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antimetabolites

Sunday 12 March 2006

Definition: An antimetabolite is a chemical with a similar structure to a substance (a metabolite) required for normal biochemical reactions, yet different enough to interfere with the normal functions of cells, including cell division.

Antimetabolites can be used in cancer treatment, as they interfere with DNA production and therefore cell division and the growth of tumors.

Anti-metabolites masquerade as purine (azathioprine, mercaptopurine)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. They also affect RNA synthesis. Due to their efficiency, these drugs are the most widely used cytostatics.

Types

- folic acid analogues
- purine analogues
- pyrimidine analogues

Folic acid analogues

- methotrexate, a folic acid analogue, prevents the formation of tetrahydrofolate, essential for purine and pyrimidine synthesis, by inhibiting dihydrofolate reductase. This leads to inhibited production of DNA, RNA and proteins (as tetrahydrofolate is also involved in the synthesis of amino acids serine and methionine).

Purine analogues

- azathioprine is the main immunosuppressive cytotoxic substance. It is widely used in transplantations to control rejection reactions. It is nonenzymatically cleaved to 6-mercaptopurine that acts as a purine analogue and an inhibitor of DNA synthesis. By preventing the clonal expansion of lymphocytes in the induction phase of the immune response, it affects both the cell and the humoral immunity. It also successfully suppresses autoimmunity.

- mercaptopurine

- fludarabine inhibits function of multiple DNA polymerases, DNA primase, DNA ligase I and is S phase-specific (since these enzymes are highly active during DNA replication).

Pyrimidine analogues

- 5-fluorouracil (5-FU) which inhibits thymidylate synthase.

See also

- anti-cancer drugs