Phase III Multicenter Randomized Clinical Trial to Evaluate the Safety and Efficacy of CoFactor®/5-Fluorouracil/Bevacizumab Versus Leucovorin/5-Fluorouracil/Bevacizumab as Initial Treatment for Metastatic Colorectal Carcinoma

Folate Metabolism Intracellular reduced folates exists as a “pool” of 6 interconvertable forms. One of these forms, 5,10 methylenetetrahydrofolate (CH2THF), is the essential 1-carbon donor and reduced folate substrate for thymidylate synthase (TS), the enzyme that catalyzes the methylation of deoxyuridine-5 -monophosphate (dUMP) to deoxythymidine-5 -monophosphate (dTMP). This enzymatic pathway is critical because it provides the necessary nucleotide substrates for DNA synthesis, and for this reason, TS has been an important target for cancer chemotherapy (Figure 1).1 The fluoropyrimidine 5-fluorouracil (5-FU) exerts its cytotoxic activity, at least in part, through inhibition of TS. Specifically, it does so through the 5-FU metabolite fluorodeoxyuridine monopohosphate (FdUMP) with formation of a ternary complex made up of FdUMP, TS, and the reduced folate CH2THF, which then results in TS enzyme inhibition. Historically, leucovorin (LV; 5-formyltetrahydrofolate) has been used as a strategy to increase the intracellular reduced folate pools and increase TS enzyme inhibition. However, LV must be metabolized within the cell through multiple intracellular enzymatic steps to form CH2THF. In general, CH2THF levels are relatively low, in the range of approximately 0.1-0.5 mol/L in normal cells and even lower in human cancer biopsy tissues. Although treatment with LV results in an approximately 2-fold increase in CH2THF, maximum ternary complex formation is generally achieved at CH2THF concentrations approaching 12 mol/L.4 Individual tumor metabolism of LV to tetrahydrofolate and, ultimately, to CH2THF is unpredictable, and CH2THF levels are typically among the lowest of the activated intracellular reduced folate forms.5 However, it is clear that high intratumoral levels of CH2THF allow for maximal inhibition of TS.6-10 Preclinical and clinical investigations have consistently shown that resistance of tumors to 5-FU is associated, at least in part, with decreased intratumoral reduced folate levels, typically through decreased folylpolyglutamylation.11,12 Thus, direct administration of the essential reduced folate CH2THF in place of LV might offer significant advantages with respect to enhancing clinical activity.