Singapore Drugs Database

Indication

Methotrexate is indicated in the treatment of gestational choriocarcinoma, chorioadenoma destruens and hydatidiform mole. In acute lymphocytic leukemia, methotrexate is indicated in the prophylaxis of meningeal leukemia and is used in maintenance therapy in combination with other chemotherapeutic agents. Methotrexate is also indicated in the treatment of meningeal leukemia. Methotrexate is used alone or in combination with other anticancer agents in the treatment of breast cancer, epidermoid cancers of the head and neck, advanced mycosis fungoides (cutaneous T cell lymphoma), and lung cancer, particularly squamous cell and small cell types. Methotrexate is also used in combination with other chemotherapeutic agents in the treatment of advanced stage non-Hodgkin’s lymphomas. Methotrexate is indicated in the symptomatic control of severe, recalcitrant, disabling psoriasis. Methotrexate is indicated in the management of selected adults with severe, active rheumatoid arthritis (ACR criteria), or children with active polyarticular-course juvenile rheumatoid arthritis.

Mechanism of Action

Methotrexate anti-tumor activity is a result of the inhibition of folic acid reductase, leading to inhibition of DNA synthesis and inhibition of cellular replication. The mechanism involved in its activity against rheumatoid arthritis is not known.

Pharmacokinetics

Absorption

Oral absorption is dose dependent in adults and leukemic pediatric patients. In adults, peak serum levels are reached within one to two hours. At doses of 30 mg/m^2 or less, methotrexate is generally well absorbed with a mean bioavailability of 60%. At doses greater than 80 mg/m^2, the absorption of the doses is significantly less due to a saturation effect.

Distribution

* 0.18 L/kg [initial volume of distribution (Vd)] * 0.4 - 0.8 L/kg [steady state Vd] Methotrexate competes with reduced folates for active transport across cell membranes by means of a single carrier-mediated active transport process. At serum concentrations greater than 100 micromolar, passive diffusion becomes a major pathway by which effective intracellular concentrations can be achieved. Methotrexate does not cross the blood-brain-barrier.

Metabolism

Methotrexate undergoes hepatic and intracellular metabolism to polyglutamated forms which can be converted back to methotrexate by hydroxylase enzymes. These polyglutamates act as inhibitors of dihydrofolate reductase and thymidylate synthetase. A small amount of metabolism to 7-hydroxymethotrexate may occur at doses commonly prescribed. Furthermore, intestinal flora partially metabolizes methotrexate after oral administration.

Elimination

Clearance

Methotrexate clearance rates vary widely and are generally decreased at higher doses. Delayed drug clearance has been identified as one of the major factors responsible for methotrexate toxicity.

Toxicity

Symptoms of overdose include bone marrow suppression and gastrointestinal toxicity. LD₅₀=43mg/kg(orally in rat).

Active Ingredient/Synonyms

4-amino-10-methylfolic acid | 4-amino-N(10)-methylpteroylglutamic acid | Amethopterin | Methotrexat | Méthotrexate | Methotrexatum | Metotrexato | MTX | N-[4-[[(2,4-diamino-6-pteridinyl)methyl]methylamino]benzoyl]-L-glutamic acid | Methotrexate |