P-1072. Evaluation of the Spontaneous Mutation Frequencies of SPR719 Alone and in Combination with Other Agents Used to Treat <i>Mycobacterium avium</i> Complex Pulmonary Disease

Abstract Background SPR720 (fobrepodacin), a novel oral benzimidazole prodrug that inhibits the ATPase activity of mycobacterial DNA gyrase B, is being developed for treatment of nontuberculous mycobacterial pulmonary disease (NTM-PD). SPR719, the active moiety of SPR720, has previously demonstrated antimycobacterial activity against nontuberculous mycobacteria (NTM), including Mycobacterium avium complex (MAC), the most common causative agent of NTM-PD. Methods In this study, the spontaneous mutation frequencies (SMFs) of MAC ATCC 700898 (macrolide-susceptible) and a clinical macrolide-resistant MAC isolate (MMX 9461) were evaluated in the presence of SPR719 alone, standard-of-care antimycobacterial agents (clarithromycin, ethambutol, and rifampin) alone, and for SPR719 in combination with these agents. The minimal inhibitory concentrations (MICs) for SPR719 and comparators against the NTM were determined by agar dilution. SMFs were determined by plating a high inoculum (108-109 colony forming units) on selective agar prepared at 2-, 4-, and 8-fold the MIC for SPR719, and at 4-fold the MIC for comparator agents. SMFs were also determined for SPR719 in combination with antimycobacterial agents where SPR719 was evaluated at 2-fold the MIC and a paired agent was evaluated at 4-fold the MIC. Putative mutants were patched from selection plates onto agar containing the selection drug, and susceptibility was then evaluated by broth microdilution alongside the parent isolate. Results All concentrations of SPR719 alone and in combination with other agents prevented growth of both isolates, resulting in low SMFs for SPR719 (&amp;lt; 2.21 × 10-9 and &amp;lt; 7.14 × 10-11). In contrast, mutants were readily selected with rifampin (SMF of 2.55 × 10-8 and 1.99 × 10-8) and clarithromycin (SMF of 1.16 × 10-8), and these mutants exhibited elevated MIC values ≥ 4-fold the MIC of the parent by broth microdilution. Colonies emerged during selection with ethambutol but were unstable and did not grow when patched onto ethambutol-containing agar. Conclusion The low propensity for the development of resistance against the novel agent SPR719 alone and in combination with other agents is important because emergence of resistance is a clinical concern in the setting of prolonged combination therapy for NTM-PD. Disclosures Nicole Cotroneo, B.S., Spero: Advisor/Consultant Ian A. Critchley, PhD, Spero: Employee Kamal Hamed, MD, MPH, Spero: Employee