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Antimetabolites

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Antimetabolites

Key Points

  • Antimetabolites are cell-cycle-specific chemotherapeutic agents active during the S-phase (DNA synthesis).
  • They structurally resemble normal metabolites and interfere with DNA and RNA synthesis.
  • Subclasses include folate antagonists, pyrimidine analogs, and purine analogs.
  • Common agents include methotrexate, 5-fluorouracil, cytarabine, and 6-mercaptopurine.
  • Bone marrow suppression and GI toxicity are the primary dose-limiting adverse effects.

Mechanism of Action

Antimetabolites mimic the structure of normal cellular metabolites (folic acid, purines, or pyrimidines). When incorporated into metabolic pathways, they block enzymes essential for DNA and RNA synthesis or produce nonfunctional nucleic acids. Because they are cell-cycle-specific (S-phase), they most effectively kill rapidly dividing cells.

Subclasses

  • Folate antagonists: methotrexate — inhibits dihydrofolate reductase.
  • Pyrimidine analogs: 5-fluorouracil (5-FU), capecitabine, cytarabine — interfere with pyrimidine nucleotide synthesis.
  • Purine analogs: 6-mercaptopurine, fludarabine — disrupt purine pathways.

Nursing Considerations

  • Monitor CBC with differential before each treatment cycle; hold treatment for critically low counts.
  • Assess for stomatitis and mucositis; implement oral care protocols.
  • Monitor hepatic and renal function because many antimetabolites require dose adjustment in organ impairment.
  • Implement neutropenic precautions when absolute neutrophil count falls below 1,000 cells per microliter.
  • Use safe chemotherapy handling procedures (closed system transfer devices, protective equipment).
  • Assess hydration status; encourage adequate fluid intake to support renal clearance.

Side Effects and Adverse Effects

  • Hematologic: Bone marrow suppression (nadir typically 7 to 14 days after treatment).
  • GI: Nausea, vomiting, diarrhea, stomatitis, mucositis, anorexia.
  • Dermatologic: Alopecia, photosensitivity, hand-foot syndrome (5-FU, capecitabine).
  • Hepatic: Elevated liver enzymes, hepatotoxicity.
  • Immunologic: Increased infection risk from immunosuppression.

Health Teaching

  • Report fever, sore throat, unusual bleeding, or bruising immediately (signs of bone marrow suppression).
  • Perform gentle oral care with soft-bristle toothbrush; avoid alcohol-based mouthwash.
  • Use sun protection due to photosensitivity risk.
  • Maintain adequate hydration during treatment.
  • Avoid live vaccines and minimize exposure to infectious contacts.
  • Use reliable contraception during and after therapy as directed.

Self-Check

  1. Why are antimetabolites most effective against rapidly dividing cells?
  2. What is the typical timing of blood count nadir after antimetabolite administration?
  3. How do the three subclasses of antimetabolites differ in their mechanism?

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