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 β-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 (CC ₅₀ : >78μM), demonstrated potent antimicrobial effects not only in a mouse skin wound infection model but also against oxacillin-resistant clinical isolate MRSA (IC ₅₀ ≈ 1μM), 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 (S _N Ar) reaction, as well as its association with the closure of the major active site of PBP2a.