OB/ PHARM | Methotrexate (WARNING)

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Methotrexate Overview and Indications Methotrexate is a folate antimetabolite used to treat malignant diseases (such as breast cancer, lymphoma, and leukemia) and nonmalignant autoimmune diseases (including rheumatoid arthritis, psoriasis, and juvenile idiopathic arthritis). Because of its high potential for severe and sometimes fatal toxicities, methotrexate must only be prescribed by an experienced clinician.

Mechanism of Action The drug competitively inhibits dihydrofolate reductase, an enzyme required to convert folic acid into reduced folates. This action severely blocks DNA synthesis, repair, and cellular replication, heavily impacting rapidly dividing tissues like cancer cells, bone marrow, and fetal cells. In autoimmune diseases, methotrexate acts as an immunosuppressant and anti-inflammatory primarily by inhibiting the enzyme AICAR transformylase, which selectively impairs T and B lymphocyte replication and alters adenosine concentrations.

Core 80/20 Safety Profile: Boxed Warnings & Severe Risks The vast majority of critical clinical management revolves around monitoring and preventing its severe, potentially fatal adverse reactions:

• Pregnancy & Teratogenicity: Methotrexate is strictly contraindicated in pregnant patients treated for nonmalignant diseases. It causes intrauterine fetal death and severe congenital anomalies, with studies showing a 42.5% rate of spontaneous abortion in exposed pregnancies. Both female and male patients must use effective contraception during treatment and for 3 to 6 months afterward due to risks of teratogenicity and chromosomal damage to sperm.
• Bone Marrow Suppression: The drug causes profound myelosuppression, including neutropenia and aplastic anemia, which can lead to fatal infections. This risk is unexpectedly heightened if administered concurrently with nonsteroidal anti-inflammatory drugs (NSAIDs).
• Hepatotoxicity: Methotrexate can trigger irreversible liver damage, fibrosis, and cirrhosis after prolonged use. It is strictly contraindicated in patients with alcoholism or chronic liver disease. Routine liver function monitoring is required, and high-risk or long-term patients (such as those with psoriasis) may require periodic liver biopsies or noninvasive fibrosis assessments.
• Pulmonary Toxicity: Potentially fatal acute or chronic interstitial pneumonitis can occur at any time and at any dose. A dry, nonproductive cough demands immediate investigation and potential drug discontinuation.
• Dermatologic Reactions: Patients may develop rapid, fatal skin reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis. It also sensitizes the skin to radiation, meaning patients can experience "recall" reactions from prior sunburns or radiation therapy.
• Renal Toxicity: Because the drug is cleared by the kidneys, impaired renal function causes dangerous drug accumulation. High-dose IV regimens can precipitate in renal tubules, causing acute renal failure; preventing this requires aggressive hydration and urine alkalinization (pH of 7 or higher).
• Gastrointestinal Toxicity: Methotrexate can cause severe ulcerative stomatitis, diarrhea, hemorrhagic enteritis, and fatal GI perforation. It must be used with extreme caution in patients with peptic ulcer disease or ulcerative colitis.