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C H A P T E R 9

Acellular Biomaterials for

Cardiac Repair

CHAPTER SUMMARY

An emerging body of evidence suggests that acellular forms of biomaterials have the capability to attenuate and prevent the deterioration in cardiac function after MI and instruct tissue repair. In this chapter, we describe the various acellular biomaterial strategies applied for inducing cardiac repair after MI in small and large animal models, from implantable porous scaffolds made from natural and synthetic polymers and decellularized organ-derived matrices, to injectable biomaterials, which present a more applicable clinical strategy. We then describe an alginate-based material, developed by our group, the first-in-man example of a biomaterial designed to be delivered as a solution via intracoronary injection and only at infarct to undergo gelation. This treatment has been proved to be safe in phase I/II clinical study. Finally, the last section speculates on the mechanism of biomaterial effects in cardiac repair.

9.1INTRODUCTION

In the applications discussed so far, biomaterials have been used either as a vehicle for cardiac cell delivery into a damaged myocardium or as a platform for engineering of a functional cardiac patch. Although showing promising results in preclinical studies, the clinical implementation of the cellular-based strategies in patients still faces several important obstacles. A major one is the lack of a relevant cell source for transplantation. The various approaches for cell differentiation of pluripotent stem cells into cardiomyocytes still suffer from issues like low efficiency, ethical and/or technical difficulties, and safety concerns. The quest for the most suitable adult autologous stem cell source as a temporary alternative is also still ongoing.

Meanwhile, the potential of biomaterials as an acellular therapeutic strategy has been continuously expanded and established over the last decade or so. The biomaterials, initially developed for exogenous cell delivery and tissue engineering, are now investigated as a provisional supporting matrix for migrating host cells after implantation onto the damaged tissue.

In cardiac tissue engineering and regeneration, biomaterials have a great potential to play an especially critical role as compared to other organs. As mentioned in Chapter 2, adverse remodeling after MI has been associated with excessive damage to the cardiac ECM, which plays a critical role in the maintenance, integrity, and function of the heart tissue. These dynamic changes and alterations

118 9. ACELLULAR BIOMATERIALS FOR CARDIAC REPAIR

in cardiac ECM composition have a detrimental effect on both systolic and diastolic function [1, 2]. Biomaterials can be designed and developed to mimic cardiac ECM properties, such as chemistry, mechanical properties, morphology, hydrogel formation, and degradability. The achievements in the last decade or so provide a proof-of-concept for this strategy, by showing that biomaterials are capable of providing temporary tissue support, preserving cardiac function, facilitating self-repair, and inducing cardiac tissue regeneration.

9.2ACELLULAR IMPLANTABLE SCAFFOLDS FOR IN SITU TISSUE SUPPORT

Implantable porous scaffolds have been investigated as a therapeutic strategy to thicken the infarct zone and by doing so to reduce the wall stress and attenuate LV remodeling after MI. When conceiving this strategy, it was hypothesized that the scaffold porous structure would enable its repopulation by infiltrating cells from the host, including vascular cells, and that after a complete integration of the patch to infarct zone, cardiac repair would be achieved. Since the inception of this strategy [3], several groups have investigated various types of porous scaffolds made of collagen or synthetic polymers [4].

Callegari et al implanted an acellular porous scaffold made of type I collagen on a rat infarcted heart, immediately after cryoinjury, and examined the degree of scaffold vascularization and tissue infiltration for up to 60 days [5]. The collagen scaffold was highly porous and with interconnected channels, and 60% of the material was resorbed within 60 days. The sponge attracted penetrating macrophages and induced strong angiogenic and arteriogenic responses at the infarct. In another study that also examined the effect of this strategy on functional activity of the heart, Goldman and co-workers used collagen type I scaffold (with a porosity of 70% and an average pore diameter of 30 to 60 μm) to repair non-transmural, not-ST-elevation MI induced by cryoinjury in rats.Three weeks after grafting, the scaffold that was integrated into the myocardium prevented adverse remodeling by reducing LV dilation, and increased angiogenesis. Six weeks after grafting, the scaffold was mostly absorbed by the underlying myocardial tissue [6].

Wagner and co-workers used an elastomeric biodegradable microporous polyester urethane urea (PEUU) scaffold for infarct repair in rats. This scaffold possessed 91% porosity and a 91 μm average pore size. The scaffold was placed and affixed on the infarct area two weeks after coronary artery ligation, and the outcome was analyzed after additional eight weeks. The patch region in the infarct was infiltrated with smooth muscle bundles with increased capillary density compared to control untreated animals. The treatment prevented LV dilation and improved cardiac function. These benefits were attributed to LV dilatation prevention, increased scar thickness, and reduced wall stress [7].

Although there are differences between the different studies, including the material in use and the extent of porosity and pore size, as well as different animal models, the strategy of implanting macroporous scaffolds to thicken the infarct zone has been proven to yield therapeutical benefits on cardiac repair.

9.3. DECELLULARIZED ECM 119

9.3DECELLULARIZED ECM

In parallel to the development of 3D matrices prepared from natural or synthetic materials, the emerging direction is the use of natural ECM as an appropriate matrix for cardiac tissue engineering through the process of organ decellularization. Decellularization is generally carried out by perfusion of the tissue with various detergents, aiming at cell removal without compromising the integrity and structure of the remaining ECM [8]. In such a way, the native structure and ECM composition is assumed to be preserved, and the matrix can be used for in vitro cell seeding or for instructive tissue restoration and support in situ [9, 10, 11]. The decellularized matrix can be lyophilized and milled to create a powder, which can be subsequently solubilized to create a liquid injectable matrix suitable for catheter delivery [12].

The main type of naturally derived decellularized ECM scaffold used for infarct or myocardial defects repair is prepared from porcine urinary bladder (UB). The ECM of this tissue represents a biologically latent membrane, which is already applied for various tissue engineering applications. Kochupura et al used this matrix for repair of full thickness defect in the right ventricle. Eight weeks after implantation, UB matrix improved regional systolic and diastolic function, compared to Dacron implantation. Importantly, cardiomyocytes were found in the ECM implant region, in contrast to Dacron implantation [9]. In another report, Badylak and co-workers evaluated the effect of UB matrix on infarcted hearts. At 6-8 weeks after the infarct, the matrix was placed as a full thickness LV wall replacement. Three months after implantation, myofibroblast infiltration associated with α- sarcomeric actin-positive cells was observed in the UB matrix group, compared to synthetic expanded polytetrafluoroethylene (ePTFE) patch. Cardiac function was not evaluated [13].

Ideally, the best source of a decellularized matrix for myocardial repair should be the matrix derived from the myocardium, which potentially possesses the composition and structure required for the specific needs of reconstructing the heart. The potential of such cardiac matrix for successful myocardial tissue engineering and bioartificial heart creation was proven in a proof-of-concept study performed by Ott et al, where whole rat hearts were decellularized and re-seeded with cardiac or endothelial cells. These hearts generated modest but detectable contractions and pump function [10]. Wainwright et al prepared intact cardiac ECM from whole porcine heart decellularization. This matrix supported the organization of chicken cardiomyocytes in a typical sarcomere structure, in vitro [14]. Eitan et al prepared cardiac ECM from LVs of porcine hearts, and showed the biocompatibility of the resulted scaffold for successful cell seeding, including cardiomyocytes, that showed functional phenotype and organization [11]. Mirsadraee et al prepared acellular matrix by decellularization of human pericardium. The matrix retained the major structural components and strength of the native tissue and showed biocompatibility when seeded with human dermal fibroblasts [15]. Although not matched to the properties of myocardial tissue, pericardium-derived matrices could represent an autologous option for tissue engineering applications aimed for myocardial support and repair.

Collectively, the use of decellularized matrices from natural ECM sources represents an attractive approach, as in this way the properties of the resulting scaffold may potentially be matched