Production of singlet oxygen on irradiation of a photodynamic therapy agent, zinc-coproporphyrin III, with low host toxicity

Abstract

Zinc-coproporphyrin III (Zincphyrin) acts efficiently as a photodynamic therapy (PDT) agent in mice, while it shows no tumor cell-killing activity in vitro and has a high LD₅₀ (low toxicity) in mice. It appears to have advantages over other porphyrins as a practical PDT reagent. In order to examine the action mechanism of Zincphyrin in PDT, we evaluated the photochemical characteristics of Zincphyrin by measurement of the near-infrared emission at 1268 nm, which provides direct evidence for formation of ¹O₂. Intense emission was observed in the presence of Zincphyrin, and was completely inhibited by NaN₃, a ¹O₂ scavenger. Based on a quenching study, the rate constant of the reaction of ¹O₂ with NaN₃ was determined to be 1.5–3.5 M⁻¹ s⁻¹, which is close to the reported value (3.8×10⁸ M⁻¹ s⁻¹). The intensity of the ¹O₂-specific emission was proportional to both the laser power and the concentration of Zincphyrin. The fluorescence quantum yield of Zincphyrin was 0.004 in phosphate buffer (100 mM, pH 7.4), which indicates that the excited state decays via other pathway(s) faster than through the fluorescence emission pathway. The lifetime of the triplet state of Zincphyrin (210 μs) was relatively long compared to that of other porphyrins, such as hematoporphyrin (Hp) (40 μs), coproporphyrin I (50 μs), or coproporphyrin III (36 μs). These results demonstrate the photodynamic generation of ¹O₂ by Zincphyrin.