Azathioprine, an immunosuppressive drug used to prevent organ transplant rejection and treat autoimmune diseases, was developed in the 1950s and approved for medical use shortly thereafter. Its history is marked by its effectiveness in suppressing the immune system, but also by the need for careful monitoring due to potential bone marrow suppression and liver toxicity. Azathioprine, a purine analog that inhibits DNA synthesis in immune cells, is widely used in organ transplantation and autoimmune conditions such as rheumatoid arthritis and inflammatory bowel disease. Its development included early clinical trials to establish dosing safety, and its use is often guided by routine blood tests to monitor for adverse effects.
BRAND NAMES
Imuran – widely used in the United States and many other countries
Azasan – another brand in the U.S., often for oral tablets
Azatioprine – generic versions are available under this name in multiple regions
MECHANISM OF ACTION
Azathioprine is an immunosuppressant that is metabolized into 6-mercaptopurine, which disrupts DNA and RNA synthesis in rapidly dividing T and B lymphocytes. This inhibits their proliferation, reducing the immune response, and making azathioprine effective for preventing organ transplant rejection and treating autoimmune diseases.
PHARMACOKINETICS
Absorption
Azathioprine is well absorbed orally, with bioavailability ranging from approximately 50% to 80%. After ingestion, it is rapidly metabolized in the liver and red blood cells into its active form, 6-mercaptopurine, which then exerts its immunosuppressive effects. Food may slightly delay absorption, but it does not significantly reduce the overall amount absorbed.
Distribution
After absorption, azathioprine is widely distributed throughout the body, including the liver, kidneys, and lymphoid tissues, where it acts on immune cells. It is moderately protein-bound and can cross the placenta and appear in small amounts in breast milk, so caution is needed during pregnancy and lactation.
Metabolism
Azathioprine is converted in the liver and blood into its active form, 6-mercaptopurine, which is then transformed into thioguanine nucleotides that block DNA and RNA synthesis in immune cells. The process is affected by TPMT enzyme activity, with low activity increasing the risk of toxicity.
Elimination
Azathioprine is mainly eliminated through the urine as inactive metabolites, with some excreted in feces. Its active metabolite, 6-mercaptopurine, has a short half-life, but immune-suppressing effects last longer due to incorporation into immune cells.
PHARMACODYNAMICS
Azathioprine works by suppressing the immune system. Once converted to 6-mercaptopurine and then to thioguanine nucleotides, it inhibits DNA and RNA synthesis in rapidly dividing T and B lymphocytes, reducing their proliferation and activity. This leads to decreased immune responses, making it effective for preventing organ transplant rejection and treating autoimmune diseases. Its effects are dose-dependent and require careful monitoring to avoid bone marrow suppression and other toxicities.
ADMINISTRATION
Azathioprine is usually administered orally in the form of tablets, but it can also be given intravenously in certain hospital settings. The dose is typically adjusted based on the patient’s condition, body weight, and response to therapy. It is often taken once daily or divided into multiple doses, and regular blood tests are required to monitor for side effects such as bone marrow suppression and liver toxicity.
DOSAGE AND STRENGTH
Azathioprine is usually given at 1–3 mg/kg per day depending on the condition. Tablets commonly come in 25 mg, 50 mg, and 100 mg strengths. Doses may need adjustment for patients with liver or kidney problems or low TPMT activity to reduce the risk of side effects.
DRUG INTERACTIONS
Azathioprine can interact with other medications, increasing the risk of toxicity or enhanced immunosuppression. Allopurinol can raise azathioprine levels, while drugs like chemotherapy agents or other immunosuppressants may increase side effects. Monitoring blood counts and liver function is important during co-administration.
FOOD INTERACTIONS
Azathioprine can be taken with or without food, but high-fat meals may slightly delay its absorption. There are no major foods known to significantly affect its effectiveness, but patients are advised to maintain a consistent diet and avoid alcohol, which can increase the risk of liver toxicity.
CONTRAINDICATIONS
Azathioprine should not be used in patients with known hypersensitivity to the drug or its components. It is contraindicated in individuals with severe bone marrow suppression, active infections, or pancreatitis. Caution is required in patients with liver disease, and it is generally avoided during pregnancy and breastfeeding unless the potential benefits outweigh the risks.
SIDE EFFECTS
Nausea, vomiting, and loss of appetite
Diarrhea
Bone marrow suppression (leukopenia, anemia, thrombocytopenia)
Liver toxicity
Rash and fever
Increased susceptibility to infections
Rarely, long-term use may increase risk of lymphoma or skin cancer
TOXICITY
Azathioprine toxicity mainly affects the bone marrow and liver, causing low blood counts and elevated liver enzymes. It can also lead to nausea, vomiting, diarrhea, and increased risk of infections. The risk is higher in patients with low TPMT activity or when combined with certain drugs, so regular monitoring is important.
