title: Pharmacologic rationale for fluoropyrimidine-leucovorin combination: biochemical mechanisms.
abstract: Fluoropyrimidines, 5-fluorouracil (5-FU), and 5-fluorodeoxyuridine (FdUrd) are effective inhibitors of DNA synthesis. Inhibitory action is potentiated both in vitro and in vivo by the addition of leucovorin (LV; either the natural [6S] isomer or the mixture of the unnatural and natural [6R,S] isomers). The inhibition of DNA is subsequent to thymidylate synthase inhibition mediated by the formation of a covalently bound ternary complex consisting of 5-fluorodeoxyuridine monophosphate, 5,10-methylenetetrahydrofolate, and thymidylate synthase. In vitro exposure of cells to varying concentrations of [6R,S]LV or [6S]LV increases intracellular pools of the reduced folate cofactor. The effect of [6S]LV is both dose and time dependent. The in vitro effective concentration of [6S]LV is approximately 1 to 10 mumol/L, depending on cell type. While HCT-8 cells demonstrated optimal modulation of 5-FU and FdUrd cytotoxicity with 1 mumol/L of [6S]LV for 5 hours, the SE cells required higher doses of LV and longer exposure time. Thus, dependent on the cell type, higher concentration (10 mumol/L) and duration of exposure (greater than 5 hours) may be essential for an effective modulation of fluoropyrimidine action. Clinically, to achieve the concentration and conditions found optimal in an in vitro system, a dose of 500 mg/m2 by 5-hour infusion appears to be the present optimal dose and schedule of [6R,S]LV administration in combination with 5-FU.
authors.affiliation: Department of Experimental Therapeutics, Roswell Park Cancer Institute, Buffalo, NY 14263.
authors.forename: Z G
authors.initials: ZG
authors.lastname: Zhang
authors.forename: Y M
authors.initials: YM
authors.lastname: Rustum
mesh_terms: D002430:Cecal Neoplasms / Q000188:drug therapy / Q000378:metabolism; D004334:Drug Administration Schedule; D004357:Drug Synergism; D005467:Floxuridine / Q000378:metabolism*; D006801:Humans; D007078:Ileal Neoplasms / Q000188:drug therapy / Q000378:metabolism; D007680:Kidney Neoplasms / Q000188:drug therapy / Q000378:metabolism; D002955:Leucovorin / Q000008:administration & dosage / Q000737:chemistry / Q000494:pharmacology*; D013237:Stereoisomerism; D013763:Tetrahydrofolates / Q000737:chemistry / Q000378:metabolism; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D014407:Tumor Cells, Cultured
chemical_list: D013763:Tetrahydrofolates; D005467:Floxuridine; C013123:5,10-methylenetetrahydrofolic acid; D013940:Thymidylate Synthase; D002955:Leucovorin