Phototransformation of sensitisers: 3. Implications for clinical dosimetry

Spectroscopic studies of aqueous solutions of haematoporphyrin-type sensiti sers reveal that photobleaching during eposure to light is followed by the formation of stable red-absorbing photoproducts. Experiments in model syste ms (sensitisers bound to human serum albumin or in a suspension of resealed erythrocyte 'ghosts') and in tumour tissue show that similar photomodifica tion takes place in all investigated environments. Loss of total absorption and emission intensities is accompanied by an increase of absorption in th e red spectral region (630-650 nm) which is used for the treatment of tumou rs because of the deeper penetration of light into tissues. This should be taken into account when the duration of illumination is chosen to reach an appropriate photodynamic dose using Hp-type sensitisers in the photodynamic treatment of tumours.