Olefinic side chain modification of fusidic acid enhances anti-MRSA activity and mitigates resistance development.

The escalating prevalence of antibiotic resistance underscores the urgent need for innovative antimicrobial agents. Fusidic acid (FA), a fungal-derived tetracyclic triterpene clinically employed against methicillin-resistant Staphylococcus aureus (MRSA), is limited by rapid resistance development and elevated MIC values in resistant strains. While previous olefinic side chain (Δ^24,25) modifications yielded FA derivatives with retained anti-MRSA activity, most analogs exhibited compromised efficacy against Gram-positive bacteria. To address this limitation, we systematically engineered the olefinic side chain through Wittig and olefin metathesis reactions, synthesizing 26 novel FA derivatives. Compound 10a emerged as a standout candidate, demonstrating MIC value lower than FA against MRSA (0.125 μg mL^-1) as well as low resistance. It also exhibited biofilm disruption capability of reducing MRSA biofilm formation by 61.4% at 0.5 × MIC, along with downregulation of biofilm-related regulators (e.g. clfA, cna, agrA, agrC). In a murine skin infection model, compound 10a significantly inhibited bacterial growth and accelerated wound healing at 2 mg kg^-1. Given these advantages, compound 10a represents a promising candidate molecule for combating multidrug-resistant Gram-positive infections.