Antifungal action of metalated and metal-free porphyrins in the photodynamic inactivation of Sporothrix brasiliensis

Abstract

The thermodimorphic fungi Sporothrix brasiliensis, a zoophilic pathogen frequents associated to domestic cats, is the most pathogenic species in Sporothrix genus. The increasing resistance of fungal strains to conventional antifungal agents, the prolonged nature and the refractory clinical of treatment underscore the need for alternative therapeutic approaches. Among these, photodynamic inactivation (PDI) has emerged as a promising strategy. PDI involves the combination of a photosensitizing molecule (PS), a light source, and molecular oxygen. Upon irradiation, the PS absorbs photons and undergoes a cascade of photophysical and photochemical reactions that lead to the generation of reactive oxygen species (ROS). At sufficiently high concentrations, these ROS induce irreversible cellular damage, ultimately resulting in cell death. In this study, the in vitro photodynamic activity of four porphyrinic photosensitizers against S. brasiliensis isolates was evaluated: two cationic porphyrins — meso-tetrakis(1-methylpyridinium-4-yl) porphyrin (TMPyP) and its zinc-complexed form (ZnTMPyP) — and two anionic porphyrins — meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS₄) and the its zinc-complexed analogue (ZnTPPS₄). Fungal susceptibility was determined using the microdilution method in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines. Both ZnTMPyP and TMPyP exhibited an estimated minimum effective concentration (MEC) of 0.8 μM against S. brasiliensis and Candida parapsilosis (control). Among the anionic PSs, TPPS₄ showed the greatest inhibitory potential, with an estimated MEC of 1.6 μM. Confocal fluorescence microscopy revealed that both cationic and anionic porphyrins were able to internalize within the cellular structures of S. brasiliensis, which may explain the observed inhibitory effects. These findings demonstrate that porphyrin-based PSs, particularly cationic derivatives, effectively inhibit the growth of S. brasiliensis isolates. Collectively, the results support the potential application of porphyrins as alternative therapeutic agents for the treatment of sporotrichosis.