Enhancement of photodynamic therapy by mitomycin C: a preclinical and clinical study

Photodynamic therapy (PDT) using Photofrin was used in combination with a hypoxic toxin (mitomycin C, MMC) to treat four patients with recurrent skin metastasis of a mammary carcinoma. In preclinical experiments an additive effect was found for the combination of MMC and PDT for treating subcutaneous RIF1 tumours in mice. When interstitial PDT was combined with a low dose of MMC (administered 15 min before illumination), the Photofrin dose or light dose could be reduced by a factor of 2 in order to obtain equivalent cure rate or growth delay. In the clinical pilot study, a low dose of Photofrin (0.75 mg kg-1) was used for PDT alone (superficial illumination) or combined with low-dose MMC (5 mg m-2). Different tumour areas were illuminated with or without a preceding infusion of MMC. Both tumour response and skin photosensitivity were scored. After 8-12 weeks of treatment, tumour cure could be achieved by administering light doses > or = 150 J cm-2 for PDT alone and similar effects were obtained when light doses of 75-87.5 J cm-2 were given after infusion with MMC. In all cases necrotic tissue of both tumour and surrounding skin was observed, which lasted for a mean of 5 months (range 2-20 months). Skin phototoxicity, tested by using a standardised illumination of skin patches on the back, lasted maximally 3 weeks. Three main conclusions could be drawn from these studies: (1) The enhanced effects of the combination of PDT and MMC observed in mouse tumours can be extrapolated to patients with mammary skin metastasis. (2) The combination of PDT and hypoxic toxins facilitates treatment by permitting lower doses of photosensitiser to be used (thereby reducing skin phototoxicity) or lower light doses (thereby reducing illumination times and allowing the possibility to treat larger tumour areas). (3) Restoration of skin after PDT in previously treated tumour areas (chemotherapy, radiation therapy and surgery) is very low.