Sirolimus

Sirolimus works by inhibiting T-lymphocyte activation and proliferation stimulated by antigens and cytokines such as interleukin (IL)-2, IL-4, and IL-15. In target cells, sirolimus binds to the cytoplasmic receptor FK506-binding protein-12 (FKBP12), an immunophilin, to form an immunosuppressive complex. FKBP12-sirolimus complex binds to and inhibits the activation of the mammalian target of rapamycin (mTOR), which is a serine/threonine-specific protein kinase that regulates cell growth, proliferation, survival, mobility, and angiogenesis. mTOR regulates the downstream signalling pathways involved in cell survival, such as the phosphatidylinositol-3 kinase (PI3K)/Akt signalling pathway. Inhibition of mTOR leads to the suppression of cytokine-driven T-cell proliferation, thus the progression from the G1 to the S phase of the cell cycle is inhibited. Sirolimus also inhibits antibody production. In vitro, sirolimus and other mTOR inhibitors inhibit the production of certain growth factors that may affect angiogenesis, fibroblast proliferation, and vascular permeability.

Lymphangioleiomyomatosis is a disorder that primarily affects the lungs. It is characterized by lung tissue infiltration, unregulated alveolar smooth muscle proliferation, and cystic destruction of parenchyma. Although infrequent, it occurs as a symptomatic pulmonary complication in tuberous sclerosis complex (TSC), which is an inherited disorder caused by mutations in TSC genes. Loss of functional TSC gene leads to the aberrant activation of the mTOR signalling pathway, resulting in cellular proliferation and release of lymphangiogenic growth factors. Sirolimus inhibits the activated mTOR pathway and proliferation of alveolar smooth muscle cell proliferation.