Development of non-β-Lactam covalent allosteric inhibitors targeting PBP2a in Methicillin-Resistant Staphylococcus aureus
Methicillin-resistant Staphylococcus aureus (MRSA), a Gram-positive bacterial pathogen, continues to pose a serious threat to the current public health system in our society. The high level of resistance to {beta}-lactam antibiotics in MRSA is attributed to the expression of penicillin-binding protein 2a (PBP2a), which catalyzes cell wall cross-linking. According to numerous research reports, the activity of the PBP2a protein is known to be regulated by an allosteric site distinct from the active site where cell wall cross-linking occurs. Here, we conducted a screening of 113 compounds containing a 1,3,4-oxadiazole core to design new covalent inhibitors targeting the allosteric site of PBP2a and establish their structural-activity relationship. The stereochemically selective synthesis of sulfonyl oxadiazole compounds identified in the initial screening resulted in a maximum eightfold enhancement in cell inhibition activity. The sulfonyl oxadiazole-based compounds formulated as PEG-based ointments, with low toxicity test results on human cells (CC50: >78M), demonstrated potent antimicrobial effects not only in a mouse skin wound infection model but also against oxacillin-resistant clinical isolate MRSA (IC50 {approx} 1M), as evidenced by the results. Furthermore, additional studies utilizing LC-MS/MS and in-silico approaches clearly support the allosteric site covalent binding mechanism through the nucleophilic aromatic substitution (SNAr) reaction, as well as its association with the closure of the major active site of PBP2a.