NCLEX Review Guide: Antimetabolite Medications in Hematological & Oncological Nursing

Mechanism of Action

• Antimetabolites are chemotherapy agents that interfere with DNA synthesis by mimicking natural metabolites required for cell division.
• These medications are cell cycle-specific, primarily targeting rapidly dividing cells during the S-phase of mitosis.
• They work by competing with normal metabolites for enzyme binding sites, ultimately preventing DNA replication and cell division.

Folate Antagonists

• Methotrexate (MTX) inhibits dihydrofolate reductase, preventing folate conversion needed for DNA synthesis.
• Used for leukemia, lymphoma, breast cancer, and autoimmune conditions like rheumatoid arthritis.
• Leucovorin (folinic acid) rescue therapy must be administered to prevent severe toxicity in high-dose protocols.

Purine Antagonists

• 6-Mercaptopurine (6-MP) and 6-Thioguanine interfere with purine metabolism and DNA synthesis.
• Primarily used in acute lymphoblastic leukemia (ALL) maintenance therapy.
• Metabolized by xanthine oxidase; avoid concurrent allopurinol use as it increases toxicity risk.

Pyrimidine Antagonists

• 5-Fluorouracil (5-FU) and Cytarabine (Ara-C) block thymidine synthesis.
• 5-FU is commonly used for colorectal, breast, and head/neck cancers.
• Cytarabine is the cornerstone treatment for acute myeloid leukemia (AML).

Critical Laboratory Values

1. Monitor complete blood count (CBC) for myelosuppression - nadir typically occurs 7-14 days post-administration.
2. Assess liver function tests (ALT, AST, bilirubin) as antimetabolites can cause hepatotoxicity.
3. Monitor renal function (BUN, creatinine) especially with methotrexate due to nephrotoxicity risk.
4. Check folate and B12 levels when administering folate antagonists long-term.

Hematological Toxicity

• Myelosuppression is the dose-limiting toxicity requiring frequent CBC monitoring and infection precautions.
• Implement neutropenic precautions when ANC <2,000/mm³ including private room, hand hygiene, and avoiding fresh flowers/fruits.
• Monitor for bleeding when platelets <50,000/mm³ and provide platelet transfusions as ordered.

Gastrointestinal Toxicity

• Mucositis and stomatitis commonly occur, requiring frequent oral care with soft-bristled toothbrush and alcohol-free mouthwash.
• Severe diarrhea can lead to dehydration and electrolyte imbalances requiring IV fluid replacement.
• Administer antiemetics prophylactically and provide small, frequent meals to manage nausea and vomiting.

Organ-Specific Toxicities

• Methotrexate pneumonitis presents as dry cough, dyspnea, and fever requiring immediate discontinuation.
• Cytarabine can cause cerebellar toxicity manifesting as ataxia, dysarthria, and nystagmus.
• 5-FU may cause hand-foot syndrome requiring dose modification and supportive care with moisturizers.