Structure-Based Design of β-Lactamase Inhibitors. 1. Synthesis and Evaluation of Bridged Monobactams

Bridged monobactams are novel, potent, mechanism-based inhibitors of class C β-lactamases, designed using X-ray crystal structures of the enzymes. They stabilize the acyl−enzyme intermediate by blocking access of water to the enzyme−inhibitor ester bond. Bridged monobactams are selective class C β-lactamase inhibitors, with half-inhibition constants as low as 10 nM, and are less effective against class A and class B enzymes (half-inhibition constants > 100 μM) because of the different hydrolysis mechanisms in these classes of β-lactamases. The stability of the acyl−enzyme complexes formed with class C β-lactamases (half-lives up to 2 days were observed) enabled determination of their crystal structures. The conformation of the inhibitor moiety was close to that predicted by molecular modeling, confirming a simple reaction mechanism, unlike those of known β-lactamase inhibitors such as clavulanic acid and penam sulfones, which involve secondary rearrangements. Synergy between the bridged monobactams and β-lactamase-labile antibiotics could be observed when such combinations were tested against strains of Enterobacteriaceae that produce large amounts of class C β-lactamases. The minimal inhibitory concentration of the antibiotic of more than 64 mg/L could be decreased to 0.25 mg/L in a 1:4 combination with the inhibitor.