Sulbactam-durlobactam susceptibility test method development and quality control ranges for MIC and disk diffusion tests

Sulbactam-durlobactam is a beta-lactam/beta-lactamase inhibitor combination developed to treat hospital-acquired and ventilator-associated bacterial pneumonia caused by Acinetobacter baumannii-calcoaceticus complex (ABC). Durlobactam is a diazabicyclooctane beta-lactamase inhibitor with potent activity against Ambler classes A, C, and D serine beta-lactamases and restores sulbactam activity against multidrug-resistant ABC. Studies were conducted to establish sulbactam-durlobactam antimicrobial susceptibility testing methods for both broth microdilution minimal inhibitory concentration (MIC) and disk diffusion tests as well as quality control (QC) ranges. To establish the MIC test method, combinations of sulbactam and durlobactam were evaluated using a panel of genetically characterized A. baumannii isolates which were categorized as predicted to be susceptible or resistant based on the spectrum of beta-lactamase inhibition by durlobactam. MIC testing with doubling dilutions of sulbactam with a fixed concentration of 4 mu g/mL of durlobactam resulted in the greatest discrimination of the pre-defined susceptible and resistant strains. Similarly, the sulbactam/durlobactam 10/10 mu g disk concentration showed the best discrimination as well as correlation with the MIC test. A. baumannii NCTC 13304 was selected for QC purposes because it assesses the activity of both sulbactam and durlobactam with clear endpoints. Multi-laboratory QC studies were conducted according to CLSI M23 Tier 2 criteria. A sulbactam-durlobactam broth MIC QC range of 0.5/4-2/4 mu g/mL and a zone diameter QC range of 24-30 mm were determined for A. baumannii NCTC 13304 and have been approved by CLSI. These studies will enable clinical laboratories to perform susceptibility tests with accurate and reproducible methods.