Adjuvant intraoperative photodynamic therapy in experimental colorectal cancer using a new photosensitizer

Photodynamic therapy is a promising anticancer modality. The effect of adjuvant intraoperative photodynamic therapy (AIOPDT) using photosensitizers Photofrin or mesotetra (hydroxyphenyl) chlorin (m-THPC) was evaluated in a mouse model of local recurrence of colorectal cancer.

Animals were injected with either photosensitizer 24 h before surgery. Standard excision included removal of all visible tumour by a surgeon blinded to both the drug and subsequent light treatment. Following surgery the tumour beds were illuminated with either 70 J cm-2 of 510-nm light (Photofrin) or 2.5 J cm-2 of 652-nm light (m-THPC). Control groups had either surgery only, surgery and Photofrin only, surgery and m-THPC only, or surgery and either 652-nm or 510-nm light.

After a 70-day observation period, the local recurrence rates were 25-26 per cent in the photodynamic therapy groups compared with 80-90 per cent in the control groups (P < 0.001).

AIOPDT using either Photofrin or m-THPC was equally effective in reducing local recurrence in this model. Since treatment time with m-THPC was only 25 s compared with 11 min with Photofrin, the use of the former new photosensitizer increases the practicality of AIOPDT and could enable the laser to be replaced with cheaper, less intense light sources.