Regeneration of tetrahydrohomofolate in cells: Possible basis for antitumor activity of homofolates

Abstract

The purpose of this study was to determine if reduction of homofolates to tetrahydrohomofolate in mice and rat tumors in vivo could form the basis for the antitumor activity of reduced homofolates. A single dose (400 mg/kg) of dihydrohomofolate from the optimal therapeutic range was given to animals bearing advanced tumors, and their spleen or tumor tissues were analyzed for dihydro- and tetrahydrohomofolate content by DEAE cellulose column chromatography. Mice bearing leukemia L1210/FR-8, which is responsive to reduced homofolates, showed a measurable reduction of dihydrohomofolate in their spleen, whereas the mice bearing a less responsive tumor (L 1210) or no tumor showed extensive metabolism and no detectable amount of dihydro- or tetrahydrohomofolate in their spleens. The drug was also extensively metabolized in rats bearing Walker carcinosarcoma 256 tumor, which might also explain the minimal response of the tumor to reduced homofolates. The reduction of the parent compound, homofolate, which is a minimally effective antitumor agent, was not detectable in mouse liver. These studies suggested an apparent correlation between the ability of the tumors to reduce homofolates to the tetrahydro level and their response to treatment with these drugs. Since the tetrahydro form of homofolate appears to be the active moiety, the levels of dihydrofolate reductase and catabolizing enzymes, and determinants of the sustained levels of the tetrahydro derivative in tumors might be the important factors in determining the responsiveness of tumors to homofolates.