Biomedical Engineering Reference
In-Depth Information
recent study has demonstrated that there appears to be no signifi cant dif-
ference in the use of polymer-free rapamycin-eluting Yukon ® stent and the
polymer-based paclitaxel-eluting Taxus stent (Ruef et al. , 2008).
As was the case with radioactive stents, DESs have been developed with
the aim of reducing the rate of ISR seen with the use of bare metal stents.
In this approach, a stent coating is loaded with a chemotherapeutic drug
able to limit early thrombogenicity and/or neointima formation. Based on
the emerging importance of cellular proliferation in the understanding of
ISR formation, drugs with anti-proliferative and anti-infl ammatory actions,
including sirolimus and paclitaxel, have been shown in preliminary ran-
domised trials to have marked effects on restenosis compared with BMS
(4.9% versus 20%) (Holmes et al., 2004). In 2003 NICE (National Institute
of Clinical Excellence) guidelines recommend DESs containing either siro-
limus or paclitaxel for use in patients with high risk restenosis profi les,
including lesions in small calibre vessels (
15 mm)
believed to be particularly prevalent in the already high risk diabetic
community (Aronson, 2002). Having the ability to target the full range of
mechanisms leading to ISR (recoil, remodelling and cellular proliferation),
DESs appeared to offer signifi cant advantages over BMS but not without
potential complications or expense (an additional £500-600 for an equiva-
lent but drug eluting stent) (NICE, 2003).
Typically the drugs are eluted from the DES during the fi rst month of
implantation. The aim is to deliver the drug directly to the vessel wall, thus
minimising the amount of drug required to provide an effective therapy
(Bazian, 2005). The advent of the DES has resulted in a wide array of
materials and coating combinations to produce the ideal system in terms of
mechanical stability, elution profi le of the drugs and in some cases degrada-
tion of polymer layer. The drug is released either through diffusion mecha-
nisms or during the degradation of the polymer coating which may be
present either as a single or multiple layers on either a stainless steel or
cobalt-chromium alloy platform. The concept of drug delivering stents has
received a huge amount of attention in the design and development of new
stents as well as the testing of their clinical effi cacy. However, the technol-
ogy has faced a number of challenges, not least identifying the correct
pharmacologic agent and ensuring its appropriate release kinetics (Ber-
trand & Sipehia, 1998). It is, however, clear that the success of a DES is
dependent upon the correct support material combined with an appropriate
polymeric coating to act as a reservoir and drug release platform, and the
use of an appropriate drug able to interfere with the development of infl am-
mation, cell migration and proliferation and secretion of extracellular
matrix.
The drugs are incorporated in a durable or biodegradable polymeric
matrix that is generally spray-coated onto the surface of a metallic stent
<
3 mm) or long lesions (
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