Drug Monograph
Full clinical overview, indications, dosage references & safety notes.
Overview
Azathioprine (Imuran®, Azasan®) is a purine-antagonist immunosuppressive agent used primarily in dogs for the management of immune-mediated and inflammatory diseases. It is most often incorporated into long-term treatment protocols when sustained immunosuppression is required.
In canine patients, azathioprine is commonly used as a second-line or adjunctive immunosuppressive drug, particularly in combination with corticosteroids. This combination allows dose reduction and decreased frequency of corticosteroid administration, helping to limit steroid-associated adverse effects.
Azathioprine has a delayed onset of action, and clinically meaningful immunosuppression may not be observed for several weeks after initiation. For this reason, it is frequently started alongside faster-acting agents, with daily dosing during the initial treatment phase.
Mechanism of Action (MOA): Azathioprine antagonizes purine metabolism by interfering with DNA and RNA synthesis, leading to impaired replication of rapidly dividing cells, particularly lymphocytes. It preferentially suppresses cell-mediated immunity and delayed hypersensitivity reactions, with comparatively less effect on humoral antibody responses.
Use of azathioprine requires careful monitoring due to its narrow safety margin and potential for serious adverse effects, particularly bone marrow suppression and hepatotoxicity. Because cats are highly sensitive to azathioprine-induced myelosuppression, its use is generally avoided in feline patients.
Indications
Azathioprine is used in dogs as an immunosuppressive agent for the management of a variety of immune-mediated and inflammatory diseases. Due to its delayed onset of action and potential toxicity, it is most commonly used as adjunctive or long-term therapy rather than as a sole initial treatment.
- Immune-mediated hemolytic anemia (IMHA): Frequently used in combination with corticosteroids as part of initial or early therapy to enhance immunosuppression and allow earlier steroid dose reduction.
- Immune-mediated thrombocytopenia (IMT): Used as an adjunct immunosuppressive agent in dogs with inadequate response to glucocorticoids alone.
- Immune-mediated polyarthritis: Used to control immune-mediated joint inflammation, often as part of a multi-drug immunosuppressive protocol.
- Inflammatory bowel disease (IBD): Used in dogs with moderate to severe disease that is refractory to dietary management and corticosteroids.
- Perianal fistulas (anal furunculosis): Used to reduce immune-mediated inflammation, typically in combination with corticosteroids.
- Myasthenia gravis (adjunctive therapy): Used in nonresponsive cases alongside anticholinesterase therapy and corticosteroids.
- Immune-mediated glomerular disease: Used as adjunctive immunosuppressive therapy to reduce immune-mediated renal injury.
- Chronic hepatitis (immune-mediated component): Used in combination with corticosteroids to reduce hepatic inflammation.
- Meningoencephalomyelitis of undetermined etiology (MUE): Used as part of combination immunosuppressive protocols with corticosteroids.
- Prevention of renal allograft rejection: Used in combination with cyclosporine in dogs receiving MHC-matched renal transplants.
Dosage (Reference)
Dog
In dogs, azathioprine is used exclusively as an extra-label immunosuppressive agent. Due to its delayed onset of action and potential for serious toxicity, careful dosing and intensive laboratory monitoring are required.
| Clinical use | Route | Dose | Notes |
|---|---|---|---|
| General immunosuppression | PO | 2 mg/kg every 24 hours |
Initial induction phase for 1–4 weeks. Clinical response may take 1–5 weeks to appear. |
| Maintenance immunosuppression | PO | 0.5–2 mg/kg every other day |
Begin after induction phase to reduce toxicity. Often alternated with corticosteroid dosing days. |
| IMHA / IMT (adjunctive therapy) | PO | 2 mg/kg or 50 mg/m² every 24 hours |
Given with immunosuppressive doses of glucocorticoids for 2–3 weeks, then reduced to every other day. |
| Inflammatory bowel disease | PO |
2 mg/kg every 24 hours × 2 weeks, then 2 mg/kg every other day, then 1 mg/kg every other day |
Improvement may take 2–6 weeks. |
| Immune-mediated polyarthritis | PO |
2 mg/kg every 24 hours × 2 weeks, then 0.5 mg/kg every other day |
Gradual taper required to avoid relapse. |
| Myasthenia gravis (adjunct) | PO | 2 mg/kg every 24 hours × 2 weeks, then every 48 hours |
CBC weekly for first month. Discontinue if WBC < 4,000 cells/µL or neutrophils < 1,000 cells/µL. |
| Perianal fistulas | PO |
2 mg/kg every 24 hours initially, then taper to every other day |
Long-term protocol may extend up to 12 months. Used with prednisone initially. |
| Chronic hepatitis (immune-mediated) | PO | 2.2 mg/kg every 24 hours |
Given with corticosteroids; reduce to every other day after 1–2 weeks. |
| MUE (with corticosteroids) | PO | 2 mg/kg every 24 hours × 2 weeks, then every other day | Part of combination immunosuppressive protocols. |
• Azathioprine has a delayed onset; do not judge efficacy too early.
• Myelosuppression often occurs after several months, not immediately.
• Dose tapering must be gradual to avoid disease relapse.
• Not recommended for use in cats due to severe myelosuppression risk.
Warnings & Precautions
Azathioprine is a potent immunosuppressive agent with a narrow therapeutic margin in dogs. Its use requires careful patient selection, strict adherence to dosing protocols, and intensive laboratory monitoring to minimize the risk of serious adverse effects.
- Species limitation: Use of azathioprine in cats is generally contraindicated due to marked sensitivity to myelosuppression related to low thiopurine methyltransferase (TPMT) activity.
- Bone marrow suppression: Myelosuppression is the principal toxicity and may manifest as leukopenia, thrombocytopenia, or poorly regenerative anemia. Effects are often delayed and may occur after weeks to months of therapy.
- Hepatic disease: Use cautiously in dogs with pre-existing liver disease. Hepatotoxicity may occur, and liver enzyme elevations can develop within the first several weeks of treatment.
- Renal disease: Use with caution in patients with renal dysfunction, as altered drug clearance may increase toxicity.
- Infection and neoplasia risk: Chronic immunosuppression increases susceptibility to opportunistic infections and may increase the risk of neoplastic disease with long-term use.
- Delayed onset of action: Clinical immunosuppressive effects may not be observed for up to 5 weeks; azathioprine should not be used as the sole initial therapy in life-threatening immune-mediated disease.
- Breed and individual variability: Dogs exhibit variable TPMT activity, which may influence both efficacy and risk of toxicity; unpredictable idiosyncratic reactions can occur.
- Drug name confusion: Prescription errors may occur due to confusion with azithromycin. Use of “tall man” lettering (azaTHIOprine vs aZITHROmycin) is recommended to reduce medication errors.
- Hazardous drug handling: Classified as a hazardous drug; appropriate personal protective equipment should be used when handling tablets or compounded formulations.
- Reproductive risk: Azathioprine is mutagenic and teratogenic and should be avoided during pregnancy whenever possible.
Drug Interactions
Azathioprine has several clinically important drug interactions in dogs, primarily related to additive immunosuppression, increased risk of hematologic toxicity, or interference with drug metabolism. When used concurrently with the following agents, careful monitoring and dose adjustment may be required.
- ACE inhibitors (e.g., benazepril, enalapril): Concurrent use may increase the risk of hematologic toxicity; monitor CBC closely.
- Allopurinol: May decrease hepatic metabolism of azathioprine, increasing toxicity. If used together, substantial dose reduction of azathioprine may be required.
- Aminosalicylates (e.g., sulfasalazine, mesalamine): Increased risk of azathioprine toxicity due to inhibition of thiopurine metabolism.
- Other myelosuppressive agents (e.g., cyclophosphamide, chlorambucil, sulfa–trimethoprim): Additive bone marrow suppression and increased hepatic toxicity may occur.
- Purine antagonists (e.g., mercaptopurine, mycophenolate, thioguanine): Increased risk of toxicity due to similar mechanisms of action.
- Vaccines (live or inactivated): Immunosuppressive effects may reduce vaccine efficacy and increase the risk of adverse vaccine reactions.
- Warfarin: Potential reduction in anticoagulant effect; monitor coagulation status if used concurrently.
- Nondepolarizing neuromuscular blockers: Azathioprine may reduce or reverse neuromuscular blocking effects.
Side Effects & Overdose
Side Effects
Adverse effects associated with azathioprine in dogs are primarily related to gastrointestinal irritation, bone marrow suppression, and hepatotoxicity. Many effects are unpredictable and may be idiosyncratic rather than strictly dose dependent.
- Bone marrow suppression: The most serious adverse effect. Manifestations include leukopenia (most common), thrombocytopenia, and poorly regenerative anemia. Myelosuppression typically develops after weeks to months of therapy.
- Gastrointestinal effects: Vomiting, anorexia, diarrhea, and general GI distress may occur, particularly early in therapy.
- Hepatotoxicity: Elevations in ALT and ALP may occur within the first several weeks of treatment. Clinical signs may include anorexia and diarrhea; most cases are reversible with drug discontinuation.
- Pancreatitis: Reported in dogs receiving azathioprine and may present with vomiting, abdominal pain, and lethargy.
- Infection risk: Immunosuppression may predispose patients to opportunistic or severe infections, especially with long-term therapy.
Overdose
Overdose or accidental ingestion of azathioprine may result in exaggerated gastrointestinal and hematologic toxicity. Reported toxicities are based on experimental data and poison control case reports.
- Clinical signs: Vomiting, anorexia, lethargy, thrombocytopenia, leukopenia, and increased liver enzyme activity.
- Initial management: Gastrointestinal decontamination (e.g., activated charcoal) may be considered following recent ingestion.
- Supportive care: Treatment is supportive and includes monitoring CBC and liver enzymes, managing GI signs, and treating secondary infections if present.
- Dialysis: Azathioprine is partially dialyzable (~45%) due to moderate protein binding.
- Consultation: For suspected overdose, consultation with a 24-hour veterinary poison control center is strongly recommended.
Key Notes
Practical clinical points specific to azathioprine use in dogs, highlighting considerations not duplicated elsewhere in this monograph:
- Slow tapering is essential: Abrupt dose reduction or discontinuation, particularly after disease control, may precipitate relapse of immune-mediated disease.
- Combination therapy advantage: Concurrent use with corticosteroids allows dose reduction of both drugs, improving long-term tolerability.
- Individual response variability: Both efficacy and toxicity vary widely between dogs, requiring individualized treatment plans.
- Long-term commitment: Therapy often extends for months, and in some cases years, necessitating consistent owner compliance and follow-up.
- Risk–benefit reassessment: Periodic evaluation of continued need for azathioprine is recommended, especially once disease remission is achieved.
- Owner safety awareness: Emphasize safe handling and storage at home due to the drug’s hazardous classification.
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