ANALOGS OF DNA PRECURSORS; GENERAL CONSIDERATIONS
The synthesis of new DNA is an essential step in the replication of normal and malignant cells. Accordingly, the four bases that comprise DNA (the pyrimidines: cytosine, thymine; and the purines: adenine, and guanine) have provided a rational target for synthesis of analogues that inhibit the function of DNA, including its replication. These bases become active substrates for DNA synthesis through the attachment of deoxyribose sugars to form a deoxynucleoside. Three phosphate molecules must then be attached to the 5′-OH position of the nucleoside's sugar, forming a metabolically active deoxynucleotide. These synthetic reactions, which lead to formation of the triphosphates required for making DNA, occur within the cancer cell, as well as within normal proliferating tissues, such as bone marrow and epithelium.
Normal as well as tumor cells do not have to synthesize bases for DNA. They can take up certain bases (guanine and uracil) as well as nucleosides (deoxycytidine, thymidine, adenosine, guanosine) from the circulation. Alternatively, these bases or their nucleosides can be synthesized by tumor cells de novo, in a complex, multistep system of reactions. Many of the earliest effective anticancer agents were designed as analogs of these bases or nucleosides. These analogs are transported into cells and converted to active triphosphates by the same transporters and enzymes that activate physiologic bases and deoxynucleosides.
5-Fluoro-uracil (5-FU) and its prodrug, capecitabine (4-pentoxycarbonyl-5′-deoxy-5′-fluorocytidine), are central agents in the treatment of epithelial cancers. They have synergistic interaction with other cytotoxic agents, such as cisplatin or oxaliplatin, and with radiation therapy. As a component of adjuvant and anti-metastatic therapy, fluoropyrimidines have improved survival in patients with colorectal cancer (1).
MECHANISM OF ACTION AND RESISTANCE
The first agent of this class, 5-FU (Figure 1-1), was synthesized in 1956 by Heidelberger, based on experiments that demonstrated the ability of tumor cells to salvage uracil for DNA synthesis. Later work showed that 5-FU is converted by multiple different routes to an active deoxynucleotide, FdUMP, a potent inhibitor of thymidylate synthase (TS), and thereby, DNA synthesis (Figure 1-1).
Routes of activation (via TP and TK) and inactivation (via DPD) of 5-fluorouracil (5-FU). Note that TP is a reversible reaction.
The active product, FdUMP, forms a tight tripartite complex with TS in the presence of the enzyme's cofactor, 5-10-methylene tetrahydrofolic acid. It thereby blocks the conversion of dUMP to dTMP, a necessary precursor of dTTP (2). dTTP is one of four deoxynucleotide substrates required for synthesis of DNA. An exogenous folic acid source such as leucovorin (5-formyl-tetrahydrofolate) enhances formation of the TS-F-dUMP-folate complex and increases the response rate in patients with colon cancer (3).
5-FU also forms 5-FUTP, and becomes ...