Valnivudine, also known as FV-100, a prodrug that has been indicated as a potential antiviral for the treatment of shingles (herpes zoster) that could both reduce the pain burden of the acute episode and reduce the incidence of post‐herpetic neuralgia compared to available treatments. Phase I clinical trial with FV100 showed safety and tolerability in healthy volunteers. Valnivudine also participated in phase III, where its efficacy was compared with valacyclovir; however, the study was terminated.

Mericitabine is a specific inhibitor of hepatitis C virus (HCV) replication that target NS5B polymerase. Mericitabine, a prodrug, is hydrolyzed in vivo to produce PSI-6130. It had been studied in phase II clinical trials for the treatment of chronic hepatitis C. However, while it showed a good safety profile in clinical trials, it was not sufficiently effective to be used as a standalone agent.

Piritrexim is a synthetic antifolate agent with antiparasitic, antipsoriatic and antitumor properties. Piritrexim inhibits the enzyme dihydrofolate reductase, thereby disrupting folate metabolism and DNA synthesis and cell division. A theoretical advantage of piritrexim over trimetrexate is a lack of any known effects on histamine metabolism, which may lower the risk of hypersensitivity reactions. Piritrexim is a nonclassical antifolate for antitumor and parasitic chemotherapy that passively diffuses into cells and hence do not have to depend on folylpoly-gamma-glutamate synthetase or the reduced folate carrier. Because piritrexim is not a substrate for polyglutamation, the drug is not selectively retained within cells for prolonged periods. Piritrexim has a reliably high oral bioavailability of about 75%, which has led to its development as an oral lipophilic antifolate. Most commonly, it has been administered in oral daily doses of 75 to 150 mg bid or tid every 5 days, with cycles repeated every 3 weeks. Oral absorption is rapid, with peak plasma levels appearing at 1.5 hours after ingestion. Elimination occurs primarily via hepatic metabolism of the drug to active metabolites, and the terminal half-life of the parent compound is about 1.5 to 4.5 hours. Single-agent oral piritrexim has clinical activity in melanoma, urothelial cancers, and head and neck cancers. Tolerable combinations of piritrexim with cisplatin, fluorouracil, and leucovorin have been tested, with promising results achieved in head and neck cancer. An interesting attempt to alternate piritrexim with methotrexate did not have any greater activity than methotrexate alone.

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.

Elacytarabine (CP-4055 or araC-5'elaidic acid ester) is a lipid-conjugated derivative of the nucleoside analog cytarabine. Elacytarabine blocks cell division and may kill cancer cells. It is a type of antimetabolite. It was studied in clinical trials for the treatment of solid tumors. In preclinical and clinical studies, elacytarabine has demonstrated both safety and efficacy in acute myeloid leukemia (AML), with noteworthy activity among the cytarabine-refractory AML population. Elacytarabine was granted orphan drug designation status from the European Commission in 2007 and from the US FDA in 2008, with a fast-track approval designation from the FDA in 2010. Elacytarabine development has been discontinued.

Troxacitabine is a synthetic nucleoside analogue. It is a poor substrate for nucleoside transporters and gains entry into cells by passive diffusion. Intracellular conversion to its active triphosphate form is via deoxycytidine kinase. Incorporation of this metabolite into DNA results in immediate chain termination and apoptosis induction. It is the first nucleoside analog with anticancer activity that has an unnatural stereochemical configuration. The dose-limiting adverse reactions were stomatitis and hand–foot syndrome.

Lometrexol, formerly known as DDATHF; LY 264618; T-64 was the first glycinamide ribonucleotide formyl transferase (GARFT) inhibitor to be investigated clinically. Lometrexol had been in phase II clinical trial for the treatment non-small cell lung cancer (NSCLC). However, the studies have been discontinued by Tularik Inc, because Company had suggested, that drug would face competition from other companies in the indication

CNDAC (TAK-109) is an analog of the nucleoside deoxycytidine with potential antineoplastic activity. CNDAC is incorporated into DNA and induces single-strand breaks, which are converted into double-strand breaks (DSBs) when cells go through a second S phase. This results in the cell cycle arrest in the S and G2/M phases, DNA fragmentation, and tumor cell apoptosis. Sapacitabine, a prodrug of CNDAC, is being developed by the US biotechnology company Cyclacel for the treatment of hemalogical cancers and solid tumors.

Valopicitabine is a nucleoside analog and the orally bioavailable prodrug of NM107 that competitively inhibits HCV NS5B polymerase, causing chain termination. Valopicitabine had been in phase II clinical trial for once-daily oral treatment of Hepatitis C virus infection. However, because of the overall risk/benefit profile of subjects undergoing clinical trials, further development of the drug has been temporarily placed on hold by the Swiss drug major Novartis and USA-based Idenix Pharmaceuticals company and the FDA.

Tezacitabine is a cytidine derivative patented by Merrell Dow Pharmaceuticals, Inc. as an antineoplastic and antiviral agent. Tezacitabine acts as irreversible ribonucleotide reductase inhibitor and DNA chain terminator. Tezacitabine shows as potent activity in a broad spectrum of tumor cell lines and in vivo tumor models, including human colon, prostate, and breast tumor xenografts, In clinical trials combination of Tezacitabine and 5-Fluorouracil exerts favor influences in patients with advanced solid tumors particularly in patients with esophageal and other gastrointestinal carcinomas.