Drug-eluting stents: what room for improvement?

Related tags Myocardial infarction Atherosclerosis

There is no doubt that the introduction of stents, and latterly
those that deliver pharmacological agents, has revolutionised the
treatment of coronary artery disease over the last 10 years and all
but consigned heart surgery to second-line use. But has the
phenomenal improvements seen with the first generation of
drug-eluting stents (DES) raised the bar too high for other
products to follow, asks Phil Taylor?

Quite possibly, according to Martin Rothman, professor of interventional cardiology at Barts and the London NHS Trust Hospital in the UK, who believes the termination or delay of a number of clinical trials of new product candidates - from the likes of Guidant - is a symptom of this lofty development hurdle.

"Other companies have a mountain to climb to prove that their candidates are as good as, let alone better than, the current generation of DES,​ he told In-PharmaTechnologist.com. And this raises questions about whether millions of research dollars should be spent on chasing small, incremental improvements in clinical efficacy.

"In an ideal world, companies would say OK, we stop now and spend the funds on areas with more potential for clinical gain,"​ he said.

The march to replace coronary artery bypass grafting (CABG) surgery in coronary artery disease began in 1977, with the first presentation of data showing that a blocked artery could be opened by inflating a balloon within it, allowing blood flow to resume. This technique, known as angioplasty, was much less invasive for the patient and was associated with a lower death rate during the procedure than CABG.

Angioplasty had problems from the outset - there was a risk of acute closure of the artery and 30 per cent of patients needed a repeat procedure because the cleared artery would close up again, a process known as restenosis. But the benefits of avoiding heart surgery prompted cardiologists to persist with the technique, and their optimism paid off in the mid-1990s with the introduction of stents, metal sleeves that were opened out in the cleared artery to form a scaffold, much like the wooden supports in a mineshaft.

These bare metal stents cut the repeat procedure rate to 15-20 per cent, but additional improvements proved hard to achieve, as the process of angioplasty and stenting - together known as percutaneous coronary intervention (PCI) - caused local damage that stimulated the formation of scar tissue and the proliferation of cells, such as smooth muscle, in the vessel. This tended to grow through the mesh of the stent, eventually re-blocking the vessel.

The solution appeared in 2000, when the first European patient received a DES - the Cypher product developed by Johnson & Johnson subsidiary Cordis. This was coated with a polymer that delivered sirolimus, a 20-year-old immune-suppressing drug that blocked the biological process underlying restenosis. This cut the average restenosis rate to just 4 per cent, and even lower in patients without complications such as diabetes, long lesions or lesions in small vessels.

Cypher has been joined on the market by Boston Scientific's Taxus stent, delivering the anticancer drug paclitaxel, which has achieved similar improvements over bare metal stents. The first a head-to-head trial of the stents, called REALITY, was reported at the American College of Cardiology meeting yesterday. This J&J-sponsored study suggested there was little to choose between the products in terms of restenosis rates, although the company claimed some superiority for Cypher in terms of thrombosis, which remains a complication of current products.

Together, these stents have revolutionised the treatment of coronary artery disease, with close to 100 per cent penetration in the US, and 50-60 per cent in the UK. Meanwhile, the market for drug-eluting stents is expected to reach $5.5 billion by the end of this year and $6.3 billion by 2007.

However, "the risk developers of new DES face is that some of the new technologies in development may not match the performance of the first-generation products,"​ said Rothman.

Nevertheless, there is a lot of research going on in the stent area as companies aim to tap into this fast-growing market. A number of companies, including Medtronic, Guidant, Abbott Laboratories and Conor MedSystems are developing first-generation products, aiming to show that they can demonstrate improvements over bare metal stents at least as good as those achieved by Cypher and Taxus.

For example Medtronic reported the results of the ENDEAVOR II trial at the ongoing ACC annual meeting yesterday, and initial reactions from the show suggest that the data is strong enough to secure approval. The company intends to launch its DES, which delivers the drug ABT-578 in Europe in 2007 and the US in 2008.

Meanwhile, Cordis and Boston Scientific are both working on ways to improve the properties of their existing stents. For example, Boston Scientific recently won approval for a variation on Taxus called the Taxus Liberte stent. This has the same drug delivery system and the same drug, but a refined design that should make delivery and deployment of the stent easier, and potentially broaden its use into blood vessels for which its forerunner would not have been suitable. And Medtronic and Guidant are also developing stents made of cobalt chromium, a lighter, thinner material than that used in the current generation.

Another area of development in the DES field is the use of novel compounds that can act on different elements of the pathway leading to restenosis. A number of compounds are being evaluated in addition to sirolimus and paclitaxel, which could be used either alone or in combination.

For example, statin drugs - already multibillion dollar sellers in oral form for lowering cholesterol, could be used to promote regression of the lipid component of an arterial plaque. And the glitazones - widely used to treat diabetes - have anti-inflammatory properties that could also help.

One approach sparking interest is being developed by a small company called Orbus, and involves coating the stent with a drug that stimulates the protective cellular lining of the artery - the endothelium - to grow around the stent. This could reduce the chances that the stent itself might stimulate the formation of a blood clot, said Rothman.

In terms of the delivery vehicle itself, Rothman also noted that work is ongoing on so-called biodegradeable stents, which would only reside in the artery for a limited period of time and then dissolve away once their drug delivery duties were discharged.

The premise is that these would make it easier for a patient to have a CABG procedure later, if required, but there is always the possibility that this approach will not match the performance of the current stents in the first instance, he suggested.

Overall, we may arrive at a situation where stents are considered not merely as support for the vessel, but as implantable drug delivery devices, according to Rothman. This could extend the use of stents into other areas, such as the treatment of peripheral artery disease and perhaps non cardiovascular applications such as oncology, he suggested.

Another project is using the stent to deliver a growth factor (VEGF) that could stimulate the production of new, healthy drug vessels to treat areas of diminished blood flow to the heart.

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