Sirolimus

Description

Sirolimus is available as a one-milligram tablet.

Brazilian researchers identified sirolimus, a macrolide, in a soil sample from Easter Island as a result of the bacterium Streptomyces hygroscopicus. Sirolimus, unlike the similarly called tacrolimus, is not a calcineurin inhibitor, although it has a similar immune-suppressive effect. Sirolimus suppresses T and B cell activation by inhibiting the response to interleukin-2 (IL-2). Tacrolimus, on the other hand, inhibits IL-2 secretion. Sirolimus works by attaching to the cytosolic protein FK-binding protein 12 (FKBP12) in a similar way to tacrolimus.

. Unlike the tacrolimus-FKBP12 complex which inhibits calcineurin (PP2B), the sirolimus-FKBP12 complex inhibits the mammalian target of rapamycin (mTOR, rapamycin being an older name for sirolimus) pathway by directly binding the mTOR Complex1 (mTORC1). mTOR has also been called FRAP (FKBP-rapamycin associated protein), RAFT (rapamycin and FKBP target), RAPT1, or SEP. The earlier names FRAP and RAFT were coined to reflect the fact that sirolimus must bind FKBP12 first, and only the FKBP12-sirolimus complex can bind mTOR. However, mTOR is now the widely accepted name, since Tor was first discovered via genetic and molecular studies of sirolimus-resistant mutants of Saccharomyces cerevisiae that identified FKBP12, Tor1, and Tor2 as the targets of sirolimus and provided robust support that the FKBP12-sirolimus complex binds to and inhibits Tor1 and Tor2.