(FDA approved in bold)

• Prophylaxis of organ rejection in patients with allogenic renal transplants
  
• Rheumatoid arthritis
  
• Myasthenia gravis (MG) (monotherapy or adjunctive)
  
• Inflammatory myopathies: polymyositis (PM) and dermatomyositis (DM)
  
• Multiple sclerosis (MS)
  
• Neuromyelitis optica

• Azathioprine, a derivative of 6-mercaptopurine, inhibits the synthesis of purine. This interferes with DNA and RNA synthesis, repair and replication cells, predominantly T leukocytes
  
• The mechanism of action in autoimmune diseases is unclear, but appears to suppress cellular cytotoxicity and blunt hypersensitivity reactions

• At least 3 months

• MG: Improves strength and muscle fatigue. Often used as an adjunctive to corticosteroids or acute treatment such as immune globulin or plasma exchange. Taper corticosteroids if clinical symptoms improve
  
• PM/DM: May allow improvement in strength and discontinuation or reduced dose of corticosteroids. (Corticosteroids are usually tapered first.) Taper slowly over 6 months if clinical remission occurs
  
• MS: May reduce relapses and new lesions on MRI

• MG: Effectiveness may not occur until 1 year. For patients with severe disability, consider more rapid-acting treatments, such as IV immune globulin
  
• DM/PM: Question the diagnosis (inclusion-body myositis, hypothyroidism, muscular dystrophy), rule out corticosteroid-induced myopathy and evaluate for undiagnosed malignancy (especially in DM). Change to methotrexate
  
• MS: If clearly not helpful, change to another agent

• MG: Usually combined with corticosteroids or other treatments in MG. Most patients also use symptomatic medication, such as pyridostigmine
  
• DM/PM: Usually used in combination with corticosteroids as a sparing agent
  
• MS: Occasionally combined with other treatments for the treatment of MS

• Obtain CBC weekly the first month, then twice monthly months 2–3, then monthly unless dose changes
  
• Before starting treatment, screen for thiopurine methyltransferase deficiency. Heterozygous patients often need a lower dose and closer monitoring. Homozygous patients are at risk for severe bone marrow toxicity