The sodium salt form of azathioprine, a pro-drug of purine analogue with immunosuppressive activity. Azathioprine is converted in vivo to its active metabolite 6-mercaptopurine (6-MP), which substitutes for the normal nucleoside and mistakenly gets incorporated into DNA sequences. This leads to inhibition of DNA, RNA, and protein synthesis. As a result, cell proliferation may be inhibited, particularly in lymphocytes and leukocytes. or A drug that is used to treat myelodysplastic syndromes and is being studied in the treatment of other types of cancer. It belongs to the family of drugs called antimetabolites. Also called Mylosar and Vidaza.