2131. Activity of the Novel Antibiotic Zosurabalpin (RG6006) against Clinical Acinetobacter Isolates from China

BACKGROUND: Zosurabalpin (RG6006) is the first representative of a novel class of tethered macrocyclic peptide antibiotics active against Acinetobacter spp., including carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex (ABC) organisms. In this study, the susceptibility testing of zosurabalpin was carried out against a panel of 150 randomly selected Acinetobacter spp. isolates (100 A. baumannii & 50 non-A. baumannii) representing 11 sites in China and a broad susceptibility profile (65% of which were multi-drug resistant [MDR]) collected in 2021. METHODS: MICs were performed using the Clinical Laboratory Standards Institute (CLSI) broth dilution method cation-adjusted Mueller Hinton broth (CA-MHB) supplemented with either 20% of human serum (HS) or 20% of horse serum (HoS). For a fraction of isolates (10-25%), MIC determination for zosurabalpin is affected by aberrant readings (trailing, multiple skipped wells) in CA-MHB. This effect complicates routine susceptibility testing. Supplementation of CAMHB with serum allows accurate MIC determinations without any major effects on the MIC distribution. RESULTS: Summary data from the study are shown in Table 1. Zosurabalpin was active against all Acinetobacter spp., with an MIC(50/90) of 0.12/0.5 μg/mL and 0.25/1 μg/mL in CA-MHB supplemented with HoS and HS, respectively (MIC range of 0.015/0.03 to 8 μg/mL). Against ABC isolates (n=133), the MIC(50/90) for zosurabalpin was 0.12/0.25 μg/mL and 0.25/0.5 μg/mL, in HoS and HS, respectively (MIC range of 0.015/0.03 to 1 μg/mL). A similar activity was observed against carbapenem-resistant ABC isolates. [Figure: see text] CONCLUSION: In addition to the potent activity observed against isolates from USA and Europe, zosurabalpin exhibited potent in vitro antibacterial activity against Acinetobacter clinical isolates circulating in China. These data support the continued clinical development of zosurabalpin for infections caused by ABC isolates, including difficult to treat carbapenem-resistant isolates. DISCLOSURES: Stephen Hawser, PhD, Allecra: study funding|Innoviva Specialty Therapeutics, Inc.: Honoraria|Roche: Honoraria|Roche: This project has been funded by BARDA (HHSO100201600038C). Nimmi Kothari, PhD, Allecra: Allecra (study funding)|Innoviva Specialty Therapeutics, Inc.: Honoraria|Roche: Honoraria|Roche: This project has been funded by BARDA (HHSO100201600038C). Thomas Valmont, BS, Roche: Honoraria|Roche: This project has been funded by BARDA (HHSO100201600038C). Séverine Louvel, PhD, F. Hoffmann-La Roche Ltd: employee of the company Claudia Zampaloni, n/a, F. Hoffmann-La Roche Ltd.: Full time employee of Roche