Thiomandelic Acid, a Broad Spectrum Inhibitor of Zinc β-Lactamases

International audienceResistance to ␤-lactam antibiotics mediated by metallo-␤-lactamases is an increasingly worrying clinical problem. Candidate inhibitors include mercaptocarboxylic acids, and we report studies of a simple such compound, thiomandelic acid. A series of 35 analogues were synthesized and examined as metallo-␤-lactamase inhibitors. The K i values (Bacillus cereus enzyme) are 0.09 M for R-thiomandelic acid and 1.28 M for the Sisomer. Structure-activity relationships show that the thiol is essential for activity and the carboxylate increases potency; the affinity is greatest when these groups are close together. Thioesters of thiomandelic acid are substrates for the enzyme, liberating thiomandelic acid, suggesting a starting point for the design of "pro-drugs." Importantly, thiomandelic acid is a broad spectrum inhibitor of metallo-␤-lactamases, with a submicromolar K i value for all nine enzymes tested, except the Aeromonas hydrophila enzyme; such a wide spectrum of activity is unprecedented. The binding of thiomandelic acid to the B. cereus enzyme was studied by NMR; the results are consistent with the idea that the inhibitor thiol binds to both zinc ions, while its carboxylate binds to Arg 91. Amide chemical shift perturbations for residues 30-40 (the ␤ 3-␤ 4 loop) suggest that this small inhibitor induces a movement of this loop of the kind seen for other larger inhibitors