Folate Analogs at Oncogenic Doses: Methotrexate

Mechanism

Pharmacological Class and Group

Structural analogue antimetabolite of folic acid, phase-specific (S Phase).

Mechanism of Action

At intermediate or high oncologic doses (500 mg/m² to 12 g/m²), methotrexate actively penetrates the tumor cell via the reduced folate transporter (RFC-1). The compound undergoes an intramolecular polyglutamylation process that prevents its cytoplasmic release.

Blocking the Synthesis of Monocarbons and Purines

1. Inhibition of DHFR: It competitively binds to the enzyme Dihydrofolate Reductase (DHFR), blocking the reduction of dihydrofolate to tetrahydrofolate (THF).

2. Reduced Folate Depletion: Disruption of the folate cycle depletes intracellular pools of monocarbon cofactors essential for the biosynthesis of purine nucleotides and thymidylate:

Methotrexate DHFR THF Depletion Stopping Thymidylate and de novo Purine Synthesis

3. Arrest of DNA Synthesis: The lack of cofactors turns off the de novo synthesis of dATP, dGTP and dTTP, blocking the action of DNA polymerase and causing the arrest of the mitotic cycle with subsequent cell fragmentation due to apoptosis.

Pharmacokinetics

High Resolution Pharmacokinetics

• Distribution: Apparent volume of distribution equivalent to total body water. It accumulates markedly and for a long time in "third spaces" (ascitic fluid, pleural effusions), from where it is slowly and continuously released into the bloodstream, increasing the risk of profound systemic toxicity in the absence of rescue.
• Metabolism: It undergoes intracellular metabolism mediated by folylpolyglutamate synthase (FPGS), transforming it into stable, highly active polyglutamylated metabolites. The circulating drug is weakly metabolized to 7-hydroxymethotrexate in the liver.
• Elimination: Predominant renal clearance (>90%) through glomerular filtration combined with active tubular secretion mediated by the transporters OAT1 and OAT3.
• Half-life (t_1/2): It has triphasic kinetics, with a terminal elimination half-life of approximately 8 to 15 hours.

Indicators and dose

Clinical Indications and Frequent Regimens

• Acute Lymphoblastic Leukemia (ALL): Consolidation and systemic intensification, in addition to meningeal prophylaxis through intrathecal administration (12-15 mg in preservative-free solution).
• High Grade Osteosarcoma: Very high dose protocols (12 g/m²) with mandatory rescue with leucovorin.
• Primary Central Nervous System Lymphoma: Chemotherapy based on high-dose methotrexate combined with cytarabine to force penetration through the blood-brain barrier.

Critical Management of High Doses: Alkalinization and Rescue with Leucovorin

The administration of methotrexate at doses greater than 500 mg/m² requires a strict metabolic support protocol to avoid death due to acute renal failure and medullary aplasia:

1. Urinary Alkalinization: An alkaline diuresis should be forced by infusing sodium bicarbonate solution to strictly maintain urinary pH > 7.5. The solubility of methotrexate increases up to 10 times by raising the pH from 5.5 to 7.5, preventing its crystalline precipitation in the renal tubules.

2. Rescue with Leucovorin (Calcium Folinate): Mandatory administration of Leucovorin intravenously, starting 24 hours after starting methotrexate. Calcium folinate exogenously provides the reduced tetrahydrofolate necessary for the survival of healthy cells by bypassing the enzymatic blockade of DHFR:

Leucovorin Conversion to THF without using DHFR Restoration of Synthesis in Healthy Cells ("Rescue")

The dose of leucovorin is adjusted based on daily monitoring of plasma levels of methotrexate, continuing until serum levels of the drug are less than 0.05 - 0.1 µ M.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Moderate to severe renal dysfunction (Cl_Cr < 45 mL/min).
• Clinically relevant presence of massive pleural effusions or ascites (active third spaces).
• Pre-existing liver dysfunction or active cirrhosis.
• Pregnancy confirmed.

Relative Contraindications

• Concomitant use of drugs that interfere with the tubular secretion of organic acids (NSAIDs, salicylates, penicillins, PPIs).
• Severe uncorrected anemia or thrombocytopenia.

Adverse Effects (ADR) and Specific Toxicity

• Myelosuppression: Dose-limiting leukopenia and thrombocytopenia with nadir after 7-10 days.
• Nephrotoxicity: Nephropathy due to intratubular crystalline precipitation of methotrexate and its metabolite 7-hydroxy-MTX under conditions of acidic urinary pH, causing tubular necrosis and anuric acute renal failure.
• Gastrointestinal: Severe ulcerative oral mucositis and exfoliative enteritis.
• Hepatotoxicity: Transient elevation of transaminases with a self-limiting course; portal fibrosis or cirrhosis in sustained chronic therapies.