Photophysical properties of Sn-porphyrins: potential clinical implications

The photophysical properties of Sn-protoporphyrin and two of its synthetic analogues, Sn-mesoporphyrin and Sn-diiododeuteroporphyrin, were examined. All three compounds are potent competitive inhibitors of heme oxygenase, the rate-limiting enzyme in the catabolism of heme to bilirubin, and can suppress completely or diminish significantly experimentally induced or naturally occurring forms of jaundice in animals or man. The results of these studies show that all three compounds have long-lived triplet states which are quenched by molecular oxygen both in solution and when incorporated in liposomes. However, the addition of quenching groups such as iodine to the porphyrin macrocycle results in a marked (approximately 60%) decrease in the triplet yield and a threefold decrease in the triplet lifetime. The triplet yield was shown to be independent of the excitation wavelength, and as a result, the metalloporphyrins were extremely poor photosensitizers when excited in the spectral region commonly used in phototherapy. In the presence of serum albumin, the triplet state of Sn-protoporphyrin was not quenched by oxygen. These results indicate that Sn-porphyrins can be custom designed with considerably reduced photosensitizing properties for potential clinical use as inhibitors of bilirubin production.