Substance that competes with, replaces, or inhibits a specific compound within a cell, whose functioning is thereby disrupted. Because its structure resembles the compound's, it is taken up by the cell, but it does not react in the same way with the enzyme that acts on the usual compound. It may inhibit the enzyme or be converted into an aberrant chemical. Many antimetabolites are useful in treating disease, including sulfa drugs, which disrupt bacterial but not human metabolism for bacterial diseases, and others (e.g., methotrexate, 5-fluorouracil) for various cancers.
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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. However, because thymidine is used in DNA but not in RNA (where uracil is used instead), inhibition of thymidine synthesis via thymidylate synthase selectively inhibits DNA synthesis over RNA synthesis.
Due to their efficiency, these drugs are the most widely used cytostatics.