Bioabsorbable stents

, Associate Editor(s)-In-Chief Raviteja Reddy Guddeti, M.B.B.S.

Overview
Bioabsorbable stents also known as the disappearing stents are a promising new discovery in the field of Interventional Cardiology.They have been an interesting field of research over the past decade and half and as the name suggests they get absorbed completely over a period of time after their work has been done. Percutaneous Coronary Intervention (PCI) with bioabsorbable stents has created interest because the need for mechanical support for the healing artery is temporary, and beyond the first few months the presence of a bare metallic prosthesis has potential disadvantages.

Types
Bioabsorbable stents can be broadly classified as two types - Polymeric and metallic types. The key features to be considered while selecting a polymer or an alloy for a bioabsorbable stent are:
 * Strength - to avoid potential immediate recoil
 * Rate of degradation and corrosion
 * Biocompatibility with the vessel wall
 * Lack of toxicity

Polymers have been widely used in Cardiovascular devices and are now primarily used as delivery vehicles for drug coatings. Among the polymers suggested for bioabsorbable stents are Poly-L-Lactic acid(PLLA), polyglycolic acid(PGA),Poly(D,L-lactide/glycolide) copolymer(PDLA) and polycaprolactone. The use of bioabsorbable polymer coating reduces the need for extended dual anti-platelet therapy and in turn late thrombotic events. Among the polymers, Poly-L-Lactic acid is widely used in medicine. It breaks down to lactic acid a natural metabolite in human body, which enters Krebs Cycle and is metabolized to carbon dioxide and water.

So far two bioabsorbable metals alloys have been proposed for this application: magnesium and iron.The factors that determine the biocompatibility of these alloys are their solubility and their released degradation products. Magnesium stents are made of 93% magnesium and 7% rare-earth-metals. Magnesium was chosen because it is an essential element in the body. The alloy induces rapid endothelialization, has low thrombogenicity and has a degradation time of 2-3 months. It has calcium antagonist and antiarrhythmic properties and is also not associated with any adverse reactions.

The following is the list of some of the bioabsorbable stents that are currently under trials.


 * The Igaki-Tamai stent was one of the first bioabsorbable stents to be tested in clinical trials and is no longer in development now.
 * Bioabsorbable therapeutics and REVA medical are testing stents coated with sirolimus and paclitaxel respectively.
 * The PROGRESS-AMS study is a prospective multicenter non-randomized study that used the Magnesium-alloy stent and was conducted on 63 patients. The results of this study are 100% device and procedural success rate and the study met it's primary endpoint of MACE(major adverse cardiac events) of <30%.
 * The ABSORB trial involving Abbott's drug eluting BVS stent is a prospective non-randomized two phase study on 131 patients. The results demonstrated that the stent successfully treated coronary artery disease(CAD) and was resorbed into the walls of treated arteries after approximately 2 years. There were no blood clots experienced by the patients and no new MACE between 6 months and 4 years ( 3.4% at 4 years). The polylactide BVS stent struts resisted the constrictive remodeling forces sufficiently, such that by 6 months on Intravascular Ultrasound(IVUS) there was no shrinkage of the vessel. Between 6 months and 2 years the IVUS showed no changes in the vessel area, but the lumen increased in size and the stent was no longer visible by 2 years.
 * In the EVOLVE trial experts compared 2 bioabsorbable polymer SYNERGY stents - one deliverig full dose of everolimus and one delivering half dose of the drug with the durable PROMUS ELEMENT metal stent delivering full dose of the drug in 291 patients. The angigraphic outcomes of the study suggest that it may be possible to achieve at least comparable efficacy with a lower dose of everolimus than is used in commercialy available everolimus-eluting stents.

Advantages
The potential advantages these Bioabsorbable stents can offer over the traditional bare metal stents are:
 * Reduced late stent thrombosis - the stent being absorbed completely leaves behind no stimulus to ignite a chronic inflammatory process and thus in turn reducing the rate of late stent thrombosis which is a major concern of the traditional stents.
 * Short duration of post-stenting use of dual anti-platelet drugs.
 * Obviates metal implants in vessels, leaves only healed natural vessel.
 * Improved lesion imaging with CT and MRI unlike their bare metal counterparts.
 * Facilitates reintervention (PCI and CABG)
 * Prevents the constrictive vascular remodeling postdilatation due to the scaffolding effect of the stent and after being absorbed completely allows late expansive luminal and vessel remodeling.
 * Increased drug loading capabilities that will enable chronic drug release strategies.
 * In the long run, eliminates mechanical stent deformity and strut fractures

Limitations
The bioabsorbable polymer stents have quite a few limitations in their own way:
 * Strength is lower when compared to metallic counterparts, which can result in early recoil post implantation.
 * Associated with a significant degree of local inflammation.
 * These stents are radiolucent which may impair accurate positioning.
 * The polymer alone has limited mechanical performance and a recoil rate of approximately 20%,which requires thick struts that impede their profile and delivery capabilities, especially in small vessels.
 * Some experts opine that these stents can not be used in calcified lesions due to their lack of mechanical strength.

Future applications
The probable future applications of the bioabsorbable stents are:
 * Can be used in peripheral arteries like that of the femoral artery, tibial artery etc.,
 * It's feasible to transfer genes that code key regulatory pathways inside the cells of the arterial wall using polymer stents as vehicles.
 * Pediatric congenital heart diseases like Pulmonary artery stenosis.