Methotrexate exerts its cytotoxic effect by inhibiting which enzyme, leading to depletion of reduced folate cofactors required for nucleotide synthesis?

Explanation

## Mechanism of Methotrexate **Key Point:** Methotrexate is a structural analogue of folic acid that binds tightly to dihydrofolate reductase (DHFR), preventing the conversion of dihydrofolate to tetrahydrofolate (THF). This blocks the one-carbon transfer reactions essential for purine and pyrimidine synthesis. ## Folate Metabolism and Antimetabolite Action ```mermaid flowchart TD A[Folic acid]:::outcome --> B[Dihydrofolate reductase]:::action B --> C[Tetrahydrofolate THF]:::outcome C --> D[One-carbon transfer reactions]:::action D --> E[Purine synthesis]:::outcome D --> F[Pyrimidine synthesis]:::outcome D --> G[Amino acid metabolism]:::outcome H[Methotrexate]:::urgent --> B H -->|Inhibits| B B -->|Blocked| C C -->|Depleted| D D -->|Impaired| E D -->|Impaired| F ``` ## Comparison of Folate Antagonists | Drug | Target Enzyme | Mechanism | Clinical Use | |---|---|---|---| | Methotrexate | Dihydrofolate reductase | Competitive inhibitor | Leukemias, lymphomas, solid tumors, autoimmune | | Trimethoprim | Bacterial DHFR | Selective for bacterial enzyme | Bacterial infections (UTI, PCP prophylaxis) | | Pemetrexed | DHFR + thymidylate synthase | Multi-target folate antagonist | Lung cancer, mesothelioma | ## High-Yield Facts **High-Yield:** Methotrexate is a cell-cycle phase-specific agent (S-phase) that is widely used in both oncology and rheumatology. At high doses (>1 g/m²), it requires leucovorin (folinic acid) rescue to prevent severe toxicity. **Clinical Pearl:** Leucovorin (5-formyl THF) bypasses the DHFR block and replenishes intracellular THF pools, rescuing normal cells from methotrexate toxicity. This is why high-dose methotrexate is always followed by leucovorin rescue. **Mnemonic:** **DHFR-MTX** — Dihydrofolate Reductase is the target; Methotrexate blocks it, preventing THF regeneration and nucleotide synthesis.