Drug-eluting Biodegradable Coronary Stent

A Drug-eluting Biodegradable Coronary Stent is an expandable wire form with a drug-coating over it that delivers medications to a specific area of the artery and is degraded into harmless products in the body after its function. Currently the types of stents used for preventing the coronary artery collapse include simple metallic stents and drug-eluting metallic stents.

One of the main reasons to develop a ‘drug-eluting biodegradable coronary stent’ is the short-term need for a stent and to avoid the potential long-term complications of metallic stents.These biodegradable coronary stents are generally made of a PLA {poly-L/D-lactic acid} polymer. They are known to deliver medications at specific sites of the arteries and they degrade into the body after 2 years of their implantation. Since they are biocompatible, they evoke minimum inflammatory response as compared to the traditional metallic stents. Moreover, they decrease the artery collapse rate (restenosis) to 10.5%

Stents are inserted in the coronary arteries (that provide blood supply to heart) and are enlarged to a diameter either greater than or equal to the diameter of the lumen so as to allow the free flow of fluid there through.

Metallic stents with/without drug-coating open up the arteries that have been blocked by fatty deposits (atherosclerosis).But the downside can be formation of thrombus and the process of restenosis(re-blockage of the artery).Metallic stents also cause infection and interference with some diagnostic procedures like MRI.

To overcome these problems, stents made up of biodegradable polymer that has the capacity to deliver drugs have been developed. These stents minimize restenosis and degrade into harmless products after a few months. They deliver medications at the local areas of the arteries and then they degrade into harmless products after a few months of their implantation.

Basically these stent are double-spiral zigzag helical structures consisting of a of 0.2 mm wire. Their outer diameter varies from 2.2 to 2.4 mm; with an initial lumen of 1.8 to 2.2 mm. The zigzag structure facilitatesreduced vessel wall injury at the time of stent implantation.

There are laminated multilayers wherein one layer addresses the structural requirements of the stent and additional layers release drugs at predictable rates that prevent proliferation of cell growth and blood clotting.

With multiple layers of polymer films, up to 20 layers of drugs can be released at varying rates. They are coated with anticoagulants, such as heparin, or platelet inhibitors, such as glycoprotein IIa-IIIB.

PLA is about 37% crystalline, with a melting point of 175—178°C and a glass-transition temperature of 60—65°C.It has lower tensile strength, higher elongation and a much more rapid degradation time, making it more attractive as a drug delivery-system. Dexamethasone[DEX]andSimvastatin[SIM]are the two drugs used in the combination with poly-96L/4D-lactic acid (PLA) stents.

These stents are designed in such a way that they can be broken down into lactic acid, which can be absorbed harmlessly into the body. The biological process of degradation begins at the 6^th month of the implantation and is known to be completed after 24 months. The main degradation end products of are normal metabolic products which are then used by the body.24months after implantation, the stent material appears largely hydrolyzed and cannot be detected in polarizing light.

On performing Angiography as a post-stenting procedure it was observed stented arteries were angiographically patent.The mean luminal diameter (3.05 mm) and area (30.36 mm2) of PLA stents was decreased as compared to other stents.

Some advantages are mentioned below:

1. These stents are biocompatible, producing negligible inflammation and provide excellent haemodynamic stability.

2. They are known to self-expand for at least 3 months and this prevents restenosis process in the specific area of artery where they are placed.

3. They can be used in the cases where repeat angioplasty or bypass grafting are required.

4. They cause minimum interaction with fibrinogen and hence can minimize blood clot formation in the arteries.

5. Moreover they are opaque to X-rays and hence can be easily visualized.

Some drawbacks are mentioned below:

1. The time for the complete degradation of these stents cannot be predicted.

2. After the stenting, the patients the patient must take an anti-clotting or antiplatelet drug, (such as clopidogrel or ticlopidine) for 6 or more months to prevent the blood from reacting to the new device by thickening and clogging up the newly expanded artery.

3. To effectively impact tissue growth, it may be necessary to add specific antiproliferative compounds to this biodegradable stent and so they are very costly.

4. High temperature (65°C to 75°C for few seconds) required during the process of insertion will result in the pathologic death of the living tissues (necrosis) of the arterial walls.

5. Moreover high temperature (around 55°C) also results in the platelet adhesion to the vessel wall seems which may cause thrombosis.

CONCLUSION

Stents and other modern endovascular devices such as stent grafts have undoubtedly changed the field of vascular interventions. Their use is an important step towards more sophisticated treatment of vascular diseases.Thus, biodegradable materials, such as PLA96, may be promising for small-vessel use as stent core materials and may prove to be an effective alternative to the metallic versions.

COMPILED BY:-

MEET OZA

JAHANVI MODI

5^TH BIOMEDICAL ENGINEERING

U.V.PATEL COLLEGE OF ENGINEERING