## Most Common Dose-Limiting Toxicity of Cytarabine **Key Point:** Myelosuppression is the dose-limiting and most common toxicity of cytarabine, occurring in nearly all patients receiving therapeutic doses. ### Mechanism of Cytarabine Toxicity Cytarabine (arabinosyl cytosine) is a pyrimidine antimetabolite that: 1. Is converted to cytarabine triphosphate by deoxycytidine kinase 2. Inhibits DNA polymerase and ribonucleotide reductase 3. Causes chain termination during DNA synthesis 4. Primarily affects rapidly dividing cells (bone marrow, GI tract) ### Toxicity Profile of Cytarabine | Toxicity | Frequency | Severity | Timing | |----------|-----------|----------|--------| | **Myelosuppression** | Very common (>90%) | Dose-limiting | 3–7 days post-dose | | Mucositis | Common (30–50%) | Moderate | 5–10 days | | Hepatotoxicity | Rare (<5%) | Mild to moderate | Variable | | Nephrotoxicity | Rare (<2%) | Mild | Variable | | Pulmonary toxicity | Rare (<1%) | Severe if occurs | Delayed | **High-Yield:** Myelosuppression manifests as neutropenia, thrombocytopenia, and anemia, requiring supportive care with G-CSF, transfusions, and prophylactic antibiotics during the nadir period (typically days 5–10). ### High-Dose Cytarabine (HD-AraC) Considerations **Clinical Pearl:** At doses ≥3 g/m² IV, cytarabine can cause **cerebellar syndrome** (ataxia, dysarthria, nystagmus) and **encephalopathy**, which are dose-limiting in high-dose regimens. However, at standard therapeutic doses used in AML induction, myelosuppression remains the primary dose-limiting toxicity. **Mnemonic:** **MACE** = Myelosuppression, Anemia, Cytopenia, Enteritis (common antimetabolite toxicities, with myelosuppression first). [cite:KD Tripathi 8e Ch 65]
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