Development of in vitro models for the investigation of stent-cell interactions

Although stenting as a medical procedure is well established, it is clear from clinical data that there is much left to learn about the long-term impact of in-stent restenosis (ISR) and how it can be prevented. Inadequacies in traditional 2D cell culture and poor in vitro and in vivo correlation contribute to this knowledge gap. In vitro cell studies of hydrophobic drugs, frequently used in drug eluting stents to combat ISR, face difficulties associated with their low aqueous solubility. Solubilization strategies that dissolve the drugs during in vitro assays result in drug concentrations that are not representative of those that will occur in vivo. This may result in non-representative changes in cell behavior, leading to further poor in vitro and in vivo correlation. Furthermore, a large share of the in vitro studies do not consider the dynamic flow of blood through a layer of cells, as occurs in the body. ISR, or re-narrowing of blood vessels, develops through a complex cascade of cellular events and is a pathological response to stent-induced vascular injury. Stent-induced vascular injury is manifested by removal of the endothelium and phenotypic changes in the underlying medial smooth muscle cells layer. This results in pathological vascular remodelling called neointimal hyperplasia (NIH). While drug-eluting stents contain anti-proliferative agents to inhibit the proliferation of smooth muscle cells (SMC), they also delay the regrowth of the intimal endothelial cells (EC) resulting in the subsequent development of late stent thrombosis. It is proposed that promoting rapid endothelial repair can minimize formation of NIH by preventing phenotypic switch in SMCs. The work presented in this thesis focuses on: i) a novel method development to investigate existing stent coatings and their impact on vascular repair, ii) evaluation of a novel bioactive stent coating candidate and its role in promoting vascular healing and iii) the establishment of a 3D cellularized tubular vascular model to bridge the gap between in vitro and in vivo studies. In the first phase of the project, a method for the preparation of stent conditioned media for in vitro evaluation of drug eluting stents was proposed and validated. In the next phase, citric acid as a novel bioactive candidate for stent coatings was investigated by studying its impact on endothelial growth and inflammation. As a last section of this project, a 3D vascular model was established and biomechanically characterized.