Biochemical modulation of fluorouracil with leucovorin and interferon: preclinical and clinical investigations.

Leucovorin and interferon are capable of modulating the cytotoxicity of fluorouracil (5-FU). Preclinical studies demonstrate that d,l-leucovorin is rapidly metabolized in human breast and colon cells into the various one-carbon substituted folate pools and to the polyglutamated state. While increases in intracellular folate pools are proportional to the exposure concentration of leucovorin, relatively large increases in leucovorin concentrations (50- to 100-fold) are required to produce small intracellular changes (twofold). Polyglutamation is favored by prolonged exposures to leucovorin. Polyglutamate forms have a prolonged intracellular retention and a higher affinity for the target enzyme, thymidylate synthase. Ratios of up to 20:1 inactive to active leucovorin stereo-isomers had essentially no effect on the intracellular metabolism of the active isomer. Interferon gamma interacts with 5-FU in H630 colon cancer cells at the level of thymidylate synthase and enhances cytotoxicity of 5-FU by eliminating the 5-FU-induced acute overexpression of the target enzyme. No alterations in the intracellular metabolism or nucleic acid incorporation of 5-FU could be demonstrated with the addition of interferon gamma. A clinical trial combining interferon-alfa-2a (IFN-alpha-2a) (subcutaneous days 1 to 7) with 5-FU and leucovorin (given IV days 2 to 6) demonstrated that these agents could be combined with acceptable toxicity. While the addition of interferon did not allow dose escalation of 5-FU, it resulted in a significant increase in drug exposure (1.5-fold) compared with matched cycles of 5-FU plus leucovorin without interferon. The overall response rate in this pilot study of 13 untreated patients with gastrointestinal adenocarcinoma was 46%, including two complete responses. There were no responses in eight patients who had previously failed therapy with 5-FU.