AY-22989 may be the USAN-assigned generic reputation for natural product rapamycin. Sirolimus is created with a strain of Streptomyces hygroscopicus, isolated from the soil sample collected from Rapa Nui generally referred to as Easter time Island. Although sirolimus was isolated being an antifungal agent with potent anticandida activity, subsequent studies revealed impressive antitumor and immunosuppressive activities. Sirolimus demonstrates activity against several murine tumors, for example B16 43 melanocarcinoma, Colon 26 tumor, EM ependymoblastoma, and mammary and colon 38 solid tumors. Sirolimus is really a potent inhibitor of antigen-caused proliferation of T cells, B cells, and antibody production. Illustration showing the potent immunosuppressive activity of sirolimus in animal types of organ transplantation brought to numerous studies and subsequent approval by regulatory government bodies for prophylaxis of kidney graft rejection. Curiosity about sirolimus being an immunosuppressive therapy in organ transplantation stems from its mechanism of action, its side-effect profile, and how it can synergize along with other immunosuppressive agents. The molecular mechanism underlying the antifungal, antiproliferative, and immunosuppressive activities of sirolimus is identical. Sirolimus forms an immunosuppressive complex with intracellular protein, FKBP12. This complex blocks the activation from the cell-cycle-specific kinase, TOR. The downstream occasions such as the following the inactivation of TOR increase the risk for blockage of cell-cycle progression in the juncture of G1 and S phase.