Molecular action mechanism and light dosimetry modeling for photodynamic therapy with dihematoporphyrin ether and hypericin

The quantum yield of singlet oxygen production was measured for solubilized dihematoporphyrin ether (DHE) and hypericin using the photosensitized inactivation of lysozyme as an internal actinometer. The singlet oxygen quantum yield for DHE is 0.82 {dollar}pm{dollar} 0.03 at 514 nm. A photosensitizing photoproduct that affects quantum yield measurements in the red spectral region is formed at higher light doses. The singlet oxygen quantum yield for hypericin is 0.49 {dollar}pm{dollar} 0.06. Hypericin also forms superoxide. Similar results were obtained with quantum yield measurements in light-scattering tissue phantoms analyzed by means of Monte Carlo simulation. In other work, in vivo skin reflectance measurements carried out on human patients using an integrating sphere spectrophotometer before and after Photofrin{dollar}sp{lcub}rm R{rcub}{dollar} (PF) administration, show that the reflection coefficient increases in a basal cell lesion containing PF and decreases in normal skin containing PF. The results indicate that increased tissue scattering is the dominant effect for PF-containing basal cell lesions, while increased absorption is the dominant effect for PF-containing normal skin. A multiple-layer Monte Carlo simulation was utilized to model these results.