Provide a brief overview of methotrexate and high‑dose methotrexate, including their indications, mechanism of action, toxicities, and toxicity management, and summarize the use of leucovorin.

Methotrexate and High-Dose Methotrexate: Clinical Overview

Mechanism of Action

Methotrexate functions as a folic acid antagonist that inhibits dihydrofolate reductase, thereby blocking purine and pyrimidine synthesis and impairing DNA production in rapidly dividing cells. 1 This mechanism selectively affects proliferating cells such as B and T lymphocytes in inflammatory conditions and malignant cells in cancer treatment. 1

• The drug interferes with nucleotide synthesis by depleting intracellular tetrahydrofolate levels, which are essential for DNA replication and repair. 2
• At high doses, methotrexate penetrates cells through passive diffusion rather than active transport, allowing for selective antitumor effects when followed by leucovorin rescue. 3

Clinical Indications

Standard-Dose Methotrexate (7.5-25 mg weekly)

• Moderate to severe psoriasis in adults (strength of recommendation: A). 1
• Psoriatic arthritis for peripheral joint involvement, though less effective than TNF-inhibitors. 1
• Atopic dermatitis as an off-label use at 10-25 mg/week. 4
• Rheumatoid arthritis starting at 10-15 mg/week, escalating by 5 mg every 2-4 weeks up to 20-30 mg/week. 4
• Inflammatory bowel disease at 25 mg/week intramuscularly for induction, 15 mg/week for maintenance. 4

High-Dose Methotrexate (>500 mg/m²)

• Acute lymphoblastic leukemia for extramedullary infiltration and systemic consolidation. 5
• Osteosarcoma and other malignancies where selective tumor cell penetration is required. 3

Dosing Considerations

Methotrexate must be administered once weekly; daily dosing is absolutely contraindicated due to markedly increased hepatotoxicity risk. 6

• The weekly dose can be given as a single administration or divided into 2-3 doses over a 24-hour period to improve gastrointestinal tolerability. 1, 6
• Subcutaneous or intramuscular administration may be used and can improve bioavailability while reducing side effects. 1, 7
• A test dose should be considered in patients with impaired kidney function. 1

Toxicity Profile

Common Toxicities (occur in 25-32% of patients)

• Gastrointestinal symptoms (nausea, vomiting, diarrhea, stomatitis) are the most frequent adverse effects. 1, 4
• Fatigue, anorexia, and malaise typically occur around the time of methotrexate administration. 6
• Oral ulceration and mucositis. 6

Life-Threatening Toxicities

Myelosuppression accounts for 67 of 164 methotrexate-associated fatalities, making hematologic toxicity the most lethal complication. 6

• Bone marrow suppression manifesting as leukopenia, thrombocytopenia, and anemia. 1
• Pulmonary fibrosis accounts for 30 of 164 methotrexate-associated deaths. 6
• Hepatotoxicity potentially leading to fibrosis and cirrhosis with long-term use. 4
• Acute interstitial pneumonitis presenting with dyspnea, dry cough, and fever. 6

Risk Factors for Toxicity

• Renal insufficiency is the primary risk factor due to 85% renal excretion and impaired drug clearance. 6
• Advanced age (>50-70 years) significantly increases toxicity risk. 6
• Lack of folate supplementation. 6
• Hypoalbuminemia. 6
• Drug interactions, particularly with NSAIDs (reduce renal elimination), sulfonamides, and trimethoprim-sulfamethoxazole (absolutely contraindicated). 6

Toxicity Management

Immediate Management of Methotrexate Overdose

Leucovorin (folinic acid) must be administered immediately upon suspicion of methotrexate toxicity, as efficacy diminishes dramatically after 24 hours and is doubtful if initiated >24 hours after the last dose. 6, 2

Leucovorin Dosing Protocol

• Initial dose: 10-15 mg/m² (approximately 15 mg for average adult) IV or PO immediately. 6
• Subsequent doses: 15 mg IV or PO every 6 hours until methotrexate levels fall below 0.05 µmol/L or toxicity resolves. 6
• For high methotrexate levels or severe toxicity: up to 100 mg/m² IV if methotrexate level is unknown. 6
• The dose should equal or exceed the possible methotrexate ingestion. 2

Supportive Measures

• For recent oral ingestion (≥1 mg/kg within 1 hour): administer activated charcoal immediately. 6, 2
• Initiate aggressive IV hydration to enhance renal elimination. 6
• Perform urinary alkalinization with sodium bicarbonate to prevent methotrexate precipitation in renal tubules. 6

Management of Specific Toxicities

Hematologic Toxicity

• Withhold methotrexate if WBC <3.5×10⁹/L, neutrophils <2×10⁹/L, or platelets <100×10⁹/L. 6
• For severe neutropenia (<1×10⁹/L): administer filgrastim (G-CSF) 5 µg/kg subcutaneously daily. 6
• Monitor for sepsis with high vigilance, as mortality risk is substantial. 6

Hepatotoxicity

• Stop methotrexate if transaminases exceed 2× upper limit of normal on repeat testing. 6
• Repeat liver function tests in 2-4 weeks for persistent elevations. 1
• For patients with BMI >28 kg/m² or alcohol intake >14 drinks/week: perform transient elastography (FibroScan) screening. 6

Pulmonary Toxicity

• Obtain baseline chest x-ray for all patients starting methotrexate. 6
• Suspect pulmonary toxicity in patients presenting with dyspnea, dry cough, and fever. 6
• Discontinue methotrexate immediately; symptoms usually resolve with discontinuation. 6

Critical Drug Interactions

Trimethoprim-sulfamethoxazole is absolutely contraindicated with methotrexate due to severe bone marrow suppression risk. 6

• NSAIDs reduce renal elimination and are particularly dangerous with any dose of methotrexate. 6
• Other high-risk interactions: salicylates, sulfonamides, penicillins, probenecid, and colchicine. 6

Prevention of Toxicity

Mandatory Folate Supplementation

All patients on methotrexate must receive folic acid supplementation (1-5 mg daily, except on the day of methotrexate administration, or 5 mg weekly on a different day) to reduce gastrointestinal, hepatic, and hematologic toxicity. 1, 6

• Large doses of folic acid may reduce methotrexate efficacy, so dosing should be balanced. 1
• Folate supplementation does not compromise therapeutic efficacy at recommended doses. 6

Monitoring Requirements

Initial monitoring: CBC, liver function tests, and creatinine at weeks 2,4,8, and 12 after initiation. 6

• After stabilization: CBC and liver function tests every 3-6 months. 1, 6
• Renal function (creatinine, eGFR) every 2-3 months. 4
• Baseline chest x-ray for all patients. 6

Dose Adjustments for Renal Impairment

• eGFR >90 mL/min: normal dose. 4
• eGFR 20-50 mL/min: reduce dose by 50%. 4
• eGFR <20 mL/min: avoid methotrexate. 4

Leucovorin (Folinic Acid): Clinical Use

Mechanism of Leucovorin Rescue

Leucovorin bypasses methotrexate's metabolic block at dihydrofolate reductase by providing reduced folate cofactors directly to cells, allowing normal cells to resume DNA synthesis while tumor cells remain inhibited. 3, 8

• Leucovorin and its metabolites compete with methotrexate at folate-dependent enzymes, restoring cellular folate pools. 8
• Only leucovorin (folinic acid) is effective for rescue; folic acid cannot bypass the metabolic block and is ineffective for acute toxicity. 6

Indications for Leucovorin

High-Dose Methotrexate Protocols

• Leucovorin rescue is mandatory following high-dose methotrexate (>500 mg/m²) to prevent life-threatening toxicity in normal tissues. 3
• Standard rescue begins 24 hours after methotrexate infusion and continues until methotrexate levels are safe. 3

Methotrexate Overdose or Toxicity

• Leucovorin is life-saving and time-critical in methotrexate overdose; administer immediately upon suspicion. 6, 2
• Efficacy is maximal when given within 24 hours of methotrexate exposure. 6

Intrathecal Methotrexate

• Oral leucovorin 10 mg twice daily may be given starting on the day of intrathecal methotrexate and continuing for 3 days to mitigate systemic toxicity. 7

Leucovorin Dosing Calculation

For critical cases with retarded methotrexate elimination, leucovorin dose should be tenfold higher than the actual amount of methotrexate in the whole body system. 3

• Formula: Leucovorin (mg) = 10 × MTX (mg/L) × 0.76 × body weight (kg). 3
• Higher doses (up to 100 mg/m²) may be required for severe toxicity or elevated methotrexate levels. 6

Prevention vs. Rescue

Folic acid (1-5 mg daily) is used for prevention of chronic toxicity in patients on regular methotrexate therapy, while leucovorin (folinic acid) is used for acute rescue in overdose or high-dose protocols. 6, 7

• Do not confuse folic acid with folinic acid; only leucovorin is effective for acute toxicity management. 6
• During high-dose methotrexate treatment for ectopic pregnancy, avoid folic acid as it may counteract the therapeutic effect. 6

Contraindications

Absolute Contraindications

• Pregnancy and breastfeeding (methotrexate is a known teratogen). 1, 4
• Significant hepatic damage or cirrhosis. 4
• Severe renal impairment (eGFR <20 mL/min). 4
• Bone marrow suppression (anemia, leukopenia, thrombocytopenia). 4

Reproductive Considerations

Women must wait at least 3 months (some guidelines recommend 6 months) after discontinuing methotrexate before attempting to conceive; men should wait 3 months after discontinuation before attempting to father children. 6, 4

• If conception occurs while taking methotrexate, stop the drug immediately and initiate or continue folic acid supplementation. 6
• Termination of pregnancy is not mandatory; refer to an obstetrician to discuss teratogenicity risks. 6

Common Pitfalls to Avoid

• Delaying leucovorin administration: efficacy is doubtful if initiated >24 hours after last methotrexate dose. 6
• Administering folic acid instead of folinic acid for acute toxicity: only leucovorin bypasses methotrexate's metabolic block. 6
• Assuming low serum methotrexate levels exclude toxicity: treat based on clinical presentation, not levels alone. 6
• Prescribing daily instead of weekly dosing: this markedly increases hepatotoxicity risk. 6
• Failing to ensure mandatory folate supplementation: this is a common preventable risk factor for toxicity. 6
• Stopping antibiotics prematurely in neutropenic patients: methotrexate should remain discontinued until infection resolves. 6