Impact of the hydrophilic-lipophilic balance of free-base and Zn(II) tricationic pyridiniumporphyrins and irradiation wavelength in PDT against the melanoma cell lines

The amphiphilic and asymmetric structure of porphyrins, when used as photosensitizers (PSs) for photodynamic therapy (PDT), has been shown through numerous previous studies to be a very important property that facilitates their entry into cells, which improves their efficiency in PDT. In this work, two groups of cationic AB₃ pyridiniumporphyrins, free-base and chelated with Zn(II), both substituted with alkyl chains of various lengths, were studied in PDT on melanoma cell lines. The aim was to investigate the impact of hydrophilic-lipophilic balance and Zn(II) chelation, and the importance of matching the irradiation wavelength to the optical properties of the PS on in vitro PDT efficiency. Therefore, spectroscopic studies, singlet oxygen production and lipophilicity as well as cellular uptake, localization and cytotoxicity studies of the two series of porphyrins were performed. In both series of porphyrins, the longest alkyl chain (17 C-atoms long) enables the greatest internalization of the PS. Chelation with Zn(II) resulted in better physicochemical properties, but slower cellular internalization. As expected, free-base porphyrins were more PDT efficient than their Zn(II) complexes after 30-minute photoactivation by low-fluence (2 mW/cm²) red light (643 nm). However the use of orange light (606 nm) with the same fluence rate was more suitable for Zn(II) porphyrins and resulted in similar overall toxicity to their free-base analogues with similar lipophilicity. Although the highest phototoxicity was achieved with the PSs carrying the longest alkyl chain, TMPyP3-C₁₃H₂₇ and Zn(II)-TMPyP3-C₁₃H₂₇ proved to be the most promising candidates for use in PDT as they exhibit high phototoxicity, but also greater selectivity towards melanoma cell lines (MeWo and A375) compared to fibroblasts (HDF).