A 52-year-old man with newly diagnosed acute myeloid leukemia (AML) is started on induction chemotherapy with cytarabine and daunorubicin. On day 5 of treatment, he develops severe mucositis, diarrhea, and a white blood cell count of 0.8 × 10⁹/L. His liver function tests show AST 180 U/L and ALT 165 U/L. A bone marrow biopsy shows hypocellular marrow with no blasts. Which of the following best explains the hepatotoxicity and gastrointestinal toxicity observed in this patient?

Explanation

## Mechanism of Cytarabine Toxicity **Key Point:** Cytarabine (arabinosyl cytosine) is a pyrimidine antimetabolite that causes dose-limiting toxicity in rapidly dividing tissues, particularly the gastrointestinal tract and bone marrow. ### Cytarabine Metabolism and Mechanism 1. **Activation pathway:** - Cytarabine is phosphorylated by deoxycytidine kinase to cytarabine monophosphate (AraCMP) - Further phosphorylation yields cytarabine triphosphate (AraCTP) - AraCTP is the active form that inhibits DNA polymerase and ribonucleotide reductase 2. **Mechanism of action:** - AraCTP inhibits DNA synthesis by blocking DNA polymerase α and δ - It also incorporates into DNA, causing chain termination - Induces apoptosis in S-phase cells (rapidly dividing cells) ### Toxicity Pattern **High-Yield:** Cytarabine causes **predictable, dose-dependent toxicity** in tissues with high cell turnover: - Bone marrow (myelosuppression) - Gastrointestinal mucosa (mucositis, diarrhea) - Hepatocytes (transient elevation of transaminases) The hepatotoxicity is reversible and occurs because hepatocytes, though not rapidly dividing, are exposed to high drug concentrations and undergo apoptosis. The GI toxicity results from destruction of the intestinal epithelium, which has a high mitotic index. ### Why This Is NOT Deamination **Clinical Pearl:** Deamination by cytidine deaminase converts cytarabine to **uracil arabinoside (araU)**, which is an **inactive metabolite**. This is actually a **resistance mechanism**, not the cause of toxicity. The active form (AraCTP) is responsible for both therapeutic and toxic effects. ### Comparison with Other Antimetabolites | Drug | Mechanism | Toxicity Pattern | |------|-----------|------------------| | Cytarabine | Inhibits DNA polymerase | Myelosuppression, mucositis, hepatotoxicity | | Methotrexate | Inhibits DHFR, blocks dTMP synthesis | Myelosuppression, mucositis, nephrotoxicity | | 5-Fluorouracil | Inhibits thymidylate synthase | Myelosuppression, mucositis, hand-foot syndrome | | Mercaptopurine | Inhibits IMPDH, blocks GTP synthesis | Myelosuppression, hepatotoxicity | [cite:KD Tripathi 8e Ch 65] ## Why the Other Options Are Wrong **Option 0 (Deamination):** While cytarabine IS deaminated by cytidine deaminase, this produces **araU (uracil arabinoside)**, which is **inactive**. Deamination is a resistance mechanism, not the cause of toxicity. The active metabolite AraCTP is responsible for the observed effects. **Option 2 (Daunorubicin ROS):** Although daunorubicin does generate ROS and cause hepatotoxicity, it is NOT the primary cause of the **gastrointestinal mucositis** seen in this case. Daunorubicin's toxicity is more related to cardiotoxicity and cumulative dose effects, not acute mucositis. **Option 3 (Thymidylate synthase inhibition):** This is the mechanism of **methotrexate and 5-fluorouracil**, not cytarabine. Cytarabine acts by a different mechanism (DNA polymerase inhibition and chain termination). This is a common trap confusing different antimetabolite classes.