Methotrexate (rINN) (pronounced /mɛθɵˈtrɛkseɪt/), abbreviated MTX and formerly known as amethopterin, is an antimetabolite and antifolate drug used in treatment of cancer, autoimmune diseases and as an abortifacient in the induction of medical abortions. It acts by inhibiting the metabolism of folic acid. Methotrexate began to replace the more powerful and toxic antifolate aminopterin starting in the 1950s, and the two should not be confused. The drug was developed by Yellapragada Subbarao.
In 1947, a team of researchers led by Sidney Farber showed that aminopterin, a chemical analogue of folic acid developed by Yellapragada Subbarao Lederle, could induce remission in children with acute lymphoblastic leukemia. The development of folic acid analogues had been prompted by the discovery that the administration of folic acid worsened leukemia, and that a diet deficient in folic acid could, conversely, produce improvement; the mechanism of action behind these effects was still unknown at the time.
Other analogues of folic acid were in development, and by 1950, methotrexate (then known as amethopterin) was being proposed as a treatment for leukemia. Animal studies published in 1956 showed that the therapeutic index of methotrexate was better than that of aminopterin, and clinical use of aminopterin was thus abandoned in favor of methotrexate. In that same year, methotrexate was found to be a curative treatment for choriocarcinoma—a solid tumor, unlike leukemia, which is a cancer of the blood. The drug was then investigated as a treatment for many other cancers, alone or in combination with other drugs, and was studied for other, non-cancer indications in the 1970s. In 1988, it was approved by the U.S. Food and Drug Administration (FDA) to treat rheumatoid arthritis.
Mode of action:
Methotrexate competitively inhibits dihydrofolate reductase (DHFR), an enzyme that participates in the tetrahydrofolate synthesis. The affinity of methotrexate for DHFR is about one thousand-fold that of folate for DHFR. Dihydrofolate reductase catalyses the conversion of dihydrofolate to the active tetrahydrofolate. Folic acid is needed for the de novo synthesis of the nucleoside thymidine, required for DNA synthesis. Also, folate is needed for purine base synthesis, so all purine synthesis will be inhibited. Methotrexate, therefore, inhibits the synthesis of DNA, RNA, thymidylates, and proteins.
Methotrexate acts specifically during DNA and RNA synthesis, and thus it is cytotoxic during the S-phase of the cell cycle. Logically, it therefore has a greater toxic effect on rapidly dividing cells (such as malignant and myeloid cells, and GI & oral mucosa), which replicate their DNA more frequently, and thus inhibits the growth and proliferation of these non-cancerous cells as well as causing the side effects listed below.
Lower doses of methotrexate have been shown to be very effective for the management of rheumatoid arthritis, Crohn’s disease, and psoriasis. For the treatment of rheumatoid arthritis, patients should supplement their diet with folate. In these cases inhibition of dihydrofolate reductase (DHFR) is not thought to be the main mechanism, but rather the inhibition of enzymes involved in purine metabolism, leading to accumulation of adenosine, or the inhibition of T cell activation and suppression of intercellular adhesion molecule expression by T cells.
In cancer chemotherapyMethotrexate was originally used as part of combination chemotherapy regimens to treat many kinds of cancers. It is still the mainstay for the treatment of many neoplastic disorders including acute lymphoblastic leukemia.
Medical termination of pregnancyMethotrexate is commonly used (generally in combination with misoprostol) to terminate pregnancies during the early stages (i.e. as an abortifacient). It is also used to treat ectopic pregnancies. In the case of early missed miscarriage (particularly a blighted ovum), in which fetal demise has occurred but the body has not expelled the fetus, methotrexate may be used to help the body begin the miscarriage process.
Other usesIt has come into use as a treatment for some autoimmune diseases, including Myasthenia Gravis, polymyositis, dermatomyositis, inclusion body myositis, ankylosing spondylitis, Crohn’s disease, psoriasis, pustular psoriasis, psoriatic arthritis, rheumatoid arthritis, Wegener’s granulomatosis, Adult-Onset Still’s Disease, and scleroderma (see disease-modifying antirheumatic drugs). A parallel use with TNFα blockers, such as adalimumab, infliximab, or etanercept, has been shown to markedly improve symptoms.
It is also sometimes used to treat a rare condition called Behçet’s disease where it is taken weekly, along with folic acid daily. In the case of immune disorders, such as Behçet’s disease and rheumatoid disorders, it is believed that the clinical goal of the low dose methotrexate regimen is to inhibit AICAR transformylase, which leads to increased AICA ribose (AICAR transformylase’s substrate). The AICA ribose inhibits adenosine deaminase, resulting in a build-up of extracellular adenosine. Extracellular adenosine inhibits the expression of IL-2 receptors on circulating T-lymphocytes, causing a suppression of the immune system, and thus ameliorating the effects of the immune disorder.