Since their emergence in 1986 for use in performing percutaneous transluminal coronary angioplasty to create patency in occluded vessels and reduce the incidence of angiographic restenosis, coronary stents have demonstrated incontestable--if not trouble-free--benefits.
Among current drug-eluting stents (DES), second-generation designs boast higher procedural success outcomes and better long-term clinical outcomes than first-generation versions, according to Syed Hossainy, Volwiler Fellow and director of the Innovation Incubator at Abbott Vascular (Santa Clara, CA). "However, there's still room for improvement in second-generation drug-eluting coronary stents, especially for patients with Type 2 diabetes," adds Hossainy, who delivered a series of well-received remarks on this topic at the recent MedTech Cardio Conference in Minneapolis. "For such patients, DES works better than bare-metal stents but not as well as in nondiabetic patients.
Another problem facing coronary drug-eluting stents involves patients with small-vessel, long, and diffuse-disease chronic total occlusions, Hossainy comments. But notwithstanding the problems associated with treating these patient cohorts, current technologies have significantly pushed the boundaries of where drug-eluting stents were six or seven years ago.
These successes should not obscure the fact that DES strategies have not been able to make much of a dent in treating the peripheral vasculature--for example, the distal superficial femoral artery (SFA) and vessels below the knee. These applications, Hossainy says, are the major unmet clinical need. "Whether DES or drug-coated balloons, none of these technologies have turned out to be effective."
Treating common pathobiologies in the peripheral vessels is complicated by the fact that the legs are subjected to chronic and cyclic extravascular forces. "When you walk, bend, sit, or stretch, a significant amount of fatigue affects stents in the SFA," Hossainy states. "Permanent implants do a great job in more unperturbed areas, but the moment they're implanted in the SFA, they cause constant trauma to the vessel. No matter how good the stent or the coating is, the trauma is going to remain because of the motion."
This is where bioabsorbable stents come in. "Initially, such implants will act as load-bearing scaffolds," Hossainy notes. "Over time--in a controlled and gradual manner--the scaffold will lose structural continuity and thus relieve the stress on the vessel. This transient scaffolding will enable the vessel to behave close to its native, unstented state, eliminating any chronic mechanical insult resulting from the extravascular forces on the otherwise stiffened, permanently stented vessel."
Although Abbott and other companies are conducting clinical trials to test the efficacy of bioabsorbable stents for peripheral applications, the time lag between initial human trial data and commercial launches in non-U.S. markets typically takes two to three years. And achieving FDA approval could take another three years. Until then, patients will have to be patient.