Bioresorbable scaffolds (BRS) were developed as an alternative to drug-eluting stents (DES) to facilitate vessel restoration and reduce the risk of future adverse events. First-generation BRS are represented by the three CE-mark approved devices: the poly-l-lactic acid (PLLA)-based Absorb Bioresorbable Vascular Scaffold (Abbott Vascular) and DESolve NX (Elixir Medical) and the magnesium-based DREAMS (Biotronik) scaffold. Designed to overcome the drawbacks of DESsuch as chronic local inflammatory reaction and late stent thrombosisfirst-generation BRS have shown an efficacy profile comparable to second-generation metallic DES in low-to-moderate-complexity angiographic scenarios.16 However, recent meta-analyses and real-world registries have raised some concern about the safety of this novel technology, especially due to an increased risk of thrombosis within the first weeks of scaffold implantation.710 These devices appear to be less forgiving to poor implantation strategies when compared to contemporary DES. Additionally, first-generation BRS have several limitations, such as (1) increased strut thickness ( 150 mm) and crossing profile; (2) relatively low resistance to overexpansion, which might result in scaffold fracture; (3) lack of radiopacity, which requires a more frequent need for intravascular imaging modalities such as intravascular ultrasound (IVUS) and optical coherence tomography (OCT) to achieve reasonable device deployment; and (4) special storage requirements, such as refrigeration, to preserve the polymer physical characteristics.
It should be noted that consistent optimized implantation strategies were not used in most of the prior reports and they had relatively low rates of postdilation and intravascular imaging use.1114 Furthermore, a recent report demonstrated that the incidence of scaffold thrombosis could be significantly reduced with an optimal implantation strategy.15 Even so, the combination of poor safety results and the above-listed limitations recently led Abbott Vascular to halt production of the Absorb Bioresorbable Vascular Scaffold. Use of the remaining first-generation devices has also been reduced and limited to simple anatomies and clinical trials. However, the concept behind the development of fully absorbable devices remains encouraging, and novel devices to overcome the current limitations are being pursued.
Based on the current literature and our center's experience with these devices, this review discusses the various tips and tricks to optimize BRS implantation, the pitfalls related to the current generation of BRS, and the potentially advantageous features of three next-generation scaffold systems (Fantom, DESolve CX, and Meres 100) that are advanced in their in-human clinical evaluations.
TIPS AND TRICKS FOR ADEQUATE BRS IMPLANTATION
Recent studies have suggested that adverse events after BRS may be more frequent with either undersizing or oversizing the scaffold relative to the vessel diameter and with implantation in small vessels and suboptimal angiographic results.15,16 Due to intrinsic properties related to its design and composition, the deployment of BRS might require some caveats. This has led to the concept of PSP (optimal predilation, vessel and device sizing, and postdilation) to optimize BRS outcomes.17
Adequate lesion preparation is an essential component when implanting current BRS and is important for both scaffold delivery and optimal scaffold expansion. BRS have larger crossing profiles, inevitably resulting in reduced deliverability compared to metallic DES. Furthermore, current BRS have less radial strength than metallic stents, which may result in greater acute recoil18 and inadequate scaffold expansion in insufficiently prepared lesions.19,20 Therefore, predilatation should be considered mandatory.
Overexpansion of the BRS, especially the first-generation ones, runs the risk of structural disruption/fracture. This has already been described with the Absorb device, which was the most frequently implanted BRS in daily practice until late 2017, when the manufacturer halted sales.21 Conversely, underexpanded scaffolds and those with incomplete strut apposition might put the patient at higher risk of negative events during follow-up, as described in different series of BRS failure.9,22
Currently, Absorb is only available in three diameters (2.5, 3.0, and 3.5 mm), and it is recommended for implantation in de novo coronary lesions with reference lumen diameters between > 2.5 mm and < 3.8 mm. Thus, the current version of this device was not designed to treat large vessels due to the risk of structural damage in case of expansion above the recommended dimensions. Although other scaffolds in bench evaluations have been shown to tolerate postdilation with bigger balloon catheters,23 the safety of these procedures has not been properly addressed in a real-world clinical scenario since the inclusion criteria in trials evaluating these devices have been very restrictive to prevent this kind of situation.
After publication of the ABSORB III trial, the use of these devices has been questioned for small vessels. In that study, BRS deployment in vessels with a reference diameter < 2.25 mm resulted in increased rates of target lesion failure (12.9 vs 8.3) and device thrombosis (4.6 vs 1.5) when compared to metallic DES.2 Part of this problem might be explained by their bulky struts (150 190 mm) limiting/disturbing the effective flow area in small coronary lumens. Reducing the strut surface in contact with the vessel wall can be achieved by reducing the strut size and/or modifying the strut shape. Plaque composition and high-pressure scaffold implantation may also play a role in strut embedment and ultimately influence the flow dynamic in the scaffolded segment. Postdilation with a high-pressure balloon, within the device's expansion limits, may help obtain a better strut embedment. Furthermore, recent studies have suggested that BRS implantation with high-pressure postdilation rates (above 90) and pressure (above 20 atm) are associated with lower rates of device thrombosis.24
In addition, while metallic stents have a smooth and antithrombogenic surface, the same electropolishing treatment cannot be applied to BRS polymeric scaffolds, which have a rough surface. Surface roughness influences the amount of protein adherence since it determines the contact area between the stent and coronary artery.25 As such, the surface properties of the stent might influence post-PCI complications such as thrombogenicity and tissue reaction.26,27 A smooth surface can help prevent the activation and aggregation of platelets, which is recognized as one component of the thrombosis process. Electropolishing effectively minimizes thrombosis and potentially reduces neointimal hyperplasia.28
Notably, visual estimation and online quantitative comparative analysis (QCA) are the most frequent tools to determine stent dimensions since they are usually available worldwide, require less-specific training, and do not add additional cost or time to the procedure. However, it is important to note that neither the Absorb device nor most BRS under clinical investigation are radiopaque; therefore, angiography is limited in identifying problems related to their deployment. The only exception is the Fantom BRS, which is developed from a proprietary, inherently radiopaque polymer composed of tyrosine analogs and other natural metabolites that allow visualization using conventional angiography. As a result, the advent of BRS has encouraged a more widespread use of intravascular imaging (IVUS/OCT) for a more accurate estimation of real vessel dimension and to identify potential mechanisms of device failure, including underexpansion, incomplete strut apposition, and structural device damage. Figure 1 illustrates our center's algorithm for BRS diameter selection based on preintervention intravascular imaging assessment. Even so, two recent surveys of operators experienced in BRS implantation indicated that they used routine intracoronary imaging in less than 20 of their cases.29,30 Therefore, even in high-volume centers that treat complex lesions, device choice is mostly based on visual estimation (> 80 of cases) or online QCA (14),30 which might explain the poorer outcomes reported with these devices in more cumbersome scenarios.
For postdilation, an accurately-sized noncompliant balloon (1:1 scaffold:balloon diameter) with high pressure (more than 20 atm) can be an appropriate initial strategy. If further postdilation is required, higher pressures with the same noncompliant balloon or a different balloon with a diameter equal to scaffold size (with a maximum of 0.5 mm) can be used. The threshold for scaffold fracture may decrease depending on lesion morphology; therefore, inflation pressure should be carefully increased when using larger balloons.
NEXT-GENERATION BIORESORBABLE SCAFFOLDS
The DESolve scaffold (Elixir Medical) is comprised of a PLLA-based backbone coated with a matrix of the drug Novolimus and a polylactide-based polymer. The drug is contained in a proprietary bioresorbable PLLA-based polymer from the same family of PLLA-based polymers contained in the scaffold backbone. The device has sinusoidal ring patterns optimized for each diameter and requires two platinum-markers at each end to facilitate positioning and postdilation (Figure 2). Inflation should be performed gradually, increasing 2 atm every 3 to 5 seconds, and the device must be stored between 0C and 8C.
The antiproliferative drug Novolimus is a metabolite of sirolimus and belongs to the family of compounds of macrocyclic lactones with a mechanism of action similar to sirolimus. Novolimus is applied to the scaffold at a dose of 5 g per mm of scaffold length, and 85 of the drug is eluted over 4 weeks.4 The polymer coating degrades within 6 to 9 months, and the PLLA-based scaffold degrades within 12 months and resorbs within 24 months.
A polymer's differing chemical properties and the way they are processed produce distinctive mechanical properties. As a result, the DESolve BRS has some unique features, including (A) the ability to self-correct to the vessel wall when expanded to nominal diameter and therefore to correct small incomplete strut apposition31; (B) the ability to tolerate overexpansion during postdilation without fracture23,31 (e.g., a 3.0-mm scaffold has been shown to expand up to 4.5 mm with no structural damage); and (C) a shorter bioresorption time (95 reduction in molecular weight by 1 year, with complete absorption by 2 years).
The DESolve NX, the first generation of this BRS, has already received CE-mark approval. It has a strut thickness of 150 m and had its efficacy evaluated in the single-arm, multicenter DESolve NX trial, which enrolled 126 patients with noncomplex coronary lesions. Device success was achieved in 97 of the cases, and acute recoil was low (6.6). A 6-month invasive assessment was obtained in 93 of the cases and showed in-scaffold and in-segment QCA late loss of 0.20 0.32 and 0.21 0.31 mm, respectively, with 3.5 binary restenosis.5 Of note, in the IVUS (n 40) and OCT (n 38) substudies, this BRS showed an early and significant area increase from postprocedure to the 6-month follow-up that was attributed to its early resorption properties, which resulted in early vessel restoration. Additionally, at 6 months, 98.8 of all struts were fully covered by OCT with a very thin layer of tissue (30.6 m). No case of scaffold thrombosis was documented in this registry.5 More recently, DESolve has undergone a few alterations in polymer processing, resulting in a 20 reduction in strut thickness while maintaining its unique features. The novel DESolve CX (120 m) is currently being evaluated in a first-in-man registry with 150 patients.
In parallel, Elixir Medical has developed a scaffold dedicated to STEMI patients. The Amity device has the same self-correction mechanism as DESolve and can expand up to 0.6 mm in diameter over 3 days, which may be useful for thrombus-containing lesions. Once the thrombus vanishes, the self-correction mechanism would then resolve any residual strut malapposition. Clinical evaluation of this device is yet to be initiated.
Finally, one of the drawbacks of current BRS is the limited variety of sizes available for commercial use. In particular, treatment of large coronary arteries has been curtailed by the lack of devices with diameters > 3.5 mm. To this end, Elixir has recently developed a scaffold dedicated to the treatment of large vessels. The DESolve XL is a polylactic acid-based device with a nominal diameter of 4.0 mm and a strut thickness of 150 m, although additional device characteristics regarding polymer and antiproliferative drug kinetics are the same as those of the other Elixir scaffolds. The first-in-man evaluation of this BRS was performed in 10 patients treated with 4.0- 18-mm devices guided by OCT imaging.32 On average, postdilation was performed with 4.8 0.3-mm balloons. No case of scaffold fracture was noticed after the procedure, and the percentage of malapposed struts was only 0.09 0.26.
A major drawback of most contemporary polymers is the lack of intrinsic radiopacity, which might result in geographical miss during implantation and require more use of intracoronary imaging to guide scaffold deployment. Recently, REVA Medical developed a proprietary, inherently radiopaque polymer composed of tyrosine analogs and other natural metabolites. The resulting copolymer is a biodegradable polyester carbonate called poly(I2DAT-co-lactic acid). Additionally, iodinated tyrosine analogues such as 3,5-di-iododesaminotyrosine were incorporated into the polymer backbone, which allows the device to be visualized using conventional angiography. Iodine atoms are covalently bound directly to the backbone of the desaminotyrosine component and, due to their greater mass, scatter x-rays and impart radiopacity.
Initial in-human studies with this polymer focused on an innovative Slide & Lock design as found in the ReZolve scaffold series. Unlike traditional deformable metal stent designs, the Slide & Lock design is deployed by sliding open and locking into place. It had a series of 15 individual components assembled into a single scaffold, and the components were made up of two primary elements that included 12 U-shaped 120-m struts and three sinusoidal backbone components of approximately 203 m thickness. In theory, this novel design would eliminate the need to significantly deform the device during deployment, making it ideally suited for use with polymers, which are inherently not as amenable to deformation as metals. However, technical difficulty related to its deliverability, documented scaffold fracture, and excessive neointimal tissue formation led the company to develop a second-generation device (the Fantom scaffold) using a traditional deformable design. The platform of the current version is a single-element device that contains a uniform thickness of approximately 125 m across the full length of the scaffold (Figure 3). Designed as circumferential supporting hoops, the scaffold is joined by a series of non-supporting connective elements, with a crossing profile of 1.25 to 1.35 mm depending on scaffold diameter (5F compatible).33 Notably, the polymer backbone is manufactured to withstand single-step inflation during deployment, mimicking the traditional method of metallic stent expansion without fracture. The modulus of elasticity ranges from 2.0 to 2.4 GPa, while the tensile strength and elongation at break are 80 to 95 MPa and > 150, respectively. Thus, it is recommended that the postdilatation balloon not overstretch the scaffold by more than 0.75 mm. The Fantom BRS does not have any special storage requirements such as refrigeration.
The device backbone is coated with a thin layer of an amorphous form of the same polymer, which carries the antiproliferative drug sirolimus in a dose of 197 g/cm2. This coating matrix promotes controlled release of sirolimus such that 60 of the total drug load is released within the first 30 days. The remaining sirolimus dose is released slowly over the next several months.
The polymer degrades via hydrolysis of the carbonate and ester bonds in the backbone, with the resulting I2DAT excreted through the kidneys and lactic acid metabolized through the Krebs cycle. According to preclinical studies, complete polymer backbone degradation is expected to occur at 36 to 48 months. The polymer-coating matrix is the same as the backbone polymer and is expected to degrade similarly.
A pilot human evaluation of this BRS was conducted in two centers (Brazil and Poland). Seven patients with single de novo lesions treated with 3.0- 18-mm devices were enrolled in the FANTOM I trial.34 Device success was achieved in all cases, with an acute recoil of 4.82. At 4-month invasive follow-up, in-scaffold late loss was 0.21 mm while neointimal hyperplasia obstruction by IVUS was 3.14 2.04. Notably, OCT evaluation revealed that 99.1 of all scaffold struts were fully covered, with no single case of incomplete apposition.34
Following the enthusiastic initial results, the FANTOM II trial recruited 240 patients with up to two de novo lesions treatable with devices of 2.5, 3.0, or 3.5 mm in diameter and up to 24 mm long. The enrolled population was divided into two cohorts with distinct invasive follow-up timelines. Cohort A (n 117) underwent invasive follow-up at 6 and 24 months while cohort B (n 123) underwent follow-up at 9 and 36 months. Short-term technical success, short-term procedural success, and clinical procedural success were achieved in 96.6, 99.1, and 99.1 of patients, respectively. Mean 6-month in-stent late lumen loss was 0.25 0.40 mm (n 100). Binary restenosis was present in two patients (2.0). Major adverse cardiac events within 6 months occurred in three patients (2.6), including no deaths, two myocardial infarctions (MI), and two target lesion revascularizations (TLRs); one patient had both an MI and TLR, and scaffold thrombosis occurred in one patient (0.9).35
The MeRes100 BRS (Meril Life Sciences Pvt. Ltd.) is a balloon-expandable PLLA polymer backbone scaffold. The top contains an active drug coating of sirolimus distributed at a dose of 1.25 g/mm2 formulated in a 1:1 mixture of biocompatible and bioabsorbable polymer poly-D, L-lactide (PDLLA), which acts as a drug reservoir and controls the drug release rate. The thin uniform coating is 3 to 4 m and does not web, crack, or lump as studied by scanning electron microscopy. Both PLLA and PDLLA undergo hydrolytic degradation of the ester bonds in the polymers, generating lactic acid that is converted to CO2 and H2O, which are eliminated from the body.
The expected degradation of the scaffold from the treatment site is within 24 to 36 months of implantation.36 The MeRes100 BRS has a hybrid cell design, close cells at the edges, and open cells along the length (Figure 4) to ensure optimal vessel wall conformability. It has a low strut thickness of 100 m, and strut width varies from 150 to 200 m, depending on scaffold diameters. The crossing profile is 1.20 mm and 1.25 mm for 3.0 mm and 3.5 mm diameters, respectively. The couplets of tri-axial platinum radiopaque markers fixed circumferentially 120 apart from each other at either end of the scaffold allow it to be viewed in two orthogonal views during its deployment.
In preclinical studies using OCT and histopathology, the MeRes100 showed equivalent in vivo acute and chronic recoil as well as similar neointimal formation and arterial healing up to 180 days when compared to the benchmark Absorb BRS up to 180 days. Preliminary evaluation of serial OCT obtained at 1 and 2 years suggests a more gradual integration of the scaffold into the arterial wall than that reported for Absorb (2 of preserved box appearance in MeRes100 vs 80.4 for Absorb at 2 years).
The MeRes-1 first-in-human trial was a single-arm prospective multicenter study that enrolled 108 patients with de novo coronary artery lesions (116 scaffolds were deployed to treat 116 lesions in 108 patients). At 6 months, quantitative coronary angiography revealed in-scaffold late lumen loss of 0.15 0.23 mm with 0 binary restenosis. Optical coherence tomography demonstrated minimum scaffold area (6.86 1.73 mm2) and 99.30 neointimal strut coverage. Quantitative intravascular ultrasound analysis confirmed a 0.14 0.16 mm2 neointimal hyperplasia area. At 1 year, major adverse cardiac eventsa composite of cardiac death, any myocardial infarction, and ischemia-driven target lesion revascularizationoccurred in only one patient (0.93) and no scaffold thrombosis was reported. At 1 year, computed tomography angiography demonstrated that all scaffolds were patent, and in-scaffold mean percentage area stenosis was 11.33 26.57.36
Despite the theoretical long-term benefits of bioresorbable scaffolds in the treatment of coronary artery disease, the first generation of these deviceswith bulky struts and high crossing prolife, prolonged resorption time (> 24 months), lack of x-ray visibility, and limited tolerance to postdilationhave restricted their clinical application and negatively impacted their short- to mid-term safety performance, with a trend to more thrombotic events compared to the current generation of metallic drug-eluting stents. At present, BRS should be avoided in very large coronary arteries (> 4.0 mm) and carefully considered in complex anatomies and clinical scenarios, including aorto-ostial and long lesions, bifurcations, and patients with acute coronary syndrome, particularly those with acute STEMI. The future of this technology as a working horse in the interventional cardiology field depends on the clinical performancesupported by a stronger body of scientific dataof the next generation of these devices.