Development of a total atherosclerotic occlusion with cell-mediated calcium deposits in a rabbit femoral artery using tissue-engineering scaffolds

Failure Analysis of TEVG's I: Overcoming the Initial Stages of Blood Material Interaction and Stabilization of the Immune Response

Vascular grafts (VG) are medical devices intended to replace the function of a diseased vessel. Current approaches use non-biodegradable materials that struggle to maintain patency under complex hemodynamic conditions. Even with the current advances in tissue engineering and regenerative medicine with the tissue engineered vascular grafts (TEVGs), the cellular response is not yet close to mimicking the biological function of native vessels, and the understanding of the interactions between cells from the blood and the vascular wall with the material in operative conditions is much needed. These interactions change over time after the implantation of the graft. Here we aim to analyze the current knowledge in bio-molecular interactions between blood components, cells and materials that lead either to an early failure or to the stabilization of the vascular graft before the wall regeneration begins.

A calcified chronic total occlusion preclinical model

OBJECTIVE

To create an experimental chronic total occlusion (CTO) model with calcification by dietary modification (cholesterol, calcium carbonate, vitamin D) and local injection of pro-calcification factors (dipotassium phosphate, calcium chloride, and bone morphogenetic protein-2 [BMP-2]).

BACKGROUND

Percutaneous revascularization of CTOs frequently fails in heavily calcified occlusions. Development of novel approaches requires a reproducible preclinical model of calcified CTO.

METHODS

CTOs were created in 18 femoral arteries of 9 New Zealand White rabbits using the thrombin injection model. Dietary interventions included a high cholesterol diet (0.5% or 0.25%), calcium carbonate (150 mg × 3-5 days/week), and vitamin D (50,000 U × 3-5 days/week). In selected animals, BMP-2 (1-4 μg), dipotassium phosphate, and calcium chloride were injected locally at the time of CTO creation. Animals were sacrificed at 2 weeks (n = 4 arteries), 6 weeks (n = 4 arteries), and 10-12 weeks (n = 14 arteries).

RESULTS

CTOs showed evidence of chronic lipid feeding (foam cells) and chronic inflammation (intimal/medial fibrosis and microvessels, inflammatory cells, internal elastic lamina disruption). In calcium/vitamin D supplemented rabbits, mineralization (calcification and/or ossification) was evident as early as 2 weeks post CTO creation, and in 78% of the overall arteries. Mineralization changes were not present in the absence of calcium/vitamin D dietary supplements. Mineralization occurred in 85% of BMP-treated arteries and 60% of arteries without BMP.

CONCLUSIONS

Complex mineralization occurs in preclinical CTO models with dietary supplementation of cholesterol with vitamin D and calcium.

Improving Surgical Methods for Studying Vascular Grafts in Animal Models

While clinical vascular grafting uses an end-to-side surgical method, researchers primarily use end-to-end implant techniques in preclinical models. This may be due in part to the limitations of using small animal models in research. The work presented here provides support and evidence for the improvement of vascular graft implant techniques by demonstrating the successful implantation of experimental grafts into both large and small animal models. Specifically, models of aortoiliac baboon (Papio anubis) bypass and common carotid rabbit (Oryctolagus cuniculus) bypass were used to test vascular grafts for thrombosis and vascular healing after 1 month using an end-to-side anastomosis grafting procedure. Patency was evaluated with ultrasound or histological techniques, and neointimal growth was quantified with histology. In the development of this procedure for small animals, both an end-to-end/end-to-side and an end-to-side/end-to-side configuration were tested in rabbits. One hundred percent of rabbit implants (2/2) with an end-to-end/end-to-side configuration were patent at explant. However, with the end-to-side/end-to-side configuration, 66% (6/9) of rabbit implants and 93% (13/14) of baboon implants remained patent at 1 month, suggesting the importance of replicating the end-to-side method for testing vascular grafts for clinical use. This study describes feasible preclinical surgical procedures, which simulate clinical vascular bypass grafts even in small animals. Widespread implementation of these end-to-side surgical techniques in these or other animals should improve the quality of experimental, preclinical testing and ultimately increase the likelihood of translating new vascular graft technologies into clinical applications.

Animal chronic total occlusion models: A review of the current literature and future goals

Coronary chronic total occlusions (CTOs) are commonly found in patients undergoing coronary angiography and is associated with poorer prognosis than in those patients with other forms of stable coronary artery disease. As such, with an increasing appreciation of this clinical entity, there is a need to identify, firstly the pathophysiological process driving its formation, as well as new percutaneous strategies for revascularisation with long term durability and improved outcomes. An appropriate, reliable and reproducible animal model is vital for both of these objectives. We review the prevalence of spontaneous collaterals in different species, as well as review the current literature with respect to animal models of CTOs, and compare and contrast the advantages and disadvantages of these differing models. Whilst both extrinsic compression models and endoluminal procedures may create situations analogous to a CTO in a human, the ideal animal model of a CTO will include an occluded artery, functional collaterals and a viable myocardium. This would allow study of the process driving collateral formation and arteriogenesis as well as percutaneous intervention strategies for both acute and long term benefits.

Animal models for vascular tissue-engineering

Because of rise in cardiovascular disease throughout the world, there is increasing demand for small diameter blood vessels as replacement grafts. The present review focuses on the animal models that have been used to test small-diameter TEVs with emphasis on the attributes of each model. Small animal models are used to test short-term patency and address mechanistic hypotheses; and large, preclinical animal models are employed to test long-term patency, remodeling and function in an environment mimicking human physiology. We also discuss recent clinical trials that employed laboratory fabricated TEVs and showed very promising results. Ultimately, animal models provide a testing platform for optimizing vascular grafts before clinical use in patients without suitable autologous vessels.