Synthesis and evaluation of boronic acids as inhibitors of Penicillin Binding Proteins of classes A, B and C

The widespread use of beta-lactam antibiotics has lead to the worldwide appearance of drug-resistant strains. Bacteria have developed resistance to beta-lactams by three main mechanisms: the production of beta-lactamases that catalyze hydrolysis of beta-lactams, the production of low-affinity, drug resistant Penicillin Binding Proteins (PBPs) and the over expression of resistant PBPs. PBPs are interesting targets because they catalyse the last steps of the biosynthesis of peptidoglycan, which is unique in bacteria and has no mammalian analogs, outside the cytoplasmic membrane. Various non-ß-lactam inhibitors of PBPs have been developed with the objective of attempting to stall the development of ß-lactam resistance.
Boronic acids are potent beta-lactamase inhibitors and have been shown to display some specificity for soluble transpeptidases and PBPs, but their potential as inhibitors of the latter enzymes is yet to be widely explored. Recently, a (2, 6-dimethoxybenzamido)methylboronic acid was identified as being a potent inhibitor of Actinomadura sp. R39 transpeptidase (IC50: 1.3 µM). Here, we will discuss the synthesis of a number of acylaminomethylboronic acids, analogs of (2, 6-dimethoxybenzamido)methylboronic acid, and their potential as inhibitors of PBPs. Several boronic acids of this library were able to inhibit PBPs of classes A, B and C from penicillin sensitive strains. Thus (2-nitrobenzamido)methylboronic acid was identified as a good inhibitor of class A PBP (PBP1b from S. pneumoniae, IC50 = 26 µM), class B PBP (PBP2xR6 from S. pneumoniae, IC50 = 138 µM) and class C PBP (R39 from Actinomadura sp., IC50 = 0.6 µM). Crystal structures of complexes of R39 and PBP1b with boronic acid analogs of our library have already been solved and allowed an interpretation of results. We believe that this work opens new avenues towards the development of molecules that will inhibit PBPs, and eventually display bactericidal effect, on distinct bacterial species.