Zotarolimus (ABT-578) is an immunosuppressant. It is a semi-synthetic derivative of rapamycin. It was designed for use in stents with phosphorylcholine as a carrier. Coronary stents reduce early complications and improve late clinical outcomes in patients needing interventional cardiology. Medtronic are using zotarolimus as the anti-proliferative agent in the polymer coating of their Endeavor and Resolute products. Zotarolimus was developed by Abbott Laboratories as the first cytostatic agent to be used solely for delivery from drug-eluting stents to prevent restenosis. The mechanism (or mechanisms) by which the Endeavor Zotarolimus-Eluting Coronary Stent System affects neointimal production as seen in clinical studies has not been established conclusively. In vitro, zotarolimus inhibited growth factor-induced proliferation of human coronary artery smooth muscle cells and also demonstrated binding affinity with FKBP 12 (binding protein). The suggested mechanism of action of zotarolimus is to bind to FKBP 12, leading to the formation of a trimeric complex with the protein kinase mTOR (mammalian target of rapamycin), inhibiting its activity. Inhibition of mTOR activity leads to inhibition of cell cycle progression from the GI to the S phase.