Methotrexate exerts its primary cytotoxic effect by inhibiting which enzyme, thereby blocking the synthesis of thymidylate and purine nucleotides?

Explanation

## Methotrexate Mechanism of Action **Key Point:** Methotrexate inhibits **dihydrofolate reductase (DHFR)**, blocking the conversion of dihydrofolate (DHF) to tetrahydrofolate (THF). This depletes the one-carbon pool required for both thymidylate and purine synthesis. ### Biochemical Mechanism ```mermaid flowchart TD A[Methotrexate]:::action --> B[Binds DHFR]:::action B --> C[Blocks DHF → THF conversion]:::action C --> D[Depletes one-carbon pool]:::outcome D --> E[Impaired dTMP synthesis]:::outcome D --> F[Impaired purine synthesis]:::outcome E --> G[DNA synthesis block]:::urgent F --> G ``` ### Why DHFR Inhibition is Central - **THF is a one-carbon donor** for both thymidylate synthase (dTMP synthesis) and AICAR transformylase (purine synthesis) - **Methotrexate polyglutamate** accumulates intracellularly and is retained, prolonging DHFR inhibition - **Selectivity:** Cancer cells have higher folate uptake and metabolism than normal cells **High-Yield:** Methotrexate is a **folate antagonist**, not a nucleotide analogue. Its effect is indirect — via enzyme inhibition, not incorporation into DNA/RNA. **Mnemonic:** **DHFR-MTX** = Dihydrofolate Reductase is the target of MethotrexAte. ### Rescue Strategy - **Leucovorin (folinic acid)** bypasses DHFR and replenishes the THF pool - Used after high-dose MTX to prevent toxicity to normal tissues **Clinical Pearl:** Resistance to methotrexate can arise from decreased DHFR expression, increased DHFR enzyme levels, or impaired cellular uptake via reduced folate carrier (RFC).