A 52-year-old woman with metastatic colorectal cancer is being treated with 5-fluorouracil. Which pharmacological feature best distinguishes 5-FU from cytarabine (ara-C) among pyrimidine antimetabolites?

## Mechanism of Action: 5-FU vs. Cytarabine **Key Point:** Although both are pyrimidine antimetabolites, they exert cytotoxicity through fundamentally different mechanisms — 5-FU inhibits an enzyme (thymidylate synthase), while cytarabine is incorporated into DNA/RNA as a chain-terminating analog. ## Mechanistic Comparison Table | Feature | 5-Fluorouracil (5-FU) | Cytarabine (Ara-C) | |---------|----------------------|--------------------| | **Mechanism** | Enzyme inhibition (thymidylate synthase) | Chain terminator (DNA/RNA incorporation) | | **Active Form** | FdUMP (after activation) | Ara-CTP (triphosphate) | | **Target** | Blocks dTMP synthesis | Incorporated into DNA strand | | **Cell Cycle Phase** | S-phase dependent | S-phase dependent | | **Primary Toxicity** | Mucositis, diarrhea, hand-foot syndrome | Cerebellar toxicity (high dose), myelosuppression | | **Tumor Types** | Solid tumors (colorectal, breast, gastric) | Hematologic (AML, ALL, lymphoma) | | **Metabolism** | Requires OPRT activation | Requires deoxycytidine kinase | **High-Yield:** 5-FU is a **metabolic inhibitor** (prevents nucleotide synthesis); cytarabine is a **nucleotide analog** (becomes part of the DNA backbone and causes chain termination). **Mnemonic:** **FUDGE** = **F**luorouracil **U**ses **D**epletion of **G**uanine/dTMP synthesis (enzyme inhibition); **ARA** = **A**nalog **R**equires **A**ctivation and incorporation. **Clinical Pearl:** Cytarabine's dose-limiting toxicity at high doses is **cerebellar syndrome** (ataxia, dysarthria, nystagmus), whereas 5-FU's classic toxicity is **mucositis and diarrhea**. This difference in toxicity profile reflects their different mechanisms. ## Why Other Options Are Incorrect - **Option 1 (Correct):** Captures the essential mechanistic distinction. - **Option 2:** While 5-FU does cause mucositis and diarrhea more prominently, cytarabine also causes myelosuppression and GI toxicity at standard doses. This is not a reliable discriminator. - **Option 3:** This is a clinical use pattern, not a pharmacological feature. Both drugs can be used in various malignancies depending on context (e.g., 5-FU in some hematologic cancers, cytarabine in some solid tumors). - **Option 4:** Dosing schedule is a pharmacokinetic consequence of mechanism, not the mechanism itself.