The interaction of hematoporphyrin derivative, light, and ionizing radiation in a rat glioma model

Abstract

The effects of hematoporphyrin derivative, light, and cobalt 60 (⁶⁰Co) irradiation were studied in a rat glioma model using an in vivo and an in vitro clonogenic assay. There was no effect on tumor growth by visible light or by a single dose of ⁶⁰Co irradiation at 4 Gy or 8 Gy, whereas 16 Gy inhibited tumor growth to 40% versus the control. Hematoporphyrin derivative alone slightly stimulated growth (P < 0.1). Light in the presence of 10 mg hematoporphyrin derivative/kg inhibited tumor growth to 32%. ⁶⁰Co irradiation in the presence of hematoporphyrin derivative produced a significant tumor growth inhibition (P < 0.02). This growth inhibition was directly related to the concentration of hematoporphyrin derivative. The addition of ⁶⁰Co to light in the presence of hematoporphyrin derivative produced a greater growth inhibition than light or ⁶⁰Co irradiation alone. This effect was most pronounced when light was applied 30 minutes before ⁶⁰Co irradiation. Our experiments in a subcutaneous rat glioma model suggest a radiosensitizing effect of hematoporphyrin derivative. Furthermore, the photodynamic inactivation is enhanced by the addition of ⁶⁰Co irradiation. These findings may be of importance in planning new treatment modalities in malignant brain tumors.