Sirolimus , also known as rapamycin, is a macrolide (one of a group of drugs containing a macrolide ring) produced by the bacterium Streptomyces hygroscopicus. It has immunosuppressant functions in humans and is used to prevent rejection in organ transplantation; it is especially useful in kidney transplants. It prevents activation ofT cells and B cells by inhibiting their response to interleukin-2 (IL-2). Sirolimus is also used as a coronary stentcoating. Sirolimus works, in part, by eliminating old and abnormal white blood cells.[citation needed] Sirolimus is effective in mice with autoimmunity and in children with a rare condition called autoimmune lymphoproliferative syndrome (ALPS).

Sirolimus was discovered by Canadian researchers in a soil sample from Easter Island,also known as Rapa Nui.It was approved by the US Food and Drug Administration in September 1999 and is marketed under the trade name Rapamuneby Pfizer (formerly by Wyeth).

Sirolimus was originally developed as an antifungal agent. However, this use was abandoned when it was discovered to have potent immunosuppressive and antiproliferative properties. It has since been shown to prolong the life of mice and might also be useful in the treatment of certain cancers.