimens, administering the same total LZD dose less frequently may be equally effective and cause less Cmin-dependent toxicity.

Preserved Efficacy and Reduced Toxicity with Intermittent Linezolid Dosing in Combination with Bedaquiline and Pretomanid in a Murine Tuberculosis Model.

The novel regimen of bedaquiline, pretomanid, and linezolid (BPaL) is highly effective against drug-resistant tuberculosis, but linezolid toxicities are frequent. We hypothesized that, for a similar total weekly cumulative dose, thrice-weekly administration of linezolid would preserve efficacy while reducing toxicity compared with daily dosing, in the context of the BPaL regimen. Using C3HeB/FeJ and BALB/c mouse models of tuberculosis disease, thrice-weekly linezolid dosing was compared with daily dosing, with intermittent dosing introduced (i) from treatment initiation or (ii) after an initial period of daily dosing. In all animals, BPa was dosed daily throughout treatment. Blood counts were used to assess hematologic toxicity. After unexpected findings of apparent antagonism, we conducted additional experiments to investigate strain-to-strain differences in the contribution of linezolid to regimen efficacy by comparing each 1- and 2-drug component to the BPaL regimen in BALB/c mice infected with Mycobacterium tuberculosis H37Rv or HN878. Giving linezolid daily for 1 to 2 months achieved the greatest efficacy but, after that, results were similar if the drug was stopped, dosed thrice-weekly, or continued daily. Erythrocyte counts were lower with daily than thrice-weekly dosing. Linezolid had additive effects with BPa against M. tuberculosis H37Rv but antagonistic effects with BPa against M. tuberculosis HN878. However, the overall efficacy of BPaL was high and similar against both strains. Dosing linezolid daily for the first 2 months and then less frequently thereafter may optimize its therapeutic margin. Linezolid's contribution to BPaL regimens may depend on the M. tuberculosis strain.

Assessment of carbapenems in a mouse model of Mycobacterium tuberculosis infection.

We present further study of a subset of carbapenems, arising from a previously reported machine learning approach, with regard to their mouse pharmacokinetic profiling and subsequent study in a mouse model of sub-acute Mycobacterium tuberculosis infection. Pharmacokinetic metrics for such small molecules were compared to those for meropenem and biapenem, resulting in the selection of two carbapenems to be assessed for their ability to reduce M. tuberculosis bacterial loads in the lungs of infected mice. The original syntheses of these two carbapenems were optimized to provide multigram quantities of each compound. One of the two experimental carbapenems, JSF-2204, exhibited efficacy equivalent to that of meropenem, while both were inferior to rifampin. The lessons learned in this study point toward the need to further enhance the pharmacokinetic profiles of experimental carbapenems to positively impact in vivo efficacy performance.

Development of a penem antibiotic against Mycobacteroides abscessus.

ß-lactams are the most widely used antibiotic class to treat bacterial infections in humans. Mycobacteroides abscessus is an emerging pulmonary pathogen resistant to most antibiotics, including penicillins and cephalosporins. With no current FDA-approved treatment and cure rates <50%, there is a pressing need for effective therapies. Here we report T405, a new ß-lactam of the penem subclass that exhibits potent activity against M. abscessus and a panel of drug-resistant strains isolated from cystic fibrosis patients. Additionally, in combination with the ß-lactamase inhibitor avibactam, the rate of spontaneous resistance of M. abscessus to T405 approached the limit of detection. Lastly, we show the favorable pharmacokinetic profile of T405 in mice and the absence of toxicity at elevated dosage, which support the clinical potential of this compound.

The root causes of pharmacodynamic assay failure.

Robust pharmacodynamic assay results are valuable for informing go/no-go decisions about continued development of new anti-cancer agents and for identifying combinations of targeted agents, but often pharmacodynamic results are too incomplete or variable to fulfill this role. Our experience suggests that variable reagent and specimen quality are two major contributors to this problem. Minimizing all potential sources of variability in procedures for specimen collection, processing, and assay measurements is essential for meaningful comparison of pharmacodynamic biomarkers across sample time points. This is especially true in the evaluation of pre- and post-dose tumor biopsies, which suffer from high levels of tumor insufficiency due to variations in biopsy collection techniques and significant specimen heterogeneity within and across patients. Developing methods to assess heterogeneous biopsies is necessary in order to evaluate a majority of tumor biopsies collected for pharmacodynamic biomarker studies. Improved collection devices and standardization of methods are being sought in order to improve the tumor content and quality of tumor biopsies. In terms of reagent variability, we have found that stringent initial reagent qualification and quality control of R&D-grade reagents is critical to minimize lot-to-lot variability and prevent assay failures, especially for clinical pharmacodynamic questions, which often demand assay performance that meets or exceeds clinical diagnostic assay standards. Rigorous reagent specifications and use of appropriate assay quality control methodologies help to ensure consistency between assay runs, laboratories and trials to provide much needed pharmacodynamic insights into the activity of investigational agents.