Evolution of Drug Coated Coronary Stents : A Major Breakthrough in Managing Coronary Artery Disease
Coronary Artery Stenosis is a common Cardiac problem leading to heart attack. Instead of Coronary Bypass Surgery, Balloon Angioplasty has become a popular treatment modalities. To keep the artery patent after angioplasty, physicians are increasingly using Mesh like metal devices called "sten". These devices help in keeping the artery open and reducing second time coronary narrowing. Modern developments include usage of drugs coated on the stents.
But these devices were limited in two ways. One was abrupt closure, which means that after the block was opened up the artery may close and the patient needed emergency surgery. The second was a problem called Restenosis or "re-blockage." That is ,in almost 30-40% of cases the arteries developed narrowing of their lumen due to growth of scar tissue . Stents are metallic scaffoldings that hold the artery open. They were introduced in the early 80s. Stents also held the artery open so the artery couldn't shrink, reduced the restenosis rate by almost half .But later it was found that , Scar tissue formation within the stent was a major problem which led to repeat narrowing within the stent (IN STENT RESTENOSIS)
Drug Coated stents inhibit this scar tissue formation, and were found to significantly reduce restenosis and hence enabled patients to avoid bypass surgery . After a Drug Coated Stent Deployment the Restenosis rate was found to be reduced to about 8 per cent which represents a Major Breakthrough in Cardiac therapy.
A wonderful advance in the battle against restenosis Drug coated stents, hence represent a paradigm shift in the therapy of Coronary artery Stenosis.
Drug coated stents basically inhibit scar formation after angioplasty. Sometimes referred to as a "coated" or "medicated" stent, a drug eluting stent is a normal metal stent that has been coated with a pharmacologic agent (drug) to interfere with the process of restenosis (reblocking). Restenosis is a very complex process and its prevention is equally complex. Drug eluting stent successfully reduces restenosis from the 20-30% range to single digits. There are three major components to a drug eluting stent:
- Type of stent that carries the drug coating
- Method by which the drug is delivered (eluted) by the coating to the wall of the artery (polymeric or other)
- The drug itself - how does it act to prevent restenosis?
In addition, there are several decisions made by the interventional cardiologist that result in a successful placement
- Correct sizing of the stent length to match the length of the lesion, or blocked area
- Correct sizing of the stent diameter to match the thickness of the healthy part of the artery
Sufficient deployment of the stent, making sure that the stent, once placed at the optimum site in the blocked artery, is expanded fully to the arterial wall underexpansion can result in small gaps between the stent and arterial wall which can lead to serious problems such as blood clots, or Sub Acute Thrombosis (SAT) Usually the sizing and the assessments of expansion are made by viewing the real-time angiogram in the cath lab, although some cardiologists also are using more detailed information obtained through intravascular ultrasound imaging (IVUS).
Finally, in addition to aspirin, the patient may need a Anticlotting Injection such as GP IIb/IIIa Receptor blocker like Abciximab (ReoPro™; Antibody to platelet GPIIb/IIIa receptor), Tirofiban (Aggrastat™) Reversible, nonpeptide GPIIb/IIIa receptor antagonist during the PCI and Clopidogrel (brand names Plavix) for up to six months after the stenting, to prevent the blood from reacting to the new device by thickening and clogging up the newly expanded artery (thrombosis). Ideally a smooth, thin layer of endothelial cells (the inner lining of the blood vessel) grows over the stent during this period and the device is incorporated into the artery, reducing the tendency for clotting.
Among the stents Boston Scientific TAXUS Paclitaxel Stent and Cypher sirolimus stent are already been approvced by FDA and used globally.CYPHER® Sirolimus-eluting Coronary Stent : Sirolimus a drug that stops cell replication, or division, by inducing cell cycle arrest. Sirolimus readily diffuses across membranes and into cells, where it halts the proliferation of smooth muscle cells by interrupting the cell cycle at a critical checkpoint in the G1 phase. Cells remain viable and return to their resting state (G0), producing a cytostatic effect. Once inside the cell, sirolimus binds tightly to a specific intracellular receptor protein. This complex inhibits a key regulatory enzyme called TOR, or "target of rapamycin," which processes biochemical signals necessary for the cell to progress from the G1 phase to the S phase. By inhibiting TOR, sirolimus shuts off the cell cycle and limits neointimal hyperplasia.
The CYPHER® Sirolimus eluting Coronary Stent effectively inhibits the growth of smooth muscle cells in the stent lumen.
Boston Scientific's Taxus Express² paclitaxel eluting coronary stent system is coated with a special polymer, a hydrocarbon based elastomer, called TransLute™, in which the drug paclitaxel has been embedded. The polymer is formulated to allow very specific time release of the drug.
The drug works by binding to parts of the cell, called microtubules, and interrupting the biochemical signaling process that causes cell migration and accumulation. In large doses, it has significant anti-tumor properties -- it prevents unwanted cells from growing. However, in smaller time-release doses, such as those eluted by the Taxus stent, paclitaxel takes on more anti-inflammatory characteristics; that is to say, the drug seems to have the effect of regulating, but not completely eliminating, cell accumulation.
Newer stents like Costar and Eurostar Cobalt Chromium stent eluted with paclitaxel have also shown significant beneficial effects in a larger number of European Hospitals . Genous Endothelial proliferating cell (EPC) which is a new generation BIOLOGICAL stent represents another major advance in interventional cardiology and will be launched early next year. Other Newer stents like 17 B Estradiol DC coated stent, Biodivysio Matrix LO stent, Avantec Mycophenolic Acid-Eluting Stent, Everolimus Eluting Stent and Biorest Bisphosphonates Stent are in trial stage.
Problems : Medical science is an evolving discipline and probably no technology marks the final Answer to all medical illnesses. In the same vein it has been found that Drug coated stents do have some problems. One of these has been that in some specific instances like Difficult coronary lesions, in the early stages of Acute heart attack or in small Coronary arteries ( AS FOUND IN SOUTH ASIA) drug coated stents lead to ACUTE THROMBOTIC STENT OCCLUSION due to clot formation. This again signifies that Drug stents cannot be used randomly in each and every case but can only be used selectively as per careful decisions made by Clinicians/cardiologists .
DR. R RAVI KUMAR, DR. AHM WALIUL ISLAM
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