## Antimetabolites: Classification and Mechanism ### Pyrimidine Antagonists **Key Point:** 5-FU and cytarabine are pyrimidine antagonists, NOT purine antagonists. | Agent | Class | Mechanism | Primary Use | |-------|-------|-----------|-------------| | 5-Fluorouracil (5-FU) | Pyrimidine antagonist | Inhibits thymidylate synthase; incorporates into RNA | GI, breast, colorectal cancers | | Cytarabine (Ara-C) | Pyrimidine antagonist | Inhibits DNA polymerase; chain termination | Acute leukemias (AML, ALL) | | Methotrexate | Folate antagonist | Inhibits dihydrofolate reductase | Leukemias, lymphomas, solid tumors | ### Purine Antagonists **Key Point:** Mercaptopurine and thioguanine are purine antagonists used primarily in hematologic malignancies. | Agent | Mechanism | Use | |-------|-----------|-----| | Mercaptopurine (6-MP) | Converted to nucleotides; incorporated into DNA | ALL (maintenance), CML | | Thioguanine (6-TG) | Converted to nucleotides; incorporated into DNA | AML, ALL | ### Why the Incorrect Option is Wrong **Cytarabine is a PYRIMIDINE antagonist, not a purine antagonist.** It is a cytidine analog that inhibits DNA polymerase and causes chain termination. However, cytarabine is used almost exclusively in **acute leukemias** (AML and ALL), not solid tumors. The statement claims it is "primarily used in solid tumors," which is factually incorrect. **High-Yield:** Cytarabine is the drug of choice for acute myeloid leukemia (AML) and is given by continuous IV infusion or high-dose regimens. It is NOT used in solid tumors like colorectal or breast cancer. **Clinical Pearl:** The confusion often arises because both 5-FU and Ara-C are pyrimidine antagonists, but their clinical applications differ: 5-FU is used in solid tumors (GI, breast, colorectal), while Ara-C is reserved for hematologic malignancies. [cite:KD Tripathi 8e Ch 65]
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