ABSTRACT Multidrug-resistant Acinetobacter baumannii has rapidly spread worldwide, resulting in a serious threat to hospitalized patients. Zidebactam and WCK 5153 are novel non-β-lactam bicyclo-acyl hydrazide β-lactam enhancer antibiotics being developed to target multidrug-resistant A. baumannii . The objectives of this work were to determine the 50% inhibitory concentrations (IC ₅₀ s) for penicillin-binding proteins (PBP), the OXA-23 inhibition profiles, and the antimicrobial activities of zidebactam and WCK 5153, alone and in combination with β-lactams, against multidrug-resistant A. baumannii . MICs and time-kill kinetics were determined for an A. baumannii clinical strain producing the carbapenemase OXA-23 and belonging to the widespread European clone II of sequence type 2 (ST2). Inhibition of the purified OXA-23 enzyme by zidebactam, WCK 5153, and comparators was assessed. All of the compounds tested displayed apparent K _i values of >100 μM, indicating poor OXA-23 β-lactamase inhibition. The IC ₅₀ s of zidebactam, WCK 5153, cefepime, ceftazidime, meropenem, and sulbactam (range of concentrations tested, 0.02 to 2 μg/ml) for PBP were also determined. Zidebactam and WCK 5153 demonstrated specific high-affinity binding to PBP2 of A. baumannii (0.01 μg/ml for both of the compounds). The MICs of zidebactam and WCK 5153 were >1,024 μg/ml for wild-type and multidrug-resistant Acinetobacter strains. Importantly, combinations of cefepime with 8 μg/ml of zidebactam or WCK 5153 and sulbactam with 8 μg/ml of zidebactam or WCK 5153 led to 4- and 8-fold reductions of the MICs, respectively, and showed enhanced killing. Notably, several of the combinations resulted in full bacterial eradication at 24 h. We conclude that zidebactam and WCK 5153 are PBP2 inhibitors that show a potent β-lactam enhancer effect against A. baumannii , including a multidrug-resistant OXA-23-producing ST2 international clone.