XATMEP
Clinical safety rating: caution
Comprehensive clinical and safety monograph for XATMEP (XATMEP).
Methotrexate is a folate analog that inhibits dihydrofolate reductase, blocking the synthesis of tetrahydrofolate and thereby inhibiting DNA synthesis and cell proliferation. It also has immunosuppressive and anti-inflammatory effects through inhibition of purine metabolism and adenosine accumulation.
| Metabolism | Primarily hepatic metabolism via aldehyde oxidase and xanthine oxidase to 7-hydroxymethotrexate; minor metabolism via CYP450 enzymes. Polyglutamation occurs intracellularly, enhancing retention and activity. |
| Excretion | Methotrexate is primarily eliminated renally via glomerular filtration and active tubular secretion. Approximately 80-90% of the dose is excreted unchanged in urine within 24 hours. Fecal excretion is minimal (<10%), with biliary elimination accounting for a small fraction. |
| Half-life | The terminal elimination half-life of methotrexate is approximately 3-10 hours for low doses (<50 mg/m²) and 8-15 hours for high doses (≥500 mg/m²). Prolonged half-life (>24 hours) is associated with renal impairment and drug accumulation, increasing toxicity risk. |
| Protein binding | Approximately 50% bound to albumin, primarily to albumin with some binding to other proteins. Binding is saturable; elevated bilirubin or uremia can displace methotrexate, increasing free fraction. |
| Volume of Distribution | Volume of distribution is approximately 0.4-0.8 L/kg for low doses and up to 0.6-1.0 L/kg for high doses. Distribution is extensive into third-space fluid collections (pleural, ascitic), which can act as reservoirs, prolonging elimination. |
| Bioavailability | Oral bioavailability is dose-dependent: ~60% for doses <30 mg/m², decreasing to <20% for doses >80 mg/m² due to saturable absorption. Subcutaneous administration yields nearly 100% bioavailability with less variability. Intramuscular and intravenous routes provide complete bioavailability. |
| Onset of Action | Oral: Clinical effect (e.g., immunosuppression) begins within 3-6 weeks. Subcutaneous: Onset within 2-4 weeks. Intravenous: Onset within hours to days for antineoplastic effect; peak plasma concentrations are achieved immediately after IV bolus. |
| Duration of Action | Duration of action depends on dose and route. For low-dose weekly therapy (rheumatoid arthritis), clinical effects persist for about 1 week. For high-dose methotrexate (cancer), antineoplastic effects last 2-4 weeks with leucovorin rescue. Accumulation can lead to prolonged toxicity. |
Methotrexate 10 mg orally once weekly; maximum 25 mg per week.
| Dosage form | SOLUTION |
| Renal impairment | CrCl 30-50 mL/min: reduce dose by 50%; CrCl <30 mL/min: contraindicated. |
| Liver impairment | Child-Pugh Class A: caution; Class B or C: contraindicated. |
| Pediatric use | Weight-based dosing: 0.5-1 mg/kg orally once weekly; maximum 25 mg per week. |
| Geriatric use | Start at lower end of dosing range (5-7.5 mg/week) with careful monitoring for toxicity. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for XATMEP (XATMEP).
| Breastfeeding | Methotrexate is excreted into breast milk in low concentrations (M/P ratio approximately 0.08:1). However, due to potential for accumulation and toxicity in the neonate (including myelosuppression), breastfeeding is contraindicated during therapy. The American Academy of Pediatrics recommends avoiding breastfeeding. |
| Teratogenic Risk | XATMEP (methotrexate) is contraindicated in pregnancy. It is an FDA Pregnancy Category X drug. First trimester exposure causes 30-40% incidence of congenital malformations (CNS, skeletal, cardiac). Second and third trimester exposure risks fetal growth restriction, renal failure, and fetal demise. Folinic acid rescue does not eliminate risk. |
■ FDA Black Box Warning
Fatal or severe toxic reactions including myelosuppression, hepatic fibrosis/cirrhosis, renal failure, acute interstitial pneumonitis, and severe dermatologic reactions have been reported. Methotrexate elimination is reduced in patients with renal impairment, ascites, or pleural effusions. Use only in life-threatening neoplastic diseases, or in patients with severe, recalcitrant, disabling psoriasis or rheumatoid arthritis not responsive to other therapy. Methotrexate can cause fetal death or congenital anomalies when administered to a pregnant woman.
| Serious Effects |
["Hypersensitivity to methotrexate","Pregnancy","Breastfeeding","Severe renal impairment (CrCl <30 mL/min)","Severe hepatic impairment","Pre-existing severe bone marrow suppression","Active alcoholic liver disease","Immunodeficiency syndromes"]
| Precautions | ["Hepatotoxicity: monitor liver function tests; avoid in hepatic disease","Pulmonary toxicity: acute interstitial pneumonitis; monitor for cough/fever","Myelosuppression: monitor blood counts","Renal toxicity: reduce dose in renal impairment","Dermatologic reactions: exfoliative dermatitis","Gastrointestinal toxicity: stomatitis, diarrhea","Immunosuppression: increased risk of infection","Carcinogenicity: lymphoma risk","Concomitant NSAIDs increase toxicity","Pregnancy: contraindicated; use contraception"] |
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| Fetal Monitoring | In cases of inadvertent exposure: serial ultrasound for fetal anatomy and growth, fetal echocardiography at 18-22 weeks, and postnatal assessment for neurodevelopmental effects. Maternal monitoring includes CBC, LFTs, creatinine, and urinalysis at baseline and monthly during pregnancy. Folinic acid supplementation should be considered. |
| Fertility Effects | Methotrexate can cause reversible oligospermia and amenorrhea. In women, it may interfere with ovulation and implantation. Preconception counseling is recommended; men should discontinue therapy at least 3 months prior to conception to allow spermatogenesis recovery. |