Other Immunosuppressants

Mycophenolate and azathioprine both suppress lymphocytes, but one has a life-threatening drug interaction with allopurinol that can cause fatal bone marrow failure. Do you know which?

Mycophenolate mofetil (CellCept) and azathioprine (Imuran) are antimetabolite immunosuppressants used to prevent organ transplant rejection and manage autoimmune conditions. Both target lymphocyte proliferation but through different pathways. Azathioprine is a purine analog converted to 6-mercaptopurine (6-MP), which is inactivated by xanthine oxidase and thiopurine methyltransferase (TPMT). Allopurinol inhibits xanthine oxidase, so concurrent use causes 6-MP to accumulate to dangerous levels — triggering severe pancytopenia. TPMT genetic testing should be done before starting azathioprine; patients with low TPMT activity are at high risk for myelosuppression even at standard doses. Mycophenolate inhibits inosine monophosphate dehydrogenase, selectively blocking de novo purine synthesis in lymphocytes. It is a known teratogen (FDA pregnancy category X equivalent); two forms of contraception are required, and a negative pregnancy test is needed before initiation. Both drugs cause dose-dependent bone marrow suppression, requiring regular CBC monitoring — typically weekly initially, then monthly. GI effects (nausea, diarrhea, abdominal pain) are especially prominent with mycophenolate. Both increase infection risk and long-term malignancy risk, particularly skin cancers and lymphoma.

Key Distinctions

Don't confuse azathioprine's allopurinol interaction with mycophenolate — only azathioprine is metabolized by xanthine oxidase. Students mix up TPMT testing (azathioprine-specific genetic screening) with trough-level monitoring used for calcineurin inhibitors. Mycophenolate is the teratogen requiring two contraceptive methods; azathioprine has reproductive risks but is sometimes cautiously used in pregnancy for autoimmune disease under close supervision.

Clinical Pearl

Allopurinol + azathioprine = bone marrow disaster. If the patient takes both, the azathioprine dose must be drastically reduced or the combination avoided entirely.