Identification of Potential PBP2a Inhibitors Against Methicillin-Resistant Staphylococcus aureus via Drug Repurposing and Combination Therapy

Methicillin-resistant Staphylococcus aureus (MRSA) achieves high-level resistance against β-lactam antibiotics through the expression of penicillin-binding protein 2a (PBP2a), which features a closed active site that impedes antibiotic binding. Herein, we implemented a strategy that combines drug repurposing with synergistic therapy to identify potential inhibitors targeting PBP2a's allosteric site from an FDA-approved drug database. Initially, retrospective verifications were conducted, employing different Glide docking methods (HTVS, SP, and XP) and two representative PBP2a structures. The combination of Glide SP and one representative PBP2a conformation showed the highest efficacy in identifying active compounds. The optimized parameters were then utilized to screen FDA-approved drugs, and 15 compounds were shortlisted for potential combination therapy with cefazolin, an ineffective cephalosporin against MRSA. Through biological assays—checkerboard, time-kill assays, and live/dead bacterial staining—we discovered that four compounds exhibited robust bactericidal activity (FICI < 0.5) compared to both untreated control and monotherapy with cefazolin alone. Scanning electron microscopy (SEM) confirmed that while cefazolin alone did not cause visible damage to MRSA cells, the combination treatment markedly induced cell lysis. Additional MM-GBSA studies underscored the strong binding affinity of mitoxantrone to the allosteric site. These findings introduce a combination therapy approach that potentially restores MRSA's susceptibility to β-lactam antibiotics.