Effect of Leucovorin on 5-Fluorouracil Induced Inhibition of Thymidylate Synthase in Patients with Colon Cancer

Abstract

5-Fluorouracil (5FU) is often used in the treatment of patients with advanced colorectal cancer [1], but most tumors show intrinsic or acquired resistance; the objective response rates are below 20%. Currently leucovorin (LV; 5-formyl-tetrahydrofolate) is added to this therapy leading to improved response rates generally between 30–40% [2]. Resistance against 5FU in patients may be related to a reduced inhibition of thymidylate synthase (TS) by FdUMP, the active metabolite of 5FU [1]. TS catalyzes the conversion of dUMP to TMP for which 5,10-methylene-tetrahydrofolate (CH2-THF) is the methyldonor. Inhibition of TS by FdUMP is mediated by the formation of a covalent ternary complex between FdUMP, TS and CH2-THF. The extent and retention of inhibition of TS, depending on the concentrations of FdUMP and CH2THF, are considered to be limiting factors. In addition, TS can have different characteristics in tumors of different patients, determining whether inhibition by FdUMP is effective or not. It has been demonstrated by others [3,4] and ourselves [5] that TS shows a very large heterogeneity in colorectal cancer; this includes a very large variation in enzyme activity (more than 20-fold) and variations in kinetic properties (affinity of FdUMP and of the natural substrate dUMP for TS). Also different affinities for the folate-cofactor have been demonstrated [6]. Spears et al [7] postulated that a number of these factors may determine resistance to inhibition by FdUMP. Several authors determined inhibition of TS in tumors of patients after treatment with 5FU [7–9]. However, generally the time of biopsies was limited to a few hr after 5FU administration, but never longer than 24 hr. Since a long retention may be a crucial factor in determining the antitumor effect we measured the inhibition of TS in tumors and normal tissues of patients 48 hr after treatment with 5FU. In addition we determined whether enhancement of the intra-tumoral folates pools by administration of LV would lead to a more pronounced inhibition of TS. We not only measured TS with the FdUMP ligand bindings assay but also with the catalytic assay, i. e. the conversion of dUMP to TMP.KeywordsLeucovorin CalciumHuman Colon Tumor CellCatalytic AssayHuman Colon Tumor Cell LineXenografted Human Colorectal CancerThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.