A Nickel N-Heterocyclic Biscarbene Complex Derived from Caffeine Enhances Fluconazole Efficacy against Candida glabrata

Invasive infections caused by Candida spp. are associated with high morbidity and mortality, particularly in immunocompromised patients, and are increasingly difficult to treat due to rising antifungal resistance. Here, we further investigate the antifungal properties of a previously reported nickel N-heterocyclic biscarbene derived from caffeine (compound 5) and describe the synthesis of an analogous nickel N-heterocyclic biscarbene based on benzimidazole (compound 8), designed to evaluate the impact of the biscarbene and xanthine frameworks on activity and toxicity. Compound 5 displayed selective activity against Candida glabrata, inhibited biofilm formation and showed greater cellular accumulation in this species. In a Galleria mellonella infection model, 5 significantly reduced fungal burden while exhibiting lower cytotoxicity than the benzimidazole analogue 8. Importantly, although compound 5 and fluconazole are individually fungistatic, their combination was fungicidal against C. glabrata. In the presence of compound 5, the minimal inhibitory concentration of fluconazole decreased against both a fluconazole-resistant petite mutant and a respiratory-competent C. glabrata strain evolved in vitro to fluconazole resistance. Compound 5 increased the frequency of petite mutants, suggesting an effect on mitochondrial function; however, its retained activity against these mutants and its synergism with fluconazole in the petite background indicate an additional mechanism of action. These findings identify 5 as a promising antifungal adjuvant and support its potential use in combination therapy to enhance azole efficacy against C. glabrata. ImportanceInvasive infections caused by C. glabrata are increasingly hard to treat because this pathogen is often tolerant to azoles and is developing resistance to echinocandins, leaving clinicians with few effective options. Our work explores a nickel-caffeine complex that shows selective activity against C. glabrata and low toxicity, features that make it attractive as a potential therapeutic lead. Notably, this compound enhances the activity of fluconazole, turning a commonly used fungistatic drug into a fungicidal combination against both susceptible and resistant C. glabrata. By also affecting mitochondrial function yet remaining active in respiratory-deficient mutants, the compound appears to act through mechanisms distinct from existing antifungals. These properties suggest that metal-based xanthine complexes could be developed as adjuvants to restore or boost azole efficacy against difficult-to-treat C. glabrata infections, addressing an important priority identified for fungal pathogens.