Download Drug-eluting stents Current research in resolving angina pectoris

Drug-eluting stents
Current research in resolving angina pectoris and avoiding MI associated with coronary artery
stenosis is focused on the design and development of drug-eluting stents in prevention of in-stent
restenosis (ISR). Coronary heart disease (CHD) is the leading cause of death in the western world.
Even though mortality from CHD has more than halved since 1961 (British Heart Foundation, 2011) it
remains a major socioeconomic burden, with the American Heart Association committed to
decreasing mortality rates in the USA by a further 20% by 2020 (AHA, 2011).
In CHD, there is a progressive build-up of atheromatous plaque lining the walls of coronary arteries
as a result from high circulating low density lipoprotein cholesterol, local inflammation and shear
stress among other factors. While plaque stability is maintained by vessel remodelling throughout a
period of many years, its encroachment (if left untreated) eventually leads to artery stenosis and
thrombus formation, ultimately leading to rupture and blockade of narrow vessels to result in acute
myocardial ischemia. This used to be – and is still sometimes – resolved by coronary artery bypass
grafting (first conducted in 1967). This procedure involves grafting another vessel (usually the
saphenous vein) to the coronary arteries to bypass the blockage, restoring perfusion to meet
metabolic demands of the myocardium and reduce the risk of CVD. However, veins are capacitance
vessels and can harden by means of atherogenesis from carrying out arterial function. This often
leads to graft failure. The first angioplasty was performed on dogs by Andreas Greuntzig and
presented to the AHA in 1976. Astonishingly, a year later, the procedure was first performed in man
in Zurich and is now much more commonly performed than grafting.
Percutaneous transluminal coronary angioplasty (PTCA)
Coronary artery stenting is performed in acute cases of CHD that cannot be managed by drug
therapy but rarely in older people. The procedure is performed by cardiologists or surgeons and
does not require general anaesthesia. A cut is made in the groin under local anaesthesia (but is not
compulsory) and a catheter is inserted into the femoral artery guided by needle. The catheter has a
tapered hydrophobic end that prevents backflow of arterial blood through the wall. It is then guided
up to the coronary arteries by fluoroscopy and a wire is passes through the catheter to the blockage.
This wire has a small balloon on the end which is blown up one or twice to press the plaque to the
artery walls and increase lumen space to re-establish efficient perfusion. A stent (a wire mesh tube)
can be inserted over the balloon and expanded to hold the artery lumen open. The balloon is then
collapsed (by applying negative pressure) and removed with the stent is left in place.
Stenting
Stents are used in more than 90% of all PTCA procedures as they have decreased the occurrence of
re-stenosis from 40% (with balloon angioplasty) to 10% by eliminating vascular recoil. To support
calcified lesions, stents have to have good durability and flexibility; also, strut thickness has to be
adjusted so as to maintain strength yet allow easy delivery. However, stenting can result in ISR which
occurs due to neointimal hyperplasia – the migration of predominantly proliferating vascular SMCs
and extracellular matrix to grow around the stent. Like vascular recoil, this process is likely to be
driven by oxidative stress from inflammation. Furthermore, autopsies revealed heterogeneity in the
aforementioned response, where some areas presented hyperplasia and some did not. ISR occurs in
20-50% of procedures using bare metal stents such as the Palmaz-Schatz stent and is a major
complication. To combat this issue, drug-eluting stents (DES) were developed that inhibit
mechanisms leading to SMC proliferation and restenosis. In order to have effect, DES require the
following: (1) must contain a sufficient amount of compound, (2) compound must be stable to
prevent washing off during delivery of stent, (3) compound must be released over a specific period
of time and (4) compound must reach target cells. It is important for the compound to be eluted
over the time it takes for hyperplasia to occur. Moreover, the drug has to be efficacious at
preventing the predominant mechanisms underlying hyperplasia to positively affect healing (possibly
including ECs). The two leading commercially available DES approved by the FDA are Cypher stent
and Taxus stent. The Cypher stent was developed first to elute the immunosuppressant drug
sirolimus (also known as rapamycin) from a non-biodegradable polymer. The Taxus stent releases
paclitaxel – a mitotic inhibitor. Pathology findings in a patient with both a BMS and Cypher stent
revealed that, due to the effectiveness of serolimus, neointimal hyperplasia was almost completely
abolished in association with the latter. Although introduction of DES correlated with a dramatic
reduction in ISR rates in several clinical studies, a major problem is reduced vascular healing
characterized by endothelial regrowth. Pathological findings of a Cypher FIM Study investigating
vascular healing 4 years after stenting revealed (by scanning electron micrographs) that >95% of
stent was endothelized but a strut was left uncovered. In animal studies, thrombosis is also seen to
occur after 30 days following stenting in 1% of cases, often resulting in death from MI. This is
associated with cessation of anti-platelet therapy in the presence of an incompletely endothelized
stented artery (as in the Cypher FIM study). When blood and metal come into contact this triggers
coagulation and increases the risk of thrombosis. For this reason, anti-coagulant therapy is required
until the endothelium regrows over the neointimal. However, it is impossible to be sure of 100%
regrowth thus there is always a risk of late-stent thrombosis which is not easily treatable as acute
thrombosis. At the same time, prolonged anti-platelet treatment can cause gastrointestinal
complications such as haemorrhage. While old stents were made from stainless steel, new ones are
cobalt/titanium alloys. Also, sirolimus analogues such as everolimus and zotarolimus are already in
use. Other developments in stenting include the use of anti-oxidants (to reduce oxidative stressmediated hyperplasia), endothelial cell seeding (by attaching molecules attracting EC adhesion),
endothelial cell progenitor cell capture (which creates a platform for EC growth) and use of
endogenous compounds such as oestradiol, VEGF and NO donors. The advantage of endogenous
compounds is lack of toxicity however toxicity is usually not an issue as there is poor systemic
distribution of eluted compounds.
Succinobucol study
Oxidative stress is seen as a key mediator of neointimal hyperplasia by inducing SMC proliferation
and apoptosis, and results from overproduction of vessel-derived ROS. Shown to be potentiated by
balloon angioplasty, ROS are derived from NADPH and NADPH oxidase reactions and induce
inflammatory MMP enzymes. Importantly, oxidative stress was shown to be toxic to ECs and impede
their migratory activity. A recent study at the UG investigated the beneficial properties of
succinobucol in reducing ISR. The compound is an anti-oxidant which has been established as a
novel vascular protectant drug with potential antiplatelet activity. The drug also showed to have
beneficial effects in a subgroup of diabetic patients. A monosuccinic acid ester of probucol,
succinobucol has a side-chain that increases lipophilicity. Probucol was previously shown to inhibit
ISR by potentiating and lower lipid levels but was toxic to the liver. The first step of the study was to
perform in vitro analysis of succinobucol, then to create a succinobucol and succinobucol/sirolimus
combination stent to test if (1) succinobucol improved the profile of the sirolimus stent, (2)
succinobucol alone can reduce ISR. BMS were also used as control. An improved profile would be
characterized by reduced platelet aggregation. Platelets are small, colourless, enucleated cells that
originate from megakaryocytes in bone marrow with a life span of 7-10 days. Platelets circulate in
blood, leading to thrombus formation.
Platelet aggregation was measured by impedance
aggregometry, where cuvettes containing electrodes are filled with a solution of platelets. Platelets
are agitated with a stir bar and aggregation is stimulated (by collagen, in this study). Current, created
at the electrodes, is impeded upon platelet aggregation to form a layer on the electrodes. Change in
impedance (Ω) was measured as it is proportional to aggregation: the higher the impedance the
greater the aggregation. Addition of succinobucol was observed to reduce platelet aggregation in
both rabbit blood and plasma in a dose-dependent manner, thereby, presenting an anti-thrombotic
efficacy. In order to determine whether platelet aggregation can be induced by oxidative stress (and
hence reduced by succinobucol), xanthine and xanthine oxidase were added to the cuvettes.
Xanthine oxidase catalyzes the conversion of xanthine to form superoxide and can therefore be used
as a model of oxidative stress in vitro. Increasing concentrations of xanthine and xanthine oxidase
significantly increased platelet aggregation, which was reduced in the presence of the enzyme,
substrate and succinobucol. This signifies that succinobucol reduces thrombosis by ROS scavenging.
Yukon stents were coated by spraying alcoholic solutions of compounds directly onto stent.
The stent has a pitted surface (visible using high power EM) that accommodates drug and eliminates
the necessity of a polymer. PTCAs were performed on large white pigs (up to 28kg) with the catheter
inserted using the Seldinger Technique into the femoral artery. Each pig received 2-3 coated stents
as well as dual antiplatelet therapy. Stents were oversized (1.2:1) on purpose so as to induce
oxidative stress and hyperplasia in vessel walls. In vivo release studies were conducted for up to 28
days, following which histology and planimetry were performed. Tissue was removed from stents,
homogenized and amount of drug released was quantified. Both in vitro and in vivo release studies
showed that compounds were indeed released into tissue; however, was this release time and rate
optimized? Surprisingly, endothelial regrowth was virtually unaltered by any DES compared to BMS
or between the different drug combinations. Succinobucol-eluting stents (SES) also appeared to
potentiate inflammation (based on the fibrin score) even though it was expected to reduce it by
inhibiting ROS synthesis. Furthermore, histological studies showed that SES was associated with an
increased neointimal area (indicating hyperplasia) and % vessel stenosis compared to BMS.
Contrarily, sirolimus-eluting stents (both dual and single) showed a decreased neointimal
hyperplasia compared to BMS (with more uncovered struts). These results correlated with a clinical
trial (Tardif et al., 2003) of oral succinobucol following PTCA where no reduction in neointimal
hyperplasia was achieved. It is postulated that succinobucol coating may impair release or effect of
sirolimus thereby reducing its anti-restenotic properties. Another reason could be inflammation or
reduced vascular healing brought on by succinobucol. It is also likely that the concentration of the
agent used may have been too high, rendering it toxic. However, due to the labor-intensive nature
of the aforementioned study it would not be feasible to repeat it using a smaller dose.
Bindarit study
The same team in Glasgow commenced a similar study using bindarit – an inhibitor of Monocyte
Chemotactic Protein-1 synthesis in vitro and in vivo without affecting other chemokines or causing
immunosuppression. Bindarit was previously found to ameliorate renal dysfunction in pig models of
renovascular hypertension and reduce neointimal formation mediated by vascular injury in rodents.
It is also currently tested in a Phase II clinical trial for rheumatoid arthritis. In this study, bindarit was
orally administered to pigs (with reported difficulty that was resolved with yoghurt) from 2 days
before stenting until the end point at 28 days. 1 or 2 stents were implanted into separate coronary
arteries using the aforementioned procedure. Adjacent arterial tissue was then removed for
biochemistry and aortic SMCs were grown for in vitro studies. Bindarit was found to significantly
decrease TNFα-mediated MCP-1 production in pig SMCs in a dose-dependent manner. The
compound also reduced TNFα-mediated SMC proliferation, showing significantly decreased
neointimal area and thickness in histological studies. Bindarit also significantly reduced the
inflammatory score compared to control in vivo. It was concluded that bindarit reduced ISR
associated with neointimal hyperplasia by apparent anti-inflammatory actions. This signifies that a
future bindarit-eluting stent could be developed.