An inhibitor of β-lactamases with greater activity against Ambler class A serine penicillinases and class C cephalosporinases, including TEM-1, SHV-1, and P99 (IC₅₀s = 97, 150, and 8.5 nM); also inhibits the class D oxacillinase OXA-2 (IC₅₀ = 10 nM).