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Biomedicine

A Disappearing Heart Stent

An absorbable coronary stent shows promise in early trials.

Coronary stents have revolutionized the treatment of heart disease, but they leave metal scaffolding inside arteries that can occasionally cause immune reactions and blood clots, and interfere with future surgeries. A new, fully absorbable stent could one day help prevent problems like these. The stent, made by Abbott Laboratories in Illinois, just finished its first test–a two-year trial involving 30 patients–and now, after a few adjustments, the company is launching its next phase of clinical tests.

Long gone: Imaged via ultrasound two years after surgery, this artery shows no trace of the bioabsorbable stent used to prop it open.

Surgeons use stents to open blocked arteries, but the devices can cause complications. The materials used incite an immune response, and damage to the artery wall can trigger the formation of blood clots that, once dislodged, can cause stroke or other damage in a process called late stent thrombosis. Over the past few years, these complications have decreased with the widespread use of anticlotting therapies and drug eluting stents coated with immune suppressants. But some cardiologists still liken the use of metal stents to leaving a cast in place once a person’s broken limb has healed.

Abbott’s new bioabsorbable everolimus-eluting stent, or BVS, is made up of two layers of a biodegradable polymer: one that contains the immunosuppressant drug everolimus, the other forming a longer-lasting backbone. Over time, the body breaks down and absorbs the polymer, ultimately leaving nothing behind. For its initial test run, the stent was placed in patients with just a single narrowed or blocked artery. According to research published this month in The Lancet, the stent appeared to be almost completely absorbed after two years, with no instances of cardiac death, no known cases of stent thrombosis, and no evidence that the arteries had begun to re-narrow.

“It really looks like the stent did its job of stopping re-narrowing, it was safe, and it disappeared,” says John Ormiston, lead researcher of the Lancet study and an interventional cardiologist at Mercy and Auckland City Hospitals in New Zealand. “We have hope that the long-term chance of thrombosis will be very low.”

Because the stent was tested on so few people, however, and because those patients had very simple problems, it is difficult to know whether it will match the long-term success of its metal predecessors. “It certainly appears to work in patients with simple lesions, but it won’t get widespread use unless it works in more-complex patients,” Ormiston says.

“If you have a stent that’s bioabsorbable, could return a vessel to its native condition and protect it from late-stent thrombosis, that’s an inherently appealing concept, especially when you talk about the fact that we’re implanting about a million patients per year here in the U.S.,” says Gregg Stone, director of cardiovascular research and education at New York-Presbyterian Hospital and the Columbia University Medical Center.

Invisible repair: Optical coherence tomography shows an artery immediately, six months, and two years after a stent is put in place. At six months, a small amount of tissue has created a bridge between two of the stent’s ribs; after two years, the bridge and the struts have largely disappeared.

“Phase A of the project was really a proof of concept, but I think the concept was very successful” says Stone, who is on Abbott’s BVS advisory board. “Why have a permanent, metallic prosthesis if you don’t have to?”

“I think it’s a wonderful concept,” says Frederic Resnic, the director of the Cardiac Catheterization Laboratory at Brigham and Women’s Hospital in Boston. “But the bar to prove that it is more than just a niche product is going to be very high, because you’re going to have to meet or exceed the results of 2009 drug-eluting stents, and it’s very hard to demonstrate that level of safety and efficacy.”

Abbott is not quite there yet. For a second trial, launching now, the company is using a slightly modified version of the earlier stent; it is easier to store (the first had to be kept at -20 degrees Celsius to prevent the polymer from cracking) and has slightly thicker, stronger struts to help maintain the arterial opening and better prevent re-narrowing. This trial will still be limited to patients with simple lesions and will be only slightly larger than the first, with planned enrollment of 80 people in Europe, Australia, and New Zealand.

Because metal stents have proven to be so effective, some question whether a bioabsorbable version is worth the research investment, especially since the bar to prove both safety and efficacy is set so high, and since there’s not yet proof that the disappearing stent doesn’t leave scarred tissue in its wake. But Ormiston and Stone point to the fact that their patients’ arteries appear to have restored elasticity and that eroding stents don’t prevent the branching-off of new vessels, unlike the metal ones. Plus, they note that patients often have to return for a second stent placement–if the first stent has been absorbed, it can’t get in the way of a second surgery.

These are all distant possibilities, however, and will remain so until patients with more-complex conditions have been treated successfully. “The current study is far from definitive from a clinical perspective,” Resnic says. “Large prospective clinical trials will be necessary to demonstrate that this bioabsorbable platform is as safe and effective as existing [and future iterations of] commercially available drug-eluting stents.” But, he adds: “if perfected, I believe it would naturally replace permanent metallic implants.”

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