Regarding antimetabolite chemotherapy agents, all of the following statements are correct EXCEPT:

Question

Accepted Answer

A. Cytarabine (ara-C) is a purine antimetabolite that is incorporated into DNA and RNA, causing chain termination, and is primarily used in acute leukemias. ## Classification of Antimetabolites

**Key Point:** Antimetabolites are divided into two main classes based on the nucleotide base they mimic: **pyrimidine antimetabolites** and **purine antimetabolites**.

### Pyrimidine Antimetabolites
- 5-Fluorouracil (5-FU)
- Cytarabine (ara-C)
- Gemcitabine
- Fluorodeoxyuridine (FdUrd)

### Purine Antimetabolites
- 6-Mercaptopurine (6-MP)
- 6-Thioguanine (6-TG)
- Pentostatin
- Fludarabine

## Analysis of Each Option

| Agent | Classification | Mechanism | Clinical Use |
|-------|----------------|-----------|---------------|
| 5-FU | Pyrimidine | Inhibits thymidylate synthase | Colorectal, breast, gastric cancers |
| Methotrexate | Folate antagonist | Inhibits DHFR → ↓ reduced folates | Leukemias, lymphomas, breast cancer |
| Cytarabine (ara-C) | **Pyrimidine** | Chain termination in DNA/RNA | Acute leukemias (AML, ALL) |
| Gemcitabine | Pyrimidine | Inhibits ribonucleotide reductase | Pancreatic, lung, breast cancers |

**High-Yield:** Cytarabine is a **pyrimidine analog**, not a purine antimetabolite. It is a deoxycytidine analog (cytidine derivative) and therefore belongs to the pyrimidine class. The stem incorrectly classifies it as a purine antimetabolite.

**Clinical Pearl:** Cytarabine is the drug of choice for acute myeloid leukemia (AML) and is often used in high-dose regimens. Its mechanism involves incorporation into DNA, causing chain termination and apoptosis of rapidly dividing cells.

**Warning:** Do not confuse purine and pyrimidine antimetabolites. Pyrimidines are 6-membered rings (cytosine, thymine, uracil); purines are 9-membered rings (adenine, guanine). Cytarabine mimics cytosine, making it a pyrimidine analog.

Answer

B. 5-Fluorouracil (5-FU) is a pyrimidine antimetabolite that inhibits thymidylate synthase and is commonly used in colorectal and breast cancers

Answer

C. Gemcitabine is a deoxycytidine analog that inhibits ribonucleotide reductase and is FDA-approved for pancreatic cancer, non-small cell lung cancer, and breast cancer

Answer

D. Methotrexate acts as a folate antagonist by inhibiting dihydrofolate reductase, leading to depletion of reduced folates required for nucleotide synthesis

Regarding antimetabolite chemotherapy agents, all of the following statements are correct EXCEPT:

Explanation

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