The Potential of Bioabsorbable Vascular Stents
Sometimes, the human body needs a little help when it comes to repairing itself. In the case of a blood vessel (such as a vein or artery) becoming damaged or blocked, a hollow tube – a stent – can be surgically inserted into the vessel to keep it open and ensure that the flow of blood can continue. Stent surgery is incredibly common – an estimated two million people alone have coronary artery stents inserted every year.
Many of these patients opt to have stent surgery in the hope of preventing a heart attack – but stents do come with their own risks. The object itself is typically made of plastic or metal, neither of which are found in the body naturally. This means that sometimes the body believes it is being invaded by a foreign object and generates an immune response, which can eventually result in the body rejecting the stent. This type of immune rejection often occurs slowly over time, so the longer a stent is in the body, the higher the risk to the patient. To work around the potential risk of immune rejection, scientists have been investigating a possible role for bioabsorbable vascular stents.
Bioabsorbable Materials – Gone Before You Know It
Bioabsorbable materials are commonly made of glycolide-lactide polymers and copolymers, or various carbon-based materials, and slowly “dissolve” in the body over a period of time, thereby reducing the likelihood of immune rejection occurring. Around a dozen bioabsorbable materials have been identified, and some are currently being utilized to create rods, pins, screws and plates that can be inserted into various parts of the body. Bioabsorbable sutures have become a popular alternative method to traditional suturing and offer the advantage of not having to be removed from the patient at a later time.
Despite the success of these items, progress has been slow when it comes to utilizing bioabsorbable materials for vascular stents. Although stent patients could certainly benefit from a bioabsorbable alternative, these materials are not easy to work with and certain functional details surrounding this technology remain unclear.
Unique Materials Bring Unique Challenges
In terms of vascular stents, one of the major challenges associated with product development has been identifying materials that are absorbed at the right speed. Different blood vessels take different amounts of time to heal and depending on the degree of damage and the individual patient’s needs, the length of time a stent needs to be functional could vary significantly. This makes it difficult to create a product that is useful for a broad range of patients, as needs vary on a case by case basis.
A second problem is that a majority of bioabsorbable materials are not heat stable, which means that they can lose their strength and durability during heat treatment. Unfortunately, heat is used in the molding and also sterilization of any medical device which is to be placed inside a patient, adding a degree of complication when it comes to designing effective items such as vascular stents from bioabsorbable materials.
Recent Attempts at Developing Bioabsorbable Stents
Abbott, a medical technology company with a vast catalog of innovative products, developed a bioresorbable vascular stent (the Absorb) which was approved for commercial sales in Europe in 2011 and in the US in 2016. However, by 2017 the product was no longer commercially available, with Abbott stating that low commercial sales were the reason for this. They are allegedly working on a second-generation stent, though information on how this is progressing is scarce.
Certainly, the cost of manufacturing bioabsorbable materials is a key factor in why the success of these products is limited and why the products themselves are few and far between. Abbott’s Absorb stent was initially considered a breakthrough in the field, but as it became more widely used, limitations to this technology became apparent. When compared to traditional metal stents, the Absorb was not as flexible, broke more easily, required very precise sizing, and was not effective for use in very small blood vessels. Combined with the substantially higher price tag, these limitations meant that the Absorb was not a viable option to replace traditional, permanent stents.
Another company, Boston Scientific, has developed the SYNERGY bioabsorbable polymer drug-eluting stent system – the first of its kind available in the United States. A drug-eluting stent is a system that slowly releases some type of medication into the bloodstream, typically to prevent another blockage from developing in the blood vessel. In the SYNERGY model, the polymer controls the rate of release of the drug, and both the drug coating and the polymer are fully absorbed once the drug release is complete, at 3 months after insertion. However, the stent itself is composed of a platinum chromium body, which does not dissolve, rendering this a partially bioabsorbable stent.
At this stage, there is limited clinical data available regarding the efficacy of the SYNERGY system, particularly in the long term. Until the production costs associated with bioabsorbable materials are reduced, it is unlikely that bioabsorbable stents will become a mainstream product that a majority of patients are able to benefit from.