Integrative Experimental and Molecular Docking Analyses Reveal Phenanthroline Derivatives as Inhibitors of Cyanide-Resistant Respiration in Candida albicans

Background: Candida albicans can adapt to inhibition of the classical mitochondrial respiratory chain by activating an alternative oxidase (AOX)-dependent pathway, whereas Candida glabrata lacks detectable AOX-mediated respiration under the conditions tested. Identifying compounds that selectively inhibit this alternative pathway may provide new therapeutic strategies against fungal pathogens. Methodology: The antifungal activity of 5-Nitro-1,10-phenanthroline (5-Nitro-Phen) and 1,10-phenanthroline (Phen) was evaluated against C. albicans and Candida glabrata using a diffusion plate assay. Oxygen consumption was measured in intact cells before and after cyanide inhibition of Complex IV, and cyanide-resistant respiration was assessed using salicylhydroxamic acid (SHAM). Metal-chelating activity was evaluated relative to EDTA. Molecular docking was performed using SwissDock, ReverseDock, and PrankDock to predict interactions with AOX1. Results: Both compounds showed greater antifungal activity against C. albicans than C. glabrata, with C. glabrata inhibited only above 350 μM. In C. albicans, cyanide revealed a SHAM-sensitive, AOX-dependent respiratory component that was selectively inhibited by Phen and 5-Nitro-Phen. In contrast, C. glabrata showed no detectable cyanide-resistant or AOX-dependent respiration. Although Phen exhibited greater metal-chelating activity (43% relative to EDTA), both compounds showed similar AOX inhibitory potency (IC50 = 2.7 and 2.9 μM, respectively). Docking predicted favorable binding to AOX1 (−7.39 to −8.395 kcal/mol), involving hydrophobic residues including Phe247, Phe251, Val254, and Phe265. Conclusion: Phen and 5-Nitro-Phen inhibit AOX-dependent respiration in C. albicans, supporting AOX as a promising antifungal target and phenanthroline derivatives as potential scaffolds for AOX-directed inhibitor development.