A drug used to keep the body from rejecting organ and bone marrow transplants. Sirolimus blocks certain white blood cells that can reject foreign tissues and organs. It also blocks a protein that is involved in cell division. It is a type of antibiotic, a type of immunosuppressant, and a type of serine/threonine kinase inhibitor. Sirolimus was previously called rapamycin. Also called Rapamune.