1. Introduction

344 M. Murakami & M. Simons

2. Concepts and Rationales

The concept of therapeutic angiogenesis is to facilitate blood vessel growth to restore the perfusion to and function of the ischemic tissue. Tissue ischemia refers to a situation when the oxygen supply does not meet the demand necessary to maintain normal tissue function and homeostasis, resulting in impaired organ function and endangered viability. In clinical situations, acute or chronic occlusion of the main feeding artery is largely responsible for development of ischemia although diffuse small vessel arterial disease is also a fairly frequent cause.

Natural biological responses to ischemia include in situ upregulation of angiogenic factors in conjunction with mobilization and recruitment of various cellular components, promoting new vessel growth and arterial remodeling. However, in most circumstances, this endogenous response does not achieve a full compensation of original blood supply, resulting in compromised tissue function and clinical symptoms. Therefore, currently the rationale of therapeutic angiogenesis resides in augmentation and manipulation of this revascularization process to gain a maximum restoration of tissue function by administrating exogenous angiogenic growth factors or cellular products.

The latest progress of vascular biology research has expanded the notion of therapeutic angiogenesis to encompass other types of vascular growth, namely, arteriogenesis and vasculogenesis, defining the term as general enhancement of blood vessel growth.1 For this reason, although the term “therapeutic angiogenesis” is still used and will continue to be used here, “therapeutic neovascularization” is, perhaps, a more appropriate term. While we use therapeutic angiogenesis in a broad sense, blood vessel growth in general is referred to as neovascularization in this chapter. As currently understood, adult neovascularization occurs as a result of several processes, including angiogenesis, arteriogenesis, and potentially vasculogenesis (Table 1).

In its strictest sense angiogenesis, defined as growth of new capillaries, takes place at the site of ischemia, by promoting formation of new capillaries from post-capillary venules. Therefore, it does not augment arterial inflow into the region. In contrast, arteriogenesis, referred to as positive remodeling of pre-existing collaterals or de novo growth of

Table 1. Types of neovascularization.

conduit arteries, typically occurs in the upstream area of ischemia in response to increased shear stress and endothelial activation coupled with the subsequent influx of blood derived-mononuclear cells. The physiological significance of arteriogenesis is well recognized clinically by the development of collateral vessels that bypass the occluded artery and supply arterial inflow to various degrees.

From the point of view of therapy in most cardiovascular settings such as coronary or peripheral arterial disease, arteriogenesis is more appealing as it can increase tissue perfusion to a greater magnitude in comparison to angiogenesis.2 Moreover, increased arterial inflow can trigger tissue regeneration efficiently coupled with concomitant

346 M. Murakami & M. Simons

angiogenesis in the ischemic area. However, the precise mechanism underlying arterial growth is less well understood with its complex nature involving many cell types and driving factors. Angiogenic growth factors are, in general, believed to be positive regulators of arteriogenesis; however, it appears that monocyte chemoattractant factors, such as MCP-1, GM-CSF and PlGF, are another entity of a potent driving force of arteriogenesis.

Vasculogenesis refers to the process of an in situ formation of blood vessels from circulating or tissue-resident endothelial progenitor cells (EPC) and vascular progenitor cells. While probably real, the frequency, feasibility and physiological significance of adult vasculogenesis in the setting of ischemic diseases have not been established conclusively.

3. Strategy

The basic strategy of therapeutic angiogenesis thus far has been reduced to administration in or recruitment to an ischemic area of an angiogenic agent or cellular products. However, the choice of a therapeutic angiogenic agent has been extensively revised as the knowledge of vascular biology grew in the last decade. The biological agents used in therapeutic angiogenesis primarily include angiogenic growth factors in the form of peptide, plasmid DNA and viral vector encoding a cognate sequence, and lately cellular components such as fractionated or unfractionated mononuclear cells. Combination of angiogenic factors and utilization of a master gene that can transcriptionally upregulate multiple angiogenic factors and their receptors have also been explored recently.

At this point, target diseases for therapeutic angiogenesis approaches are limited to peripheral artery disease (critical limb ischemia and claudication), ischemic cardiomyopathy and chronic coronary artery disease, including acute myocardial infarction in case of certain cell therapies. Stroke and its less severe form such as brain hypoperfusion due to carotid occlusion can theoretically be proximal candidates for therapeutic angiogenesis as well. Cell therapy for cardiac repair, in which stem cells are used with the intention of the functional recovery of infracted heart, has lately drawn considerable attention and shown some evidence of improvement of cardiac function.3,4 Apart from the

authenticity of the original concept where transplanted cells transdifferentiate to functioning cardiomyocytes, the alternative mechanism also suggests that these cells stimulate angiogenesis by secreting various growth factors, thus facilitating vascularization in the hibernated myocardium and improving functionality of the heart.

In the last couple of decades, we have experienced three strategic phases of therapeutic angiogenesis.5 With the discovery of angiogenic growth factors in the late 70s and 80s, the early attempts to perform therapeutic angiogenesis have been initiated with the assumption that these growth factors are capable of enhancing vascular growth in the ischemic area. The first phase of therapeutic angiogenesis, the angiogenic approach, involved administration of a single angiogenic growth factor such as VEGF and FGF in the form of protein therapy and gene therapy. The methods of delivery included direct injection in the ischemic or periischemic area or a catheter-based infusion. To achieve sustained local levels of a growth factor, a heparin-alginate formulation had also been tested. Although results of initial animal experiments and open label clinical trials were encouraging, double-blind, placebo-controlled, randomized trials failed to show definitive functional improvement in the patients with coronary heart disease and peripheral arterial disease. A number of issues need to be considered in reaching a conclusion regarding the failure of these early approaches. However, it appears that while such an approach was valid in healthy young animals, it is probably not applicable in older end-stage ischemic disease patients.

The second strategic phase of therapeutic angiogenesis began shortly after the identification of circulating bone marrow-derived endothelial progenitor cells (EPC) with their possible contribution to adult vasculogenesis. The understanding and clarification of these progenitors which express markers of both hematopoietic (CD133, CD34, c-kit) and endothelial (VEGF-R2) lineage have accelerated the shift in the strategy of therapeutic angiogenesis: the angiogenic approach to the vasculogenic approach. In this approach, bone marrow-derived or circulating endothelial progenitor cells were administrated or recruited to the site of ischemia in expectation of not only efficient vascular growth, but also transdifferentiation of these cells into other tissue-specific cell types,