Medical Device Daily Washington Editor
SAN FRANCISCO – Bioresorbable stents may seem like the latest thing, but according to a panel moderator speaking during a session at Transcatheter Cardiovascular Therapeutics (TCT), the effort to bring them from a concept to reality has a history spanning more than two decades.
Despite the seemingly long timeline, Tuesday's session on this class of stents strongly suggested that most of them still have a distance to cover before they are ready for a FDA advisory committee. This is largely due to the fact that the best feature of such a device is also its Achilles' heel; a stent designed to disappear in mere months is more prone to structural integrity problems than one designed to persist. Still, neither doctor nor patient is interested in a lifetime with a metal stent, so the market for this type of device is a sure bet.
Giving an overview of the difficulties and the progress in developing a bioresorbable stent, Robert Schwartz, MD, deputy editor of the New England Journal of Medicine, said bioresorbables present fewer problems where imaging with CT and MRI are concerned, but that in general they need "improved deliverability." He also reminded the audience of an obvious fact, that degradation characteristics are pivotal.
The ReZolve, made by Reva Medical (San Diego), generates "benign breakdown products," Schwartz said, but faces deliverability problems largely because its crossing profile is "still not a profile as low as current bare metal stents." Schwartz added that degradation rates for the tyrosine-derived polycarbonate stent are "also an issue."
The stent, also known as BVS, Schwartz said, "has received a lot of good press and is a good platform," but he noted that the device "requires special storage in some cases, although this has been changed."
The comment about storage of the BVS, made by Abbott (Abbott Park, Illinois), was in reference to the device's need to be kept at subfreezing temperatures, which was due to the fact that the stent is constructed from a variant of polylactic acid.
"There is now also a series of metal stents ... with absorbability as well," Schwartz said in reference to the absorbable metal stent (AMS) made by Biotronik (Berlin). The AMS uses a magnesium alloy that the company assures will not interfere with MRI, and has been tested in other anatomical locations.
Several of these units incorporate a trade-off between radial resistance to pressure and the ability to decompose and vanish. This radial strength problem for the BVS was highlighted in one of the Absorb trials, but the Biotronik product was not immune to this problem either.
Most of these "concerns seem to be answered as the technology progresses," Schwartz observed, including radial "strength and how long that strength lasts. But the question of resorption and resorption rates" is still problematic. However, as soon as manufacturers answer the questions of "strength, resisting remodeling, the polymer itself, the rate of degradation" and one or two other key questions, the bioresorbable stent "will in fact be a reality," Schwartz stated. "The question is where will it be in comparison to metal devices," he concluded.
Offering three-year data on the BVS stent – the device in this group with the most developmental progress behind it – was John Ormiston, MD, of Mercy Hospital (Auckland, New Zealand). Ormiston was the first to implant the device in a human, an event that took place at his institution.
The BVS is made from polylactic-L-lactide, a variant of polylactic acid often derived from cornstarch, and elutes everolimus, the same drug used by Abbott for its Xience series of stents. Ormiston said the strut for the BVS is about 150 microns thick and noted that the "release [of everolimus] is similar to that of the Xience stent."
Ormiston said the Absorb A study was a hypothesis generating trial and that the device, "originally a 3 x 12 mm stent," later became 3 x 18 mm unit. Abbott recruited 30 patients and implanted in 28, all of who were available at two years for follow-up. Outcomes "were outstanding; only one major adverse event" was recorded, he said, a non-Q wave infarct. As for lumen loss, the six-month average was .4 mm, and at two years was "similar to that of Taxus" at .47 mm. Still, Ormiston cautioned, "these were simple lesions." On the other hand, he also said that intravascular ultrasound imaging indicated that lumen diameter was better at two years than at six months.
"There were concerns about performance in more complex disease," Ormiston acknowledged, partly because of radial strength issues. "The strut absorption time I'm told is still two years," he said, a feature that was affected by the redesign of the struts into what he described as "a linked zigzag hoop" configuration.
The initial design was more like a conventional strut design, similar to those that have landed both Abbott and Boston Scientific (Natick, Massachusetts) in court for patent infringement at the hands of Bruce Saffran, MD, who won a judgment for more than $400 million against Boston Scientific. Saffran's suit against Abbott was filed last month.
The new design, Ormiston observed, is a "more uniform strut" design that is radiolucent with two imaging markers at strut junctures. Regarding restenosis, he said this was a problem in the first iteration "mainly due to negative remodeling" of the target lesion. This updated design, dubbed BVS 1.1, offers radial strength roughly equivalent to that of the company's Multilink stent. This radial strength "maintained out to six months in preclinical testing," Ormiston said.
However, the 1.1 device exhibited late lumen loss (.44 mm) greater than that of the Xience V, which Ormiston said could be chalked up either to bioactive remodeling of the lumen or to device recoil. As to rumors that the stent is hard to deliver, he said, "It's not true. In bench testing, generation 1.1 is as easy to deliver as a Xience."
Ormiston said that data for the third cohort will be available for the annual scientific sessions held by the American Heart Association (Dallas) later this year.
Mark McCarty, 703-966-3694;