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Methotrexate

Brand names: Bendatrexat®, Lantarel®, Metex®, Neotrexat®, and numerous generics
Synonyme: MTX
German: Methotrexat

1 Definition

Methotrexate is an active substance that is part of the class of folic acid-antagonists which belong to the cytostatic agents.

2 Chemistry

The empirical formula for Methotrexat is: C20H22N8O5. It has a molar mass of 454,44 g/mol.

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3 Mechanism of Action

Being a folic acid antagonist, Methotrexate competitvely and reversibly inhibits the dihydrofolate reductase (DHFR) enzyme. MTX has close structural similarity to folic acid, the difference being a 4-NH2 group instead of a 4-OH group. After making contact with the dihydrofolate reductase enzyme, MTX acts as a false substrate – inhibiting the conversion of dihydrofolic acid to tetrahydrofolic acid.

Organisms need tetrahydrofolic acid in order to synthesize purine and thymidine so they can eventually perform the de novo synthesis of DNA and RNA. In addition methotrexat inhibits further enzymes, such as the thymidilate synthase which catalyzes metabolic steps of the pyrimidine-synthesis.

4 Indications

The therapeutic use of methotrexat is based on its ability to inhibit the proliferation of cells. Low dosages of MTX can be used to achieve immunosuppresion. For that use it ist the gold standard fort he basic therapy of rheumatoid arthritis (RA). Further autoimmune diseases for which MTX is administered are:

Significantly higher dosages are administered if MTX is used as a cytostatic agent, as is done for following malign tumors:

5 Undesired side effects

Methotrexate is mostly excreted via the kidneys. As long as the renal elimination is not limited, mucositis and myelosuppression normally should regress within two weeks after start of the therapy. When administering a high dosage therapy usually folic acid is added in order to antagonize the effect of MTX and reduced the undesired side effects.

6 Contraindications

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