By David N. Leff
Though declining in recent years, heart attack remains the number one killer-disease of Americans. Despite "lite" diets, cholesterol monitoring, even exercise, coronary arteries, which feed blood to the heart, keep blocking up with plaques - clots. Short of an arterial bypass operation, the preferred treatment for keeping blood pumping to the cardiac muscle is coronary catheterization.
"With probably more than a million a year done in the U.S.," observed clinical and research cardiologist Robert Levy, "cardiac catheterization is the most frequent surgical procedure performed in this country." Levy, who directs the Pediatric Cardiology Research Laboratory at the Children's Hospital of Philadelphia, describes how the operation is done:
"In cardiac catheterization, the patient's arteries are entered with a long flexible tube, which is manipulated under X-ray guidance through the blood stream to the point where it actually enters the heart. Once inside the heart, it can be used to measure all sorts of important physiological data - such as blood pressure and flow. Then dyes may be injected through the catheter to image the cardiovascular anatomy, and show the coronary obstruction that is present.
"When they see an obstruction," Levy went on, "a separate catheter is substituted for doing what's called an angioplasty - interventional therapy to modify the blood vessel. Once the obstructed area is identified and pinpointed, another catheter is put in that has a stent on the end of it, with a small balloon underneath. The stent, a steel structure that expands and locks in place, overstretches the arteries, so it mechanically relieves obstruction. The hydraulic pressure in the balloon," he explained, "causes the stent to expand.
"A stent," Levy went on, "is a network of carefully structured steel wires - essentially a cylindrical mesh. Its length is usually 1.5 centimeters; and diameter in unexpanded state between 1 and 2 millimeters. In expanded state, diameter usually goes to 4 or 5 millimeters."
Thumbnail-Size Wire Mesh Costs $1K to $5K
He added that the cost of a single steel or titanium alloy stent - aside from all other expenses of hospitalization and care - "typically ranges from a low of about $1,000 all the way up to $5,000." He added, "A tremendous amount of engineering has gone into it."
Stenting owes its name to a 19th-century English dental surgeon, Charles Thomas Stent (1807-1885). "His specialty," Levy said, "was what we'd now call restorative dentistry. He developed a substance called Stent's compound," which is available to this day in English apothecaries. The modern idea of stenting came up during World War I," he noted, "where there were some terrible facial injuries. Stent's compound was used as a temporary scaffold to keep a wound open during reconstructive surgery.
"Long before their use in coronaries, these steel stents were applied in gastrointestinal surgery, especially pancreatic cancer. Coronary stents became available at the technical high level they're at now in emergency circumstances. Cardiologists were using bile-duct stents to widen coronary arteries - usually for severe indications such as abrupt, partial obstruction of a blood vessel."
Nowadays, nearly 1 million Americans a year require coronary stents. But despite the obvious and immediate relief that angioplasty stenting affords a blood-starved heart, Levy observed, "Clinicians are now focusing on the percentage of patients who develop restenosis - artery reblockage - after they've had stenting. That statistic says that within six months of stenting, 20 percent of those patients are symptomatic, due to the stent having damaged the blood vessel - with ongoing return of coronary disease.
"These symptoms," Levy said, "are those of obstructive coronary disease - angina-related chest pain and cardiac arrhythmas. In that 20 percent, they are the same symptoms that led to the stenting in the first place."
Numerous researchers have sought to apply gene therapy to this frequent failure of stenting. "Many genes in experimental animals," Levy said, "have been successful in treating restenosis, and reducing its severity. And none of them has been used successfully in humans yet. What we decided to do was focus on a delivery system that could be used with anything."
Levy is senior author of a paper in the November issue of Nature Biotechnology. Its title: "Gene delivery from a DNA-controlled-release stent in porcine coronary arteries."
"It reports for the first time," he told BioWorld Today, "that it's been possible to deliver a sustained-release coating of DNA from a stent - and show gene transfer in the arterial wall."
As their demonstrator-model marker DNA, he and his co-authors chose the gene for green fluorescent protein (GFP), which expresses a product that glows in telltale color under a fluorescence microscope. To deliver this sequence, they incorporated the GFP gene in a biodegradable polymer emulsion coating of polylactic-polyglycolic acid copolymer. "It maintains the DNA in a functional state," Levy pointed out, "and is different from the usual film coating that's used in a lot of drug-delivery stents."
Porkers Prove Preclinical Protocol
In an in vivo experiment, he recounted, "six pigs underwent cardiac catheterization, just as humans do. The coronary arteries on one side of each animal were entered with angioplasty catheters tipped by stents carrying the DNA-polymer construct. The six control blood vessels on the other side received untreated stents.
"After one week," he said, "we euthanized the pigs, retrieved the stents, and analyzed their arteries for the presence of the GFP marker gene's expression. We found that 1.4 percent of the cells in the DNA-stented porcine arterial walls had positive evidence of gene transfer - but no sign in the controls. Having done that," Levy observed, "we should now be able to develop higher-level systems that completely localize an appropriate gene or genes to the artery in humans."
Among the many candidate genes proposed by others for treating restenosis, Levy and his team have chosen two they are now screening: the fas ligand gene and the herpes simplex virus thymidine kinase gene. "We will need to do a major preclinical in vivo study in pigs to select a therapeutic gene for submission to the FDA. It would take two or three years," he concluded, "before this clinical data could be put in order, and clinical trials begin."