Drug-eluting, balloon-expandable, bioresorbable vascular scaffolds reduce neointimal thickness and stenosis in an animal model of percutaneous peripheral intervention.

Objective: Recanalization with balloon angioplasty and/or self-expanding stents (SES) has become the endovascular treatment of choice for symptomatic femoropopliteal occlusive disease. These strategies generate suboptimal clinical results, however, because they fail to expand the artery fully and ineffectively prevent recoil, neointimal hyperplasia, and restenosis. Balloon-expandable stents, given their greater radial force and rigid structure, represent a more effective treatment strategy, but only short lengths can be implanted safely in arteries that deform and bend with skeletal motion. The purpose of this preclinical experiment was to test the hypothesis that simultaneous implantation of a series of short, resorbable, balloon-expandable, paclitaxel-eluting scaffolds would prevent neointimal hyperplasia and stenosis compared with SES in an animal model of percutaneous femoropopliteal intervention. Methods: We extruded 6 × 60 mm Efemoral Vascular Scaffold Systems (EVSS) from copolymers of poly-L-lactic acid, coated with paclitaxel 3 µg/mm2, crimped onto a single delivery balloon, and implanted percutaneously into the iliofemoral arteries of eight Yucatan mini-swine. We implanted 7- to 8-mm × 60 mm SES into the contralateral experimental arteries. The animals were serially imaged with contrast angiography and optical coherence tomography after 30, 90, 180, 365, and 730 days. The primary end point of this study was neointimal morphometry over time. Secondary end points included acute deformation and angiographic and optical coherence tomography-derived measurements of chronic vascular response. Results: Over the 2-year study period, one SES was found to be completely occluded at 90 days; all EVSS were widely patent at all time points. Arteries treated with SES exhibited profound neointimal hyperplasia with in-stent stenosis. In contrast, arteries treated with EVSS exhibited only modest vascular responses and minimal stenosis. After 2 years, the mean neointimal thickness (0.45 ± 0.12 vs 1.31 ± 0.91 mm; P < .05) and area (8.41 ± 3.35 vs 21.86 ± 7.37 mm2; P < .05) were significantly decreased after EVSS implantation. By 2 years, all scaffolds in all EVSS-treated arteries had resorbed fully. Conclusions: In this preclinical animal model of peripheral endovascular intervention, the EVSS decreased neointimal hyperplasia and stenosis significantly compared with SES, then dissolved completely between the first and second years after implantation.

Intravascular treatment of long segments of experimental peripheral arteries with multiple, serial, balloon-expandable, resorbable scaffolds.

Symptomatic femoropopliteal occlusive disease has been increasingly treated using endovascular methods. However, restenosis, especially after implantation of permanent metallic stents, has remained common. To date, resorbable scaffolds have failed to achieve sufficient radial strength to enable the successful treatment of long, mobile, peripheral arteries. In the present nonsurvival, large animal experiment, a novel device consisting of multiple, short, serial, balloon-expandable, bioresorbable scaffolds was deployed in arteries subjected to supraphysiologic deformation. Compared with native vessels, the scaffolded arteries continued to bend (113° ± 19° vs 110° ± 20°; P = .10) and shorten (15% ± 15% vs 20% ± 14%; P = .16), unencumbered by the placement of the investigational device. The mean luminal diameter of the scaffolded arteries was preserved without kinks or occlusions in exaggerated flexion (4.7 ± 0.7 vs 4.7 ± 0.5 mm in extension vs flexion; P = .80). Arterial deformation was borne by shortening of the interscaffold spaces (2.2 ± 0.8 mm vs 1.9 ± 0.7 mm in extension vs flexion; P < .01) and the scaffolds themselves (10.7 ± 1.4 mm vs 9.9 ± 1.1 mm in extension vs flexion; P < .01). The results from the present study challenge the perceived limitations of balloon-expandable devices implanted in peripheral mobile arteries. We have presented a bioresorbable scaffold that combines sufficient radial strength to preserve the mean luminal diameter with movement and the flexibility to accommodate femoropopliteal deformation.

An Animal Model of Human Peripheral Arterial Bending and Deformation.

BACKGROUND: Designing peripheral arterial stents has proved challenging, as implanted devices will repetitively and unpredictably deform and fatigue during movement. Preclinical testing is often inadequate, given the lack of relevant animal models. The purpose of this study was to test the hypothesis that deformation of the human peripheral vasculature could be qualitatively and quantitatively modeled using an experimental animal. METHODS: Anteroposterior contrast angiography was performed in domestic Landrace-Yorkshire farm pigs. Images were obtained with the hind limbs naturally extended then repeated, (1) flexed approximately 90° at the hip and knee, (2) overflexed in a nonphysiological fashion. Quantitative vascular angiographic analysis was utilized to measure arterial diameter, length, and deformation. Percent axial arterial compression and bending were assessed. RESULTS: Eight iliofemoral arteries in four animals were imaged. Mean luminal diameters of the iliac and femoral segments in the neutral position were 5.4 ± 0.5 mm and 4.6 ± 0.5 mm. Hind limb physiologic flexion induced profound arterial compression, 17 ± 8% and 29 ± 6% and bending, 36°±10° and 76° ± 13° within the iliac and femoral segments, respectively. With extreme flexion, the femoral artery could be reliably bent >90°. The observed findings exceeded the deformation observed historically within the human superficial femoral (∼5% compression and 10° bending) and popliteal artery (∼10% compression and 70° bending). CONCLUSIONS: Significant nonradial deformation of the porcine iliofemoral arteries was observed during manual hind limb flexion and exceeded that typically observed in humans. This model constitutes a "worst case" scenario for testing deformation and fatigue of intravascular devices indicated for the human peripheral vasculature.

Evaluation of Magnesium-based Medical Devices in Preclinical Studies: Challenges and Points to Consider.

Absorbable metallic implants have been under investigation for more than a century. Animal and human studies have shown that magnesium (Mg) alloys can be safely used in bioresorbable scaffolds. Several cardiovascular and orthopedic biodegradable metallic devices have recently been approved for use in humans. Bioresorbable Mg implants present many advantages when compared to bioabsorbable polymer or nonabsorbable metallic implants, including similar strength and mechanical properties as existing implant-grade metals without the drawbacks of permanence or need for implant removal. Imaging visibility is also improved compared to polymeric devices. Additionally, with Mg-based cardiovascular stents, the risk of late stent thrombosis and need for long-term anti-platelet therapy may be reduced as the host tissue absorbs the Mg degradation products and the morphology of the vessel returns to a near-normal state. Absorbable Mg implants present challenges in the conduct of preclinical animal studies and interpretation of pathology data due to their particular degradation process associated with gas production and release of by-products. This article will review the different uses of Mg implants, the Mg alloys, the distinctive degradation features of Mg, and the challenges confronting pathologists at tissue collection, fixation, imaging, slide preparation, evaluation, and interpretation of Mg implants.

In vitro and in vivo degradation of microfiber bioresorbable coronary scaffold.

The degradation of Mirage Bioresorbable Microfiber Scaffold was evaluated in vitro and in vivo. The degradation in polymer molecular weight (MW), strut morphology, and integrity was accessed using gel permeation chromatography (GPC), X-ray micro-computed tomography (micro-CT) evaluation. To simulate the physiological degradation in vitro, scaffolds were deployed in silicone mock vessels connected to a peristaltic pumping system, which pumps 37°C phosphate-buffered saline (PBS, pH 7.4) at a constant rate. At various time points (30D, 60D, 90D, 180D, 270D, and 360D), the MW of microfibers decreased to 57.3, 49.8, 36.9, 13.9, 6.4, and 5.1% against the baseline. The in vivo degradation study was performed by implanting scaffolds in internal thoracic arteries (ITAs) of mini-swine. At the scheduled sacrifice time points (30D, 90D, 180D, 270D, 360D, and 540D), the implanted ITAs were excised for GPC analysis; the MW of the implanted scaffolds dropped to 58.5, 34.7, 24.8, 16.1, 12.9, and 7.1, respectively. Mass loss of scaffolds reached 72.4% at 540D of implantation. Two stages of hydrolysis were observed in in vitro and in vivo degradation kinetics, and the statistical analysis suggested a positive correlation between in vivo and in vitro degradation. After 6 months of incubation in animals, significant strut degradation was seen in the micro-CT evaluation in all sections as strut fragments and separations. The micro-CT results further confirmed that every sample at 720D had X-ray transmission similar to surrounding tissue, thereby indicating full degradation within 2 years. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1842-1850, 2018.

Vascular response and mechanical integrity of the new biodegradable polymer coated sirolimus-eluting PROLIM stent implanted in porcine coronary arteries.

BACKGROUND: Although durable polymer coated drug-eluting stents (DES) are standard care in percutaneous coronary interventions, new stent platforms employing biodegradable polymer based drug delivery are increasingly being used in clinical practice. AIM: To evaluate the short- (28 days) and medium-term (90 days) vascular effects of the new biodegradable polymer coated sirolimus-eluting stent - the PROLIM stent. METHODS: The objectives of the study were evaluated using standard angiographic and histological methods. In addition, the mechanical integrity of tested stents was assessed using Faxitron imaging. A total of 12 PROLIM stents, 11 biodegradable polymer only coated stents (BPCS), and 12 bare metal stents (BMS) were implanted in the coronary arteries of 16 female non-atheroslerotic domestic swine using an overstretch of 1.1:1.0. RESULTS: At 28 days, neointimal proliferation was significantly lower in the PROLIM and BMS stents compared to the BPCS stents (p ≤ 0.05). Interestingly, despite thin neointima found at this time in the PROLIM group, there was a further significant decrease in neointimal formation between 28 and 90 days (p = 0.04). Although a statistically bigger neointima was found in BPCS stents at 28 days compared to the PROLIM and BMS stents, there was a 50% decrease in the neointimal area at 90 days follow-up (p = 0.02) which reached the level seen in other groups. The endothelialisation was completed in all tested stents after 28 days. There was a significant increase of fibrin depositions in the PROLIM treated arteries at 28 days which were resorbed nearly completely at 90 days follow-up. At 28 days, the inflammatory response was found to be numerically higher in the BPCS stents (p = NS) compared to other tested groups. On the contrary, at 90 days follow-up when the degradation process of the polymer had been completed, the inflammatory reaction decreased substantially to the level seen in the PROLIM and BMS stents. Faxitron analysis of the stented arteries revealed no major abnormalities except for isolated strut fractures observed in the mid portions of two BMS stents and one BPCS stent. CONCLUSIONS: The PROLIM - a biodegradable polymer coated sirolimus-eluting stent - demonstrates very good short-term and medium-term angiographic and histological results. The lack of 'catch-up phenomenon', fast endothelialisation process, and minimal inflammatory reaction may contribute to favourable clinical outcomes using PROLIM stents.

Comparative vascular responses three months after paclitaxel and everolimus-eluting stent implantation in streptozotocin-induced diabetic porcine coronary arteries.

BACKGROUND: Diabetes remains a significant risk factor for restenosis/thrombosis following stenting. Although vascular healing responses following drug-eluting stent (DES) treatment have been characterized previously in healthy animals, comparative assessments of different DES in a large animal model with isolated features of diabetes remains limited. We aimed to comparatively assess the vascular response to paclitaxel-eluting (PES) and everolimus-eluting (EES) stents in a porcine coronary model of streptozotocin (STZ)-induced type I diabetes. METHOD: Twelve Yucatan swine were induced hyperglycemic with a single STZ dose intravenously to ablate pancreatic ß-cells. After two months, each animal received one XIENCE V® (EES) and one Taxus Liberte (PES) stent, respectively, in each coronary artery. After three months, vascular healing was assessed by angiography and histomorphometry. Comparative in vitro effects of everolimus and paclitaxel (10-5 M-10-12 M) after 24 hours on carotid endothelial (EC) and smooth muscle (SMC) cell viability under hyperglycemic (42 mM) conditions were assayed by ELISA. Caspase-3 fluorescent assay was used to quantify caspase-3 activity of EC treated with everolimus or paclitaxel (10-5 M, 10-7 M) for 24 hours. RESULTS: After 3 months, EES reduced neointimal area (1.60 ± 0.41 mm, p < 0.001) with trends toward reduced % diameter stenosis (11.2 ± 9.8%, p = 0.12) and angiographic late-loss (0.28 ± 0.30 mm, p = 0.058) compared to PES (neointimal area: 2.74 ± 0.58 mm, % diameter stenosis: 19.3 ± 14.7%, late loss: 0.55 ± 0.53 mm). Histopathology revealed increased inflammation scores (0.54 ± 0.21 vs. 0.08 ± 0.05), greater medial necrosis grade (0.52 ± 0.26 vs. 0.0 ± 0.0), and persistently elevated fibrin scores (1.60 ± 0.60 vs. 0.63 ± 0.41) with PES compared to EES (p < 0.05). In vitro, paclitaxel significantly increased (p < 0.05) EC/SMC apoptosis/necrosis at high concentrations (≥ 10-7 M), while everolimus did not affect EC/SMC apoptosis/necrosis within the dose range tested. In ECs, paclitaxel (10-5 M) significantly increased caspase-3 activity (p < 0.05) while everolimus had no effect. CONCLUSION: After 3 months, both DES exhibited signs of delayed healing in a STZ-induced diabetic swine model. PES exhibited greater neointimal area, increased inflammation, greater medial necrosis, and persistent fibrin compared to EES. Differential effects of everolimus and paclitaxel on vascular cell viability may potentially be a factor in regulating delayed healing observed with PES. Further investigation of molecular mechanisms may aid future development of stent-based therapies in treating coronary artery disease in diabetic patients.

Inhibition of hypoxia-induced angiogenesis by cigarette smoke exposure: impairment of the HIF-1alpha/VEGF pathway.

Smoking is a major risk factor for atherosclerotic diseases. However, the impact of cigarette smoke exposure on neovascularization that develops in response to tissue ischemia is unknown. Here we demonstrate that cigarette smoke extracts inhibit hypoxia-induced in vitro angiogenesis (matrigel assay) in human umbilical vascular endothelial cells. In vivo, mice exposed to cigarette smoke (MES) were shown to have a significant impairment of angiogenesis following surgically induced hindlimb ischemia. The reduced angiogenic response in MES was documented by Laser Doppler flow perfusion studies and capillary density analyses in ischemic hindlimbs. Inhibition of angiogenesis by cigarette smoke in vitro and in vivo was associated with a reduced expression of hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) in hypoxic conditions. Administration of an adenoviral vector encoding for HIF-1alpha/VP16, a hybrid transcription factor that is stable in hypoxic and normoxic conditions, restored VEGF expression and completely reversed the cigarette smoke inhibition of angiogenesis in hypoxic conditions. Taken together, these results suggest that cigarette smoke exposure impairs angiogenesis by inhibiting VEGF through decreased expression of HIF-1alpha in hypoxic conditions.