Therapeutic angiogenesis: another passing phase?

Efforts to stimulate tissue vascularization in patients with ischemia resulting from coronary or peripheral arterial disease (therapeutic angiogenesis1,2) have passed through several scientific phases over the last dozen years, based on our increased understanding of the molecular mechanisms underlying vascular homeostasis (Table). In the first phase, a single angiogenic growth factor, or its cognate DNA sequence, was injected into ischemic tissue.2,3 In the case of angiogenic growth factor gene therapy, the goal was to augment production of the angiogenic factor by the patient’s own cardiac or skeletal muscle cells. The angiogenic factor, as exemplified by vascular endothelial growth factor (VEGF), was believed to act by binding to receptors that were expressed on the surface of endothelial cells, activating the cells to migrate, proliferate, and form new or large vessels, thus allowing increased perfusion of tissue surrounding the injection site. The administration of these factors worked well in accelerating the recovery of perfusion in healthy young laboratory animals subjected to an acute interruption of coronary or femoral arterial blood flow. Encouraging results were also obtained in early clinical trials that lacked control populations. However, the double-blind placebo-controlled VIVA study of VEGF in patients with myocardial ischemia showed no significant objective benefit of therapy but instead revealed a placebo effect that was remarkable in its magnitude.4 The second phase of therapeutic angiogenesis began with the identification of circulating bone marrow–derived cells, which expressed markers of both hematopoietic progenitor cells (CD34, c-kit) and endothelial cells (VEGFR2) and thus were given the name endothelial progenitor cells (EPCs) and shown to participate in ischemia-induced angiogenesis.5,6 In this phase, the cells, which were mobilized from the bone marrow and recruited to sites of ischemia, were thought to differentiate into the additional endothelial cells and pericyte/ smooth muscle cells that were required for new blood vessel formation. In cardiac infarction models, even more amazing feats of transdifferentiation were ascribed to these cells, leading to the regeneration of myocardium by cells which were purported to have only recently arrived from the marrow.7,8 Many of the “angiogenic growth factors” (such as VEGF) that were identified in the first phase were shown to function as “vasculogenic cytokines” in the second phase.3,6 A hybrid between the first and second phases of therapeutic angiogenesis involved the administration of cytokines, such as granulocyte-macrophage colony-stimulating factor (GMCSF), that promote the mobilization of bone marrow cells. The START trial, in which patients with limb ischemia were treated with GM-CSF, failed to demonstrate efficacy.9 In the third phase, careful cell labeling experiments using confocal microscopy confirmed that bone marrow–derived cells were indeed recruited to sites of ischemia in laboratory animals, but indicated that these cells took up residence in a perivascular location where, it was suggested, their participation in tissue vascularization was limited to an instructive role, through the production of “paracrine factors”10,11 corresponding to the “vasculogenic cytokines” of the second phase and the “angiogenic growth factors” of the first phase. Similarities between the functional and structural characteristics of EPCs and peripheral blood mononuclear cells (PBMNCs) were also reported.12,13 Despite continued uncertainty regarding the mechanisms by which these cells promote vascularization after administration to laboratory animals, the TOPCARE-AMI trial, which was performed in Europe and involved autologous transplants of bone marrow MNCs or circulating progenitor cells, demonstrated encouraging results.14 Double-blind placebo-controlled studies have not yet definitively established whether this approach is efficacious. We may have entered another phase with the publication of an interesting study by Tateno et al in this issue of Circulation Research.15 The authors enrolled 29 patients with critical limb ischemia, the majority of whom (21 of 29) were facing amputation as the only therapeutic option, in a study designed to explore the utility of autologous PB-MNC transplantation. Approximately 10 billion PB-MNCs were collected from each patient by peripheral blood apheresis which, unlike bone marrow collection, does not require general anesthesia. The PB-MNCs were transplanted into the ischemic leg divided among 100 injection sites that were spaced several centimeters apart. The procedure was repeated approximately one month later. Patient status was assessed 2, 9, and 12 months after treatment. Maximum walking distance, healing of ischemic ulcers, and ankle:brachial blood pressure ratios were increased, whereas rest pain and amputation were decreased, with 21 of 29 patients manifesting improvement in at least one of these categories. The results reported by Tateno et al15 stand in contrast to the results of a study by Tateishi-Yuyama et al in which injection of PB-MNCs was performed as a negative control in the contralateral limb of patients injected The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Vascular Biology Program, Institute for Cell Engineering; Departments of Pediatrics, Medicine, Oncology, and Radiation Oncology; and McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, Baltimore. Correspondence to Gregg L. Semenza, MD, PhD, Broadway Research Building, Suite 671, The Johns Hopkins University School of Medicine, 733 North Broadway, Baltimore, MD 21205. E-mail [email protected] (Circ Res. 2006;98:1115-1116.) © 2006 American Heart Association, Inc.