By David N. Leff

Almost everyone, it seems, knows - or knows of - somebody who has undergone a coronary bypass operation. With 600,000 or more "coronaries" a year, it's the most frequent form of major surgery in the U.S.

The procedure performs an end run around the coronary arteries that pump blood to the heart, when these vessels get plugged up by atherosclerotic plaque. As narrowing of those arteries begins to cut off oxygen to the heart muscle, ischemia sets in. The angina pectoris that this threat signals consists of a characteristic pattern of tight chest pain that may radiate out to the left shoulder and arm. People with this condition pop nitroglycerin pills to control the intermittent attacks.

The bypass surgery usually involves cannibalizing lengths of great saphenous vein from the patient's leg, reversing them end to end, and stitching that replacement tubing to the coronary arteries, above and below their stretch of occlusive plaque sludge. That end-to-end switch is necessary because veins are studded with one-way valves that keep their outflow blood from backing up.

That's all very well, but coronaries aren't the only arteries that get occluded with atherosclerotic plaque, and require venous bypass. Another important surgical site concerns the femoropopliteal arteries that run from the groin to below the knee, and feed blood to the muscles of the calf, thigh, hip, buttocks and foot. Instead of angina pectoris, the typical warning pain of this limb ischemia consists of intermittent nocturnal cramps.

Typically, a person is awakened by a sharp, unrelenting twinge in some leg muscle. Preferred first aid is to swing the offending leg off the edge of the bed, and let it hang there till the pain goes away - usually in a few minutes. But as these arteries fill with atherosclerotic plaque, more drastic measures are required to forfend the ultimate threat of ulcerated extremities and eventual amputation. Diabetic patients are particularly threatened.

But a particularly insidious Catch-22 often vitiates peripheral as well as coronary venous bypass. Arteries are conduits that pump blood under pressure - as the spurts of a cut artery testify - so their walls are made of stern stuff. Not so veins, which flow placidly, and consist of thinner, less resistant material. And there's the rub. A saphenous vein pressed into involuntary servitude as a replacement artery tries to beef up the structure of its concentric cellular layers - neointima, endothelia and adventitia, plus smooth-muscle fibers. Result: Its walls thicken and thicken until they close the vessel lumen and stop blood from reaching the limb muscles. This is for all the world like the atherosclerotic plaques that the surgery was designed to relieve - and which sooner or later recur.

Arterial Bypass Often Bites Its Own Tail

This irony was not lost on cardiovascular researcher Victor Dzau, who chairs the department of medicine at Harvard-affiliated Brigham and Women's Hospital in Boston. After extended preclinical experiments in rats and rabbits, he has just reported a controlled, randomized, fully blinded clinical trial of his gene therapy strategy in 41 men and women undergoing femoropopliteal artery bypass grafts. His paper appears in this week's issue of The Lancet, dated Oct. 30, 1999. Its title: "Ex vivo therapy of human vascular bypass grafts with E2F decoy: the PREVENT single-center, randomized, controlled trial."

"Neointimal graft-wall thickening," Dzau pointed out in his paper, "is an adaptive reaction that provides hemodynamic stability to the graft, but leads to up-regulated expression of growth factors, cytokines and adhesion molecules by the vascular smooth-muscle cells." It's this reaction that fuels the unwelcome thickening of vein graft walls.

E2F is the earliest transcription factor that turns on the multiple genes involved in cell division. The DNA component, oligodeoxynucleotide, is designed to bind to E2F and act as a decoy, preventing it from getting to its target, namely the cells in the vein wall progressing through their cell cycle into division.

To prevent that cell-cycle buildup, Dzau mobilized a gene-therapy strategy called PREVENT - "Project of Ex-vivo Vein graft Engineering via Transfection." In the 10 minutes between excision of the donor saphenous vein and its anastomosis to the occluded artery, this protocol bathed the vein in a naked DNA construct based on a solution of antisense oligodeoxynucleotide inside a tubular pressure vessel surrounding the vein. This ex vivo treatment took place on a sterile table in the OR.

The 21 men and 20 women, aged 35 to 96, who enrolled in this Phase I/II trial were randomized to three treatment arms: 17 received grafts that had been transfected with pressurized DNA, 16 got untreated graft veins and eight were assigned to control grafts treated with scrambled oligodeoxynucleotide sequences.

Gene Therapy Worked, Even In High-Risk Patients

Vascular surgeon Michael Mann is lead author of the Lancet paper, and co-director with Dzau of the clinical trial. "We chose at the beginning of the study," Mann told BioWorld Today, "to include even very high-risk grafts, particularly those that are referred to Brigham and Women's Hospital. These are patients who have no other options, although their veins are really of very poor quality for bypass. Because of this very high-risk population, we saw an event rate of 70 percent in our controls, which gave us the opportunity to do a statistical comparison of that group with the group of patients treated with the genetic engineering. In fact, the overall event rate allowed us to observe a greater than 50 percent reduction in graft occlusions and revisions in our treated segment."

Just before handing the donor veins over to the bypass surgeons, the gene therapists removed 2-centimeter snippets from each, to analyze the penetration of nucleotide transfection. The box score was 89 percent showing "successful delivery and nuclear localization of cells throughout the vessel wall." It was "delivered homogeneously to cells in the intimal, medial and adventitia layers."

Vascular surgeon Michael Conte is one of the co-authors in this Lancet article, which means some of his patients were enrolled in the trial. "It's a promising finding," he told BioWorld Today, "but the number of patients is way too small to say that this is actually having an effect. This study shows that what's needed is scale-up - a much larger number of patients to be enrolled in a future clinical trial."