|Slideshow | Bioabsorbable Stents: Design Considerations|
|Robert S. Schwartz, MD |
Minneapolis Heart Institute
The first-generation bioresorbable stent design has increased options for retreatment, such as in-stent restenosis, distal lesion and CABG; has increased CT/MR imaging capability; has appealed to concern over permanent implant; and has improved deliverability.
Therefore, according to Schwartz, the new rationale and goal for bioabsorbable stents (based on the BVS platform) are to revascularize the vessel like a metallic drug-eluting stent (DES), then resorb naturally into the body, leaving no permanent metallic implant and no permanent scaffold. Therefore, researchers are seeking to understand if these new stents can restore natural vascular response to physiological stimuli and potentially permit late lumen expansion.
“The physiologic occurrences as the vessel heals and understanding the time course of this have informed us that within six months the vessel has begun to heal,” Schwartz said. “Also, it seems that six months is the timeframe in which support may be allowed to disappear with this technology.”
With everolimus elution, revascularization at four to eight weeks “seems to work well,” he noted. “Early on in the research process, we also learned that if mass loss happens too soon, it will likely generate too much inflammation, and therefore lead to neointimal.”
Therefore, no stimulus for chronic inflammation could potentially reduce the need for long-term dual-antiplatelet therapy. “Patients who have unfortunately died from late stent thrombosis—after undergoing PCI with a DES or with a bare-metal stent—show that late inflammation can lead to stent thrombosis,” Schwartz said.
Preliminary analysis shows that CT might be the best noninvasive diagnostic imaging modality to analyze these patients at follow-up, he postulated.
Schwartz also reviewed the various current platforms in various research phases. (See attached slide show.)
He characterized Abbott Vascular’s everolimus-eluting bioresorbable vascular scaffold (BVS) platform as being the furthest along in its research, and it stands as a “significant goal for other programs that are attempting to develop these technologies.”
The BVS stent and the coating are made from polylactic acid (PLA), a material that is absorbed by natural processes in the body, Schwartz explained. The polymer is mixed with everolimus to form a matrix. The drug on the BVS stent is everolimus—the same drug used in the Xience V stent. “Although the drug is eluted from a bioabsorbable coating, the release rate is similar to that of Xience V,” he added.
The new BVS design has a “more uniform strut distribution, which should lend itself to a more even support of the vessel wall,” he said. “The regions of unsupported surface area within the cells are also considerably smaller. The company has also made some processing enhancements that have contributed to yielding a stent that demonstrates less late stent area loss and higher radial strength, in bench and pre-clinical testing.”
The trade-off for bioreabsorbable stents are degradation profile and vascular compatability, compared with profile, radial strength, recoil, vessel conformability and standard implant/storage.
For Reva Medical’s platform, the polymer is tyrosine-derived polycarbonate, formulated for bioresorbable stent, which has benign breakdown products (amino acids, ethanol, and CO2) and the ability to vary degradation rate. “The polymer is impregnated with iodine, making the entire stent radiopaque,” Schwartz explained.
The stent also is estimated to lose radial strength in about six months and completely degrade in two years.
Biotronik’s absorbable metal stent (AMS) features metal alloy consisting of 93 percent magnesium and 7 percent rare elements. It has a conventional stent design that allows for standard balloon deployment, and there are no radiopaque markers on the stent only on the balloon. “The AMS stent degrades much quicker than the polymer technologies. The AMS loses its radial strength in a couple of weeks and completely degrades in three months,” he explained.
“We have learned thus far that bio-polymers are possible, absorbing polymers are possible and timeframe appears to be mass loss at 12 months, and perhaps out to as long as two years,” Schwartz concluded.