Thymidylate synthase inhibitors.

Abstract

Thymidylate synthase (TS, EC 2.1.1.45) is an ?#1 essential enzyme in pyrimidine biosynthesis. TS catalyses the methylation of deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP), using 5,10-methylenetetrahydrofolate as a co-substrate to donate methyl groups. TS is a cytoplasmic enzyme which consists of a dimer of two identical subunits of MW 38.5 kDa [1]. Two binding sites on the enzyme are recognized — the pyrimidine binding site and a pocket of the surface of the enzyme in which the folate co-substrate binds. The enzyme transfers a methyl group from the bound folate to the pyrimidine to form dTMP, followed by release of both products into the cytoplasm. This reaction provides the only endogenous source of dTMP available to cells and is the rate-limiting enzyme for the de novo synthesis of thymine nucleosides and nucleotides. Thus, TS plays an essential role in the synthesis and repair of DNA. The TS gene is located on chromosome 18p11.32 and consists of seven exons and eight introns. The expression of this gene is cell cycle dependent, in that the level increases sharply as cells enter S-phase, but it may also be proliferation dependent [2]. Thus, tumor cells have TS activities much above the level found in nonmalignant cells (see below). A high percentage of monosomic loss of the TS gene has been shown in some cancers, most notably colon carcinoma [3]. Because of its pivotal role in DNA metabolism TS, has long been a target for anticancer chemotherapy. Inhibition of TS leads to major disturbances in pyrimidine and folate metabolism. The desire to exploit this axis has led to the development of a number of TS inhibitors over several decades, which are either analogues of the pyrimidine substrate dUMP, or analogues of the folate cofactor. 5-Fluorouracil (5FU) is the classical pyrimidine analogue, which for several decades has been the mainstay of the treatment of various types of cancer including gastrointestinal (GI), breast and head and neck cancers [4]. Increasing investigation and development of these agents has occurred in recent years, which has maintained this group of drugs in the forefront of anticancer drug pharmacology and therapeutics. There are now a large range of TS inhibitors undergoing evaluation and clinical development (Figs. 1, 2 and 5). This chapter reviews recent developments in the understanding and clinical applications of FP and folate-based TS inhibitors. Readers requiring more background and historical data are referred to previous reviews of these agents [4–6].