Influence of Dose of [6RS]Leucovorin on Reduced Folate Pools and 5-Fluorouracil-mediated Thymidylate Synthase Inhibition in Human Colon Adenocarcinoma Xenografts

Abstract

Using preclinical models of human colon adenocarcinomas in immunedeprived mice, the influence of dose of [6RS]leucovorin ([6RS]LV, 20 to 1000 mg/m²) administered by 24-h i.v. infusion was determined on the following parameters: (a) plasma concentrations of the active [6S] and inactive [6R] isomers of [6RS]LV and the biologically active diastereoisomer of 5-methyltetrahydrolate (5-CH₃-H₄PteGlu); (b) expansion of intratumor pools of 5,10-methylenetetrahydrofolates (CH₂-H₄PteGlu_n) and tetrahydrofolates (H₄PteGlu_n), that may influence the binding of 5-fluorodeoxyuridylate to thymidylate synthase; (c) the distribution of polyglutamate forms of CH₂-H₄PteGlu_n and H₄PteGlu_n; and (d) (5-fluorouracil (FUra)-mediated thymidylate synthase inhibition in HxELC₂, HxGC₃, HxVRC₅, and HxHC₁ tumors. Folylpolyglutamate synthetase activities were also determined in each line. Linear increases in plasma concentrations of [6R]LV, [6S]LV, and 5-CH₃-H₄-PteGlu were determined over the complete range of [6RS]LV doses examined. However, in neoplastic tissues three patterns of biochemical modulation by [6RS]LV were evident. (a) In HxELC₂ and HxVRC₅ tumors, pools of CH₂-H₄PteGlu_n and H₄PteGlu_n were elevated in proportion to the dose of [6RS]LV between dose levels of 50 and 200 mg/m². Subsequent expansion of these pools continued that was disproportionate to the dose of [6RS]LV until no further increase was observed beyond 800 mg/m² [6RS]LV, at which point pools were maximally expanded by 4- to 4.5-fold. The extent of retardation of recovery of thymidylate synthase activity increased as the dose of [6RS]LV was increased in both tumors, when FUra (15 or 50 mg/kg), was administered by i.v. bolus injection 3 h into the 24-h infusion of [6 RS]LV. This was related to the increase in predominance of CH₂-H₄PteGlu_2–5 with increasing dose of [6RS]LV. (b) For HxHC₁ tumors, little expansion of CH₂-H₄PteGlu_n and H₄PteGlu_n pools (maximum, 137% of control) was detected at the highest dose levels of [6RS]LV, and no significant modulation of FUra-inhibited thymidylate synthase activity was detected, even at 1000 mg/m² [6RS] LV. CH₂-H₄PteGlu₅ remained similar or decreased as the dose of [6RS] LV was increased. (c) For line HxGC₃, pools of CH₂-H₄PteGlu_n and H₄PteGlu_n increased gradually from 169% of control at 20 mg/m² [6RS] LV to 233% of control at 1000 mg/m² [6RS]LV, and were intermediate between the expansion observed in HxHC₁ in comparison to HxELC₂ and HxVRC₅ tumors. CH₂-H₄PteGlu_3–5 were elevated at low dose levels of [6RS]LV. A greater extent of retardation of the recovery of thymidylate synthase following FUra administration was observed at a lower dose of [6RS]LV. No relationship was detected between the activity of folylpolyglutamate synthetase and the pattern of modulation of CH₂-H₄PteGlu_n species in tumors by [6RS]LV. The metabolic characteristics of modulation by [6RS]LV thus differed among the four colon xenograft lines providing a spectrum from little or no modulation to tumors in which pools could be markedly enhanced.