Gene Therapy and Vein Graft Patency in Coronary Artery Bypass Graft Surgery

A major and much-needed advance for cardiac surgeons and patients requiring coronary artery bypass graft (CABG) surgery would be the availability of an easily applied intervention that would improve long-term patency rates for saphenous vein grafts to those achieved with internal thoracic artery grafts. With newer endoscopic techniques, the saphenous vein can be harvested with minimal trauma and without compromising short-term patency rates.^{1 - 2} While saphenous vein grafts are the easiest conduits to use technically, especially for sequential grafting, the internal thoracic artery, with its excellent long-term patency (especially when used to the left anterior descending vessel), will remain the preferred primary conduit in the vast majority of cases.

Saphenous vein graft failure has a trimodal distribution. Early failure occurs within the first 1 to 2 months, probably from primary thrombosis due to technical factors, poor runoff into small or severely diseased distal coronary arteries,^{3 - 4} or unrecognized intrinsic saphenous vein disease. Late failure occurs after 3 to 5 years and results from the known tendency for saphenous veins to develop accelerated atherosclerosis.⁵ These processes may be considerably modified by currently available antiplatelet and lipid-lowering drug therapy.^{6 - 7} Intermediate failure of saphenous vein grafts is due to the development of neointimal hyperplasia, which is most prominent in the first year after CABG surgery, but can occur up to 3 years after implantation.^{8 - 9}

Several factors act in concert to influence the development of neointimal lesions. These include ischemia of the venous wall, hemodynamic stress and mechanical trauma during vein harvesting and preparation, and inherent abnormalities within the vein exposed to arterial pressure, particularly in patients with multiple risk factors for atherosclerosis.¹⁰ Intimal hyperplastic lesions are composed of mesenchymal cells embedded in the extracellular matrix and result from complex interactions between endothelial cells, smooth muscle cells, inflammatory cells, and extracellular matrix macromolecules.^{11 - 12} Smooth muscle cells, the major component of these neointimal lesions, are believed to arise from phenotypically altered medial smooth muscle cells or possibly from CD34+ cells present in peripheral blood.¹¹

The initial response of a harvested saphenous vein to an operatively induced endothelial injury is smooth muscle cell proliferation. Although this process has been extensively studied in rodents, pigs, and nonhuman primates, little is known about the occurrence, duration, or importance of this proliferative response in humans.¹¹ An increase in cell number and phenotypical alterations in the cytoskeleton of smooth muscle cells seem to be essential for their migration from the media into the intima and produce intimal thickening.

Delineating signaling pathways implicated in smooth muscle cell proliferation, migration, and extracellular matrix production is critically important for designing novel therapies for inhibiting or limiting neointimal thickening. Gene profiling of culprit lesions may offer some insight into these events. Although the complementary DNA arrays of saphenous vein explanted after a mean of 94 months have been analyzed,¹³ few human graft explants are available for analysis within the proliferative phase of lesion formation.

The E2F family of transcription factors consists of 8 known members and 2 partner proteins, DP1 and DP2, essential for E2F activity.¹⁴ Members 1, 2, and 3 of the E2F family have high-transcription activity and are responsible for G1/S progression of cells through the cell cycle and thus regulate smooth muscle cell proliferation.¹⁵ Other members of the E2F family have poor transcription activity and inhibit cellular proliferation. Targeting strategic transcriptional control of gene expression such as the G1/S transcription phase of the cell cycle with edifoligide, an oligonucleotide decoy, is a logical concept based on preliminary in vitro and in vivo studies.^{16 - 18}

In this issue of JAMA, the PREVENT (Project of Ex-vivo Vein Graft Engineering via Transfection) IV investigators report the results of a large phase 3 multicenter randomized clinical trial evaluating edifoligide, which has been shown in experimental and preliminary clinical studies to inhibit neointimal hyperplasia in vein bypass grafts.¹⁹ In this study of 3014 patients undergoing primary CABG surgery with at least 2 planned saphenous vein grafts, edifoligide had no effect on the primary end point of per patient graft failure (45.2% in the edifoligide group vs 46.3% in the placebo group) or on the incidence of major adverse cardiac events at 1 year. All but 3% of vein graft failures were due to occlusions. Many of these occlusions were clinically important because the incidence of myocardial infarction early after operation was more than twice as high in the vein graft failure group and the incidence of subsequent cardiac events of death, myocardial infarction, or need for subsequent revascularization was 26% in the vein graft failure group compared with 1.8% in patients who did not have vein graft failure.

The authors acknowledge that their vein graft failure rate is higher than what would be expected when compared with other published studies, but might still be representative of the true rate of vein graft failure in clinical practice. They discuss the possibility that the vein treatment regimen in the placebo group could possibly have been harmful. However, it is not clear from their description what specific vein preparation techniques were used in addition to that described for the placebo group and whether this approach was different from standard vein preparation. If the method of preparation of the grafts did differ substantially, one could argue that the study should have included a third group of controls with standard graft preparation techniques.

In another large study of saphenous vein graft patency involving nearly 4000 grafts placed between 1969 and 1994, 88% were patent within about the first month after operation and 81% were patent at 1 year.²⁰ In contrast, patients in the PREVENT IV trial were not studied early postoperatively; however, if half of the 1-year failure rate was from early occlusion, this would leave about 15% of grafts in which neointimal hyperplasia could have been the major cause of graft failure and potentially could have been modified by agents such as edifoligide. A more recent randomized study comparing radial artery graft and saphenous bypass grafts demonstrated an 86% saphenous vein patency rate at 1 year.²¹ Imbalances in risk factors between the 2 experimental groups were not expected in PREVENT IV because of the randomization of such a large study population. However, given the wealth of data available on the determinants of vein graft patency, the authors should explore whether variables previously associated with vein graft failure were similar in their patients.

As the PREVENT IV investigators also point out, the changing nature of the patient population referred for surgical myocardial vascularization might account for an increased incidence of these risk factors for vein graft failure compared with previous studies. These include severe distal coronary disease, myocardial scar, small distal vessels, or less significant proximal native coronary disease in the distribution grafted with a saphenous vein,²² which was evidently the case for many of the bypassed vessels in this study. In addition, more than 20% of patients in PREVENT IV had CABG surgery without cardiopulmonary bypass. Although previous studies have not shown a difference in graft patency rates, it would be reassuring to know that off-pump procedures were not independently associated with lower graft patency in comparable quality target vessels.²³

The negative results of PREVENT IV raise a number of interesting issues. One concern is whether the dose and duration of exposure of the cells lining the vein grafts to the decoy oligonucleotide were sufficient to allow efficient transfection. As the PREVENT IV investigators note, edifoligide is not a selective inhibitor of members 1, 2, and 3 of the E2F family, making it possible that the balance between smooth muscle cell proliferation and inhibition may have shifted, thus neutralizing the effects of the oligonucleotide.

Saphenous vein grafts are ideally suited for gene therapy because they are easily accessible and only temporary suppression of the target gene may be necessary to inhibit neointimal thickening. Although gene therapy continues to show promise, a number of difficulties remain. Transfection efficiency in the target tissue needs to be improved. It must also be determined if targeted delivery of gene therapy to the area of injury (thus minimizing systemic toxicity) will improve efficacy or whether interference with specific properties of the endothelium or newly formed microvessels could be achieved with gene transfer vectors.

Perhaps one of the most important lessons from the PREVENT IV study was that its planning, organization, and execution resulted in a conclusive result. A promising therapeutic agent was identified and tested in the experimental laboratory and preliminary clinical studies. The structure and size of the clinical study was established in collaboration with leading academic cardiac surgeons, and participating academic and private centers were identified and contacted through the Society of Thoracic Surgeons’ National Database participation list. This ensured that the study would be adequately powered and that data analysis, which was centralized, would be consistent and accurate. Although results were negative for this agent, this large, adequately powered, and well-executed clinical study established this conclusion with some finality. The study also was powered for long-term clinical events, and the results of that analysis, when available, should be of interest.

As a result of PREVENT IV, more has been learned about the signaling mechanisms involving neointimal hyperplasia since edifoligide was developed and tested experimentally. Further studies specifically targeting the precise mechanisms operative in vein graft failure still hold promise for improving intermediate and perhaps long-term vein graft patency.