5-Fluoroindole-3-acetic acid: a prodrug activated by a peroxidase with potential for use in targeted cancer therapy.

Indole-3-acetic acid and some derivatives are oxidized by horseradish peroxidase, forming a radical-cation that rapidly fragments (eliminating CO(2)) to form cytotoxic products. No toxicity is seen when either indole-3-acetic acid or horseradish peroxidase is incubated alone at concentrations that together form potent cytotoxins. Unexpectedly, 5-fluoroindole-3-acetic acid, which is oxidized by horseradish peroxidase compound I 10-fold more slowly than indole-3-acetic acid, is much more cytotoxic towards V79 hamster fibroblasts in the presence of peroxidase than the unsubstituted indole. The fluorinated prodrug/peroxidase combination also shows potent cytotoxic activity in human and rodent tumor cell lines. Cytotoxicity is thought to arise in part from the formation of 3-methylene-2-oxindole (or analogues) that can conjugate with thiols and probably DNA or other biological nucleophiles. Levels of the fluorinated prodrug in the murine carcinoma NT after intraperitoneal administration of 50 mg/kg were about 200 microM. Although these were 4-5-fold lower than plasma levels (which reached 1mM), the integrated area under the concentration/time curve in tumors over 2 hr was approximately 20 mM min, almost double the exposure needed to achieve approximately 90-99% cell kill in human MCF7 breast or HT29 colon tumor cell lines and CaNT murine cells in vitro, although the human bladder T24 carcinoma cell line was more resistant. The high cytotoxicity of 5-fluoroindole-3-acetic acid after oxidative activation suggests its further evaluation as a prodrug for targeted cancer therapy involving antibody-, polymer-, or gene-directed delivery of horseradish peroxidase or similar activating enzymes.