We evaluated the in vitro potency of cefepime combined with AAI101, a novel extended-spectrum β-lactamase inhibitor, against a population of clinical Escherichia coli and Klebsiella pneumoniae collected from USA hospitals. Of the 223 cefepime non-susceptible isolates, 95% were ceftazidime non-susceptible, 49% ertapenem non-susceptible, 57% piperacillin/tazobactam non-susceptible, 90% were multidrug-resistant (resistant to ≥3 drug classes), 22% produced carbapenemases, and 67% produced ESBLs. Addition of AAI101 restored the activity of cefepime such that the MIC50 was reduced from >64 mg/L for cefepime to 0.13 mg/L for cefepime/AAI101, supporting its continued development treatment for infections caused by these organisms.