Comparative In Vitro Activities Of Several Oxazolidinones Against Mycobacterium Kansasii

Background: Oxazolidinones were found to be active in vitro against mycobacteria particularly M. tuberculosis soon after their discovery. Linezolid (LZD) has been effective in the treatment of patients with multidrug resistant tuberculosis, however, significant toxicity issues limit its use in tuberculosis and other mycobacterial diseases. Newer oxazolidinones, which may have reduced toxicity, are at various stages of development. The purpose of the present study was to evaluate the in vitro activities of MRX-1, sutezolid, and tedizolid compared to linezolid against clinical isolates of M. kansasii (MK). Methods & Materials: Linezolid and MRX-1 were provided by MicuRx Pharmaceuticals (Hayward, CA), tedizolid was provided by Trius Pharmaceuticals (San Diego, CA) and sutezolid was provided by NIAID (Bethesda, MD) Each of the drugs were dissolved in 100% dimethyl sulfoxide (DMSO) at a concentration of 5 mg/ml prior to freezing at -20 °C. MK isolates (N = 20) were grown in Middlebrook 7H9 broth with 10% ADC. The organisms were diluted in Middlebrook 7H9 broth to about 5 × 105 CFU/ml for use in the broth dilution assay. The actual inocula used for each isolate was determined by titration and plating on 7H10 agar plates which were incubated at 370 C for 2 weeks prior to counting. Polystyrene 96-well round-bottom microtiter plates were filled with 50 μl of broth. The compounds were prepared at 4 times the maximum concentration at which they were to be tested and added to the first well prior to being serially diluted 2-fold. Fifty microliters of the inocula was added to each well. The microtiter plates were sealed and incubated at 37 °C in ambient air for 7 days prior to reading. Each isolate was tested in duplicate. The MIC was defined as the lowest concentration of antimicrobial agent yielding no visible turbidity. Results: Sutezolid was the most active agent with MIC50 and MIC90 respectively of 0.125 μg/ml and 0.25 μg/ml compared to 0.5 μg/ml and 0.5 μg/ml for tedizolid, 1 μg/ml and 1 μg/ml for MRX-1 and 1 μg/ml and 2 μg/ml for linezolid. Conclusion: These newer oxazolidinones should be studied in a murine model of pulmonary MK infection to determine if subsequent clinical evaluation is warranted.