698. Nacubactam Inhibits Class A β-lactamases

BACKGROUND: Nacubactam, formerly RG6080 and OP0595 (Figure 1A), is a bridged diazabicyclooctane (DBO) that inactivates class A and class C β-lactamases. Unlike avibactam, the DBO that is approved for use in combination with ceftazidime, nacubactam also inhibits penicillin binding proteins (i.e., PBP2) in Enterobacteriaceae. We set out to determine the effectiveness of meropenem-nacubactam against Klebsiella pneumoniae clinical strains and to elucidate the structure–function relationships. METHODS: Minimal inhibitory concentration (MIC) measurements using broth microdilution according to Clinical and Laboratory Standards Institute for meropenem (MERO) ± nacubactam (fixed concentration of 4 mg/L or fixed 1:1 ratio) was performed on 50 clinical K. pneumoniae strains (6 having OXA-48-like β-lactamases and 44 harboring KPC-2 or KPC-3) and 47 isogenic Escherichia coli strains harboring bla genes encoding K. pneumoniae carbapenemase (KPC) variants with single amino acid substitutions in residues that are involved in catalysis. IC(50)s for selected KPC-2 variants were determined on periplasmic extracts with varying concentrations of nacubactam using nitrocefin as a reporter substrate. RESULTS: The MERO combinations with either 4 mg/L or a 1:1 ratio of nacubactam effectively lowered the MERO MICs of K. pneumoniae strains (Figure 1B). Similarly, all E. coli strains expressing bla(KPC-2) variants were susceptible to the MERO-nacubactam combinations based on the breakpoint of MERO. The strains harboring K73R, S130G, and K234R had slightly elevated MERO-nacubactam MICs relative to wild type but did not have corresponding increases in MERO MICs. Strains with pBC SK-KPC2, K73R or S130G had 0.015 mg/L MERO MICs. The pBR322-K234R strain had a twofold lower MERO MIC than pBR322-KPC-2 (Figure 1C). The IC(50) of cell extracts containing the K234R variant is 781 µM, which is 12-fold higher than that for KPC-2 (66 µM) (Figure 1C). Extracts containing the S130G variant were not inhibited by nacubactam (IC(50) > 2.6 mM). CONCLUSION: Meropenem-nacubactam is an effective β-lactam β-lactamase inhibitor combination for Enterobacteriaceae with KPC or OXA-48 β-lactamases. The single amino acid substitutions K73R, S130G, and K234R in KPC-2 affect the inactivation mechanism. [Image: see text] DISCLOSURES: M. R. Jacobs, F. Hoffmann-La Roche Ltd.: Grant Investigator, Research grant. K. M. Papp-Wallace, F. Hoffmann-La Roche Ltd.: Grant Investigator, Research grant. R. A. Bonomo, F. Hoffmann-La Roche Ltd.: Grant Investigator, Research grant.