1447. Emergence of Avibactam Resistance in Multidrug-Resistant Enterobacteriaceae

BACKGROUND: Avibactam (AVI) is a non-β-lactam β-lactamase inhibitor used clinically to inhibit bacterial β-lactamase activity against the β-lactam antibiotic ceftazidime. We previously observed intrinsic in vitro antibacterial activity of AVI against multidrug-resistant Enterobacteriaceae. Here we characterize the rapid emergence of AVI resistance following AVI exposure. METHODS: We grew two carbapenem- and colistin-resistant isolates (E. coli ARLG 2829/MCR1_NJ and Klebsiella pneumoniae AR-0636) in liquid culture containing 16x the AVI minimum inhibitory concentration (MIC) for 24 hours. We then tested the AVI MIC of each strain daily for 17 days following serial passage on antibiotic-free media. We also tested MICs of AVI and 6 β-lactam antibiotics against broadly susceptible E. coli and K. pneumoniae isolates following growth with AVI. Finally, we tested in vivo activity of AVI using a mouse thigh infection model in which groups of 5 mice infected with 1x10(8) CFU/thigh of AR-0636 were treated with AVI 250 mg/kg or saline every 8 hours for 24 hours. RESULTS: Following growth in AVI 128 μg/mL, the AVI MIC of both strains increased from 8 to > 256 μg/mL and remained ≥ 256 μg/mL for 17 days of serial passage on antibiotic-free media. Following AVI treatment, MICs were also elevated for mecillinam, which, like AVI, targets penicillin-binding protein 2 (PBP2), but not for drugs with different PBP affinities. In a mouse thigh infection model, AVI treatment resulted in an average 1.4 log(10) decrease in CFU/thigh compared to placebo. AVI MICs in bacteria recovered from treated mouse thighs were unchanged from initial MIC. CONCLUSION: AVI resistance emerged rapidly in vitro and persisted for over two weeks in the absence of selective pressure. The co-emergence of mecillinam resistance suggests that AVI resistance may reflect PBP2 alterations. Development of resistance was not observed in a mouse model. These results have important implications for new non-β-lactam β-lactamase inhibitors (nacubactam, zidebactam) with structural similarities to AVI and known intrinsic antibacterial activity that have recently completed Phase I trials in combination with β-lactam drugs and are likely to play an important future role in CRE treatment. DISCLOSURES: Thea Brennan-Krohn, MD, D(ABMM), Tecan (Other Financial or Material Support, HP D300 digital ​dispenser and its consumables were provided by Tecan. Tecan had no role in study design, data collection/interpretation, or poster preparation.) Shade Rodriguez, BA, Tecan (Other Financial or Material Support, HP D300 digital ​dispenser and its consumables were provided by Tecan. Tecan had no role in study design, data collection/interpretation, or poster preparation.) James Kirby, MD, D(ABMM), AstraDx (Advisor or Review Panel member, Other Financial or Material Support, Co-founder)First Light Biosciences (Advisor or Review Panel member)Tecan (Other Financial or Material Support, HP D300 digital ​dispenser and its consumables were provided by Tecan. Tecan had no role in study design, data collection/interpretation, or poster preparation.)