Aztreonam-Avibactam Combination Restores Susceptibility of Aztreonam in Dual-Carbapenemase-Producing Enterobacteriaceae

LETTER

Newly approved Food and Drug Administration (FDA) β-lactam–β-lactamase inhibitor (βL-βLI) combinations, such as ceftazidime-avibactam and meropenem-vaborbactam, are potent inhibitors of class A carbapenemases (typified by Klebsiella pneumoniae carbapenemase-like [KPC-like] genotypes) (1, 2) but are ineffective against carbapenemase-producing Enterobacteriaceae (CPE)-producing class B metallo-β-lactamases (MβLs), such as New Delhi metallo-β-lactamases (NDMs) (1–4). Hence, these βL-βLI combinations are expected to have less utility in geographical regions where NDMs predominate, namely, the Asian continent, which has an estimated abundance of 60% of global NDM producers (5, 6).

Locally in Singapore, NDM, KPC, and OXA-48-like are the predominant circulating carbapenemase genotypes (5). The carriage of dual carbapenemases, typically NDM cooccurring with KPC or OXA-48-like carbapenemases, is not uncommon (7). MβLs such as those of the IMP and VIM genotypes are infrequently encountered in our epidemiologic context (5). The monobactam aztreonam is refractory to MβL hydrolysis (2, 8, 9); hence, combining aztreonam with a βLI can restore the activity of aztreonam due to inhibition of the cocarried non-MβL. The aztreonam-avibactam combination has been demonstrated to be inhibitory to MβLs (NDM, IMP, or VIM MβLs) cocarrying a KPC or OXA-48-like carbapenemase (10–13). The largest test set of isolates in these studies contained 47 isolates (12). Here, we present in vitro susceptibility testing data on, to our knowledge, one of the largest collections of NDMs in combination with KPC or OXA-48-like carbapenemases.

A total of 70 phenotypically carbapenem-resistant, dual-carbapenemase Enterobacteriaceae isolates (13 Escherichia coli, 44 Klebsiella pneumoniae, 7 Citrobacter freundii, and 6 Enterobacter cloacae complex isolates) were obtained from the reference National Public Health Laboratory, Singapore, and their carbapenemase genotypes were confirmed by PCR and sequencing (14) (see Table S1 in the supplemental material). MICs were determined using the reference broth microdilution method (15). All antibiotic powders were purchased from MedChemExpress (Princeton, NJ, USA). Aztreonam and meropenem were tested alone and in combination with avibactam. The final concentration of aztreonam and meropenem tested ranged from 0.06 mg/liter to 64 mg/liter. For combination testing, avibactam concentrations were fixed at 4 mg/liter. Quality control isolates E. coli ATCC 25922 and K. pneumoniae ATCC 700603 were used for susceptibility testing, and the results were considered valid when they fell within the expected ranges for aztreonam and meropenem (ATCC 25922) and aztreonam-avibactam (ATCC 700603). The testing was performed in duplicate. Susceptibility was interpreted based on Clinical and Laboratory Standards Institute (CLSI) breakpoints for aztreonam and meropenem (15). For aztreonam, susceptibility was indicated by a MIC of ≤4 mg/liter, intermediate susceptibility by 8 mg/liter, and resistance by ≥16 mg/liter. For meropenem, susceptibility was indicated by a MIC of ≤1 mg/liter, intermediate susceptibility by 2 mg/liter, and resistance by ≥4 mg/liter.

Susceptibility testing was initially performed for various βL-βLI combinations (aztreonam [or meropenem] with avibactam [n = 23]), followed by testing of the remaining isolates (n = 47) with aztreonam-avibactam only. Initially, 23 dual-carbapenemase producers were evaluated against combinations of aztreonam and meropenem with avibactam. Six isolates cocarried bla_KPC-2 and bla_NDM-1, while 17 isolates cocarried either bla_NDM-1 or bla_NDM-5 in combination with bla_OXA-48-like. All isolates were nonsusceptible to meropenem and aztreonam, with the MIC_90s for them being >64 mg/liter. Avibactam restored the susceptibility of 22 (95.7%) isolates to aztreonam and of one (4.3%) isolate to meropenem. Following the promising initial results for the aztreonam and avibactam combination, the remaining 48 dual-carbapenemase-producing Enterobacteriaceae were tested against aztreonam and avibactam, alone and in combination (Table 1; Fig. 1). All isolates were resistant to aztreonam but had their susceptibility restored by avibactam. The combined MIC₉₀s of all 70 isolates were >64 mg/liter for aztreonam and 2 mg/liter for aztreonam-avibactam (Fig. 1). The aztreonam-avibactam combination was largely inhibitory to our collection of dual-carbapenemase CPE, with 98.6% (69/70) of isolates having their susceptibility to aztreonam restored by the addition of avibactam.

TABLE 1.

Susceptibilities of CPE carrying dual carbapenemases to aztreonam and avibactam singly and in combination

Carbapenemase genes (isolates)	Avibactam (mg/liter)			Aztreonam (mg/liter)			Aztreonam-avibactama (mg/liter)
	MIC or range	MIC50	MIC90	MIC or range	MIC50	MIC90	MIC or range	MIC50	MIC90
NDM-1 + IMP-4 (1 K. pneumoniae isolate)	>64			32			0.25
NDM + KPC-2 (11 K. pneumoniae isolates, 2 E. cloacae complex isolates, 1 C. freundii isolate)	16 to >64	64	>64	32 to >64	64	>64	≤0.06 to 4	0.12	1
NDM + OXA-48-like (32 K. pneumoniae, 13 E. coli, 4 E. cloacae complex, 6 C. freundii isolates)	4 to >64	32	>64	32 to >64	>64	>64	≤0.06 to 8	0.5	4
All isolates (n = 70)	4 to >64	64	>64	32 to >64	>64	>64	≤0.06 to 8	0.5	2

^aThe avibactam concentration was constant at 4 mg/liter.

FIG 1.

Aztreonam and aztreonam-avibactam MIC distribution of CPE carrying dual carbapenemases.

Avibactam is currently marketed in combination with ceftazidime and has demonstrated high rates of activity against non-MβL carbapenemases, such as KPC and OXA-48-like carbapenemases (3). However, the overall effectiveness of ceftazidime-avibactam is affected by the local epidemiology of CPE. Molecular identification of MβL carbapenemases precludes the use of ceftazidime-avibactam alone. Where MβLs and CPE predominate, aztreonam-avibactam rather than ceftazidime-avibactam should prove to be more effective therapeutically, and its efficacy further extends to CPE possessing KPC or OXA-48-like carbapenemases. The administration of ceftazidime-avibactam in combination with aztreonam separately may be useful for treating MβL-CPE infections and can be guided by phenotypic susceptibility testing results.