Antitumor effect of 5-aminolevulinic acid-mediated photodynamic therapy can be enhanced by the use of a low dose of photofrin in human tumor xenografts

Practically all of the exogenous photosensitizers used for clinical photody namic therapy (PDT) target mainly vasculature. Although effective in tumor destruction, they also, unavoidably, induce phototoxicity of normal tissues . Porphyrins synthesized endogenously from 5-aminolevulinic acid (ALA) accu mulate within cells. Tumor eradication would be more efficient if both cell ular components and vascular stroma of a tumor could be targeted. Thus, PDT with a mixture of ALA and Photofrin (Pf, a vessel-targeted sensitizer) may simultaneously destroy the two elements. Using chemical extraction assays, pharmacokinetics of ALA and ALA-induced porphyrins were studied in the pla sma and tumors of nude mice bearing human WiDr and KM20L2 colonic carcinoma s after an i.p. injection of 250 mg/kg body weight of ALA. Subsequently, PD T efficacy of the two tumor models with ALA, Pf, or with the two drugs in c ombination was evaluated. The phototoxic effects on tumor cells in vitro wi th the combined drugs was also determined. Moreover, histological and ultra structural alterations of the treated tumors were investigated, and tumor c ell clonogenicity was assessed as a function of time after in vivo PDT usin g an in vitro colony formation assay. Finally, the photosensitivity of norm al skin tissue treated according to various protocols was compared. The amo unts of ALA peaked at 0.5 h after administration in both plasma and WiDr tu mor. The rates of ALA clearance seemed to follow a one-compartment model wi th half-lives of similar to 18 and 58 min in the plasma and tumor, respecti vely. About 100 and 60 times higher concentrations of ALA were needed to in duce a given concentration of porphyrins in the plasma and tumor, respectiv ely, although the plasma porphyrins may not only be released from blood cel ls but also from other organs. Similar kinetics of distribution patterns of ALA- and ALA methylester-induced porphyrins were found in the plasma and t umors, and the elimination rates were consistent with a two-compartment mod el. ALA induced much more porphyrins than ALA methylester in both plasma an d tumors. Tumors PDT-treated with ALA plus Pf at a low dose (I mg/kg) grew significantly more slowly than those treated with either of the drugs in bo th WiDr and KM20L2 models. However, the enhanced antitumor effect was not f ound in the tumor cells under in vitro conditions. Morphological studies de monstrated that PDT with the combined regimen resulted in necrosis of neopl astic cells and severe disruption of tumor microvasculature. This was suppo rted by the findings obtained from the studies of in vivo PDT and in vitro clonogenic assay that showed a progressive reduction in tumor cell viabilit y with times following PDT. Such a combined PDT protocol did not induce any phototoxicity in normal skin tissue. These data indicate that targeting bo th neoplastic cells and stroma with ALA and Pf (a low dose) can potentiate antitumor PDT effect with no risk of prolonged skin photosensitivity.