Transition metal-dependent heavy-atom effect of metallochlorin photosensitizers for enhanced photodynamic therapy

In the present study, a chlorin derivative photosensitizer (PS), purpurin-18 methyl ester (P18ME), and its transition metal complexes were synthesized and evaluated for their in vitro photodynamic therapy (PDT) effi-cacy. The effect of introducing transition metals into the free-base PS was assessed, and the heavy-atom effect of two divalent transition metals, palladium(II) and cobalt(II), was compared. All metal complexes exhibited hypsochormic shifts of the Q-bands and demonstrated significantly improved photostability. In addition, the palladium(II) complex of P18ME (Pd-P18ME) exhibited increased singlet oxygen (1O2) photogeneration and enhanced efficacy in vitro, owing to the heavy-atom effect. However, the cobalt(II) complex of P18ME displayed reduced 1O2 photogeneration and decreased photodynamic activity. These results showed that the heavy-atom effect might depend on the compound structure and the transition metal used. Therefore, Pd-P18ME displayed enhanced photodynamic activity based on the heavy-atom effect to induce increased photostability and 1O2 photogeneration, indicating a potential PS candidate for PDT.