Pemetrexed is a new-generation antifolate, approved for the treatment of mesothelioma and non-small cell lung cancer, currently being evaluated for the treatment of a variety of other solid tumors. Pemetrexed, is a folate analog metabolic inhibitor that exerts its action by disrupting folate-dependent metabolic processes essential for cell replication. In vitro studies have shown that pemetrexed inhibits thymidylate synthase (TS), dihydrofolate reductase (DHFR), glycinamide ribonucleotide formyltransferase (GARFT) and and to a lesser extent aminoimidazole carboxamide ribonucleotide formyltransferase (AICARFT), which are folate-dependent enzymes involved in the de novo biosynthesis of thymidine and purine nucleotides. Pemetrexed is taken into cells by membrane carriers such as the reduced folate carrier and membrane folate binding protein transport systems. Once in the cell, pemetrexed is converted to polyglutamate forms by the enzyme folylpolyglutamate synthetase. The polyglutamate forms are retained in cells and are inhibitors of TS and GARFT. Polyglutamation is a time- and concentration-dependent process that occurs in tumor cells and, is thought to occur to a lesser extent, in normal tissues. Polyglutamated metabolites are thought to have an increased intracellular half-life resulting in prolonged drug action in malignant cells.

Capecitabine is a fluoropyrimidine carbamate with antineoplastic activity. It is an orally administered systemic prodrug which is converted to 5-fluorouracil (5-FU). Both normal and tumor cells metabolize 5-FU to 5-fluoro-2’-deoxyuridine monophosphate (FdUMP) and 5-fluorouridine triphosphate (FUTP). These metabolites cause cell injury by two different mechanisms. First, FdUMP and the folate cofactor, N5-10-methylenetetrahydrofolate, bind to thymidylate synthase (TS) to form a covalently bound ternary complex. This binding inhibits the formation of thymidylate from 2’-deoxyuridylate. Thymidylate is the necessary precursor of thymidine triphosphate, which is essential for the synthesis of DNA, so that a deficiency of this compound can inhibit cell division. Second, nuclear transcriptional enzymes can mistakenly incorporate FUTP in place of uridine triphosphate (UTP) during the synthesis of RNA. This metabolic error can interfere with RNA processing and protein synthesis. Most common adverse reactions (≥30%) were diarrhea, hand-and-foot syndrome, nausea, vomiting, abdominal pain, fatigue/weakness, and hyperbilirubinemia. The concentration of 5-fluorouracil is increased and its toxicity may be enhanced by leucovorin.

Flucytosine (5-flucytosine, Ancobon) is an antifungal agent used for treatment of serious fungal infections caused by Candida or Cryptococcus. A fluorinated cytosine analog it was originally developed as an anti-tumor agent, but was found to be non-effective against tumors. Monotherapy with 5-FC is limited because of the frequent development of pathogens resistance. It is often used in in combination with amphotericin B. The severe side effects of 5-flucytosine include hepatotoxicity and bone-marrow depression. 5-fluorocytosine is a prodrug to the cytotoxic compound 5-fluorouracil. Although the exact mode of action is unknown, it has been proposed that flucytosine acts directly on fungal organisms by competitive inhibition of purine and pyrimidine uptake and indirectly by intracellular metabolism to 5-fluorouracil. Flucytosine is taken up by fungal organisms via the enzyme cytosine permease. Inside the fungal cell, flucytosine is rapidly converted to fluorouracil by the enzyme cytosine deaminase. Fluorouracil exerts its antifungal activity through the subsequent conversion into several active metabolites, which inhibit protein synthesis by being falsely incorporated into fungal RNA or interfere with the biosynthesis of fungal DNA through the inhibition of the enzyme thymidylate synthetase.

Floxuridine is a pyrimidine analog that acts as an inhibitor of the S-phase of cell division. This selectively kills rapidly dividing cells. Floxuridine is an anti-metabolite. Anti-metabolites masquerade as pyramidine-like molecules which prevents normal pyrimidines from being incorporated into DNA during the S phase of the cell cycle. Flurouracil (the end-product of catabolism of floxuridine) blocks an enzyme which converts cytosine nucleosides into the deoxy derivative. In addition, DNA synthesis is further inhibited because fluoruracil blocks the incorporation of the thymdine nucleotide into the DNA strand. Floxuridine is used for palliative management of gastrointestinal adenocarcinoma metastatic to the liver, when given by continuous regional intra-arterial infusion in carefully selected patients who are considered incurable by surgery or other means. Also for the palliative management of liver cancer (usually administered by hepatic intra-arterial infusion).Floxuridine first gained FDA approval in December 1970 under the brand name FUDR. The drug was initially marketed by Roche, which also did a lot of the initial work on 5-fluorouracil. The National Cancer Institute was an early developer of the drug. Roche sold its FUDR product line in 2001 to F H Faulding, which became Mayne Pharma.

Tegafur (INN, BAN, USAN) is a chemotherapeutic fluorouracil prodrug used in the treatment of cancers. It is a component of the combination drugs tegafur/uracil and tegafur/gimeracil/oteracil. UFT is an anticancer medication composed of a fixed molar ration (1:4) of tegafur and uracil. This drug is commonly used in the treatment of head and neck cancer, gastric cancer, colorectal cancer, hepatic cancer, gallbladder cancer, bile-duct cancer, pancreatic cancer, lung cancer, breast cancer, bladder cancer, prostatic cancer, or uterine cervical cancer. In the body, tegafur is converted into 5-fluorouracil (5-FU), the active antineoplastic metabolite. The mechanism of cytotoxicity of 5-FU is thought to be derived from the fact that 5-fluoro-deoxyuridine-monophosphate (FdUMP), the active metabolite of 5-FU, competes with deoxyuridine-monophosphate (dUMP), thereby inhibiting thymidylate synthase and subsequently DNA synthesis. Another active metabolite of 5-FU, 5-fluorouridine-triphosphate (FUTP) is integrated into cellular RNA, inhibiting RNA function. Uracil, when combined with tegafur, enhances the antitumor activity of 5-FU due to higher 5-FU concentrations in the tumor tissue versus normal surrounding tissue compared with tegafur alone. Uracil inhibits degradation of the released 5-FU. The combination of these two drugs enhances the antitumor activity of Tegafur.

Leucovorin is a compound similar to folic acid, which is a necessary vitamin. It has been around and in use for many decades. Leucovorin is a medication frequently used in combination with the chemotherapy drugs fluoruracil and methotrexate. Leucovorin is not a chemotherapy drug itself, however it is used in addition to these chemotherapy drugs to enhance anticancer effects (with fluorouracil) or to help prevent or lessen side effects (with methotrexate). Leucovorin is also used by itself to treat certain anemia problems when folic acid deficiency is present.

The dihydrochloride salt of nolatrexed, a water-soluble lipophilic quinazoline folate analog with antineoplastic activity. Nolatrexed occupies the folate binding site of thymidylate synthase, resulting in inhibition of thymidylate synthase activity and thymine nucleotide synthesis with subsequent inhibition of DNA replication, DNA damage, S-phase cell cycle arrest, and caspase-dependent apoptosis. This agent also exhibits radiosensitizing activity. Orphan designation of nolatrexed was granted in the Unites States of America for treatment of hepatocellular carcinoma.

1-(2-DEOXY-2-FLUORO-β-D-ARABINOFURANOSYL)URACIL (FAU) is a thymidine analog. In several cancer cell lines, FAU was phosphorylated intracellularly to its monophosphate, 1-(2-deoxy-2-fluoro--Darabinofuranosyl) uracil monophosphate (FAUMP), by thymidine kinase and methylated in the 5-position by thymidylate synthase to form the product, 1-(2-deoxy-2-fluoro- -D-arabinofuranosyl) 5-methyluracil monophosphate (FMAUMP). FAU strongly inhibits the growth of tumor cells with high thymidylate synthase activity. FAU had been in phase I clinical trial for the treatment of advanced solid tumors.

Folitixorin, a thymidylate synthase inhibitor is a substrate used by the enzyme methylenetetrahydrofolate reductase (MTHFR) to generate 5-methyltetrahydrofolate. Folitixorin was studied in clinical trials for the treatment of breast cancer, metastatic colorectal cancer and for the treatment of advanced pancreatic cancer. Folitixorin had been granted orphan drug status for the treatment of pancreatic cancer in both the U.S. and EU. However, further development of this drug was discontinued.

OSI-7904L is a liposomal formulation of the highly specific, noncompetitive thymidylate synthase inhibitor OSI-7904 (also known as GW1843, BW1843U89, and GS7904). The liposome formulation was developed to enhance the therapeutic index and dose schedule convenience of this potent antifolate compound. This drug was studied in phase II clinical trial in patients to treat head and neck cancer, gastroesophageal adenocarcinoma and advanced biliary cancer, but these studies were discontinued. As an example in case of OSI-7904L, was revealed that its activity was below a level of clinical relevance in advanced biliary tract cancer, providing only a small degree of disease stabilization.