Effect of photodynamic therapy in combination with ionizing radiation on human squamous cell carcinoma cell lines of the head and neck

Photodynamic therapy (PDT) is a promising treatment modality for head and n eck, and other tumours, using drugs activated by light. A second generation drug, 5-aminolaevulinic acid (5-ALA), is a precursor of the active photose nsitizer protoporphyrin IX (PpIX) and has fewer side-effects and much more transient phototoxicity than previous photosensitizers. We have investigate d the effect of 5-ALA mediated PDT in combination with gamma-irradiation on the colony forming ability of several human head and neck tumour cell line s. The effect of treatments on the DNA cell cycle kinetics was also investi gated. Our results indicate that the combination of 5-ALA mediated PDT and gamma-irradiation results in a level of cytotoxicity which is additive and not synergistic. 5-ALA mediated PDT had no discernible effect on DNA cell c ycle distributions. gamma-irradiation-induced cell cycle arrest in G2 did n ot enhance the phototoxicity of 5-ALA, (C) 2000 Cancer Research Campaign.