No abstract

Bioabsorbable drug-eluting stents (BDES) offer multiple advantages over a permanent bare metal stent (BMS) for coronary artery disease (CAD). However, current BDES remains two major issues: inferior radial strength and biocompatibility. PowerStent Absorb BDES, fabricated by co-formulating amorphous calcium phosphate (ACP) nanoparticles with poly-L-lactic acid (PLLA/ACP, 98/2, w/w) and 2% Paclitaxel (PAX, w/w) was designed to address these issues. Two cohorts of 6 miniature pigs were each implanted with PLLA/PAX (control, 2% PAX, w/w) or PowerStent Absorb BDES. After 1 month in-vivo study, histological analyses showed significantly reduced restenosis in the PowerStent Absorb BDES cohort relative to the control cohort (44.49 +/- 410.49% vs. 64.47 +/- 16.2%, p < 0.05). Stent recoil (21.57 +/- 5.36% vs. 33.81 +/- 11.49, P < 0.05) and inflammation (3.01 +/- 0.62 vs. 4.07 +/- 0.86, P < 0.01) were also obviously decreased. From in-vitro studies, PLLA/ACP/PAX stent tube maintained significantly greater radial strength than control group during 6 months in-vitro degradation (PLLA/ACP/PAX vs. PLLA/PAX: before hydrolysis: 82.4 +/- 1.9 N vs.74.8 +/- 3.8 N; 6 weeks: 73.9 +/- 1.8 N vs. 68.0 +/- 5.3 N; 3 months: 73.5 +/- 3.4 N vs.67.2 +/- 3.8 N; 6 months: 56.3 +/- 8.1 N vs. 57.5 +/- 4.9 N). Moreover, ACP facilitated the hydrolytic degradation of PLLA compared with control one (62.6% vs. 49.8%), meanwhile, it also increased the crystallinity of PLLA (58.4% vs. 50.7%) at 6 months. From SEM observations, ACP created nanometer pores that enlarge gradually to a micrometer scale as degradation proceeds. The changes of the porosity may result in greatly promoting re-endothelialization.

Biodegradable polymers used as vascular stent coatings and stent platforms encounter a major challenge: biocompatibility in vivo, which plays an important role in in-stent restenosis (ISR). Co-formulating amorphous calcium phosphate (ACP) into poly(lactic-co-glycolic acid) (PLGA) or poly-L-lactic acid (PLLA) was investigated to address the issue. For stent coating applications, metal stents were coated with polyethylene-co-vinyl acetate/poly-n-butyl methacrylate (PEVA/PBMA), PLGA or PLGA/ACP composites, and implanted into rat aortas for one and three months. Comparing with both PEVA/PBMA and PLGA groups after one month, the results showed that stents coated with PLGA/ACP had significantly reduced restenosis (PLGA/ACP vs. PEVA/PBMA vs. PLGA: 21.24 +/- 2.59% vs. 27.54 +/- 1.19% vs. 32.12 +/- 3.93%, P < 0.05), reduced inflammation (1.25 +/- 0.35 vs. 1.77 +/- 0.38 vs. 2.30 +/- 0.21, P < 0.05) and increased speed of re-endothelialization (1.78 +/- 0.46 vs. 1.17 +/- 0.18 vs. 1.20 +/- 0.18, P < 0.05). After three months, the PLGA/ACP group still displayed lower inflammation score (1.33 +/- 0.33 vs. 2.27 +/- 0.55, P < 0.05) and higher endothelial scores (2.33 +/- 0.33 vs. 1.20 +/- 0.18, P < 0.05) as compared with the PEVA/PBMA group. Moreover, for stent platform applications, PLLA/ACP stent tube significantly reduced the inflammatory cells infiltration in the vessel walls of rabbit iliac arteries relative to their PLLA cohort (NF-kappaB-positive cells: 23.31 +/- 2.33/mm2 vs. 9.34 +/- 1.35/mm2, P < 0.05). No systemic biochemical or pathological evidence of toxicity was found in either PLGA/ACP or PLLA/ACP. The co-formulation of ACP into PLGA and PLLA resulted in improved biocompatibility without systemic toxicity.

Our previous studies have confirmed the superior biocompatibility of the poly-L-lactic acid/amorphous calcium phosphate (PLLA/ACP) scaffolds (PowerScaffold) compared to PLLA scaffolds and their similar 6-month radial strength compared with TAXUS stents. In order to conduct further dynamic observations on the performance of the PowerScaffold after 12-month implantation compared with the TAXUS stents. Twenty PowerScaffold and 20 TAXUS were implanted in porcine coronary arteries. At 12-month follow-up, Quantitative Coronary Angiography showed that the stent reference vessel diameter (3.19 ± 0.25 mm vs. 2.75 ± 0.22 mm, p < 0.05), the mean lumen diameter (3.07 ± 0.22 mm vs. 2.70 ± 0.17 mm, p < 0.05) and the late lumen gain (0.45 ± 0.07 mm vs. 0.06 ± 0.06 mm, p < 0.01) were all significantly greater with the PowerScaffold than the TAXUS. As well, Intravascular Ultrasound showed the stent reference vessel area (7.74 ± 0.48 mm2 vs. 6.96 ± 0.51 mm2, p < 0.05), the mean stent area (7.49 ± 0.46 mm2 vs. 6.53 ± 0.47 mm2, p < 0.05) and the mean lumen area (7.22 ± 0.50 mm2 vs. 6.00 ± 0.48 mm2, p < 0.01) were all significantly greater with the PowerScaffold than the TAXUS. The luminal patency rate of the PowerScaffold significantly increased from 72.45 ± 6.84% at 1 month to 93.54 ± 8.15% at 12 months (p < 0.01) while the TAXUS stents were associated with a non-significant decreasing trend (89.44 ± 8.44% vs. 86.53 ± 8.22%). Pathology indicated the average thickness of the struts degraded by 14.25 ± 3.04 μm at 1 month, 23.39 ± 2.45 μm at 6 months and 35.54 ± 2.20 μm at 12 months. Immunohistochemical examination showed that the expression of inflammatory factors NF-κB gradually decreased from 1-month to 12-month (36.79 ± 4.78 vs. 5.79 ± 2.85, P < 0.01). As the late lumen gain of arteries implanted with the PowerScaffold increases over time with the growth of vessels, it effectively reverse the late vascular negative remodeling observed with the TAXUS stents, providing a better option for lumen restoration treatment in clinical practice.

At six-month post-implantation, the PowerStent Absorb stents maintained their structural strength and functional performance. The development of restenosis was controlled, no stent thrombosis was observed and the stents were fully re-endothelialized. These results suggest the PowerStent Absorb stent is safe and effective for up to 6 months when implanted in porcine coronary arteries.