By David N. Leff

A heart transplant is the last resort of cardiac disease therapy ¿ but it¿s not the only transplantation resort.

Upwards of 600,000 Americans a year receive a coronary artery bypass graft, in which the donor organ comes from their own bodies. When the arteries that feed the heart with blood-borne oxygen and nutrients clog up with atherosclerotic clots, the cardiac surgeon replaces them with saphenous veins dissected from hip to ankle out of the patient¿s legs.

Arteries are designed for high-flow pumping of blood to the heart; veins for relatively low-pressure return of the life-giving fluid. ¿The engrafted veins fail at the rate of 10 percent up to as high as 50 percent a year,¿ observed cardiovascular researcher Victor Dzau. ¿So if you take some middle-aged man at age 50 who goes to bypass, by age 60, many of his grafts will have failed. Say he got a triple coronary bypass ¿ three veins ¿ at 60 he¿s still a very viable and vibrant youthful man ¿ but he will have at least one or two of those grafts fail.

¿The nature of that failure,¿ Dzau went on, ¿is mechanical injury to the graft because the vein is a thin-walled structure. When put into the artery side, it gets acute mechanical stretch, then endothelial dysfunction and intimal [in the vein¿s lining] hyperplasia, leading on to atherosclerosis again, and blood clots.¿

Since 1996, Dzau has chaired the department of medicine at Harvard-affiliated Brigham and Women¿s Hospital in Boston. Before that, he headed cardiovascular medicine at Stanford University. While there, he recalled, ¿we were looking at the delivery of DNA to the venous bypass graft. We, like other gene therapists, were using either viral vectors to deliver genes, or lipid formulations for oligonucleotides.

¿But because the vein is a collapsible structure,¿ he went on, ¿I asked two lab colleagues, Michael Mann and Gary Gibbons, to find a condition whereby, if they used either construct for delivery of a viral vector, to determine that the variable in this system is not the amount of pressure on the vessel.

¿When Mann and Gibbons started to standardize the pressure, they noticed a difference in degree of vector uptake by the variation of pressure that would distend the vein, or be contained within its lumen.

¿At that point,¿ Dzau recounted, ¿we decided to do naked DNA ¿ both plasmid and oligo ¿ under pressure. And sure enough, it seemed that pressure did an equally good job in the vein with or without the viral, lipid or other vector accessories.

¿We also realized that if we over-stretched a vein or any tissue with pressure, it could injure that tissue. So we then developed a device where we have the vein in a tubing, filled with fluid containing DNA in solution. Thereby the blood vessel could sense pressure on the outside and inside, and there was in fact an enclosed tubing so it could be distended ad infinitum, to minimize the injury.

¿And we found,¿ Dzau continued, ¿that even in five minutes under pressure ¿ though we now standardize it for 10 minutes ¿ with no venous distention for that short period of time, we can get very good, consistently reproducible, high-efficiency uptake of our DNA vector. So the origin of the system was serendipitous.¿

Putting The Pressure On DNA

Dzau is senior author of a paper in today¿s Proceedings of the National Academy of Sciences (PNAS), dated May 25, 1999. It bears the title: ¿Pressure-mediated oligonucleotide transfection of rat and human cardiovascular tissues.¿

¿In a nutshell,¿ Dzau explained, ¿what we did was discover that using synthetic DNA ¿ short oligo sequences ¿ a host of such genes can be introduced into rat and human tissues by means of mechanical pressure. We can do this effectively ex vivo,¿ he added, ¿which means that when a tissue is outside the body for purposes of harvesting and reimplantation ¿ not necessarily for a heart transplant, but for bypass surgery using autologous veins and so forth ¿ we have developed a device whereby under controlled conditions for 10 minutes, we can effectively introduce DNA into the vein bypass graft, or the rat heart.

¿We haven¿t tried human hearts yet,¿ Dzau pointed out, ¿but we have followed up with studies demonstrating that if we put the right DNA into the vessel or the heart, we can have therapeutic outcomes, by expressing a beneficial gene within these tissues.¿

Besides rat veins and hearts, they tested 60 discarded human saphenous vein segments, obtained from the hospital operating room.

Dzau and his co-authors already have carried out a human clinical trial of their pressure system, he told BioWorld Today, but, ¿This is a study we are now writing up and submitting, so we feel a little reluctant to talk too much about it.¿

In that unpublished study, they used their pressure device to engineer the bypass venous graft. In similar preclinical trials, Dzau observed, ¿we published that we can target cell-cycle regulatory genes, whereby the intimal hyperplasia [atherosclerotic growth] in response to bypass graft injury, is not developed. We could arrest the cell cycle. And now,¿ he allowed, ¿we have finished that human trial, whereby we used the device to introduce short DNA sequences that blocked cell-cycle regulatory genes.¿

Like Pressure Cooking The Living Heart

¿This type of gene therapy,¿ Dzau pointed out, ¿besides being applied to bypass grafts, may also be used for cardiac transplant tissues, and as a means of delivering genes into a beating heart during catheterization. I can imagine,¿ he went on, looking to the future, ¿that for the heart, our procedure can be done in vivo in situ, either during cardiac surgery, when the chest is open, and you can put something around it ¿ in a sense like a pressure cooker ¿ or in the catheterization lab, where we¿ve already partially proven that you can temporarily occlude the coronary, to develop sufficient pressure to deliver genes across that vessel¿s vasculature.

¿This way,¿ Dzau concluded, ¿what you can potentially do is put a device around the heart in the OR and also infuse the coronary at the same time, as we did in the isolated perfused rat heart described in PNAS. Thus, under pressure for a short time you get a more homogenous delivery of DNA across the myocardium.¿ n