Aztreonam-Avibactam Susceptibility Testing Program for Metallo-Beta-Lactamase-Producing Enterobacterales in the Antibiotic Resistance Laboratory Network, March 2019 to December 2020

ABSTRACT Aztreonam-avibactam is a drug combination pending phase 3 clinical trials and is suggested for treatment of severe infections caused by metallo-beta-lactamase (MBL)-producing Enterobacterales by combining ceftazidime-avibactam and aztreonam. Beginning in 2019, four Antibiotic Resistance Laboratory Network regional laboratories offered aztreonam-avibactam susceptibility testing by broth microdilution. For 64 clinical isolates tested, the MIC50 and MIC90 values of aztreonam-avibactam were 0.5/4 μg/ml and 8/4 μg/ml, respectively. Aztreonam-avibactam displayed potent in vitro activity against the MBL-producing Enterobacterales tested.

Enterobacterales isolates that met $1 of the following criteria were eligible for aztreonamavibactam AST at the AR Lab Network regional laboratories: (i) PCR-positive for $1 MBL gene (bla NDM , bla VIM , or bla IMP ) or (ii) not susceptible to all beta-lactams tested by the submitting laboratory, including at least ceftazidime-avibactam and/or meropenem-vaborbactam.
The presence of carbapenemase genes was confirmed using CDC lab-developed realtime PCR methods or the GeneXpert Carba-R assay (Cepheid, Sunnyvale, CA). AST was performed in compliance with Clinical Laboratory Improvement Amendments guidelines for aztreonam, ceftazidime-avibactam, and aztreonam-avibactam using BMD panels prepared by the D300e digital dispenser (HP, Corvallis, OR) (11). The CDC provided laboratories with drug stock aliquots of aztreonam, ceftazidime, and avibactam and tubes containing 11 ml of cation-adjusted Mueller-Hinton broth. Quality control strains used were Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853), and Klebsiella pneumoniae (ATCC 700603). Participating laboratories also performed AST using Sensititre GNX2F BMD panels (Thermo Fisher Scientific, Waltham, MA) validated to have a final CFU/ml of approximately 5 Â 10 5 . Interpretive criteria were applied according to Clinical and Laboratory Standards Institute guidelines when available (13). Due to lack of interpretive criteria for aztreonamavibactam, only an MIC was reported to submitters. Previous studies demonstrated that aztreonam-avibactam provided equivalent in vitro susceptibility for ceftazidime-avibactam plus aztreonam in highly resistant Enterobacterales; therefore, although patients receive ceftazidime-avibactam plus aztreonam, MICs for the triple combination were not reported (14). Results were typically reported to submitters within three working days.
Laboratories reported results to the CDC using a Research Electronic Data Capture (REDCap) database (Vanderbilt University, Nashville, TN). Results from one isolate per species per patient were included. If multiple isolates of the same species were submitted, the isolate with the earliest collection date was included. Data were analyzed using Statistical Analysis Software (v9.4; SAS Institute, Cary, NC).
When examining MIC distributions by organism, E. coli demonstrated higher MIC 50 and MIC 90 results (4/4 mg/ml and 8/4 mg/ml, respectively) than others (Table 1). A similar observation was made in a large study of 275 NDM-producing Enterobacterales; the MIC 50 and MIC 90 were higher in 115 E. coli isolates (2/4 mg/ml and 8/4 mg/ml, respectively) than the MIC 50 and MIC 90 observed in 125 K. pneumoniae isolates (0.25/4 mg/ml and 0.5/4 mg/ml, respectively) (15). In our study, all 9 isolates for which avibactam did not restore aztreonam susceptibility were E. coli. This phenomenon could be explained by polymorphisms in the penicillin-binding protein 3 (16,17); additional studies are needed to confirm their presence and role in our isolates.
One limitation of our study is the sample size-a small convenience sample of 64 highly resistant NDM-producing Enterobacterales. While our findings may not be generalizable to other organisms or other MBL carbapenemases circulating in Enterobacterales, this subset of isolates represents a rich collection of very rarely detected mechanisms, and the strict inclusion criteria employed for testing are consistent with the clinical and microbiological characteristics of infections for which aztreonam-avibactam (i.e., aztreonam and ceftazidime-avibactam) is an option for therapeutic consideration. Another limitation is that no patient outcome data were collected to ascertain whether aztreonam and ceftazidime-avibactam were subsequently coadministered for treatment and whether such treatment was effective. Finally, aztreonamavibactam clinical breakpoints have not yet been established; therefore, the MIC data reported here must be interpreted cautiously.
In summary, organisms harboring MBLs confer resistance to many available antimicrobial agents and present clinicians with few, if any, effective treatment options. While our in vitro data demonstrate that aztreonam-avibactam has considerable activity against Enterobacterales coharboring MBL carbapenemases, specifically NDM, and certain other beta-lactamases capable of hydrolyzing aztreonam, more studies are necessary to assess the in vivo efficacy of aztreonam-avibactam. As of 2021, all seven AR Lab Network regional laboratories now offer aztreonam-avibactam AST for MBL-producing Enterobacterales, filling a critical gap by providing rapid results to help inform clinical treatment decisions (12). Interested health care and public health professionals can contact their AR Lab Network regional laboratory for more information.

SUPPLEMENTAL MATERIAL
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ACKNOWLEDGMENTS
The findings and conclusions in this report are those of the author(s) and do not represent the official position of the Centers for Disease Control and Prevention. This work was supported by the Centers for Disease Control and Prevention's internal funding.