<p>Everolimus, a proliferation signal inhibitor, prevents allograft rejection in rodent and nonhuman primate models of allotransplantation. It exerts its immunosuppressive effect by inhibiting the proliferation and thus, clonal expansion, of antigen-activated T cells which is driven by T cell-specific interleukins eg, interleukin-2 and interleukin-15. Everolimus inhibits an intracellular signaling pathway that normally leads to cell proliferation when triggered by the binding of these T cell growth factors to their receptors. The blockage of this signal by everolimus causes cells to be arrested at the G1 stage of the cell cycle.<br />
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At the molecular level, everolimus forms a complex with the cytoplasmic protein FKBP-12. In the presence of everolimus, the growth factor-stimulated phosphorylation of the p70 S6 kinase is inhibited. Since p70 S6 kinase phosphorylation is under the control of FRAP (also called m-TOR), this finding suggests that the everolimus-FKBP-12 complex binds to and thus, interferes with the function of FRAP. FRAP is a key regulatory protein which governs cell metabolism, growth and proliferation; disabling FRAP function thus explains the cell cycle arrest caused by everolimus.</p>