Resistance development profiling of piperacillin in combination with the novel β-lactamase inhibitor BLI-489

Objectives To evaluate development of resistance to the piperacillin/BLI-489 combination. Methods BLI-489 was used at a constant concentration of 4 mg/L. Spontaneous mutation frequency was measured on piperacillin/BLI-489-containing agar plates. Five β-lactamase-producing strains were exposed to a serial dilution of piperacillin/BLI-489, and the highest concentration allowing growth was used to inoculate subsequent serial passage for 10 days. Mutation stability was monitored in drug-free medium for 10 days. Results Escherichia coli (OXA-3, OXA-7, ACT-1, SHV-1 or none), Salmonella enterica serovar Typhimurium (CTX-M-5), Klebsiella pneumoniae (SHV-1 and SHV-5) and Enterobacter cloacae (AmpC) had a spontaneous mutation frequency of ≤1.0 × 10−9. Two AmpC-producing Pseudomonas aeruginosa strains had a mutation frequency of 6.52 × 10−6 and 1.0 × 10−7; a β-lactamase-negative P. aeruginosa strain had a mutation frequency of 2.68 × 10−8. The mutant prevention concentration (MPC) was ≤32 mg/L. During serial passages, the MIC increased 64- and 128-fold for S. enterica serovar Typhimurium (CTX-M-5) and E. cloacae (AmpC), respectively, to ≥512 mg/L. The MIC reverted to ≤64 mg/L after serial passages in drug-free medium. The MICs increased only 4-fold for K. pneumoniae (SHV-1 and SHV-5), E. coli (OXA-3) and E. coli (SHV-1). Conclusions Piperacillin/BLI-489 demonstrated a low probability of spontaneous resistance development in vitro for all of the strains tested with the exception of P. aeruginosa. The MPC value for all strains was ≤32 mg/L. Resistance developed during serial passage for two of the five strains tested; however, this resistance phenotype was unstable as MIC values reverted to ≤64 mg/L after propagation in drug-free medium.