Antimetabolites are a class of chemotherapy drugs that masquerade as the building blocks of DNA and RNA (purines and pyrimidines). Once the cancer cell incorporates these "false" molecules into its metabolic pathways, the synthesis of new genetic material is halted, or the resulting DNA is non-functional. These drugs are S-phase specific, meaning they are most effective during the active synthesis of DNA.

Common examples include 5-Fluorouracil (5-FU), used extensively in colorectal cancer, and Gemcitabine, a cornerstone in pancreatic cancer treatment. Because these drugs interfere with basic metabolic processes, they are often administered as continuous infusions or in cycles to ensure they are present when the maximum number of cancer cells are entering the S-phase. Technical data on the pharmacokinetics and half-lives of these agents can be found in the Chemotherapy Market analysis.

A notable advancement in this field is the use of oral prodrugs like Capecitabine, which is converted into 5-FU within the tumor itself by the enzyme thymidine phosphorylase. This "Intratumoral Activation" allows for higher concentrations of the drug at the cancer site while reducing systemic gastrointestinal toxicity. As precision medicine evolves, genetic testing for enzymes like DPD (Dihydropyrimidine dehydrogenase) is becoming standard to identify patients at high risk for severe adverse reactions to antimetabolites.