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When to Use Bioresorbable Vascular Scaffolds

Access at: www.USCjournal.com Over the past 35 years, treatment of coronary artery disease and acute coronary syndrome has changed drastically. Balloon angioplasty offered an early mechanical solution but carried a high risk for acute complications and subsequent restenosis, mainly from recoil. The bare metal stent (BMS) improved restenosis because of recoil, but still carried a relatively high rate of restenosis because of excessive neointimal hyperplasia. The drug eluting stent (DES) further decreased restenosis by eluting anti-proliferative drugs over time and reducing neointima. However, the first generation of DES raised concerns of increased late and very late stent thrombosis, which were mitigated by prolonged duration of antiplatelet drugs. Newer generations of DES now have lower stent thrombosis risk (<1 % at 1 year), particularly when combined with dual antiplatelet therapy (DAPT), but the permanent metal structure in the coronary artery continues to create an ongoing risk of restenosis and stent thrombosis. Late events are related to persistent inflammation, neoatherosclerosis within the stented segment, and inability to restore normal vessel architecture and physiologic function in the presence of a metal structure.

Interventional Cardiology impact on early and late outcomes of the thicker struts of BVS require consideration. The engineering of the current Absorb BVS allows for an early phase post-percutaneous coronary intervention (PCI; 0-6 months), during which the Absorb BVS provides enough radial strength for mechanical support to maintain vessel patency, while antirestenotic drugs are eluted to prevent restenosis. By 6-12 months, the BVS begins to lose radial strength and reduce mechanical restraints on the vessel to allow healing of the native vessel and restoration of vasoregulation.
Once completely resorbed by 3 years, late adaptive remodeling will accommodate new intramural plaque. The restoration of vasomotion and pulsatility has been demonstrated by response to acetylcholine and nitrates. 4,5 Optical coherence tomography (OCT) performed 5 years post-BVS implantation demonstrates normal intima, media, and adventitia of the coronary artery, the so-called 'golden tube.' 6,7 Other potential practical benefits after the BVS is fully resorbed include the ability to perform coronary artery bypass, 'unjailing' of side branch vessels, better coronary computerized tomography imaging without artifacts from metallic stents, and treating restenosis without multiple layers of stents.

Historical Background and Early Data
The first successful use of a fully degradable stent in human was the poly-L-lactic acid based non-drug-eluting Igaki-Tamai scaffold (Kyoto Medical Planning Co., Ltd.). 8

Randomized Trials
Five trials in the ABSORB series were randomized with Absorb BVS versus Xience (Abbott; metallic second-generation DES), both of which elute everolimus (see Table 1). All trials except TROFI II enrolled mostly stable coronary artery disease patients with some unstable angina patients, and excluded ST-elevation myocardial infarction (STEMI) and non-STEMI patients. ABSORB II immediately followed ABSORB Cohort B, and was powered to evaluate vasomotor function and late lumen loss at 3 years.
ABSORB III was the largest randomized trial, designed for US approval and was powered for clinical endpoints. ABSORB TROFI II uniquely included only STEMI patients.
ABSORB II randomized 501 patients. Initial post-PCI imaging showed lower acute lumen gain in the Absorb group based on quantitative coronary angiography (1.15 mm versus 1.46 mm, p<0.0001), and IVUS (2.85mm 2 versus 3.6mm 2 , p<0.0001). MACE rates in the two groups were similar (5 % versus 3 % at 1 year and 7.6 % versus 4.3 % at 2 years). 14,15 Likewise, TLF did not significantly differ between the two groups (7.0 % versus 3.0 %, p=0.07 at 2 years). In a substudy, more complex lesions had higher target vessel revascularization (TVR). 16 Three-year results of ABSORB II were disappointing for the primary endpoints of vasomotion and late lumen loss. 14

Bioresorbable Vascular Scaffolds
Definite or probable device thrombosis was also not different (0.4 % versus 0.0 %). 17 ABSORB JAPAN was a randomized trial of 400 patients to support approval in Japan based on clinical non-inferiority. The primary endpoint was TLF. 18 At 1 year, TLF was similar for Absorb and Xience (4.2 % versus 3.8 %; P noninferiority < 0.0001, based on a prespecified 8.6 % non-inferiority margin). Definite or probable device thrombosis was slightly higher than other contemporary DES studies at 1.5 % in both groups. Intracoronary imaging was utilized in approximately 70 % of patients in each group and post-dilation was performed in approximately 80 % in each group. As with ABSORB II and ABSORB CHINA, ABSORB JAPAN showed higher in-device diameter stenosis and lower in-device acute gain in the Absorb group immediately post procedure.
ABSORB III was the pivotal randomized trial with 2,008 patients that led to Additionally, only 10 % of STEMI patients during enrollment period were randomized. Therefore, although STEMI lesions are an important target for BVS, we do not currently have enough data to support its use.

US Applications
Absorb is the only first-generation BVS approved in the US by the

Bioresorbable Vascular Scaffolds
There is evidence that these strategies of careful lesion selection and deployment technique can improve outcomes. For example, one mechanism for very late ScT appears to be because of resorption of initially malapposed scaffold struts, causing thrombosis because of mechanical disruption. 24,25 In a large registry study of 1,305 consecutive syndrome. 27 Longer duration is also reasonable based on results from the DAPT trial. 28 There are no specific guidelines for DAPT duration after BVS implantation, although with concerns for higher very late ScT rate in clinical trials combined with the duration of scaffold resorption, prolonged DAPT should be strongly considered. It seems reasonable that initial experience in the US should be used in patients with relatively low bleeding risk and eligible for prolonged DAPT.

Lesion Complexity and Characteristics
Lesion complexity is a major consideration for BVS implantation.
Exclusion criteria in the ABSORB randomized trials were numerous, including, but not limited to, heavily calcified lesions, extreme angulation or tortuosity, left main or ostial lesions, bifurcation lesions, thrombotic lesions, and chronically occluded lesions. Although there are reports of real-practice experience and registry data on these more complex lesions from outside the US, these were usually reported by operators with substantial device experience and with meticulous technique. [29][30][31][32] It seems advisable for the initial US experience to avoid more complex lesions such as heavily calcified lesions, bifurcation lesions, and chronic total occlusions. There should be a strategy to advance to higher complexity procedures after demonstrating favorable outcomes with simple lesions and with a commitment for systematic collection of outcomes data in the US.

Conclusion
The BVS is a new category of device that has the potential for improving the treatment of coronary artery disease. The technology offers considerable promise, but as a first-generation device, BVS has been hampered with safety concerns. Additionally, the theoretical longterm benefits have yet to be proven. Adverse outcomes in early trials and practice appear to be related to implantation in smaller vessels or inadequate vessel preparation and lack of post-dilation. It seems reasonable that if the safety concerns can be mitigated by careful lesion sizing and selection in addition to improved implantation technique, the long-term benefit of a fully resorbed stent would become evident.
During initial BVS adoption in the US, physicians should be conservative with careful patient and lesion selection and follow meticulous and regimented implantation techniques involving frequent intracoronary imaging for sizing, and pre-and post-dilation. This should include full adoption of the 4-step BVS specific implantation protocol as described above. Physicians should be cautious not to overextend treatment