Design
Drug-eluting stents consist of three parts. Stent platform, coating, and drug.
The stent itself is an expandable metal alloy framework. Many DES are based on a bare-metal stent (BMS). The stents have elaborate mesh-like designs to allow expansion, flexibility and in some cases the ability to make/enlarge side openings for side vessels. Cobalt chrome alloy is stronger (and more radio-opaque) than the usual 316L stainless steel so the struts can be thinner which seems to reduce the degree of restenosis. (The L605 CoCr alloy has less nickel than 316L stainless steel and so may cause less allergy.)
A coating, typically of a polymer, holds and elutes (releases) the drug into the arterial wall by contact transfer. The first few DES licenced used durable coatings, but some newer coating are designed to biodegrade after or as the drug is eluted. Coatings are typically spray coated or dip coated. There can be one to three or more layers in the coating e.g. a base layer for adhesion, a main layer for holding the drug, and sometimes a top coat to slow down the release of the drug and extend its effect.
The drug is mainly to inhibit neointimal growth (due to proliferation of smooth muscle cells) which would cause restenosis. Much of the neointimal hyperplasia seems to be caused by inflammation. Hence immunosuppressive and antiproliferative drugs are used. Both sirolimus and paclitaxel were previously used for other medical applications; new drugs are being evaluated for coronary stents.
Examples (approved for clinical use in the U.S.) :
- Cypher (J&J, Cordis ) uses a 316L stainless steel BxVelocity stent (140 µm struts) and adds a 12.6 µm 3 layer coating (2 µm Parylene C base coat, 10 µm main coat of PEVA, PBMA and sirolimus, and a 0.6 µm top coat of PBMA). The sirolimus elutes over a period of about 30 days.
- Taxus (Boston Scientific) uses a 316L stainless steel Express2 stent (132 µm struts) and adds a 16 µm single layer Translute SIBS copolymer coating containing paclitaxel which elutes over a period of about 90 days.
- Endeavour (Medtronic) uses a cobalt chrome Driver stent (91 µm struts) and adds a 4.3 µm phosphorylcholine coating that includes zotarolimus, on a 1 µm base coat.
- Xience V (Guidant, Abbott) uses an L605 cobalt chrome ML Vision stent (81 µm struts) and adds a 7.6 µm fluropolymer multilayer coating with drug everolimus.
Examples approved outside the US :
- Zilver PTX (Cook Medical) Nitinol stent, no polymer, coated with paclitaxel, for use in peripheral arteries
- TaxCor (EuroCor GmbH) Highly Flexible Cobalt Chromium Stent Platform coated with fully biodegradable polymer as a carrier for Paciltaxel
- Axxion (Biosensors Int) Stainless steel stent, Synthetic Glycocalix coating with paclitaxel.
- BioMatrix (Biosensors Int) S stent platform, bioabsorbable PLA coating with Biolimus A9 drug.
- ARTAX (Aachen Resonance) double helix stainless steel platform, without polymer, metal coated with paclitaxel drug.
Read more about this topic: Drug-eluting Stent
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