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facilitating functional improvement and regeneration of the compromised tissue. Attempts to apply these cells in experimental and preclinical settings confirmed beneficial effects in many cases. It is, nonetheless, still controversial whether the effects are truly attributed to the “progenitorship” of these cells or a bystander effect that can be achieved by non-progenitor cells.

That is, although it has been observed that these bone marrowderived cells, under some circumstance, indeed contribute to functional recovery of ischemic tissue, this contribution may not be due to cell incorporated into the vasculature or tissue as structural or functional components. Instead, such cells may reside in the perivascular region as paracrine cells, facilitating vascular growth by secreting angiogenic and chemoattracting factors. Furthermore, the entity of these cells may not necessarily be “endothelial” precursors. Therefore, the third phase of therapeutic angiogenesis explored the possibility of non-progenitor cells. This paracrine approach involves organizer cells, for instance, bone marrow-derived or peripheral blood-derived mononuclear cells (BM-MNC or PB-MNC), capable of regulating various aspects of vascular growth.

4. Clinical Trials

4.1. Growth factor-based, angiogenic approach

Phase II/III clinical trials that address the efficacy of therapeutic angiogenesis using the growth factor-based, angiogenic approach are summarized in Table 2. This approach has been focused mainly on VEGF165, VEGF121and FGF2 with limited data available on HGF. As delivery strategies, protein therapy including heparin-alginate formulation and gene therapy have been tested in these trials. Among them, VIVA, FIRST and TRAFFIC trials are well designed and adequately powered, suggesting several important lessons despite overall disappointing results. One of them is the difficulty in translating animal models in which most of the growth factors work efficiently in clinical settings where the patient population is more heterogeneous and refractory to angiogenesis. This will require careful patient selection and ideal development of biomarkers that enable us to predict neovascularization

responsiveness. Furthermore, we have learned the importance of randomization. Most open label Phase I trials claimed significant improvement in patient’s symptoms as well as objective measurement of cardiac function. However, the same agents were not effective in the Phase II trials partially because of the unexpected placebo effect. Spontaneous improvement in the end-stage patients is highly significant, which can overshadow the agent’s true effect. It is also noted that fluctuation of the end-point is often observed in the patient population. This is not only manifested in subjective, soft end-points, but also in hard end-points such as MR and PET perfusion.

Gene therapy is an alternative to protein therapy with its ability to provide more sustained presence of the desired agent in the target tissue. While some of gene therapy trials including Phase I open label trials suggested potential beneficial effects, none of double-blind, adequately powered trials demonstrated definitive benefit as we experienced in the protein therapy trials. Conclusively, we have learned from growth factor-based studies that oneor two-time administration of a single angiogenic growth factor, regardless of the delivery strategy, is not sufficient to provide therapeutic effect.

4.2. Cell therapy-based, vasculogenic and paracrine approach

The next generation of therapeutic angiogenesis is cell-based therapy which includes the vasculogenic and paracrine approaches, although the difference of these two is practically not discernable in many cases. The vasculogenic approach utilizes progenitor cells which are, in principal, fractionated before administration by using progenitor cell markers.6 Although there is no consensus with regard to the surface marker representing genuine endothelial progenitor cells, many studies use CD34 and/or CD133 as “EPC markers.” However, in reality most large clinical trials thus far use the crude bone marrow mononuclear cell population that is heterogeneous and presumably contain a significant number of progenitor or stem cells in the preparation.

In contrast, the paracrine approach uses unfractionated mononuclear cells either from bone marrow or more recently from peripheral blood. This approach does not rely on the progenitor ability of the cell