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Pharmacology · single question

What is the mechanism of action of Methotrexate?

  1. Inhibits cell wall synthesis
  2. Intercalates into DNA strands
  3. Blocks DNA and RNA synthesis by inhibiting dihydrofolate reductase
  4. Blocks purine synthesis directly
Show answer and explanation

Correct answer: C. Blocks DNA and RNA synthesis by inhibiting dihydrofolate reductase

  • A. INCORRECT — Inhibition of cell wall synthesis is the mechanism of beta-lactam antibiotics (penicillins, cephalosporins), not Methotrexate.
  • B. INCORRECT — DNA intercalation is the mechanism of anthracyclines (doxorubicin), not Methotrexate.
  • C. CORRECT — Methotrexate is a folate antagonist that inhibits dihydrofolate reductase (DHFR), thereby blocking the synthesis of tetrahydrofolate, which is required for nucleotide synthesis. This ultimately inhibits DNA and RNA synthesis and halts cell division.
  • D. INCORRECT — While folate is needed for purine synthesis, Methotrexate's primary target is DHFR; direct purine synthesis inhibition describes drugs like 6-mercaptopurine.

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