3'-C-ethynylcytidine (Ethynylcytidine, TAS-106) is a synthetic cytidine nucleoside containing a covalently bound ethynyl group with potential antineoplastic and radiosensitizing activities. 3'-C-ethynylcytidine is metabolized in tumor cells to ethynylcytidine triphosphate (ECTP), which inhibits RNA synthesis by competitive inhibition of RNA polymerases I, II and III; subsequently, RNase L is activated, resulting in apoptosis. RNase L is a potent antiviral and antiproliferative endoribonuclease that cleaves singled stranded RNA, causes 28s rRNA fragmentation, and activates Janus Kinase (JAK), a mitochondrial-dependent apoptosis signaling molecule. Development of a cytosine derivative of 3'-ethynyl ribonucleoside, TAS-106, as an intravenously administered treatment for solid tumours was discontinued. A phase II trial in patients with head and neck cancer was terminated in the US.

THIOINOSINE (Methylmercaptopurine riboside, NSC- 40774) is a purine derivative with antineoplastic and anti-angiogenic properties. THIOINOSINE is readily converted in cells to its active form, 6-methylmercaptopurine ribonucleoside 5'- monophosphate (MMPR-P), by the enzyme adenosine kinase. THIOINOSINE inhibits amidophosphoribosyltransferase, the first committed step in de novo purine synthesis, and inhibits fibroblast growth factor-2 (FGF2)-induced cell proliferation. It has been used similarly to Mercaptopurine in the treatment of Leukemia. It has being tested in clinical trials for advanced pancreatic cancer.

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.

Talotrexin (also known as PT-523) was developed as a nonpolyglutamatable antifolate drug for the treatment of various types of tumors. It is known that antifolates are a class of cytotoxic or antineoplastic agents, which inhibit or prevent the maturation and proliferation of malignant cells. Talotrexin was studied in clinical trials for the treatment of brain and central nervous system tumors, leukemia, lymphoma, unspecified childhood solid tumor. However, this study was withdrawn because of toxicity. In addition, was studied in phase I/II multicenter clinical trial in patients with non-small-cell Lung carcinoma, this study was also withdrawn. The withdrawal was related to incidences of dose-limiting mucositis and myelosuppression. However, on May 22, 2006, was announced that the U.S. Food and Drug Administration has granted orphan drug designation for talotrexin in patients with acute lymphoblastic leukemia (ALL).

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.

5-Fluoro-2-deoxycytidine is a fluorinated pyrimidine analog antimetabolite with potential antineoplastic activity. As a prodrug, 5-fluoro-2-deoxycytidine is converted by intracellular deaminases to the cytotoxic agent 5-fluorouracil (5-FU). 5-FU is subsequently metabolized to active metabolites including 5-fluoro-2-deoxyuridine monophosphate (FdUMP) and 5-fluorouridine triphosphate (FUTP). FdUMP binds to and inhibits thymidylate synthase, thereby reducing the production of thymidine monophosphate, which leads to depletion of thymidine triphosphate and the inhibition of DNA synthesis and cell division. FUTP competes with uridine triphosphate (UTP) for incorporation into the RNA strand, which results in the inhibition of RNA and protein synthesis and cell proliferation. 5-Fluoro-2-deoxycytidine undergoing trials to test its effectiveness in treating cancer that has not responded to standard therapies.

Plevitrexed is a rationally designed, orally bioavailable, nonpolyglutamatable quinazoline antifolate that is a selective inhibitor of thymidylate synthase with potent antineoplastic activity and the ability to overcome antifolate resistance due to decreased folylpolyglutamate synthetase activity. It also features a lower toxicity than polyglutamatable thymidylate synthase inhibitors, presumably due to a lesser cellular retention. Plevitrexed is primarily transported into the cells via reduced folate carrier and selectively binds to the folate-binding site of thymidylate synthase with high affinity. Plevitrexed was assessed and in preclinical studies against a panel of human ovarian cancer cell lines and in several phase II clinical studies for the treatment of various solid cancers including colorectal, gastric, pancreatic and ovarian cancer. The combination of plevitrexed and carboplatin is well tolerated with no significant pharmacokinetic interaction between the two drugs - antitumor activity in platinum-pretreated gynecological malignancy was demonstrated.

Torcitabine is the beta-L-enantiomer of the natural nucleoside D-cytidine. The drug was under development as an antiviral agent for the treatment of chronic hepatitis B virus infection. Torcitabine has poor oral bioavailability, but its 3’,5’-derivative ester (val-L-dC) and the 3’-monovaline ester, valtorcitabine dihydrochloride, have excellent oral bioavailability and consequently the torcitabine prodrug, valtorcitabine, has replaced torcitabine in clinical development. Torcitabine is active against hepadnaviruses, specifically human hepatitis B virus (HBV), duck hepatitis virus (DHBV) and woodchuck hepatitis virus (WHV). Torcitabine triphosphate is a selective inhibitor of the polymerase enzyme of HBV.

KETOTREXATE is an antifolate developed to overcome methotrexate (MTX) resistance. However, it demonstrated such potential only in MTX-resistant sensitive L1210/FR8 leukemia cells and its clinical development was discontinued. Unlike MTX, KETOTREXATE exhibited minimal inhibition of purified dihydrofolate reductase, which implies that it does not act as a classical antifolate.

Lodenosine is the experimental HIV reverse transcriptase inhibitor. It was designed as a chemically and enzymatically stable anti-AIDS drug. A phase II trial of the nucleoside analog lodenosine was suspended after one participant died and others showed signs of liver or kidney damage.