Bioabsorbable stents have been billed as a "now-you see-it-now-you-don't"
solution to problems like stent thrombosis and delayed vessel healing: two-year
imaging results from the ABSORB study of Abbott's everolimus-eluting
bioabsorbable stent, using some of the most advanced intravascular-imaging
techniques, indicate that at least one-third of the stent has been absorbed by
the vessel wall [1]. Moreover, write Dr Patrick Serruys (Thoraxcenter,
Rotterdam, the Netherlands) and colleagues in the March 14, 2009 issue of the
Lancet, all signs point to normal healing of the vessel wall and restored
vasomotion; only one major adverse cardiac event (MACE) occurred over the
follow-up period.

Commenting on the two-year results for heartwire, study coinvestigator Dr John
Ormiston (Auckland City Hospital, New Zealand) pointed out that with the
disappearance of the stent comes some peace of mind for operators. "Most of the
struts had definitely gone by two years," he said. "On optical coherence
tomography [OCT], the position of some struts could be identified, but it is
uncertain if there is remaining strut material here or not."

In preclinical studies, he explained, the spaces previously occupied by struts
are filled with proteoglycan, and this can lead to a "strutlike appearance on
OCT," even when the strut is gone. "If there are remnants of struts, they are
covered by tissue and not exposed to the bloodstream," he said. "I am very
hopeful that the risk of late thrombosis--I do not say late stent thrombosis
because the stent is gone--will be very low, lower than after even bare-metal
stenting and possibly lower than after balloon angioplasty. The stent is almost
gone and vasomotion restored. There is no significant foreign body remaining and
none exposed to the bloodstream. The drug and polymer are gone."

As previously reported by heartwire, ABSORB was the first-ever human trial of a
fully bioabsorbable stent, tested originally in just 26 patients. Published
results at the six- and 12-month mark showed low rates of MACE and no stent
thrombosis. However, late loss and higher-than-expected restenosis
rates--believed due to "shrinkage" of the new stent--led to speculation that the
bioabsorption rate might be occurring too fast for the necessary radial strength
of the stent to keep restenosis at bay.

In their latest paper, the ABSORB investigators report 18-month results using CT
angiography, 24-month clinical results, and invasive-imaging findings
(intravascular ultrasound [IVUS] and OCT).

According to Serruys et al, no stent thromboses have occurred during the course
of two-year follow-up, and only one MACE, the non-Q-wave MI reported previously,
out of 30 patients. Multislice CT, conducted in 25 patients, pointed to diameter
stenosis of 19%, while two-year angiography suggested an in-stent late loss of
0.48 mm and diameter stenosis of 27%--similar to that reported at six months.
Somewhat discordantly, however, OCT and IVUS results, in seven and 18 patients,
respectively, pointed to luminal area enlargement, a finding that the authors
speculate was due to decreasing plaque size, with no change in the size of the
vessel, or possibly merely reflects the variability inherent in a study of such
a small number of patients.

"I put much more weight on the IVUS and OCT findings because they measure what
is happening over the entire stent lumen and not just at one point, as late loss
does," Ormiston said. He points out that other research has shown increased
lumen diameter after balloon angioplasty and after bare-metal stenting over
longer follow-up.

An exciting, though preliminary, finding was that stented segments appeared to
have functionally active endothelium on vasomotion testing. "This finding is a
seminal observation that clearly needs to be confirmed," the authors write.

From Promise to Reality?

In an editorial accompanying the study [2], Drs Antonio Colombo and Andrew Sharp
(both from San Raffaele Scientific Institute, Milan, Italy) write that the
promising findings suggest "that the theoretical aims of the bioabsorbable stent
are close to becoming a reality."

But they also point to the shortcomings of the study, among them the small
number of patients and the very simple nature of the lesions studied. They also
note that while the stent has been recently redesigned in the hopes of improving
its radial strength, it's not known yet whether these changes will reduce the
bioabsorbability of the stent.

"Despite these key limitations, these new data suggest that a major objective
has been achieved, and we should indeed be looking at a major step forward in
coronary stenting," Colombo and Sharp conclude.

Also commenting on the study for heartwire, Dr Renu Virmani (CVPath Institute,
Gaithersburg, MD) stated that "bioabsorbable stents may hold the future for
[drug-eluting stents] DES."

Since the dawn of the DES-era, Virmani has raised alarm bells over the signs of
delayed healing and hypersensitivity reactions she's seen in autopsy results and
animal studies--likely in response to either the drugs or the polymer on
metallic DES. Only in recent years has her theory linking these effects to stent
thrombosis gained traction among clinicians. Now, she cautiously acknowledges
that bioabsorbable stents may be the way forward.

"We have observed in animal models--both rabbit and swine--up to three years,
total resolution of the stent with an absence of inflammation," she told
heartwire. "Although the animal data together with the two-year outcome results
from the ABSORB trial by Serruys et al suggest a clinical benefit, the overall
safety of these devices requires further conformation in large future
event-driven clinical trials."

Ormiston and one other study author are members of the Abbott Vascular advisory
board and three study authors are Abbott Vascular employees. Virmani has
reserved research support from and consulted for Abbott Vascular.


Serruys PW, Ormiston JA, Onuma Y, et al. A bioabsorbable everolimus-eluting
coronary stent system (ABSORB): 2-year outcomes and results from multiple
imaging methods. Lancet 2009; 373:897¡V910.
Ormiston J, Serruys PW, Regar E, et al. A bioabsorbable everolimus-eluting
coronary stent system for patients with single de-novo coronary artery lesions
(ABSORB): a prospective open-label trial. Lancet 2008; 371:899-907.
Colombo A, Sharp ASP. The bioabsorbable stent as a virtual prosthesis. Lancet
2009; 373:869-870.