Photodynamic therapy referred by us as photosensitized tumour therapy (PTT) has attracted interest as a new technique with the potential for selective local destruction of malignant tumours. It is based on systemic administration of photosensitizers, which may be retained selectively in tumours relatively to the surrounding normal tissue and can be activated by light to produce a local cytotoxic effect (Fig.1).

Efficacy of PTT depends on both the concentration of a photosensitizer in cancerous tissue and the illumination dose, since only the excited photosensitizer can initiate destruction of malignant cells. The photochemical definition of photochemical dose (PD) is:

Geometrically, the photochemical dose is defined as an area under the curve of photosensitizer's concentration plotted as a function of light dose (Fig.2). The exact concentration of photosensitizer in cancerous tissue it is difficult to measure therefore the concentration of injected photosensitizer in mg per body kilogram (mg/kg) in generally is used in experiments in vivo. A more general way in which the geometric definition of PD can be expressed is

where j is a light dose in J cm^-3 absorbed at any point of the tissue, and D(j) is a concentration of photosensitizer at the same point which varies as a function of j. [1]. If photosensitizer is stable and no degradation takes place on light illumination the PD can be estimated by a simple multiplication of D and J₀.

It is well known that most of photosensitizers used in PTT are not stable under illumination with visible light [1-15]. Thus, the influence of photobleaching of photosensitizer on the photochemical dose needed for full destruction of tumour should be taken into account [1, 5-8]. If the bleaching follows exponential decay:

where D₀ is a concentration of the photosensitizer at the beginning of illumination, J₀ is the incident light dose, J cm^-2, b is a rate constant of the decrease of photosensitizer concentration, J^-1cm^-2, then PD after integration can be expressed:

On the other hand, the photodestruction of most porphyrin-type photosensitizers is accompanied by the formation of the photoproducts [9-15], which can be important in PTT.

In this paper we present our studies on the light-induced spectral changes of different photosensitizers used in PTT. These changes occurring during PTT could influence sensitization process [16] and should be taken into account in estimation of photochemical doses.