Wiktionary
thiopurine
n. (context organic chemistry English) Any of several types of sulfur derivative of a purine, some of which have medical applications
Wikipedia
Thiopurine
The thiopurine drugs are purine antimetabolites widely used in the treatment of acute lymphoblastic leukemia, autoimmune disorders (e.g., Crohn's disease, rheumatoid arthritis), and organ transplant recipients.
Metabolism is catalyzed by S-methyltransferase.