Methotrexate (MTX) is an established pharmaceutical agent used both as a potent chemotherapy drug and as a disease-modifying agent for chronic inflammatory conditions. Classified as an antimetabolite, its core action involves interfering with the body’s use of folic acid, a B vitamin necessary for cell growth and repair. In high doses, MTX targets rapidly multiplying cells, treating various cancers. At significantly lower, weekly doses, MTX is a standard treatment for autoimmune conditions like rheumatoid arthritis and severe psoriasis. This difference in therapeutic application is explained by two distinct mechanisms of action that depend heavily on the drug’s concentration.
Primary Mechanism: Folate Antagonism and Cell Division Inhibition
Methotrexate is a structural analog of folic acid, designed to mimic the vitamin’s molecular shape and disrupt its function. The drug’s primary cytotoxic effect is exerted through competitive inhibition of the enzyme Dihydrofolate Reductase (DHFR). DHFR converts inactive dihydrofolate (DHF) into its active form, tetrahydrofolate (THF).
MTX binds to DHFR with high affinity, blocking the enzyme’s active site and preventing this conversion. This blockage starves the cell of necessary THF cofactors, which are central to metabolic pathways. THF is essential for transferring single carbon units needed to synthesize purine and pyrimidine nucleotides.
Since purines and pyrimidines are the building blocks of DNA and RNA, preventing their synthesis interferes with the cell’s ability to replicate its genetic material and divide. This anti-proliferative effect is why high-dose MTX is used in oncology; it targets and destroys rapidly dividing cells, such as those found in tumors and certain leukemias.
Healthy cells that multiply quickly, including bone marrow cells, hair follicles, and the gastrointestinal lining, are also susceptible to this cytotoxic effect. The high-dose regimen is carefully controlled to maximize the impact on malignant cells while minimizing damage to healthy tissues. The overall effect is a halt in the cell cycle, particularly the S-phase where DNA synthesis occurs, leading to cell death.
Secondary Mechanism: Immunomodulation and Anti-inflammatory Effects
The mechanism by which low-dose Methotrexate treats chronic inflammatory diseases is largely independent of its cytotoxic action. In lower concentrations, MTX still interacts with the folate pathway, but its main therapeutic benefit stems from influencing the body’s inflammatory response. This secondary mechanism involves the accumulation of a specific metabolite that dampens immune cell activity.
Low-dose MTX indirectly leads to the buildup of aminoimidazole carboxamide ribonucleotide (AICAR) inside cells. The drug inhibits the enzyme AICAR transformylase (ATIC), a downstream enzyme in the folate pathway that utilizes AICAR. The resulting accumulation of AICAR triggers the anti-inflammatory effect.
Elevated intracellular AICAR levels inhibit other enzymes, notably adenosine deaminase, which regulates the body’s adenosine levels. This inhibition leads to a significant increase in the release of adenosine, a naturally occurring molecule, into the extracellular space. Adenosine acts as a powerful anti-inflammatory signaling molecule.
Once outside the cell, adenosine binds to specific receptors, primarily the A2A and A2B receptors, found on the surface of various immune cells. Activation of these receptors suppresses the function and proliferation of T-lymphocytes and other inflammatory cells. This binding reduces the production of pro-inflammatory cytokines, which drive the inflammation and joint damage seen in autoimmune diseases. This immunomodulatory effect is the dominant mode of action for MTX when used at low, weekly doses.
Mechanism-Based Toxicity and Management
The mechanisms that make Methotrexate effective also dictate its predictable side effects. Since its high-dose function inhibits cell division, the drug naturally targets the body’s most rapidly multiplying healthy cells, leading to toxicities in the bone marrow, gastrointestinal lining, and skin.
Toxicity often manifests as myelosuppression, which is the decreased production of blood cells in the bone marrow, leading to low white blood cell counts and platelet counts. Damage to the GI tract lining can cause mucositis, which is painful inflammation and ulceration of the mucous membranes. These effects are a direct consequence of DHFR inhibition, which prevents these cells from synthesizing new DNA.
Leucovorin Rescue
To manage and prevent severe toxicity, particularly following high-dose cancer therapy, a therapeutic strategy known as Leucovorin rescue is employed. Leucovorin, also known as folinic acid, is a reduced form of folate that is chemically distinct from the folic acid MTX competes with. Critically, Leucovorin does not require the DHFR enzyme to be converted into its active form.
By administering Leucovorin, clinicians bypass the MTX-blocked DHFR enzyme, providing healthy cells with the necessary active folate cofactors to resume DNA synthesis. This selective rescue allows normal cells to recover and proliferate, mitigating the toxic effects of MTX, while the drug continues to exert its anti-proliferative effect on tumor cells. In chronic, low-dose therapy for autoimmune diseases, standard folic acid supplementation is used to offset mild folate depletion and reduce common side effects like mouth sores and gastrointestinal distress.