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Update in Techniques of Myocardial Revascularization: Therapeutic Angiogenesis

Frank W. Sellke, M.D.

Division of Cardiothoracic Surgery
Beth Israel Deaconess Medical Center
Harvard Medical School
Boston, USA

Myocardial ischemia and can be treated with medications such as calcium channel blockers, ▀-blockers, or organic nitrates that reduce myocardial oxygen consumption and / or improve myocardial perfusion. In severe cases, patients can undergo coronary artery bypass grafting (CABG) or percutaneous transluminal coronary angioplasty (PTCA) in an effect to improve myocardial perfusion. However, a group of patients remains who are not candidates for these conventional methods of treatment, due to inoperable disease or very small blood vessels.

Angiogenesis, or the development and growth of new blood vessels, has been known to scientists for many years, but has only recently been ýrediscoveredţ as a potential method to improve perfusion to ischemic myocardium or to treat other illnesses. Since collateral vessels develop and grow to the ischemic myocardium distal to a gradual coronary occlusion, and certain mitogenic growth factors and cytokines can induce angiogenesis, it follows that the administration of angiogenic substances may have a potential role in the management of patients with otherwise inoperable coronary artery disease. Recently, therapeutic angiogenesis, in the form of growth factor protein administration or gene therapy, has emerged as a method for the treatment of patients not amenable to more conventional methods of revascularization, such as CABG.

Mechanisms of angiogenesis

Endothelial and vascular smooth muscle cells are mitotically inactive in normal adult coronary arteries (1). However, during organ and vascular development and under pathologic conditions such as hypoxia, inflammation or other stresses, these cells may begin to migrate and divide. This eventually leads to the development of new intramuscular blood vessels. This process passes through several steps including the dissolution of the bond between the endothelium and the underlying basement membrane, migration, adhesion and reattachment of the endothelial cells, proliferation and tube formation culminating in the development of a new capillary or rudimentary blood vessel. The cellular and molecular changes required for this process to occur are numerous, complex and have only recently become understood. The development of larger, muscular epicardial arteries in the face of ischemia, hypoxia, or inflammation or during vascular development is technically referred to neo-arteriogenesis, and the de novo embryological formation of blood vessels from angioblasts is generally known as vasculogenesis ( 1 , 2 , 3). While angiogenesis, vasculogenesis and arteriogenesis are distinct developmental events with distinct regulation, these processes may all lead to the same ultimate end result; namely, increased perfusion to the ischemic myocardium.

Angiogenesis in the heart is most often associated with acute or chronic occlusion of a major coronary artery. If the occlusion occurs gradually, sufficient time may exist for the development of an extensive collateral network with relatively few clinical manifestations of ischemia. Hypoxia and ischemia certainly play major roles in the angiogenic process, but the relative contributions of these factors and interdependence of hypoxia with other influences such as inflammation are not known with certainty . The presence of an interface between the normally perfused and the ischemic or hypoxic myocardium may contribute significantly to or be critical to the angiogenic process. Thus, other processes besides tissue ischemia or hypoxia, such as inflammation, must be involved in the process of collateral vessel growth and development.

Almost universally, myocardial ischemia and angiogenesis are associated with the production and release of protein growth factors. This suggests that these protein substances are mediators of angiogenesis and critical for the formation of new vascular networks. Many protein factors have been found to have the ability to initiate an angiogenic response. Figure 1. Not only is the presence of the growth factors critical in the initiation of angiogenesis, but the actions of inhibitory factors must also be overcome. Figure 2. These inhibitory influences are important in regulating the angiogenic process in health and disease and during fetal development.

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Fig. 1

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Fig. 2

Therapeutic Angiogenesis

Studies in animals

Improved myocardial perfusion and function after the administration of angiogenic growth factors has been demonstrated in animal models of chronic myocardial ischemia. Calcium alginate slow release devices containing FGF-2 have been used to deliver the growth factor over several weeks in the porcine ameroid model of chronic myocardial ischemia. In this pre-clinical animal study, improved myocardial perfusion and contractility (4) and normalization of endothelium-dependent relaxation of the collateral-dependent microcirculation (5) were observed in the ischemic zone with FGF-2 treatment. Attempts at the intravascular infusion of acidic FGF, or FGF-1, have generally met with minimal success (6), but the chronic epicardial application of a longer half-life mutant form of FGF-1, in which one of the cysteines is replaced with a serine residue, resulted in improved coronary blood flow, myocardial function, and normalized vasomotor regulation in the chronically ischemic territory (7). The perivascular or intravascular administration of VEGF has been demonstrated to improve perfusion and function in the chronically ischemic myocardium. Gene therapy has been used to deliver angiogenic factors to the ischemic myocardium. The delivery of FGF by an adenoviral - mediated gene transfer has been reported to augmented myocardial perfusion and increased capillary density (8). The transfection of DNA encoding VEGF has been performed in animal models of chronic myocardial ischemia. This can be accomplished with gene transfection with the use of adenoviral vectors, adenoviral-associated vectors, liposomal vectors or with the injection of naked DNA (9). Thus, the feasibility of growth factor delivery by both perivascular or intravascular delivery of the protein or by gene therapy has been demonstrated.

Clinical experience

The results of several relatively small clinical trials have demonstrated the potential utility of growth factor angiogenesis or gene therapy for the treatment of ischemic conditions. A preliminary report by Losordo et al (10) described the use of gene therapy for myocardial angiogenesis. Five patients who failed conventional therapy were treated with naked plasmid DNA encoding VEGF165. The plasmid was injected directly into the myocardium through a small left thoracotomy. All patients had significant reduction in angina and postoperative ejection fraction was either unchanged or improved. Objective evidence of reduced ischemia was demonstrated by dobutamine SPECT-sestamibi imaging in all patients and coronary angiography showed improved Rentrop score in all patients.

Recently, Schumacher et al (11) reported the results of a randomized trial in which patients undergoing CABG were given an intramyocardial injection of FGF-1 lateral to the left anterior descending artery. Computer assisted analysis of digital subtraction angiographic images showed neovascularization in the region of injection compared to that in the same region of patients given a heat inactivated form of FGF-1. No adverse effects were evident.

In a Phase 1 clinical trial recently completed at our institution, basic-FGF (FGF-2) protein was administered with the use of a calcium alginate polymer slow release devices to a non - bypassable myocardial region during otherwise conventional CABG surgery. Patients were evaluated by nuclear magnetic resonance perfusion imaging and resting and stress SPECT-sestamibi imaging before and 3 months after initiation of growth factor treatment. Preliminary results have been encouraging (12, 13) with greater recovery of myocardial function and perfusion in those hearts receiving 100 Ág FGF-2 compared to those patients in whom placebo or a lower dose of FGF-2 was administered. Of those patients receiving the 100 Ág dose of FGF-2, no patients returned with angina, while of those patients receiving placebo beads not containing FGF-2, one half returned with angina within 6 months. Clinical trials testing the intracoronary or intravenous infusion of VEGF, FGF-1 or other growth factors have generally met with limited success.

Clinical considerations

The optimal method or route of administration of growth factor protein or gene therapy is not known presently. The direct administration of the angiogenic protein has a simplicity in that it does not require the incorporation of a gene into the nucleus. However, gene therapy encoding the protein may be more preferable for several reasons. The cost involved in scaling up from research grade to human quality recombinant protein and the reimbursement for recombinant protein therapies in the future remain uncertain. The potential to maintain a desired concentration of VEGF or other growth factor over a period of time by direct application of the protein may be less than that by the prolonged endogenous local production of the growth factor resulting from gene transfection in the arterial wall. However, the prolonged production of VEGF, FGF-2 or other growth factors after gene therapy also is in doubt, as is the ability to ýturn offţ production in some cases once it is initiated. Figures 3 and 4.

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Fig. 3

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Fig. 4

Although improvement in myocardial perfusion has been documented in patients given either angiogenic proteins or genes encoding the proteins, the ability of angiogenic therapy to normalize perfusion, and to maintain normal perfusion is in serious doubt, at least at present. Also, it is unrealistic to expect that a single administration of an angiogenic substance would optimally increased myocardial blood flow. Therefore, repeated administrations during cardiac catheterization, open invasive surgery or during thoracoscopy or other minimally invasive techniques may be required. It is also possible that the beneficial effects of growth factor administration or the administration of the genes encoding the growth factors with be short lived once the angiogenic stimulus is no longer present or being produced.

As with all new forms of therapy, therapeutic angiogenesis is associated with potential problems and complications that need to considered before the treatment can be expanded in use. The delivery of the growth factor protein or genes encoding the protein both have relative advantages and disadvantages. Although few problems have been reported with gene transfer using viral vectors in clinical trials, events could occur within normal cells that take up foreign DNA that allow then to be transformed and become abnormal. It is anticipated that ongoing large clinical trials will determine the practical feasibility, investigate the bioactivity the growth factors, establish safety parameters and determine the optimum delivery method of this new promising therapy.

Which growth factor, combination of growth factors, or inducer of growth factors (such as hypoxia inducing factor-1a ) or ýmaster switchţ genes leads to optimal, clinically relevant angiogenesis for the treatment of myocardial ischemia needs to be established. Most animal studies have found little direct advantage of one growth factor over another when administered alone. As eluded to above, options for growth factor delivery are numerous. Growth factor proteins or genes may be applied to the myocardium in conjunction with CABG surgery as is being performed presently at our institution. Hybrid forms of treatment consisting of a minimally invasive CABG (MIDCAB) along with local injection of growth factor protein or DNA may become acceptable standard treatment for patients with co-morbid conditions excluding them from traditional redo CABG or from transmyocardial laser revascularization. Alternatively, growth factors may be injected directly in paste or liquid form through a sternotomy, by thoracoscopic methods, or percutaneously with a long needle. Several methods of catheter-based delivery are being investigated, such as bolus intracoronary infusion and transvascular injection using either a needle or balloon catheter with side perforations.

In summary, the investigation of angiogenesis in the heart is an exciting area of research on a purely scientific basis. More importantly, it has the potential for improving and reducing the cost of care of patients with ischemic coronary artery disease. It is not likely that therapeutic angiogenesis will significantly reduce the need for CABG or PTCA in the near future. However, therapeutic angiogenesis has the potential to augment these treatment modalities and may extend treatment options to patients who are not otherwise candidates for conventional methods of myocardial revascularization. The actual role growth factor induced angiogenesis will have will need to be evaluated with larger, randomized clinical trials.




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