Fibrin-based delivery strategies for acute and chronic wound healing

Abstract

Fibrin, a natural hydrogel, is the end product of the physiological blood coagulation cascade and naturally involved in wound healing. Beyond its role in hemostasis, it acts as a local reservoir for growth factors and as a provisional matrix for invading cells that drive the regenerative process. Its unique intrinsic features do not only promote wound healing directly via modulation of cell behavior but it can also be fine-tuned to evolve into a delivery system for sustained release of therapeutic biomolecules, cells and gene vectors. To further augment tissue regeneration potential, current strategies exploit and modify the chemical and physical characteristics of fibrin to employ combined incorporation of several factors and their timed release. In this work we show advanced therapeutic approaches employing fibrin matrices in wound healing and cover the many possibilities fibrin offers to the field of regenerative medicine.

Introduction

Chronic wounds represent a significant and often underestimated worldwide socio-economic burden to both patients and healthcare systems. In the United States only, approximately 6.5 million patients are affected by chronic wounds, with projected annual healthcare costs of more than US$25 billion [1], [2]. Often disguised as a co-morbidity, chronic wounds represent a silent epidemic and exact numbers on global prevalence and incidence are still missing [3]. In Denmark, for example, it is estimated that 1% of the population in developed countries will suffer from a chronic wound [4] and the direct and indirect socio-economic impacts are growing rapidly due to global population aging and the sharp rise in the incidence of diabetes and obesity. In general, every disruption of a previously normal anatomic tissue structure with consecutive loss of function can be classified as a wound. Normally, these wounds are repaired through a distinct timely and spatially orchestrated wound healing process. Chronic wounds, however, are defined as wounds that fail to proceed through this orderly and timely reparative process with full restoration of continuity within 3 months [5]. These full-thickness chronic wounds, also referred to as ulcers, often develop in patients with diabetes or vascular diseases and are attributed with slow or non-healing tendency, especially in the elderly [6]. Some common features of these wounds include prolonged or excessive inflammation, persistent infections, formation of microbial biofilms and the inability of dermal or epidermal cells to respond to reparative stimuli, which altogether results in the failure of the wound to heal [7]. In this respect, a high microbial load can severely impair acute and chronic wound healing, which is especially challenging in oral cavity wound care [8].

Wound healing is a dynamic and highly conserved process that involves the complex and coordinated interplay of multiple cell types, growth factors, chemo-/cytokines and components of the extracellular matrix (ECM). Together, these elements drive the overlapping stages of wound healing: hemostasis, inflammation, reepithelialization, neovascularization, collagen deposition and matrix remodeling with scar formation. Today it is well known that fibrin, the end product of the physiological blood coagulation cascade, plays a pivotal role in many of these processes [9], [10]. During the final step of this cascade, circulating fibrinogen is cleaved by the action of the serine protease thrombin, resulting in the formation of an insoluble fibrin clot. In addition, thrombin activates the transglutaminase Factor XIII (FXIIIa) which further stabilizes the clot by cross-linking of fibrin polymers [11], [12]. While hemostasis is the primary function of fibrin in wound healing, the blood clot also serves as a provisional matrix for many cell types involved in later stages of tissue repair, making fibrin one of the most important ECM molecules in the wound bed. Under physiological conditions, the blood clot is not a permanent structure. To preserve the hemostatic balance, fibrinogenesis and blood coagulation are counteracted by the fibrinolytic system. In the course of fibroplasia and matrix remodeling, fibrinolysis occurs through the actions of plasminogen (PLG) which is activated by tissue-type plasminogen activator (tPA) or urokinase-type plasminogen activator (uPA). These activators are secreted by several cells in the wound bed and convert PLG to plasmin, leading to fibrin degradation [13], [14], [15]. Maintenance of the hemostatic balance is of great significance for tissue repair, as both fibrin(ogen) and its degradation products exert multiple regulatory functions in wound healing such as immunomodulation [16], growth factor sequestration [17], [18] and modulation of cell phenotype, function and migration [19], [20], [21], [22].

The necessity to effectively manage hemostasis and wound healing during surgical interventions has led to the development and commercialization of fibrin sealants (fibrin glues). These two-component plasma-derived products consist of fibrinogen/FXIII and thrombin concentrates which, when mixed, mimic the final stage of blood clotting, that is, the formation of a fibrin clot [23]. Owing their efficiency, biocompatibility and -degradability, commercially available fibrin sealants have been granted clinical approval in a variety of surgical settings [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. More recently, fibrin sealants have emerged as scaffold materials in the rapidly expanding field of tissue engineering and regenerative medicine. The vast amount of studies has demonstrated the feasibility of fibrin as a carrier for a variety of cell types [34], [35], [36], [37]. Furthermore, diverse fibrin formulations have been successfully utilized for the localized and controlled delivery of drugs and growth factors [38], [39], [40], [41]. In addition, fibrin-based gene therapy approaches have attracted attention and the viral and non-viral delivery of therapeutic gene vectors has been explored [39], [42], [43]. This review highlights the advantageous intrinsic qualities of fibrin for tissue engineering-based wound healing approaches, sustained delivery of therapeutic biomolecules and for efficient transplantation of cells. Furthermore, strategies to tailor and control growth factor release kinetics for optimized therapeutic outcome will be addressed. Since wound healing requires the spatiotemporal provision of several stimuli, recent advances in combinational delivery approaches such as dual/sequential growth factor release or release of cells and growth factors will also be discussed.