eG Coated Stents Exhibit Enhanced Endothelial Wound Healing Characteristics

Abstract

Purpose: Despite their widespread use, a significant fraction of coronary stents suffer from in-stent restenosis and stent thrombosis. Stent deployment induces extensive injury to the vascular endothelium. Rapid endothelial wound closure is essential for the success of a stenting procedure. A recent study has demonstrated that the BuMA Supreme® sirolimus-eluting stent exhibits particularly attractive strut coverage characteristics. A unique feature of this stent is the presence of a thin brush layer of poly-butyl methacrylate (PBMA), covalently bonded to the stent's cobalt-chromium frame via electro-grafting (eG™). The present study aimed to determine whether the PBMA coating has an effect on endothelial cell wound healing and stent strut coverage.

Methods: We used an in vitro coronary artery model whose wall consisted of an annular collagen hydrogel and whose luminal surface was lined with a monolayer of endothelial cells. Mechanical wounding of the endothelial lining was preformed prior to deployment of a bare cobalt-chromium stent either with or without the PBMA layer. The migration of fluorescently labeled endothelial cells was monitored automatically over a period of 48 h to determine endothelial wound healing rates.

Results: Quantitative assessment of endothelial wound healing rates within the simulated arterial model is achievable using automated image analysis. Wound healing is significantly faster (44% faster at 48 h) for stents with the PBMA eG Coating™ compared to bare metal stents.

Conclusion: The PBMA eG Coating™ has the effect of promoting endothelial wound healing. Future studies will focus on elucidating the mechanistic basis of this observation.

Keywords: Automated image analysis; BuMA; Polymer brush coating; Simulated arterial model; Stent; Wound healing.

Figure 1

(a) Schematic of a cross-sectional representation of PBMA, covalently bonded to the Co–Cr surface via electro-grafting (eG™), and therapeutic coating of PLGA and sirolimus interdigitated into the eG Coating™. (b) SEM image of a BuMA Supreme SES strut cross-section, showing the 200 nm electo-grafted layer and 4–10 µm therapeutic coating.

Figure 2

Schematic representation of the in vitro simulated coronary artery model.

Figure 3

Phase contrast image of a stent implanted in the simulated arterial model. Note the slight deformation of the collagen after balloon inflation and stent deployment (due to stent over-expansion).

Figure 4

ROI selection in the in vitro simulated arterial model for endothelial wound healing quantification. Four ROIs were selected upstream of the stent, and 8 ROIs were defined in the stented region. ROIs were maintained over time to track the wound healing in the same regions. The green arrow denotes the direction of flow. The green box is an example of an area of collagen imperfection (air bubble in this case) which was not included in the analysis.

Figure 5

(a) Flow chart illustrating the Matlab-based process used to analyze the wounded area in each ROI. (b) Histogram-based threshold selection.

Figure 6

Evolution of the wound and the mask from which the wounded area is calculated over time. Images follow an enhancement process (Original ROI) prior to histogram analysis to obtain the threshold value. Images are divided by this threshold value (Threshold). A final step of filtration produces the final image (Final mask) from which the area covered (and not covered) by cells is calculated to obtain the wound healing rates. The fourth column (Contour) shows the contour (in red) of the final mask overlayed onto the Original ROI. Comparison of the final mask to the original ROI demonstrates the capability of the Matlab routine to accurately capture the wounded area on the original ROI.

Figure 7

Mean values of normalized wound area for each stent type over time. Bare metal stents (in blue) provided a lower healing rate in comparison with the BuMA eG-BMS stents (in purple). Data are based on five experiments for each type of stent. *Statistically significant differences (p < 0.05) which were observed for all time points after t = 24.5 h.