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Bioabsorbable Stents

Cardiologist Q&A:

Where do you see the role of bioabsorbable stent technology in contemporary coronary intervention and what are your main concerns about it?


Dr Iqbal Malik
Consultant Cardiologist
Imperial College Healthcare NHS Trust
St. Mary’s Campus, Department of Cardiology
LONDON

Most stent companies are now trying to develop bioabsorbable stents. Why? The increase in the use of percutaneous coronary intervention (PCI) as apposed to surgery (CABG)continues. Although CABG has also developed, the use of drug eluting stents (DES) and powerful antiplatelet agents have meant that for most patients, PCI gives equivalent death and myocardial infarction rates even in the medium to long term. The downsides of PCI are :

  1. Restenosis: Restenting an in-stent restenosis is never as gratifying as the original procedure, and usually results in a narrower lumen that the first time, raising the risk of restenosis even if you optimize the deployment with intravascular ultrasound (IVUS).
  2. Thrombosis: this occurs either due to poor deployment- a mechanical problem, or due to poor endothelialisation due to the powerful drugs used on drug eluting stents, or inflammation related to the polymers used to bind the drug on the stents.
  3. There is also the issue that if you stent a long segment, you take the option of later CABG away- where will the surgeon stick his/her graft?

Perhaps bioabsorbable stents (BAS) can help. If all the struts are gone, then stent thrombosis would be less likely to occur. BAS provide the mechanical scaffold whilst the artery heals and then disappear leaving a vessel similar to the original, with hopefully more normal fluid dynamics, compliance and endothelial function. This would allow the artery to remodel as it wills-without any problem of loss of apposition of the stent. The vessel would be more receptive to further revascularization with either stents or, dare I say it, surgery!

So what is out there? Polymer based BAS have the disadvantage of needing fairly thick struts to give strength- and so reduce deliverability; a slow resorption rate-many months; and being radiolucent. Metallic based ones may give greater radial strength, but need to be made visible on X-ray also, and need to be shown to be non-toxic when 80mm of stent resorb!.

Both types will need drug delivery to reduce restenosis, radial strength to reduce early recoil, and to resorb at the “ideal” rate. What rate is that? Probably over about 6 months to allow full healing to occur. If all of that happens, then we have an ideal stent. But only ideal if we can deliver it in the chalky long lesions that we currently end up treating, and perhaps not for bifurcations, and perhaps not if we want to expose them to the high pressures to flare one end, one a strut in a side branch etc. Perhaps in the acute MI setting, BAS might work well, but they will probably not see the end of the metallic DES, just as the metallic DES did not see the end of the bare metal stent.

 

Dr Nick Curzen
BM(Hons) PhD FRCP
Consultant Cardiologist & Hon. Senior Lecturer
Wessex Cardiothoracic Unit,
Southampton University Hospital
SOUTHAMPTON

It is very early days for this technology and there are not yet enough data to truly answer the first part of this question. However, whilst I recognise the potential of an absorbable stent, I suspect that it will offer most to certain sub-groups rather than as a universal solution in PCI.  My reason for saying this is that I am not completely convinced by the concept that a disappearing scaffold offers more than the traditional rigid DES structure. Specifically, the vast majority of well-deployed DES provide an excellent clinical outcome for the patient. Once the metallic scaffold is buried and incorporated into the vessel wall, I am especially unconvinced that there is a disadvantage to its continued presence. By contrast, I perceive important potential disadvantage to the assumption that a disappearing scaffold is universally a good solution to the biologically and mechanically dynamic (healing) coronary artery wall.  I accept that we cannot be complacent when there is apparently an attrition rate of around 0.6% per year for very late DES thrombosis. My contention is that we are most likely to “solve” this challenging clinical dilemma by diverting some of the massive resource that is currently being invested in the development of stent technologies into understanding the mechanisms behind the apparently idiosyncratic nature of very late ST…. does overlying endothelial function vary?  Does our response to aspirin vary over time according to vascular inflammatory or other modifying factors?  Would patients be better off with long-term P2Y12 inhibitors than aspirin?

However, there are certainly some scenarios in which I can envisage niche indications for the absorbable stent: (i) Patients who cannot take dual antiplatelet therapy; (ii) patients requiring urgent non-cardiac procedures who have important myocardial ischaemia, just as 2 examples.

In the meantime, we need a whole lot more data to answer this question properly. Characteristically, many interventional cardiologists are attracted to this concept like moths to a flame! But then many were also almost pathologically attracted to other stent designs in the past that created (inexplicable) enthusiasm (think struts with wells…?). For those of you who think I’m being a stick-in-the-mud, let me leave you with 1 question: would YOU volunteer to have a bioabsorbable stent at the moment?

 

Dr Andreas Baumbach
Consultant Cardiologists
Division of Specialised Services
University Hospitals Bristol
BRISTOL

It has been a long time coming but now for the first time we do have results for a completely absorbable stent that suggest not only feasibility but also efficiency and safety comparable, if not better, than the current metal stent platforms. Absorb Cohort A showed very good longterm patency and freedom from adverse events in 30 patients. Cohort B, with an improved stent also shows excellent results. These initial studies were conducted in highly selected patients with simple lesions and in a small number of patients. They prove the principal of vascular restoration therapy.  Absorb extend, the randomized trial and the registries will now provide more data on the use of this 
technology in more complex lesions.

However, I do think it is too early to predict the role of bioabsorbable stents in the near future. It would be wrong to assume that they can replace our workhorse stents for all clinical scenarios. The current stent platforms are highly sophisticated metal scaffoldings that can be implanted into difficult lesions, through extreme tortuosity and within heavily calcified vessels of a wide range of sizes.

The bioabsorbable stents are only just emerging as a relevant clinical tool and will almost certainly initially be used in less complex scenarios. The use of a completely absorbable scaffolding in various clinical presentations from stable angina to acute coronary syndromes and ST elevation MI needs to be assessed in clinical trials. I am, however, convinced that the benefit of a completely disappearing stent that leaves behind a healed and functioning vessel with normal endothelial function must be the ultimate goal.  We will see a series of clinical trials that will define their 
ultimate benefit for patients.

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