AAI101 is an extended-spectrum β-lactamase inhibitor, against many resistant Gram-negative pathogens. In Vitro: AAI101 shows potent activity against specific resistance phenotypes with MIC50 and MIC90 of 0.125 mg/L and 64 mg/L. Cefepime-AAI101 MICs decreases with increasing concentrations of AAI101 (over the range from 1 to 16 mg/L) for most strains, demonstrating a concentration dependence of AAI101 on restoration of the antibacterial activity of the cephalosporin. In Vivo: In neutropenic animals, cefepime-AAI101 treatment results in reductions in bacterial density of ≥0.5 log10 CFU for 12 of the 20 strains tested and reductions of ≥1 log10 CFU for 6 of these. Increases in bacterial density are for only four strains, three of which have cefepime-AAI101 MICs of ≥64 mg/L, irrespective of the AAI101 concentration.