Antifungal Activities of Multi-Halogenated Indoles Against Drug-Resistant Candida Species

The emergence of drug-resistant Candida species has created an urgent need for non-toxic molecules that inhibit fungal growth, biofilm development, and hyphal formation. In this study, fifty multi-halogenated indole derivatives were screened against ten Candida species, including azole-resistant C. albicans, C. auris, C. glabrata, and C. parapsilosis. Among them, 4,6-dibromoindole and 5-bromo-4-chloroindole exhibited the strongest antifungal and antibiofilm effects, with minimum inhibitory concentration (MIC) values of 10–50 µg/mL, outperforming ketoconazole and comparable to miconazole. Both di-halogenated indoles markedly inhibited cell aggregation, yeast-to-hyphae transition, and induced reactive oxygen species (ROS) accumulation, contributing to fungicidal activity. Microscopic analyses revealed the disruption of hyphal networks and reduced biofilm biomass. They showed moderate cytotoxicity in human hepatocellular carcinoma (HepG2) cells (median lethal dose, LD50 = 35.5 µg/mL and 75.3 µg/mL) and low phytotoxicity in plant assays. The quantitative structure–activity relationship (QSAR) model identified halogen substitution at C4, C5, and C6 positions as optimal for antifungal activity due to enhanced hydrophobic and electron-withdrawing effects. Together, these findings demonstrate that di-halogenated indoles serve as potent, low-toxicity inhibitors of Candida growth, biofilms, and morphogenesis, providing a promising scaffold for next-generation antifungal agents targeting drug-resistant Candida species.