Evaluation of Amlodipine and Imipramine Efficacy to Treat Galleria mellonella Infection by Biofilm-Producing and Antimicrobial-Resistant Staphylococcus aureus

<b>Background/Objectives:</b> Antimicrobial-resistant <i>Staphylococcus aureus</i> is a growing threat to human health for which alternative therapeutic options are needed. In this study, we aimed to evaluate the efficacy of amlodipine (AML) and imipramine (IMI) to treat <i>S. aureus</i> infection in the <i>Galleria mellonella</i> larval model by targeting efflux and biofilms, which are relevant contributors to antimicrobial resistance and virulence in <i>S. aureus</i>. <b>Methods:</b> In-house reared <i>G. mellonella</i> were used in virulence assays to determine the infective dose of two <i>S. aureus</i> strains differing in the expression of <i>norA</i> (gene encoding the native NorA efflux pump). Toxicology assays were conducted to determine the drugs' LD₅₀ for <i>G. mellonella</i>. Drug efficacy assays were performed to evaluate the potential of amlodipine, imipramine and the control drugs ciprofloxacin (CIP) and enalapril (ENA) to clear <i>S. aureus</i> infection in <i>G. mellonella</i>. <b>Results:</b> Survival analysis defined the infective dose as 1 × 10⁷ CFU/larva for both strains. High LD₅₀ values were determined (CIP: >1000 mg/kg; AML: >640 mg/kg; IMI: 1141 mg/kg; ENA: >1280 mg/kg), revealing a high tolerance of <i>G. mellonella</i> to these drugs. AML at 15 mg/kg and IMI at 100 mg/kg increased the larvae survival by 20% (<i>p</i> = 0.04) and 11% (<i>p</i> = 0.11), respectively, also positively affecting health score indexes. In agreement with the literature, ciprofloxacin at >100 mg/kg promoted larvae survival by >73%. <b>Conclusions:</b> Amlodipine and imipramine show mild potential as new therapeutic options for managing <i>S. aureus</i> infections but are promising as new lead molecules. This study also reinforces <i>G. mellonella</i> as a sustainable, reliable model for drug evaluation.