Mechanism of action of Methotrexate

 Methotrexate (MTX) is an antimetabolite and antifolate drug primarily used to treat cancer, autoimmune diseases, and ectopic pregnancies. Its mechanism of action primarily involves inhibiting the enzyme dihydrofolate reductase (DHFR), which is crucial for DNA synthesis and cell division.

Mechanism of Action:

1. Inhibition of Dihydrofolate Reductase (DHFR):

   • Methotrexate competes with dihydrofolic acid for DHFR, an enzyme that reduces dihydrofolate (DHF) to tetrahydrofolate (THF).
   • Tetrahydrofolate is essential for the synthesis of purine nucleotides and thymidylate, both of which are necessary for DNA replication and cell division.
   • By inhibiting DHFR, methotrexate depletes tetrahydrofolate stores, thus preventing the synthesis of purines and pyrimidines, effectively halting DNA, RNA, and protein synthesis, particularly in rapidly dividing cells (such as cancer cells, immune cells, and fetal cells).

2. Inhibition of Thymidylate Synthase:

   • Methotrexate also inhibits thymidylate synthase, which is essential for converting deoxyuridine monophosphate (dUMP) to thymidine monophosphate (dTMP), another step critical for DNA synthesis.

3. Accumulation of Polyglutamated Methotrexate:

   • Methotrexate is converted into polyglutamate derivatives inside cells, which are more potent inhibitors of DHFR and also inhibit other enzymes involved in purine and pyrimidine metabolism.
   • These polyglutamated forms remain trapped inside cells, prolonging methotrexate’s effect and making it especially effective in tissues with rapid cell turnover (e.g., cancer cells, inflammatory cells).

Impact:

• Cancer Treatment: By inhibiting DNA synthesis, methotrexate prevents the proliferation of cancerous cells, particularly in rapidly growing cancers like leukemia, lymphoma, and solid tumors.
• Autoimmune Diseases: In lower doses, methotrexate suppresses immune cell activity, which is helpful in conditions like rheumatoid arthritis and psoriasis, where the immune system is overactive.
• Ectopic Pregnancy: Methotrexate can terminate ectopic pregnancies by inhibiting rapidly dividing trophoblastic cells.

Side Effects:

• Due to its effects on rapidly dividing cells, methotrexate can also affect normal cells, particularly in the bone marrow, gastrointestinal tract, and hair follicles, leading to side effects like myelosuppression, mucositis, and hair loss.