A change in the quantum yield of photoinactivation of cells observed during photodynamic treatment

Abstract

When tumours and cells containing photofrin II (PII) are exposed to light at wavelengths and exposures relevant for photodynamic therapy (PDT), changes in the shape of the prophyrin fluorescence spectra, as well as a decay of the fluorescence intensity, can be observed. These changes are due to (a) photodestruction of porphyrin macrocycles resulting in loss of absorbance and fluorescence; (b) a photoinduced chemical modification of the porphyrins, leaving the porphyrin macrocycle intact; or (c) a photoinduced displacement of the porphyrins to different locations in the cells. The two processes (b) and (c) may lead to changes in the shape of the fluorescence spectra as observed. In this study survival curves were determined for cells grown in vitro, exposed to various concentrations of PII and irradiated with light. It was concluded that the quantum yield for photoinactivation of the cells increased during light exposure under the conditions where changes in the spectral shape of the fluorescence was seen. However, the photodegradation of PII resulted in the need for larger light fluences for cell inactivation at low PII concentrations than could be expected on the basis of a reciprocal dependence between light exposure and applied drug dose. © 1988 Baillière Tindall.