In Vitro Heavy-Atom Effect of Palladium(II) and Platinum(II) Complexes of Pyrrolidine-Fused Chlorin in Photodynamic Therapy

Introduction of a heavy atom into photosensitizers generally facilitates intersystem crossing and improves the quantum yield (ΦΔ) of singlet oxygen (1O2), which is a key species in photodynamic therapy (PDT). However, little information is available about the physiological importance of this heavy-atom effect. The aim of this study is to examine the heavy-atom effect in simple metallochlorins in vitro at the cellular level. 1,3-Dipolar cycloaddition of azomethine ylide to 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato palladium(II) and platinum(II) afforded metallochlorins 4b and 4c in yields of 17.1 and 12.9%, respectively. The ΦΔ values increased in the order of 4a (0.28) < 4b (0.89) < 4c (0.92) in C6D6. The photocytotoxicity of 4a, 4b, and 4c was evaluated in HeLa cells at a light dose of 16 J·cm−2 with λ > 500 nm and increased in the order of 4a < 4b < 4c at the concentration of 0.5 μM. The photocytotoxicity of 4b and 4c was significantly inhibited by addition of sodium azide, but not d-mannitol, suggesting that 1O2 is the major species causing cell death. Our results clearly indicate that 4b and 4c act as efficient 1O2 generators due to the heavy-atom effect in a cellular microenvironment as well as in nonphysiological media.