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Folic acid antagonist. Other names: Folex, Mexate, Trexall.
Normal range: TDM generally performed to ensure plasma/serum concentration <1 μmol/L at 48 hours postinfusion and <0.1 μmol/L at 72 hours postdose
Methotrexate is an antineoplastic drug used solely or in combination with other antineoplastic drugs for the treatment of leukemia and other diseases.
Severe psoriasis, sarcoidosis, and granulomatosis have been treated with methotrexate in relatively low doses.
High-dose methotrexate (greater than approximately 20 mg/kg body weight) with citrovorum factor rescue have been used with favorable results in the treatment of osteogenic sarcoma, leukemia, non-Hodgkin lymphoma, lung cancer, carcinoma of the head and neck, and breast cancer.
The efficacy of methotrexate in the treatment of other tumors such as prostate cancer is being investigated.
Potential toxicity: Reported therapeutic and toxic ranges are dependent on dose and the time the sample is drawn postdose. Consult protocol to assess toxicity.
24 hours: >10 μmol/L
48 hours: >1.0 μmol/L
72 hours: >0.1 μmol/L
Immunoassay-based tests (EMIT, FPIA) for serum/plasma
Serum must be collected in tubes without serum separator gel.
Cells must be separated as soon as possible after collection.
Heparinized, EDTA, and fluoridated collection tubes for plasma are acceptable.
This assay measures the total (protein bound and free) levels of methotrexate in serum and plasma.
It is known that aminopterin and APA (a metabolite of methotrexate) cross-react significantly with the EMIT assay and less so with the FPIA.
Patients should be rebaselined when changing the methodology.
Samples are stable for up to 24 hours when refrigerated and protected from light.