Azathioprine (Imuran)
A steroid-sparing immunosuppressant for autoimmune eye disease; carries an FDA Boxed Warning for malignancy. Many prescribers test TPMT and NUDT15 before starting to flag patients at high risk of severe myelosuppression.
Drug Class: Immunosuppressants
Azathioprine (brand name Imuran) is an immunosuppressant medication that reduces immune system activity. It is often used as a steroid-sparing agent, allowing patients to reduce their steroid dose while maintaining disease control.
FDA Boxed Warning - malignancy
Long-term immunosuppression with azathioprine increases the risk of malignancy, including lymphoma, skin cancers, and (in young patients with inflammatory bowel disease combined with anti-TNF therapy) hepatosplenic T-cell lymphoma. Use the lowest effective dose for the shortest time appropriate, and follow these precautions:
- Sun protection and annual skin checks for non-melanoma skin cancer
- Age-appropriate cancer screening as recommended by your primary care clinician
- Report any persistent swollen lymph nodes, unexplained weight loss, fevers, or night sweats
Critical drug interaction - allopurinol / febuxostat
Xanthine oxidase inhibitors (allopurinol, febuxostat) block the breakdown of azathioprine and can cause life-threatening bone marrow failure. If azathioprine and allopurinol must be combined, the azathioprine dose is typically reduced to about 25% of the usual dose - and only with very close monitoring. Tell every clinician about every medication you are on.
Key Takeaways
- Steroid-sparing immunosuppressant that takes 3-6 months for full effect
- FDA boxed warning for malignancy (lymphoma, skin cancer)
- TPMT and NUDT15 testing is commonly done before starting to flag high-risk patients; the FDA label recommends considering it (and a known deficient genotype mandates dose change). It does not replace ongoing CBC monitoring.
- Avoid combining with allopurinol or febuxostat without major dose reduction
- Regular blood monitoring (CBC, LFTs) is required for the duration of treatment
How It Works
Azathioprine is converted to active metabolites that interfere with DNA synthesis, particularly in rapidly dividing cells like immune cells. This reduces immune activity and inflammation.
Common Uses
- Myasthenia gravis
- NMO and MOGAD
- Giant cell arteritis-steroid-sparing
- Other autoimmune conditions
Dosing
- Usually 1-3 mg/kg daily
- Start low, increase gradually
- Takes 3-6 months for full effect
- TPMT testing recommended before starting
TPMT and NUDT15 Testing
TPMT and NUDT15 enzyme testing
Two genetic tests are commonly used to flag patients at high risk of severe bone marrow suppression on azathioprine:
- TPMT (thiopurine S-methyltransferase) - low or absent activity is more common in patients of European ancestry; can cause severe bone marrow suppression at standard doses.
- NUDT15 - variants are more common in patients of East Asian, South Asian, and Hispanic ancestry; predicts severe myelosuppression and is a CPIC Tier 1 actionable variant.
The FDA label recommends considering TPMT and NUDT15 evaluation - particularly in patients who develop severe myelosuppression or in those at higher risk by ancestry. CPIC and many specialists recommend testing before starting. Patients known to be deficient in either enzyme need substantial dose reduction or an alternative drug. Genetic testing does not replace ongoing CBC monitoring.
Side Effects
Common
- Nausea, vomiting
- Diarrhea
- Fatigue
- Increased infection risk
Serious (Monitor For)
- Bone marrow suppression (low blood counts)
- Liver problems
- Increased cancer risk (long-term)
- Severe infections
Monitoring
- Complete blood count-regularly (weekly initially, then monthly)
- Liver function tests
- Watch for signs of infection
- Report unusual bruising or bleeding
Frequently Asked Questions
Why does it take so long to work?
Azathioprine works by gradually reducing the immune cells that cause your condition. This process takes time-usually 3-6 months for significant effect.
Can I stop my steroids once I start this?
Not immediately. Azathioprine takes time to work. Your doctor will gradually taper steroids once azathioprine is effective.
Is this the same as chemotherapy?
Azathioprine is used in cancer treatment at higher doses, but for autoimmune disease, doses are lower. It still requires careful monitoring.
Medical Disclaimer: This information is for educational purposes only and does not replace professional medical advice.
References
For current U.S. drug labeling, contraindications, boxed warnings, pregnancy/lactation language, and formulation-specific dosing, check the official label databases and your prescriber's instructions.