Azathioprine Sodium is 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.