PEG-m-THPC-mediated photodynamic effects on normal rat tissues

Photodynamic therapy (PDT) of malignancies uses light to activate a photosensitizer preferentially accumulated in cancer cells. The first pegylated photosensitizer, tetrakis-(m-methoxypolyethylene glycol) derivative of 7,8-dihydro-5,10,15,20-tetrakis(3-hydroxyphenyl)-21-23-[H]-porphyrin (PEG-m-THPC), was evaluated in non-tumor-bearing rats. The aim of this study was to assess the photodynamic threshold for damage and its sequelae in normal rat tissue. Thirty-five Fischer rats were sensitized with 3, 9 or 30 mg/kg body weight PEG-m-THPC. Colon, vagina and perineum were irradiated with laser light of 652 nm wavelength and an optical dose of 50, 150 or 450 J/cm fiber length. Temperature in the pelvis was measured during PDT. Three days following PDT the effect on skin, vagina, colon, striated muscle, connective tissue, nerves and blood vessels was assessed by histology. The healing of the above-mentioned tissues was assessed on two rats 3 and 8 weeks after PDT using 9 mg/kg PEG-m-THPC activated with 450 J/cm laser light. No dark toxicity was observed. PDT using 30 mg/kg PEG-m-THPC induced severe necrosis irrespective of the optical dose. Body weight of 9 or 3 mg/kg activated with less than 450 J/cm induced moderate or no damage. No substantial increase in body temperature was seen during PDT. Tissues with severe PDT-induced damage seem to have a good tendency to regenerate. We conclude that within the dose required for tumor treatment PEG-m-THPC is a safe photosensitizer with promising properties. PDT of the colon mucosa below 9 mg/kg PEG-m-THPC and 150 J/cm seems to be safe. All other tissues can be exposed to 9 mg/kg PEG-m-THPC activated with less than 450 J/cm laser light with little side effects.