Sirolimus (INN/USAN), also known as rapamycin, is an immunosuppressant drug used to prevent rejection in organ transplantation; it is especially useful in kidney transplants. A macrolide, sirolimus was first discovered as a product of the bacterium Streptomyces hygroscopicus in a soil sample from Easter Island - an island also known as "Rapa Nui", hence the name. It is marketed under the trade name Rapamune by Wyeth.
Sirolimus was originally developed as an antifungal agent. However, this was abandoned when it was discovered that it had potent immunosuppressive and antiproliferative properties.
The anti-proliferative effects of sirolimus may have a role in treating cancer. Recently, it was shown that sirolimus inhibited the progression of dermal Kaposi's sarcoma in patients with renal transplants. Other mTOR inhibitors such as temsirolimus (CCI-779) or everolimus (RAD001) are being tested for use in cancers such as glioblastoma multiforme and mantle cell lymphoma.
Combination therapy of doxorubicin and sirolimus has been shown to drive AKT-positive lymphomas into remission in mice. Akt signalling promotes cell survival in Akt-positive lymphomas and acts to prevent the cytotoxic effects of chemotherapy drugs like doxorubicin or cyclophosphamide. Sirolimus blocks Akt signalling and the cells lose their resistance to the chemotherapy. Bcl-2-positive lymphomas were completely resistant to the therapy; nor are eIF4E expressing lymphomas sensitive to sirolimus. Rapamycin showed no effect on its own.