Experimental photodynamic therapy with a copper metal vapor laser in colorectal cancer

In an attempt to define the best conditions for an adjunctive treatment of residual colonic microtumors by photodynamic therapy (PDT), an experimental model has been defined. S.c. HT29 colonic-cancer-cell tumors grown in nude mice were used and, 48 hr after i.p. administration of 30 mg/kg Photofrin (PH), laser illumination was performed with 75 or 150 Joules/cm2. The efficiencies of 2 lasers, the classically used rhodamine laser (RL) and a copper metal vapor laser (CMVL), were compared. The effects of PDT were assessed by histological and immunocytochemical (detection of a digestive enzyme, dipeptidyl-peptidase IV, as a marker of cell viability) follow-up and by the growth curve of the tumors after illumination. We conclude that, although the depth of necrosis resulting from PDT was nearly 3 mm at 75 J/cm2 and nearly 4-5 mm at 150 J/cm2 with both lasers, complete necrosis was obtained only with the CMVL at 150 J/cm2 (in 50% of the tumors). Under the other conditions, a layer of unaffected cells persisted at the pole opposite to laser illumination, resulting in growth curves lower than but parallel to those of the controls. Analysis of drug concentrations in the tumors and various organs, 48 hr after injection, i.e., at the time of laser illumination, revealed the presence of 21 micrograms/g dry weight PH in the tumors. The tumor vs. host-organ ratios were equal to or higher than 1 for the small bowel, colon, stomach, lung, skin and muscle. In contrast, the ratios were below 1 for the spleen, pancreas, kidney and liver.