Tubercidin, an adenosine analogue, is a nucleoside antibiotic. It is incorporated into DNA and inhibits polymerases, thereby inhibiting DNA replication and RNA and protein synthesis. This agent also exhibits antifungal and antiviral activities.

Trimetrexate, a second-generation folate antagonist which was used under brand name NEUTREXIN with concurrent leucovorin administration (leucovorin protection) was indicated as an alternative therapy for the treatment of moderate-to-severe Pneumocystis carinii pneumonia (PCP) in immunocompromised patients, including patients with the acquired immunodeficiency syndrome (AIDS). Nevertheless, this product was discontinued. In present time, trimetrexate with a different combinations is in the phase II of clinical trial for the treatment the following cancer diseases: pancreatic cancer and colorectal cancer (in combination with fluorouracil and leucovorin) and to treat a refractory acute leukemia in combination with leucovorin. Trimetrexate is a competitive inhibitor of dihydrofolate reductase (DHFR) from bacterial, protozoan, and mammalian sources. DHFR catalyzes the reduction of intracellular dihydrofolate to the active coenzyme tetrahydrofolate. Inhibition of DHFR results in the depletion of this coenzyme, leading directly to interference with thymidylate biosynthesis, as well as inhibition of folate-dependent formyltransferases, and indirectly to inhibition of purine biosynthesis. The result is disruption of DNA, RNA, and protein synthesis, with consequent cell death.

The nucleoside analog 2',3'-dideoxycytidine (ddCyd), also known as Zalcitabine is a nucleoside analog reverse transcriptase inhibitor (NRTI) sold under the trade name Hivid. HIVID is indicated in combination with antiretroviral agents for the treatment of HIV infection. It is used as part of a combination regimen with antiretroviral agents. But it was discontinued by Roche Pharmaceuticals on December 31, 2006 due to the availability of newer HIV medicines. Within cells, zalcitabine is converted to the active metabolite, dideoxycytidine 5'-triphosphate (ddCTP), by the sequential action of cellular enzymes. Dideoxycytidine 5'-triphosphate inhibits the activity of the HIV-reverse transcriptase both by competing for utilization of the natural substrate, deoxycytidine 5'-triphosphate (dCTP), and by its incorporation into viral DNA. The lack of a 3'- OH group in the incorporated nucleoside analogue prevents the formation of the 5' to 3' phosphodiester linkage essential for DNA chain elongation and, therefore, the viral DNA growth is terminated. The active metabolite, ddCTP, is also an inhibitor of cellular DNA polymerasebeta and mitochondrial DNA polymerase-gamma and has been reported to be incorporated into the DNA of cells in culture.

Azaribine (2′, 3′, 5′-triacetyl-6-azauridine) is the orally absorbable triacetylated derivative of the pyrimidine analog 6-azauridine. Ribonucleoside of 6-azauracil, which can be derived in the tissues by the deacetylation of azaribine, has been shown, after its intracellular conversion to 6-azauridine-5'-monophosphate, to serve as an inhibitor of the activity of a key enzyme, orotidine-5'-monophosphate (OMP) decarboxylase, critically concerned with the biosynthesis de novo of pyrimidines essential for the formation of nucleic acids. Azaribine exerts antineoplastic action. In 1975 azaribine was approved for the treatment of psoriasis. The drug was withdrawn because it may cause life-threatening or fatal blood clots in the veins and arteries.

Thioguanine is an antineoplastic anti-metabolite used in the treatment of several forms of leukemia including acute nonlymphocytic leukemia. Anti-metabolites masquerade as purine or pyrimidine - which become the building blocks of DNA. They prevent these substances becoming incorporated in to DNA during the "S" phase (of the cell cycle), stopping normal development and division. Thioguanine was first synthesized and entered into clinical trial more than 30 years ago. It is a 6-thiopurine analogue of the naturally occurring purine bases hypoxanthine and guanine. Intracellular activation results in incorporation into DNA as a false purine base. An additional cytotoxic effect is related to its incorporation into RNA. Thioguanine is cross-resistant with mercaptopurine. Cytotoxicity is cell cycle phase-specific (S-phase). Thioguanine competes with hypoxanthine and guanine for the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) and is itself converted to 6-thioguanilyic acid (TGMP), which reaches high intracellular concentrations at therapeutic doses. TGMP interferes with the synthesis of guanine nucleotides by its inhibition of purine biosynthesis by pseudofeedback inhibition of glutamine-5-phosphoribosylpyrophosphate amidotransferase, the first enzyme unique to the de novo pathway of purine ribonucleotide synthesis. TGMP also inhibits the conversion of inosinic acid (IMP) to xanthylic acid (XMP) by competition for the enzyme IMP dehydrogenase. Thioguanine nucleotides are incorporated into both the DNA and the RNA by phosphodiester linkages, and some studies have shown that incorporation of such false bases contributes to the cytotoxicity of thioguanine. Its tumor inhibitory properties may be due to one or more of its effects on feedback inhibition of de novo purine synthesis; inhibition of purine nucleotide interconversions; or incorporation into the DNA and RNA. The overall result of its action is a sequential blockade of the utilization and synthesis of the purine nucleotides. Thioguanine is used for remission induction and remission consolidation treatment of acute nonlymphocytic leukemias. It is marketed under the trade name Lanvis and Tabloid among others.

Idoxuridine is an antiviral agent use in keratitis caused by herpes simplex virus. As a prescription drug it comes as a 0.1% ophthalmic solution/drops (Herplex and Dendrid). The first studies of the compound for treatment of human herpes simplex started in early 1960s. Being a structural analog of thymidine idoxuridine inhibits viral DNA replication by substituting thymidine. The effect of idoxuridine results in the inability of the virus to reproduce and/or infect tissues. Idoxuridine also blocks viral thymidine kinase as its substrate analog.

Aminopterin is a synthetic derivative of pterins with antineoplastic and immunosuppressive properties. As a folate analog, aminopterin competes for the folate binding site of the enzyme dihydrofolate reductase, thereby blocking tetrahydrofolate synthesis, and resulting in depletion of nucleotide precursors and inhibition of DNA, RNA and protein synthesis. Aminopterin was marketed by Lederle Laboratories (Pearl River, New York) in the United States from 1953 to 1964 for the indication of pediatric leukemia. The closely related antifolate methotrexate was simultaneously marketed by the company during the same period. Aminopterin was discontinued by Lederle Laboratories in favor of methotrexate due to manufacturing difficulties of the former. During the period Aminopterin was marketed, the agent was used off-label to safely treat over 4,000 patients with psoriasis in the United States, producing dramatic clearing of lesions. The use of aminopterin in cancer treatment was supplanted in the 1950s by methotrexate due to the latter's better therapeutic index in a rodent tumor model. Now in a more pure preparation and supported by laboratory evidence of superior tumor cell uptake in vitro, aminopterin is being investigated in clinical trials in leukemia as a potentially superior antifolate to methotrexate.