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 x 10(8) 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 micros) was relatively long compared to that of other
porphyrins, such as
hematoporphyrin (Hp) (40 micros),
coproporphyrin I (50 gs), or
coproporphyrin III (36 gs). These results demonstrate the photodynamic generation of 1O2 by
Zincphyrin.