Methotrexate Sodium — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.3-0.5 mg/kg	q48h	Duration: Varies; often used in combination protocols (e.g., COP protocol: 0.5-0.7 mg/kg PO once weekly) Notes: For autoimmune diseases, lower doses (0.1-0.3 mg/kg) may be used once weekly. Adjust based on response and toxicity.
Dog	IV	0.3-0.5 mg/kg	q48h or weekly	Duration: Varies; as part of chemotherapy protocols Notes: For lymphoma, often used at 0.5-0.7 mg/kg IV once weekly in combination protocols.
Cat	PO	0.5-0.75 mg/kg	q48h or weekly	Duration: Varies; as part of chemotherapy protocols Notes: Cats may be more sensitive to toxicity; start at lower end and monitor CBC.
Cat	IV	0.5-0.75 mg/kg	q48h or weekly	Duration: Varies; as part of chemotherapy protocols Notes: For lymphoma, often used at 0.5-0.7 mg/kg IV once weekly in combination protocols.
Horse	PO	0.1-0.2 mg/kg	q48h or twice weekly	Duration: Varies; for autoimmune diseases Notes: Limited data; use with caution and monitor for toxicity.
Horse	IV	0.1-0.2 mg/kg	q48h or twice weekly	Duration: Varies; for autoimmune diseases Notes: Limited data; use with caution and monitor for toxicity.

Clinical Indications & Species Uses

General Indications

Antineoplastic agent for various neoplasms
Immunosuppressive agent for autoimmune diseases

Dog (Canine)

Lymphoma (as part of combination chemotherapy protocols)
Leukemia
Osteosarcoma (adjunctive therapy)
Autoimmune diseases (e.g., immune-mediated polyarthritis, pemphigus, inflammatory bowel disease) as a steroid-sparing agent
Rheumatoid arthritis (rarely used)

Cat (Feline)

Lymphoma (as part of combination chemotherapy protocols)
Leukemia
Autoimmune diseases (e.g., immune-mediated hemolytic anemia, inflammatory bowel disease) as a steroid-sparing agent

Horse (Equine)

Autoimmune diseases (e.g., equine pemphigus foliaceus, immune-mediated keratitis) - limited use
Neoplasia (rarely used due to toxicity and cost)

Pharmacology & Mechanism of Action

Drug Class: Antimetabolite | Pharmacological Group: Folic acid antagonist

Mechanism of Action: Methotrexate is a structural analog of folic acid that competitively and reversibly inhibits the enzyme dihydrofolate reductase (DHFR). This inhibition prevents the reduction of dihydrofolate to tetrahydrofolate, a crucial cofactor in the synthesis of thymidylate, purines, and amino acids. Consequently, DNA, RNA, and protein synthesis are disrupted, leading to cell death, particularly in rapidly dividing cells. At higher doses, methotrexate also inhibits thymidylate synthetase directly and may exert immunosuppressive effects by inhibiting T-cell activation and cytokine production.

Pharmacodynamics: Methotrexate is cell-cycle specific, acting primarily during the S-phase of the cell cycle. Its cytotoxic effects are most pronounced in tissues with high rates of proliferation, such as bone marrow, gastrointestinal epithelium, and neoplastic cells. In addition to its antineoplastic activity, methotrexate has anti-inflammatory and immunomodulatory properties at lower doses, which are utilized in the treatment of autoimmune diseases. These effects are mediated through the release of adenosine, which has anti-inflammatory actions, and by altering cytokine profiles.

⚡ Pharmacokinetics Summary

Absorption: Oral absorption is dose-dependent and variable; at low doses (e.g., ≤15 mg/m²), absorption is generally good, but at higher doses, absorption becomes saturated and incomplete. In dogs, oral bioavailability is approximately 75% at low doses but decreases with higher doses. In cats, oral absorption is also variable. Parenteral administration (IV, IM, SC) ensures complete bioavailability.

Distribution: Methotrexate distributes widely into body tissues, with highest concentrations in the kidneys, liver, and spleen. It does not cross the blood-brain barrier in significant amounts at standard doses, but higher doses may achieve therapeutic concentrations in the CNS. It is approximately 50% protein-bound in plasma. It accumulates in third-space fluid collections (e.g., pleural effusions, ascites), which can prolong its half-life and increase toxicity.

Metabolism: Methotrexate undergoes minimal hepatic metabolism; it is primarily converted to 7-hydroxymethotrexate in the liver, which is less active. Intracellularly, it is polyglutamated, a process that retains the drug within cells and prolongs its action. Polyglutamated forms are particularly important in the therapeutic effect and toxicity.

Excretion: The primary route of excretion is renal, with approximately 80-90% of the drug excreted unchanged in the urine via glomerular filtration and active tubular secretion. Biliary excretion accounts for a small fraction. Renal impairment significantly reduces clearance and increases the risk of toxicity.

Half-Life: Dogs: 2-4 hours (low dose), 8-10 hours (high dose); Cats: 2-3 hours; Horses: 1-2 hours; Humans: 3-10 hours (low dose), 8-15 hours (high dose).

Bioavailability: Oral: 60-75% in dogs at low doses; variable in other species. Parenteral: 100%.

Protein Binding: Approximately 50% bound to plasma proteins, primarily albumin.

Available Formulations & Strengths

Oral Tablet 2.5 mg, 5 mg, 10 mg, 15 mg (PO)

Injectable Solution 25 mg/mL (2 mL, 4 mL, 8 mL, 10 mL vials) (IV, IM, SC, intrathecal (not commonly used in veterinary))

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to methotrexate or any component
Severe renal impairment
Severe hepatic impairment
Pregnancy or lactation (teratogenic and embryotoxic)
Bone marrow suppression (severe leukopenia, thrombocytopenia, anemia)
Active infections (immunosuppression may worsen)
Pleural effusions or ascites (may increase toxicity due to third-space accumulation)

Warnings & Clinical Precautions:

Methotrexate is a potent immunosuppressant; use with caution in animals with pre-existing infections.
Monitor complete blood count (CBC) and serum biochemistry regularly, especially renal and hepatic function.
Folic acid supplementation may reduce some toxicities (e.g., gastrointestinal) but may also reduce efficacy; use only if directed.
Nonsteroidal anti-inflammatory drugs (NSAIDs) may increase methotrexate toxicity; avoid concurrent use unless absolutely necessary.
Ensure adequate hydration to prevent renal toxicity.
Use with caution in geriatric animals or those with compromised organ function.
Handle with care; wear gloves when administering to avoid skin contact.
In food animals, observe withdrawal times; extra-label use is prohibited in some countries.

Adverse Effects & Reactions

Common:

Gastrointestinal signs: vomiting, diarrhea, anorexia
Bone marrow suppression: leukopenia, thrombocytopenia, anemia
Elevated liver enzymes
Oral ulceration (especially in cats)

Serious / Severe:

Severe myelosuppression leading to sepsis or hemorrhage
Hepatotoxicity (fibrosis, cirrhosis with chronic use)
Nephrotoxicity (acute renal failure)
Pulmonary toxicity (interstitial pneumonitis, pulmonary fibrosis)
Neurotoxicity (seizures, ataxia, especially with high doses or intrathecal administration)

Rare:

Dermatologic reactions (rash, alopecia)
Photosensitivity
Opportunistic infections (e.g., fungal, viral)
Teratogenicity
Anaphylaxis

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
NSAIDs (e.g., carprofen, meloxicam)	Reduced renal clearance of methotrexate, increasing risk of toxicity.	High
Trimethoprim-sulfonamide combinations	Additive antifolate effects, increasing risk of bone marrow suppression.	High
Penicillins	May reduce renal tubular secretion of methotrexate, increasing plasma levels.	Moderate
Probenecid	Decreases renal excretion of methotrexate, increasing toxicity.	High
Aspirin	May displace methotrexate from protein binding and reduce renal excretion.	Moderate
Folic acid supplements	May reduce efficacy of methotrexate; use with caution.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Severe bone marrow suppression (pancytopenia)
Gastrointestinal ulceration and hemorrhage
Acute renal failure
Hepatotoxicity
Neurological signs (seizures, coma)

Emergency Treatment Protocol: Immediate discontinuation of the drug. Administer leucovorin (folinic acid) rescue: typically 10-15 mg/m² IV or PO every 6 hours for 24-48 hours, or until serum methotrexate levels fall below 0.05 µmol/L. In severe cases, higher doses may be needed. Provide aggressive intravenous fluid therapy to enhance renal excretion. Consider alkalinization of urine (sodium bicarbonate) to increase methotrexate solubility. Monitor CBC, renal function, and liver enzymes closely. In cases of severe myelosuppression, consider colony-stimulating factors (e.g., filgrastim) and supportive care (antibiotics, transfusions).

Food Animal Withdrawal Times

Methotrexate is not approved for use in food animals. Extra-label use in food animals is prohibited in many countries due to the lack of established withdrawal times and potential for residues. If used, a conservative withdrawal period of at least 30 days for meat and 7 days for milk is suggested, but this is not officially established.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20°C to 25°C (68°F to 77°F); excursions permitted between 15°C and 30°C.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light. Keep in original container. Do not freeze. Store away from moisture.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; used extra-label. Human FDA-approved for various cancers and autoimmune diseases.

Extra-Label (Off-Label) Use: In the US, extra-label use of methotrexate in animals is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) provided there is a valid veterinarian-client-patient relationship, and the drug is not prohibited in food animals. However, methotrexate is not approved for veterinary use, and its use in food animals is generally discouraged due to lack of residue data. In the EU, similar regulations apply under Regulation (EC) No 470/2009.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Methotrexate is a potent antineoplastic and immunosuppressive agent used in veterinary medicine primarily for the treatment of lymphoma and autoimmune diseases. It is often used as part of combination chemotherapy protocols (e.g., COP: cyclophosphamide, vincristine, prednisone). Due to its narrow therapeutic index, careful patient selection and monitoring are essential. Baseline CBC, serum chemistry, and urinalysis should be performed before initiation and repeated regularly during therapy. Folic acid supplementation may be considered to reduce gastrointestinal and hematologic toxicity, but it may also reduce efficacy. In cats, methotrexate should be used with caution due to increased sensitivity to toxicity. Always ensure adequate hydration and avoid concurrent use of nephrotoxic drugs. In cases of overdose, prompt leucovorin rescue is critical. Due to its teratogenic effects, handling by pregnant individuals should be avoided. For food animals, extra-label use is not recommended due to lack of withdrawal data.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)