Synthesis and Structure–Activity Relationship Study of 2-(amino)quinazolin-4(3H)-one Derivatives as Potential Inhibitors of Methicillin-Resistant Staphylococcus aureus (MRSA)

<b>Background/Objectives:</b> The rise in methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) demands new therapeutic strategies. In this study, a series of 2-(amino)quinazolin-4(3<i>H</i>)-one derivatives were synthesized and evaluated for antistaphylococcal activity. <b>Methods/Results:</b> Through screening against <i>S. aureus</i> ATCC25923 and USA300 JE2, several submicromolar inhibitors were identified. Among them, compound <b>6l</b>, which contains a 7-chloro substituent on the key parental scaffold, exhibited strong overall antibacterial activity (MIC₅₀: 1.0 µM, ATCC25923; 0.6 µM, JE2) and served as a lead for further structural optimization. Structure-activity relationship analysis showed that substitution at the 2-position was critical, with its optimized analog <b>6y</b> (3,4-difluorobenzylamine) exhibiting the highest potency (MIC₅₀: 0.36 µM, ATCC25923; 0.02 µM, JE2). Cytotoxicity assays in HepG2 cells revealed six compounds with IC₅₀ values above 20 µM, yielding efficacy windows greater than 10. Compound <b>6y</b> exhibited an exceptional index (~885). Consistently, in an H460 lung epithelial infection model mimicking MRSA pneumonia, <b>6y</b> significantly reduced intracellular bacterial loads with minimal host cell damage, outperforming comparator compounds. <b>Conclusions:</b> These findings highlight 2-(amino)quinazolin-4(3<i>H</i>)-one derivatives, particularly <b>6y</b>, as promising leads for the development of new antistaphylococcal agents.