The goal of therapeutic angiogenesis is to “biologically bypass” an occluded vessel in coronary artery disease (CAD ) or peripheral arterial disease (PAD) by forming collateral blood vessels, thereby relieving ischemia. The notion of therapeutic angiogenesis for coronary and peripheral arterial diseases originates from the seminal work by Dr. Judah Folkman in the early 1970s, who demonstrated that growth factors were responsible for tumor angiogenesis [19]. These growth factors are most commonly identified today as the angiogenic peptides vascular endothelial growth factor (VEGF ) and fibroblast growth factor (FGF ) [9, 10].

Not long following Folkman’s discovery, compelling preclinical data in animal models demonstrated that angiogenic protein or gene delivery improved peripheral and myocardial collateralization, as well as peripheral perfusion and cardiac function [20, 21]. Since then, numerous trials involving over 2000 patients and utilizing administration of angiogenic proteins and their genes (e.g., VEGF-A 121, 165, and 189, FGF-1, and FGF-4) for the treatment of coronary and peripheral vascular disease have yielded mixed results [10, 22–24].

Likely, the disappointing outcomes of angiogenic therapy trials were often due to inappropriate or ineffective routes of administration (e.g., intravascular vs. intramyocardial) that provided inadequate tissue concentrations of angiogenic agents and/or were due to inappropriate end points (e.g., relatively low-resolution myocardial perfusion studies vs. PET or treadmill exercise testing) [9, 10, 13, 14]. These limitations frequently confounded data interpretation, leading many to dismiss the promise of angiogenic (gene) therapy. The unfortunate death of study patient Jesse Gelsinger, who received an abnormally high dose of adenovirus therapy for treatment of ornithine transcarbamylase deficiency, likewise led to a brief moratorium on all gene therapy in the United States, which nevertheless led to a prolonged period of diminished enthusiasm for gene therapy trials [25, 26].

Despite these challenges in this new area of drug development, retrospective analyses of the angiogenic gene therapy clinical trials have yielded useful insights that offer new opportunities for advancing this field. First, it appears clear that the type of angiogenic vector utilized (i.e., protein, plasmid, or virus) is critical to achieving an appropriately therapeutic angiogenic factor “dose” sufficient to induce angiogenesis [27, 28]. In this regard, angiogenic protein delivery typically produced disappointing results likely due to the relatively short half-life and the dose-limiting side effect of hypotension associated with large systemic dosing. Plasmid delivery likewise seems to be ineffective due to its low transduction efficiency into cells and transient expression. In contrast, adenoviral delivery of angiogenic factors allows for a higher transduction efficiency of cells and prolonged expression of genes up to weeks after administration without integration into the genome. Accordingly, trials incorporating adenoviral-mediated delivery were and may prove to be more effective in inducing angiogenesis than are those testing other agents [10, 29–33].

The use of intracoronary vs. direct intramyocardial delivery of angiogenic factors also likely undermined therapeutic efficacy, as did the choice of appropriate tissue treatment targets [15, 28]. More specifically, direct intramyocardial delivery generally yielded more favorable outcomes compared to intracoronary delivery likely because of the more favorable pharmacokinetics of “drug delivery” via this more localized approach compared to the more diffuse, systemic delivery of drug associated with intracoronary or intravascular delivery (Table 16.1) [15]. Parenthetically, the latter also confers the additional potential risk of disseminating angiogenic factors throughout the body, potentially producing “off-target” effects.

Likewise, the application of angiogenic therapies to peripheral vascular vs. coronary disease creates the challenge of effectively treating vascular obstruction at multiple levels over a relatively vast tissue territory (i.e., central inflow disease, diffuse intermediate-level vascular obstruction, and peripheral outflow obstruction), compared to more “geographically” localized coronary disease requiring less total amounts and distribution of drug delivery. The treatment of coronary disease may thus represent an ideal target for angiogenic therapy. While these considerations may seem obvious in retrospect, the failure to consider these basic pharmacokinetic principles often led to analytic generalizations about negative trial outcomes to the entire field and (mis)perceptions that angiogenic therapy was ineffective .

A number of the early angiogenesis trials were also dismissed because they lacked placebo controls and objective, clinically relevant end points, such as electrocardiographic changes associated with exercise treadmill testing [16]. Paradoxically, while inclusion of such placebo controls was relatively easy with (pharmacokinetically ineffective) intravascular trials, performing sham surgery to allow (more effective) direct intramyocardial delivery in patients with end-stage heart disease poses an ethical dilemma [15]. Unfortunately, this led many observers to discredit the positive findings demonstrated by (non-placebo) intramyocardial-based trials , despite positive objective data such as changes in ischemia levels based on ECG data. While a randomized, placebo-controlled trial may ultimately be necessary to conclusively verify the clinical benefit of angiogenic therapy, the development of a uniform study protocols with objective, validated end points, more efficient methods of gene delivery, and improved methods to determine total gene delivery into the target tissue may well offer a pathway to effectively test this new biologic treatment for patients with vascular disease [9, 10, 15, 27, 28].

Given the low regenerative capacity of cardiomyocytes (<1% per year), replacement of cardiomyocytes with exogenous stem cells (embryonic, bone marrow-derived, or induced pluripotent) following myocardial infarction has received much attention over the past two decades. Bone marrow-derived hematopoietic progenitor cells were the first to be implanted, resulting in over 100 phase I-II trials including thousands of patients. Despite the promising safety profile of these cell implants, several meta-analyses revealed that bone marrow-derived progenitor cell implantation therapy led only to a non-clinically relevant increase (i.e., not greater than 2–4%) in left ventricular ejection fraction [7, 8, 34, 35]. This negative outcome has been attributed to the low survival of implanted cells, poor engraftment into the native myocardium, and the formation of immature cardiomyocyte structures (Table 16.2) [36]. The modest improvement in ejection fraction seen in these trials has more recently been alternatively ascribed to the secretion by implanted cells of paracrine signals that might support resident myocyte survival, neovascularization, recruitment of endogenous cardiac progenitor cells, or cardiac remodeling induced by these implants rather than their differentiation into nascent cardiomyocytes [12, 35, 37].

The use of embryonic stem (ES) cells , which possess a potentially more favorable degree of plasticity and proliferative capacity than bone or circulation-derived stem cells, is not ideal as a stem cell therapy for obvious ethical reasons, as well as the risk of immune-mediated rejection necessitating immunosuppression and the formation of heterogeneous cardiac cell subtypes (ventricular cardiomyocytes, atrial cardiomyocytes, or pacemaker cells) or other aberrant tissues such as teratomas [14, 38].

Given these concerns, induced pluripotent stem (iPS) cells , which can be derived directly from the patient and reimplanted into infarcted myocardium, have emerged as potentially more suitable candidates for cell-based regeneration. The advantages of using iPS cells include their similarity to ES cells, unlimited proliferative capacity, reduced risk of immune-mediated rejection, and, of course, more ethically sound procurement [38, 39]. The disadvantages of iPS cells include the cost needed to generate patient-specific iPS cells and their immature development of sarcomeric structures following implantation, immature electrophysiologic properties, electrical ectopy, and the formation of a heterogeneous mixture of cell types , as well as the same implant survival and integration challenges associated with all cell implants into the hostile milieu of the infarcted myocardium [14, 38]. While the implantation of iPS cells into animal infarct models has been shown to lead to 10–15% increases in left ventricular ejection fraction, these results, as with other cell implants, may thus likely not be translatable to clinical efficacy [40–45].

In the context of these mixed results, efforts are currently underway to improve implant cell survival through improved mechanical adherence strategies (e.g., using engineered tissue scaffolds and adhesive biogels) and improved vascularization of the tissue bed to enhance implanted cell survival (e.g., using angiogenic pretreatment) [39, 46–50].

Given the challenges associated with cell implantation therapies for cardiac regeneration, the advent of direct cardiac cellular reprogramming represents a significant potential advancement for this field as it capitalizes on the presence of endogenous, quiescent cardiac fibroblasts to generate induced cardiomyocytes (iCMs) , avoiding nearly all of the challenges posed by exogenous cell implant strategies [6]. In general, this entire field seeks to capitalize upon the identification and administration of the differentiating factors associated with early embryonic cardiac development.

The first iterations of direct cardiac reprogramming using administration of the cardio-differentiating transcription factors Gata4, Mef2c, and Tbx5 (GMT) yielded reprogramming efficiencies of about 7%, as evidenced by expression of the cardiomyocyte marker cardiac troponin T (cTnT) , with an even smaller subset of contractile iCMs [11]. In order to enhance the reprogramming efficiency and the maturation of iCMs, several modifications have been proposed, including the improvement of the cardio-differentiating gene cocktail and incorporation of downregulation of native fibroblast gene expression (Table 16.3).