Uridine triacetate (formally PN401) is an acetylated prodrug of uridine. Following oral administration, uridine triacetate is deacetylated by nonspecific esterases present throughout the body, yielding uridine in the circulation. Uridine triacetate under VISTOGARD trade name is a uridine replacement agent approved for the emergency treatment of fluorouracil or capecitabine overdose (regardless of the presence of symptoms) or early-onset severe or life-threatening cardiac or central nervous system (CNS) toxicity and/or early-onset unusually severe adverse reactions (eg, gastrointestinal [GI] toxicity and/or neutropenia) within 96 hours following the end of fluorouracil or capecitabine administration in adult and pediatric patients. Uridine competitively inhibits cell damage and cell death caused by fluorouracil. Fluorouracil is a cytotoxic antimetabolite that interferes with nucleic acid metabolism in normal and cancer cells. Cells anabolize fluorouracil to the cytotoxic intermediates 5-fluoro-2’-deoxyuridine-5’- monophosphate (FdUMP) and 5-fluorouridine triphosphate (FUTP). FdUMP inhibits thymidylate synthase, blocking thymidine synthesis. Thymidine is required for DNA replication and repair. Uridine is not found in DNA. The second source of fluorouracil cytotoxicity is the incorporation of its metabolite, FUTP, into RNA. This incorporation of FUTP into RNA is proportional to systemic fluorouracil exposure. Excess circulating uridine derived from VISTOGARD is converted into uridine triphosphate (UTP), which competes with FUTP for incorporation into RNA. Uridine triacetate is also approved for the treatment of hereditary orotic aciduria under XURIDEN trade name. Uridine triacetate provides uridine in the systemic circulation of patients with hereditary orotic aciduria who cannot synthesize adequate quantities of uridine due to a genetic defect in uridine nucleotide synthesis.

Cytarabine is a pyrimidine nucleoside analog. Cytarabine or cytosine arabinoside (Cytosar-U or Depocyt) is a chemotherapy agent used mainly in the treatment of cancers of white blood cells such as acute myeloid leukemia (AML) and non-Hodgkin lymphoma. It also has antiviral and immunosuppressant properties. Cytarabine is an antimetabolite antineoplastic agent that inhibits the synthesis of DNA. It is a cell cycle phase-specific, affecting cells only during the S phase of cell division. Intracellularly, cytarabine is converted into cytarabine-5-triphosphate (ara-CTP), which is the active metabolite. The mechanism of action is not completely understood, but it appears that ara-CTP acts primarily through inhibition of DNA polymerase. Incorporation into DNA and RNA may also contribute to cytarabine cytotoxicity. Cytarabine is cytotoxic to a wide variety of proliferating mammalian cells in culture.The drug has a short plasma half-life, low stability and limited bioavailability. Overdosing of patients with continuous infusions may lead to side effects. Thus, various prodrug strategies and delivery systems have been explored extensively to enhance the half-life, stability and delivery of cytarabine. Alternative, delivery systems of cytarabine have emerged for the treatment of different cancers. The liposomal-cytarabine formulation has been approved for the treatment of lymphomatous meningitis.

Cytidine is a substrate of the uridine-cytidine kinase and is a part nucleic acids. It can serve as a substrate for the salvage pathway of pyrimidine nucleotide synthesis, as a precursor of the cytidine triphosphate (CTP) needed in the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthetic pathway. Cytidine was also used under the brand name posilent in Germany for the treatment of muscular, accommodative, and nervous eye disorders.

5-methyl cytidine has been indicated in the controlling mechanism for the genetic change, which leads to cancer. Human APOBEC3A (A3A), a polynucleotide cytidine deaminase (PCD) with specificity for single stranded DNA, can extensively deaminate human nuclear DNA. It was discovered that A3A among all human PCDs can deaminate 5-methylcytidine in a variety of single stranded DNA substrates both in vitro and in transfected cells almost as efficiently as cytidine itself. As 5MeCpG deamination hotspots characterize many genes associated with cancer it was made suggestion that A3A is a major player in the onset of cancer.

Cytosine is a pyrimidine nucleobase, one of the five main bases of nucleic acids. In DNA and RNA cytosine is paired with guanine. Only small amounts of cytosine administered with food are incorporated in DNA. The majority of cytosine is synthesized de-novo starting from carbamoyl phosphate.

Cytarabine ocfosfate (commercial name: Starasid) is a prodrug having stearyl group attached to phosphoric acid at 5' position of arabinose moiety of cytosine arabinoside (Ara-C). This drug is given orally. The mode of action is in the inhibition of DNA synthesis after conversion to Ara-CTP as in Ara-C. The drug is metabolized in the liver, producing the intermediate metabolite, C-C3PCA which is converted to Ara-C gradually. This property results in the maintenance of relatively long time the blood Ara-C levels. This was proved to be active clinically against acute leukemia and MDS.