In vitro activity of avibactam (NXL104) in combination with β-lactams against Gram-negative bacteria, including OXA-48 β-lactamase-producing Klebsiella pneumoniae

The objective of this study was to investigate the in vitro antibacterial activity of avibactam (formerly NXL104) in combination with imipenem, cefepime or ceftazidime against Gram-negative bacteria. Bacterial isolates included: Pseudomonas aeruginosa harbouring PER-1 β-lactamase (n = 14); Acinetobacter baumannii harbouring PER-1, OXA-51 and OXA-58 (n = 20); carbapenem-non-susceptible Klebsiella pneumoniae (n = 25) and Escherichia coli (n = 1) harbouring OXA-48; carbapenem-non-susceptible E. coli (n = 1) harbouring both IMP-1 metallo-β-lactamase and extended-spectrum β-lactamase (ESBL); carbapenem-non-susceptible Serratia marcescens (n = 1); and carbapenem-susceptible E. coli (n = 20) and K. pneumoniae isolates (n = 12) with CTX-M-15 ESBL. Minimum inhibitory concentrations (MICs) of imipenem, cefepime and ceftazidime were determined in combination with 4 mg/L avibactam by the Clinical and Laboratory Standards Institute (CLSI) method on Mueller–Hinton agar. Imipenem/avibactam and ceftazidime/avibactam displayed limited potency against A. baumannii isolates, whereas cefepime/avibactam and ceftazidime/avibactam were active against P. aeruginosa. Klebsiella pneumoniae isolates with OXA-48 β-lactamase were resistant to imipenem [MIC for 90% of the organisms (MIC90) ≥4 mg/L]. MIC90 values for the combination of avibactam 4 mg/L with imipenem, cefepime and ceftazidime were in the susceptible range for all strains (MIC90 ≤ 0.5 mg/L). All E. coli and K. pneumoniae isolates with CTX-M-15 β-lactamase were inhibited at ≤1 mg/L for combinations with avibactam and 100% were susceptible by CLSI breakpoint criteria to imipenem, cefepime and ceftazidime. In conclusion, combinations of imipenem, cefepime and ceftazidime with avibactam may present a promising therapeutic strategy to treat infections due to K. pneumoniae with OXA-48 enzyme as well as K. pneumoniae and E. coli with CTX-M-15 enzyme.