Structurally similar porphyrins and porphyrazines perform differently under green or red light irradiation against non-melanoma skin tumor cells

A porphyrin (H₂P) and a porphyrazine (H₂Pz) and their respective zinc complexes (ZnP and ZnPz), with the substituent N-ethylpyridyl-3-yl, were synthesized and their photophysical and phototoxic properties were compared. These substances showed high solubility in water due to the cationic moieties and without notable aggregation in this solvent. Photophysical studies showed that H₂P and H₂Pz have fluorescence quantum yields (Φ_F) of 2.04 % and 0.48 % in water, respectively, that, upon metalation, decreases to 1.4 % for the metalloporphyrin (ZnP) and increases to 6.1 % for the metalloporphyrazine (ZnPz). Regarding the singlet oxygen production efficiency (Φ_Δ) of the substances in water, a considerable increase from 16.2 % to 41 % was obtained for the porphyrin upon coordination to zinc, while the porphyrazines practically do not produce singlet oxygen (<1 %) in this solvent. When tested against human squamous cell carcinoma cells (A-431) and immortalized human keratinocytes (HaCaT), the toxicity of all compounds increased using green (520 ± 5 nm) and red (625 ± 5 nm) radiation when compared to the non-irradiated cells, however, phototoxicity was higher for the porphyrins using green light, while the porphyrazines performed better under red radiation. Furthermore, porphyrins had a higher phototoxic index than porphyrazines and greater selectivity for A-431 cancer cells compared to HaCaT cells. To the best of our knowledge, this is the first work that compares the photophysical response and in vitro anticancer activity of porphyrins and porphyrazines with the same substituent.