Molecular basis of different outcomes for drug-eluting stents that release sirolimus or tacrolimus.

Sirolimus and tacrolimus are potent immunosuppressants that are delivered by drug-eluting stents (DES) for the prevention of in-stent restenosis. Balloon angioplasty with stent implantation has emerged as a successful treatment for coronary stenoses; angioplasty dilates the vessel lumen and the stent prevents elastic recoil of the vessel walls. However, angioplasty and stent placement both produce vascular injuries that potently stimulate the proliferation of smooth muscle cells, resulting in a thickening of the vascular wall. The purpose of DES is to deliver pharmacological agents that counteract neointimal hyperplasia. The sirolimus-eluting-stent reduces the incidence of in-stent restenosis significantly, whereas the tacrolimus-eluting-stent demonstrates no improvement in clinical benefit compared with a bare stent. Although sirolimus and tacrolimus have similar molecular structures, these drugs regulate immune activation via different mechanisms of action. The effects of this class of drugs are mediated by binding to the FK-506-binding proteins (FKBPs), which are highly evolutionarily conserved across species. This review highlights the structure and function of sirolimus, tacrolimus and FKBPs, with particular focus on recent observations that the two drugs target signaling pathways involved in the control of vascular smooth muscle apoptosis and proliferation directly.