Sirolimus: A naturally occurring substance discovered in a soil sample from Easter Island. Sirolimus was initially thought to hold promise as an antifungal antibiotic but this idea was dropped when sirolimus was unexpectedly found to have immunosuppressive activity. The US Food and Drug Administration in 1999 approved the use of sirolimus as an immunosuppressant agent. But earlier in the nineties, evidence had been uncovered that sirolimus was also a potent inhibitor of the growth of smooth muscle cells in blood vessels. The idea was then "hatched" that sirolimus might be used to inhibit the restenosis (reclosure) of coronary arteries. (Today, after a balloon angioplasty has been done to open a clogged coronary artery, a mesh tube called a stent is often inserted to keep the artery open. However, restenosis occurs in up to a third of cases when smooth muscle cells migrate from the vessel wall into the stent. The muscle cells proliferate there and narrow the interior diameter of the stent.) To prevent restenosis, stents coated, or "medicated", with sirolimus came into use. The sirolimus is eluted continuously from the stent and deters or slows restenosis.