While the buzz surrounding bioabsorbable stents continues to build, there are new evolutions within drug-eluting stent (DES) technology that vendors say will soon (and already may) be available for the shelves of your cath lab
Following the Guidant acquisition, Boston Scientific had access to two pharmacological platforms—paclitaxel or everolimus. While Taxus Liberté uses paclitaxel, Promus has the same drug platform as Abbott’s Xience stent with everolimus; the new Taxus/Promus Element stents deliver paclitaxel and everolimus, respectively.
Besides the improved drug performance, the newer-generation stents also capitalize on improved platform design, according to Keith Dawkins, MD, associate chief medical officer at Boston Scientific. Geometrically, stent platforms have changed “to deliver the drug in a more uniform manner” and architecturally, “strut thickness has been reduced to cause fewer complications during the procedure,” Dawkins says.
In the fourth quarter, the company will release Taxus Element with paclitaxel and Promus Element with everolimus in Europe. These stents utilize a platinum chromium alloy, which helps them maintain “high radial strength and allow for good visualization,” Dawkins says. The PERSEUS trial, investigating Taxus Element, has completed enrollment and results will be presented at the 2010 ACC conference. The PLATINUM trial, assessing Promus Element, is currently enrolling patients.
Through its acquisition of Labcoat Limited in January, Boston Scientific can utilize a technology that coats DES with metered droplets of a bioerodable polymer. “A small dose of bioerodable drug is placed on the outside of the stent using inkjet technology, so we end up with a bare-metal stent [BMS],” Dawkins explains. The nine-month data of Labcoat’s JacTax stent (Taxus Liberté BMS coated with an abluminal biodegradable polymer containing paclitaxel), presented at the 2008 TCT conference, showed promise for restenosis and strut coverage.
“If we can return to a BMS state, we can reduce the chance of late stent thrombosis, while also reducing the requirement of long-term dual-antiplatelet therapy,” Dawkins says.
Boston Scientific is pursuing bioabsorbable stents through two avenues. The company acquired the BVS technology, along with Abbott, through the Guidant acquisition. Since then, however, both companies have pursued different paths, resulting in independent platforms. Through its 2004 acquisition of REVA Medical, Boston Scientific is developing balloon-expandable, bioresorbable DES that combine geometric and bioabsorbable polymer properties. However, Dawkins acknowledges that the pursuit of bioabsorbable stents will clinically follow Taxus Element and Labcoat technologies.
Cy Wilcox, PhD, vice president of science and technology for Medtronic’s cardiovascular division, says that the Endeavor zotarolimus-eluting stent uses phosphorylcholine (PC) as a binder to hold the drug onto the stent, not as a drug delivery polymer. The drug layer of the Endeavor stent is 90 percent zotarolimus and 10 percent PC; with full drug elution, this layer disappears, leaving behind only a 1-micron PC base coat. Also, zotarolimus was developed for use on a DES.
“The BioLinx polymer system used in our Resolute zotarolimus-eluting stent is a blend of three polymers: the drug is mixed with two hydrophilic polymers and one hydrophobic polymer, and sprayed as a uniform layer onto the stent,” Wilcox says. “With the BioLinx polymer system, we are able to present a hydrophilic outer surface to the body, which provides biocompatibility, but retain a hydrophobic core, to enable us to control drug delivery over longer periods of time,” he says. Some studies have shown the BioLinx polymer results in low inflammatory scores.
The Resolute DES system, which is available in about 100 countries outside the U.S., utilizes BioLinx and the Driver BMS platform. “The only difference between Endeavor and Resolute is the polymer on top of the stent—with Endeavor, there is a 1-micron PC basecoat with a 4.3-micron polymer-drug layer; with Resolute, there is a 1.4-micron parylene basecoat with a 4.2-micron coat of the drug mixed with the BioLinx polymer,” Wilcox says. At the 2008 TCT meeting, the first clinical study of the stent, the original RESOLUTE trial, showed a 1.5 percent rate of target lesion revascularization and no instances of stent thrombosis in 130 patients at two years.