What is the mechanism of action of methotrexate?

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Methotrexate Mechanism of Action

Methotrexate primarily works by inhibiting dihydrofolate reductase (DHFR), which blocks DNA synthesis and cellular replication, but its anti-inflammatory effects in rheumatic diseases operate through distinct mechanisms, most notably adenosine accumulation and JAK/STAT pathway inhibition. 1

Primary Mechanism: DHFR Inhibition

Methotrexate inhibits dihydrofolic acid reductase, preventing the reduction of dihydrofolates to tetrahydrofolates. This blocks one-carbon group transfer needed for:

  • Purine nucleotide synthesis
  • Thymidylate synthesis (dUMP to dTMP conversion)
  • DNA synthesis, repair, and cellular replication

This mechanism preferentially affects rapidly proliferating tissues (malignant cells, bone marrow, intestinal mucosa, keratinocytes in psoriasis), explaining its efficacy in cancer and psoriasis. 1

Anti-Inflammatory Mechanisms (Distinct from DHFR Inhibition)

The anti-inflammatory effects in rheumatoid arthritis and other inflammatory conditions do NOT primarily depend on folate antagonism, as evidenced by the fact that folic acid supplementation does not diminish therapeutic efficacy. 2, 1

Key Anti-Inflammatory Pathways:

1. Adenosine Accumulation (Most Widely Accepted)

  • After cellular uptake via SLC19A1 transporter, methotrexate undergoes polyglutamation by folylpolyglutamate synthase (FPGS)
  • Polyglutamated methotrexate inhibits aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC)
  • This causes intracellular adenosine accumulation
  • Adenosine engages extracellular receptors (ADORA A1 and A2a), triggering anti-inflammatory cascades 2

2. JAK/STAT Pathway Inhibition

  • Methotrexate suppresses STAT activation independently of DHFR
  • Comparable potency to ruxolitinib in suppressing JAK2 V617F-associated STAT5 phosphorylation
  • This likely explains immunosuppressive effects at low doses used in rheumatic diseases 3

3. Suppression of T Cell Activation and Adhesion Molecules

  • Dose-dependent suppression of ICAM-1 (adenosine and folate-dependent)
  • Suppression of cutaneous lymphocyte-associated antigen (CLA) (adenosine-independent)
  • Not mediated by lymphocyte apoptosis 4

4. Additional Mechanisms

  • Inhibition of NF-κB translocation
  • Altered cytokine profiles
  • Decreased nitric oxide production
  • Polyamine inhibition
  • Inhibition of DNA precursor uptake by mononuclear cells 1, 5

Clinical Implications by Indication

Rheumatoid Arthritis

The mechanism remains incompletely understood. Effects on articular swelling/tenderness appear within 3-6 weeks, but methotrexate does not induce remission or prevent radiologic progression/bone erosions—it only ameliorates inflammatory symptoms. 1

Psoriasis

The differential proliferation rate between psoriatic and normal skin makes keratinocytes particularly susceptible to methotrexate's antiproliferative effects via DHFR inhibition and thymidylate synthase inhibition. 2, 1

Osteosarcoma (High-Dose)

High-dose methotrexate may overcome resistance through:

  • Bypassing impaired active transport
  • Overcoming decreased DHFR affinity
  • Compensating for DHFR gene amplification
  • Addressing decreased polyglutamation 1

Important Caveats

  • Polyglutamated metabolites remain in tissues for extended periods, providing prolonged drug action
  • The relative contribution of each mechanism varies by tissue type and disease context
  • At low doses (rheumatic diseases), adenosine-mediated and JAK/STAT inhibition mechanisms predominate over folate antagonism
  • The complex, multifaceted mechanisms explain both therapeutic efficacy and diverse toxicity profile 5, 6

References

6
Research

Methotrexate--how does it really work?

Nature reviews. Rheumatology, 2010

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