What is the mechanism of action of methotrexate?

• A. Inhibits DHFR
• B. Increases DHFR activity
• C. Inhibits thymidylate synthase
• D. Activates DNA polymerase

Answer: (A)

Methotrexate acts by depriving cells of folate cofactors needed to make DNA. It competitively inhibits the enzyme dihydrofolate reductase (DHFR), preventing conversion of dihydrofolate to tetrahydrofolate. Without sufficient tetrahydrofolate, one-carbon transfer reactions necessary for thymidylate and purine synthesis falter, so DNA replication in rapidly dividing cells is slowed or halted. This is the reason methotrexate is effective in conditions with high cell turnover, including certain dermatologic diseases like psoriasis. The effect on thymidylate synthesis is a consequence of THF depletion rather than direct inhibition of thymidylate synthase. Activating DNA polymerase or increasing DHFR activity would actually promote DNA synthesis, not inhibit it, so those mechanisms don’t describe how methotrexate works.