Methotrexate

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Methotrexate.png
Methotrexate chemical structure


Methotrexate

N-[4-[[(2,4-diamino-6-pteridinyl)methyl]
methylamino]benzoyl]-L-glutamic acid
CAS number
59-05-2
ATC code
L01BA01 & L04AX03
Chemical formula C20H22N8O5
Molecular weight 454.4
Bioavailability variable
Metabolism hepatic
Elimination half-life 3-15 hours
(dose dependent)
Excretion renal
Pregnancy category X
Legal status Schedule 4 (Aust)
PoM (UK)
Delivery oral, IM, IV, intrathecal

Methotrexate (abbreviated MTX; formerly known as amethopterin) is an antimetabolite drug used in treatment of cancer and autoimmune disease. It acts by inhibiting the metabolism of folic acid.

Contents

Uses

Methotrexate 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.

More recently it has come into use as a treatment for some autoimmune diseases, including psoriasis, psoriatic arthritis, rheumatoid arthritis, and crohn's disease. In the case of rheumatoid arthritis, parallel use with infliximab or etanercept has been shown to markedly improve symptoms.

Although not licensed for this indication, methotrexate is also sometimes used (generally in combination with misoprostol) to terminate early pregnancies, particularly ectopic pregnancies.

It can be taken orally or administered by injection (intramuscular, intravenous or intrathecal). Although daily preparations are occasionally used, most patients take weekly doses, which decreases the risk of certain side-effects.

Adverse effects

Possible side effects can include anemia, neutropenia, increased risk of bruising, and nausea. A small percentage of patients develop hepatitis, while there is an increased risk of pulmonary fibrosis.

The higher doses of methotrexate often used in cancer chemotherapy can cause toxic effects to the rapidly-dividing cells of bone marrow and gastrointestinal mucosa. The resulting myelosuppression and mucositis are often prevented (termed methotrexate "rescue") by using folinic acid supplements (not to be confused with folic acid).

Methotrexate is a highly teratogenic drug and categorized in Pregnancy Category X by the FDA. Women must not take the drug during pregnancy, if there is a risk of becoming pregnant, or if they are breastfeeding. Men who are trying to get their partner pregnant must also not take the drug. To engage in any of these activities (after discontinuing the drug), women must wait until the end of a full ovulation cycle and men must wait three months.

Mode of action

Methotrexate inhibits dihydrofolate reductase, an enzyme that is part of the folate synthesis metabolic pathway. 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). Methotrexate, therefore, inhibits the synthesis of DNA, RNA, thymidylates, and proteins.

Methotrexate is cell cycle S-phase selective, and has a greater negative effect on rapidly dividing cells (such as malignant and myeloid cells), which are replicating their DNA, and thus inhibits the growth and proliferation of these cells.

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