Novel Cephalosporin Conjugates Display Potent and Selective Inhibition of IMP Type Metallo-β-Lactamases

In an attempt to exploit the hydrolytic mechanism by which β-lactamase enzymes degrade cephalosporins, we designed and synthesized a series of novel cephalosporin prodrugs aimed at delivering thiol-based inhibitors of metallo-β-lactamases (MBLs) in spatiotemporally controlled fashion. Notably, while enzyme-mediated hydrolysis of the β-lactam ring was found to occur, it was not accompanied by release of the thiol-based inhibitors. Nonetheless, the cephalosporin prodrugs, especially thiomandelic acid conjugate (8), demonstrated potent inhibition of IMP-type MBLs, with IC₅₀ values in the nanomolar range. In addition, conjugate 8 was also found to greatly reduce the MIC of meropenem against an IMP-28 producing clinical isolate of K. pneumoniae. The results of kinetic experiments indicate that these prodrugs inhibit IMP-type MBLs by acting as slowly turned-over substrates. Structure-activity relationship studies revealed that both phenyl and carboxyl moieties of 8 are crucial for its potency. Furthermore, modeling studies indicate that productive interactions of the thiomandelic acid moiety of 8 with residues Trp28 and Lys161 within the IMP active site may contribute to the observed inhibitory potency and selectivity.