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Background SPR719 (the active metabolite of phosphate prodrug SPR720) belongs to a novel class which targets the ATPase subunits of gyrase by a mechanism distinct from fluoroquinolones. SPR719 has potent antibacterial activity against nontuberculous mycobacteria strains (NTM), including Mycobacterium avium, and is under development for treatment of NTM pulmonary disease. Oral efficacy of SPR720 was evaluated alone and in combination treatment in the C3HeBFeJ chronic mouse infection model which produces necrotic granulomas, similar to humans. Methods Mice were infected with a pulmonary aerosol of 1x108.5 CFU of M. avium ATCC 700898, (SPR719 MIC = 2 mg/mL). Treatment started on day 28 for 8 weeks with: saline, clarithromycin 250 mg/kg (CLR) QD, SPR720 at 10, 30 and 100 mg/kg QD, or SPR720 at 50 mg/kg BID. SPR720 at 30 mg/kg QD was also combined with CLR +/- ethambutol at 100 mg/kg (EMB), or CLR + rifabutin at 100 mg/kg (RFB) +/- EMB. Mice were evaluated for bacterial burden (CFU) on days 1, 27 and 60 after infection by plating serial dilutions of organ homogenates on nutrient 7H11 and charcoal agar plates. Lung pathology was evaluated by assessing prevalence and size of pulmonary lesions. Results CLR treatment for 28 days showed a significant reduction in the bacterial burden in the lung, spleen, and liver compared to the untreated control. SPR720 demonstrated a dose dependent reduction in bacterial burden, including at 100 mg/kg which showed a statistically significant reduction in the bacterial burden in the lung, spleen, and liver. CLR + EMB + SPR720 at 30 mg/kg reduction in the bacterial burden in the lung, spleen, and liver. RFB when added to the treatment regimen did not demonstrate enhanced efficacy compared the additive effect of EMB + CLR +/- SPR720. Lung pathology showed that lesions were less numerous and smaller in infected mice treated with all regimens. Conclusion Oral administration of SPR720 demonstrated a statistically significant reduction in the bacterial burden in all tissues with concomitant improvement in lung pathology, both alone and in combination with standard of care agents. These results support the continued development of SPR720 for treatment of NTM pulmonary infections. Disclosures Nicole S. Cotroneo, BS, Spero Therapeutics (Employee, Shareholder) Suzanne Stokes, PhD, Spero Therapeutics (Employee, Shareholder)

open forum infectious diseases

1637. SPR720, A Novel Benzamidazole Gyrase Inhibitor, Demonstrates Potent Efficacy Against Mycobacterium avium ATCC 700898 in a Chronic C3HeBFeJ Mouse Infection Model