Development of more biocompatible polymers, as well as thinner stent struts and polymer coatings, is improving efficacy and safety of second-generation drug-eluting stents (DES). Given the link between certain polymers and chronic inflammatory responses to DES, it is a natural next step to develop either a third-generation bioresorbable polymer on a durable platform or a fourth-generation bioresorbable polymer on a bioresorbable platform.

There are many potential advantages to a bioresorbable scaffold:

• May reduce late “stent” thrombosis and limit duration of dual antiplatelet therapy (DAPT) since there is no foreign body—just a normal healed vessel without chronic inflammation
• Might restore normal vasomotion so the vessel can respond to physiological need
• May aid vessel repair because exposure to sheer stress and pulsatile forces is needed for healthy vessels
• Could allow expansive remodeling to accommodate plaque
• Might limit “late catch-up” as there is no chronic inflammation, perhaps allowing for vessel expansion
• No “full metal jacket” would facilitate repeat intervention and mean no need to avoid computed
  tomography (CT) or magnetic resonance imaging (MRI)
• Could eliminate side-branch “jail”
• Eliminate metallic strut fracture

Does a bioresorbable option still provide the necessary support to reduce restenosis? After all, stenting is needed to withstand negative remodeling or vessel shrinkage post-PCI. Such mechanical support is needed for perhaps 3 to 6 months, but thereafter the same protective support may become detrimental.

Also, an anti-proliferative drug is useful to limit an excessive healing response to stenting.

ABSORB

Recent evidence continues to suggest that bioresorbable scaffolds may be just what the interventionalist ordered, hoping to avoid a permanent implant while still providing the short-term support necessary to reduce restenosis.

John A. Ormiston, MB, ChB, and colleagues used quantitative coronary angiography, intravascular ultrasound (IVUS), virtual histology (VH), and optical coherence tomography (OCT) to study the second generation ABSORB everolimus-eluting bioresorbable scaffold (Abbott Vascular, Santa Clara, California).¹

There were early signs of resorption and an absence of late recoil. The different intravascular imaging
techniques were used for monitoring and demonstrated that, despite the fact that the process of bioresorption had been initiated, the polymeric struts were still easily detectable by IVUS and OCT. This allowed accurate measurement of scaffold areas over time and confirmed the absence of late or very late recoil.

While the scaffold area remained unchanged with IVUS as well as with OCT, with 97% of the struts covered at 12-month follow-up, VH radiofrequency backscattering and the echogenicity of the struts showed reduction in the strut core area. Also, the mechanical integrity and radial force of the scaffold must have substantially subsided because the scaffolded segments exhibited clear signs of pharmacologically-induced vasomotion.

In terms of complete disappearance, the stent takes about 2 years to disappear completely, although a newer version may take 3 years or longer to disappear completely.

The restenosis rate was low (3.5%), as was the 1-year major adverse CV event rate (7.1%), which is comparable to that observed in historical series of metallic everolimus-eluting stents.

The Magic of Disappearing Stents

In an accompanying editorial, Harold L. Dauerman, MD, noted that the magic of disappearing stents takes a slight hit in this study.² The results give pause to the appealing notion that bioresorbable stents will not require a full 12 months of DAPT.

Magic, he said, would be a DES that removes the clinical correlates of late stent thrombosis seen with the earlier generation of DES, including uncovered stent struts beyond 6 months, localized inflammation, and endothelial dysfunction. Although in the ABSORB trial stent integrity was lost by 12 months, the response to acetylcholine injection at the prior stent site was variable, with some showing vasoconstriction, others vasodilatation, and one no response at all.

In its worst light, he said, the results are consistent with only 42% of patients demonstrating a healthy endothelial response at 1-year follow-up. In its best light, it’s possible to imagine a substudy of the planned phase III trial where second-generation DES show enhanced levels of endothelial dysfunction compared with fourth-generation bioresorbable scaffolds, and thus provide a rationale for choosing a
bioresorbable vascular scaffold in patients unlikely to complete 12 months of DAPT.

Dr. Ormiston noted that the new technology does provide new challenges that are unique to bioresorbable platforms, but it is likely that these challenges can be overcome, making way perhaps for a fourth revolution in interventional cardiology.

References 

1. Serruys PW, et al. J Am Coll Cardiol. 2011;58:1578-88.
   http://content.onlinejacc.org/cgi/content/abstract/58/15/1578
 

2. Dauerman HL. J Am Coll Cardiol. 2011;58:1589-91.
   http://content.onlinejacc.org/cgi/content/full/58/15/1589#SEC2
   

Keywords: Inflammation, Vasodilation, Follow-Up Studies, Drug-Eluting Stents, Tomography, Optical Coherence, Sirolimus, Vasoconstriction, Magnetic Resonance Imaging, Coronary Angiography, California, Thrombosis, Polymers, Foreign Bodies

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