Beyond Tuberculosis: Unlocking the Antifungal Potential of Novel Isoniazid Derivatives against Non- Candida albicans Candida Species

Abstract The increasing prevalence of drug-resistant non-CandidaalbicansCandida (NCAC) species has created an urgent need for novel antifungal agents with improved efficacy and selectivity. In this study, eight modified isoniazid derivatives were synthesized through hydrazone formation and structurally characterized using single-crystal X-ray diffraction (SC-XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and nuclear magnetic resonance (NMR) analysis. The antioxidant potential of the derivatives was evaluated using the DPPH radical scavenging assay, while antifungal activity was assessed against Candida auris, Candida glabrata, and Candida parapsilosis using minimum inhibitory concentration (MIC) assays. Several derivatives demonstrated enhanced antifungal activity relative to the parent compound, with Compounds II, V, and VI exhibiting the strongest activity against C. auris (MIC = 15.6 μg/mL). Compound II also displayed broad-spectrum activity across all tested Candida species. Flow cytometry using propidium iodide staining revealed increased membrane permeability and loss of cellular integrity following treatment, supporting membrane disruption as a potential contributor to antifungal activity. Antioxidant evaluation showed that Compound VII (17.61 μg/mL) possessed the strongest radical scavenging activity among the derivatives, with an IC50 value approaching that of ascorbic acid (9.7 μg/mL). Cytotoxicity assessment using Vero cells demonstrated concentration-dependent effects, with variable LC50 values observed across the series. Real-time cell analysis (RTCA) further revealed dynamic differences in cellular responses following compound exposure, with Compound III exhibiting comparatively lower effects on mammalian cell viability. Structure–activity relationship analysis suggested that phenolic functionalities, lipophilic substituents and hydrazone-linked modifications contribute to both antifungal and antioxidant behaviour.