Toxicological evaluation in vitro of novel photosensitizers intended for photodynamic therapy.

Three selected photosensitizers (PS) with potential applications in PDT (ZnPcS2, TMPyP, PPIX) were evaluated for skin irritation, phototoxicity and sensitization using exclusively non-animal test methods. The experimental approach was based on OECD TG with protocol modifications to accommodate the specific requirements of photosensitizers testing. Reconstructed human epidermis (RhE) based assays were performed using adapted protocols derived from OECD TG 439 and OECD TG 498, while skin sensitization was assessed using methods based on OECD TG 442C and OECD TG 442D. TMPyP and PPIX showed no evidence of skin irritation, phototoxicity, early skin inflammation, or skin sensitization under the experimental conditions. In contrast, ZnPcS2 at higher concentrations (50 and 100 μM) induced a phototoxic response following 18 h exposure of the RhE skin model EpiDerm™ and subsequent red light irradiation (2 × 75 J cm-2). Importantly, despite its phototoxic activity, ZnPcS2 did not induce skin irritation, sensitization, or increase of inflammatory biomarkers associated with early skin inflammation. Overall, the investigated photosensitizers were well tolerated in the applied non-animal test systems. These findings demonstrated the suitability of OECD TG-derived methods with protocol adaptations for preclinical safety assessment of PS under conditions relevant to PDT and supported their further development for biomedical applications.