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Journal of Postgraduate Medicine, Vol. 49, No. 3, July-Sept, 2003, pp. 291-295 Technology Review Drug-Eluting Intra-Coronary Stents: Have We Got the Magic Bullet? Bhatia V, Bhatia R, Dhindsa S
Department of Internal Medicine, SUNY, Buffalo, NY Code Number: jp03083 Much research has been done on many mechanical devices and drugs to prevent restenosis, providing the rationale for an enormous number of clinical trials, but none have been proven to be effective.[1],[2],[3],[4] Despite the use of multiple percutaneous revascularization techniques, including balloon angioplasty, repeated stenting, laser therapy, platelet inhibitors, heparin-coated stents and atheroablation, approximately half of the 30 % of patients in whom restenosis occurs after coronary stenting, have recurrent restenosis.[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] In the present paper, we briefly describe the mechanisms of in-stent restenosis and have reviewed the literature to discuss the novel strategy of coating stents with anti-proliferative drugs to prevent this complication. We have also addressed the benefits and shortcomings of using these drug-eluted stents. The initial events immediately after stent placement result in de-endothelialization and the deposition of a layer of platelets and fibrin at the injured site in the coronary artery. Activated platelets express adhesion molecules such as P-selectin and glycoprotein (GP) Ib [alpha], which attach to circulating leukocytes via platelet receptors such as P-selectin glycoprotein ligand and begin a process of rolling along the injured surface. Under the influence of cytokines, leukocytes bind tightly to the leukocyte integrin (i.e., Mac-1) class of adhesion molecules via direct attachment to platelet receptors such as GP Ib[alpha] and through cross-linking with fibrinogen to the GP IIb/IIIa receptor. The migration of leukocytes across the platelet-fibrin layer and into the tissue is driven by chemical gradients of cytokines released from smooth muscle cells (SMCs) and resident leukocytes. Growth factors are released from platelets, leukocytes, and SMCs, which influence the proliferation and migration of SMCs from the media into the neointima. The resultant neointima consists of SMCs, extracellular matrix, and macrophages recruited over several weeks. Over even longer periods of time, there is a shift to fewer cellular elements with greater production of extracellular matrix. In addition, there is eventual re-endothelialization of at least part of the injured vessel surface. Experience with systemically administered drugs, such as antiplatelet agents, anticoagulants, calcium-channel blockers, angiotensin-converting-enzyme inhibitors, cholesterol-lowering agents, and antioxidants, has proven almost universally negative. These agents were previously tested in animal models and found to be beneficial. The lack of efficacy in human studies may be in part due to insufficient concentration of the drug at the injury site or lack of chronic dosing. In general, although animal models provide new insights into the mechanism of restenosis, biological and mechanical differences mean that therapeutic success of anti-restenotic therapies has not been achieved in human beings. The recent introduction of intracoronary radiation has emerged as a promising modality to attenuate the intimal hyperplastic reaction.[16],[17],[18] Despite the lack of benefit for preventing restenosis in de-novo lesions, brachytherapy was shown to be effective in reducing recurrent restenosis. However, larger studies and long-term follow-up showed alarming long-term sequelae such as edge restenosis and late thrombosis, raising some concerns about the potential toxicity of a cytotoxic approach.[16],[19],[20] Similarly, the results with oral administration of an anti-proliferative agent, sirolimus, have failed to show any benefit and in fact there was a higher incidence of adverse events in the recipients of such a therapy.[21] The potential usefulness of immunosuppressive agents in the treatment of restenosis arises from parallels between tumour cell growth and the benign tissue proliferation, which characterizes intimal hyperplasia. With drug-coated stents, there is site-specific drug delivery, which reduces systemic toxicity and thus is an attractive therapeutic method to achieve an effective local concentration of a drug for a designed period. The safety and efficacy of such an approach critically depends on the delicate combination of drug, polymer, and the kinetics of release.[22] Drug-eluting stent is a device that releases single or multiple bioactive agents into the bloodstream that can deposit in or around tissues adjacent to the stent. The drug can be simply linked to the stent surface, embedded and released from within polymer materials, or surrounded by and released through a carrier. The carrier can coat (strut-adherent) or span (strut-spanning) the stent struts. Sirolimus is a natural macrocyclic lactone with potent immunosuppressive and antimitotic action, which was approved in 1999 as an anti-rejection drug in renal transplant recipients. The cellular action of rapamycin (sirolimus), a natural fermentation product produced by Streptomyces hygroscopicus, is mediated by binding to the FK506 binding protein. By inhibiting a kinase known as the target of rapamycin, it restricts the proliferation of smooth-muscle cells by blocking cell-cycle progression at the G1/S transition. The finding that rapamycin possesses both anti-proliferative and anti-migratory activity suggests that it could contribute to the control of arterial re-narrowing after percutaneous intervention. Marie Claude Morice and colleagues report the first randomised double-blind trial (RAVEL study) comparing the coronary stent coated with sirolimus with a standard uncoated stent.[23] The trial included 238 patients with single coronary lesions at 19 different medical centres. Patients with complex coronary lesions were excluded. Stents were coated with the agent prior to insertion. This was achieved by applying a mixture of synthetic polymers blended with sirolimus with a second coat of drug-free polymers to serve as a diffusion barrier. The polymers act as a drug reservoir and permit the gradual elution of sirolimus. The stent was designed to release 80% of the drug within 30 days after implantation. The angiographic rate of restenosis at 6 months was 26.6% in the standard-stent group and 0% in the drug-stent group. There were no reported cases of subacute thrombosis. The mean late luminal loss was ZERO in the sirolimus-eluting stent group and 0.80 mm in the conventional stent group. During a follow-up period of up to 1 year, the overall rate of major cardiac events (MACE) was 5.8% in the sirolimus-stent group and 28.8% in the standard-stent group. The results of this trial generated a lot of enthusiasm and many had started believing that it was the end of restenosis after percutaneous coronary interventions. Two-year follow-up findings indicated that these results had been sustained at 2-year follow-up with an event-free survival of 90%, compared with 80.5% in the control arm, and a 2.5% target lesion revascularization rate. Additionally, there was no stent thrombosis observed during the study years. In another study done by Maurice MC et al, the beneficial impact of sirolimus-coated stents on neointimal growth inhibition was persistent at 2 years' follow-up.[24] These results have been further tested in a large US multicentre randomised trial, called SIRIUS (Data unpublished, results declared at the 14th annual TCT symposium, September 2002 by Moses JW et al). In this trial 1,058 patients were randomised to 2 treatment arms: 533 patients received a CYPHER™stent (rapamycin-coated) and 525 received a bare metal Bx VELOCITY™coronary stent (standard-stent). Eight-month angiographic follow-up showed minimal in-stent late lumen loss (0.17 mm) in patients treated with the sirolimus-coated stents. The 3.2% rate of angiographic in-stent restenosis represents a 91% reduction vs. the control arm (bare metal stent), and the 8.9% angiographic in-lesion restenosis (including a 5-mm area at each end of the stent) represents a 75% reduction vs. the control arm. At 9-month follow-up, the event-free survival rate was 92.7% in the sirolimus-treated cohort vs. 80.7% in the bare metal stent cohort (p<0.001). The SIRIUS patient population included substantial numbers of patients at significant risk of restenosis, including patients with diabetes mellitus (26.4%), longer lesions (average 14.4 mm), hyperlipidaemia (73.6%), hypertension (67.7 %), and multivessel disease (41.6 %), as well as patients who had previously undergone percutaneous coronary interventions or coronary artery bypass surgery (34.2 %). Thus a 0% restenosis rate with sirolimus-eluting stents is unlikely, as these devices are used in more complex and challenging coronary lesions. One-year clinical follow-up results from SIRIUS trial (Data unpublished, declared at ACC, 2003) showed that the favourable findings reported at 9-month follow-up after insertion of the sirolimus-eluting stent remained robust at 12-month follow-up. Clinical restenosis as defined by TLR rates remained markedly reduced in the sirolimus arm of the study. In addition, there was an increase in the absolute reduction in TLR rates from 12.5% at 270 days to 15.1% at 360 days. Sousa and colleagues recently conducted a trial using slow-release and fast-release sirolimus-eluting stents on 30 patients and followed them clinically, angiographically and using intra-vessel ultrasound (IVUS) for 2 years.[25] Twenty-eight patients underwent 2-year angiographic and IVUS follow-up. No patient had in-stent restenosis. At 2-year follow-up, only 1 patient had a 52% diameter stenosis within the lesion segment, which required repeat revascularization. The target-vessel revascularization rate for the entire cohort was 10% (3/30) at 2 years. All other patients had < or =35% diameter stenosis. This study demonstrates the safety and efficacy of sirolimus-eluting stents 2 years after implantation in humans for the first time. The same group of investigators demonstrated the safety and the potential utility of sirolimus-eluting stents for the treatment of in-stent restenosis.[26] Guagliumi et al described the pathological findings at autopsy in a sirolimus- stent recipient in the RAVEL trial who died after 16 months.[27] This sirolimus-eluting stent was widely patent at 16 months with > 80% endothelial coverage. Neointimal healing was nearly complete, with only rare fibrin deposits. Results of another trial called the C-Sirius trial were declared recently.[28] It was a Canadian, multicentre, randomised, double-blind trial enrolling 100 patients and it showed the safety and efficacy of sirolimus-eluting stents in maintaining in-stent minimal lumen diameter (MLD) at 8 months in de novo native coronary lesions. No in-stent stenosis was reported in this trial in sirolimus group and in-stent late loss was 91% less in the sirolimus group (0.09 mm vs. 1.01 mm) as compared with the uncoated stent group. The positive results of the RAVEL and SIRIUS trials can now be extended to patients with long lesions in smaller vessels. All the major trials of the sirolimus-eluting stents showed benefits in diabetic patients also. The total lack of restenosis in RAVEL trials in diabetic patients is striking. Data from the SIRIUS trials shows 83% reduction of in-stent restenosis and 65% reduction of in-segment restenosis in diabetic patients, and bypass surgery is often performed in this group instead of primary angioplasty or stenting. Diabetes is a formidable limitation to the success of conventional percutaneous coronary interventions (PCI). If the data from these trials are borne out, it will have a great impact on how diabetics are managed with coronary revascularization. In a preliminary early cost-effectiveness sub-study of the SIRIUS trial, the use of a drug-eluting stent, compared to a standard bare-metal stent, added more than $2,000 to the procedure's initial cost. The drug-eluting stent reduced the need for later medical care, but the resulting cost savings only partly erased its extra initial cost. The final results of the cost-effectiveness of these stents in the SIRIUS trial showed that during initial hospitalisation, costs were higher in the drug-eluting group, which were explained by the cost of the device itself. [29] During the 12-month follow-up period, there were substantial reductions in the need for repeat revascularization. Despite higher in-hospital costs, the group receiving the sirolimus stent showed cost savings of about $2,500 at 12-months follow-up. Given the initial cost differential of about $2,800 higher associated with the drug-eluting stent, the difference at one year was only about $300 per patient. PACLITAXEL-ELUTING STENTS (TAXOL-COATED STENTS) The taxanes (for example, Paclitaxel) are potent anti-proliferative agents used in cancer chemotherapy. Paclitaxel promotes polymerisation of the alpha and beta subunits of tubulin by reversibly and specifically binding the beta subunit of tubulin, and thus stabilizes microtubules. A stent coated with paclitaxel is also safe and effective for decreasing neointimal proliferation within the stented segment and reducing the incidence of clinically significant in-stent or edge restenosis. Three randomised trials (ASPECT,[30] ELUTES[31] and TAXUS I[32]) found that the paclitaxel-coated stent significantly reduced late lumen loss, neointimal volume index, and angiographic restenosis at 6 months (0 to 4 vs. 10 to 27 % for a bare stent). Paclitaxel-coated stents dramatically inhibited neointimal hyperplasia, as evidenced by angiography and IVUS evaluations at 6 months, according to the ASian Paclitaxel-Eluting stent Clinical Trial (ASPECT) study.[30] The binary restenosis rate was reduced significantly from 27% in patients receiving control, bare metal stents to 12% in those receiving low-dose paclitaxel-coated stents and to just 4% in patients receiving higher dose paclitaxel-coated stents, demonstrating an important dose-dependent relationship. Mean diameter stenosis was reduced from 38% in the control arm to 24% in the low-dose group, and to only 12% in patients treated with higher dose paclitaxel-coated stents. The TAXUS program is a series of clinical studies designed to collect data on the TAXUS paclitaxel-eluting stent (Boston Scientific, Natick, Massachusetts) for the reduction of restenosis after angioplasty and stenting. The TAXUS II trial consists of 2 sequential cohorts: (1) a slow-release (SR) formulation, and (2) a moderate-release (MR) formulation. Both delivery systems employ the same total loaded dose (1 microgram/mm 2), with different biphasic release rates. The primary difference between the 2 systems takes place within the first 48 hours following implantation, during which the amount of drug released is substantially greater (8- to 10-fold) with the MR than with the SR formulation. After 48 hours, there is a sustained release during the course of the next 10 days in both groups. The control stent used in this trial was the NIRx Conformer stent (Medinol, Tel Aviv, Israel), which is a 15-mm stent premounted on a 20-mm balloon delivery system in available diameters of 3.0 mm and 3.5 mm. Six-month quantitative coronary angiographic analysis revealed a significantly lower binary restenosis rate in the stented segment for both the SR and MR arms compared with control (2.3% vs. 4.7% vs. 19.0%, respectively, P < .0001). This significant difference was noted only in the stented segment; there was no difference noted in the restenosis rates for the proximal or distal edges among the groups. Twelve-month follow-up was declared for 96% of TAXUS II patients (Data unpublished, ACC 2003). It revealed a significantly lower MACE rate in the TAXUS arm of the study compared with the control. This finding was mostly driven by significantly lower TLR and TVR rates. There was only 1 stent thrombosis in each of the TAXUS groups, and there were no deaths reported. Importantly, these benefits were sustained between 6 and 12 months follow-up. The 12-month results of the TAXUS II study confirm the study's favourable 6-month findings. Interestingly, these results were sustained even after the discontinuation of 6 months of clopidogrel therapy. Favourable preliminary results have also been reported in TAXUS II, III[33] and the pivotal randomised TAXUS IV trial. A prospective, randomised, single-blind, multicentre trial called Deliver -1, showed no significant effect of paclitaxel-coated vs. metallic stents for treatment of coronary lesions at 9 months. The Deliver trial used the ACHIEVE™stent that did not employ a polymer coating to elute paclitaxel, leading many to speculate that it was the lack of polymer in Deliver that foiled the trial. Others feel that paclitaxel is very fat-soluble and it can be retained in the tissue, especially the atherosclerotic plaque, for a long time after it is delivered. There may have been an early loss during insertion, or there may have been variability from stent to stent and that could have been the reason for the failure of the Deliver trial. The outcomes have also not been so good with the paclitaxel derivative (7-hexanoyltaxol, QP2)-eluting stent, in which late lumen loss has been described. [34],[35],[36] The Study to COmpare REstenosis rates between QueST and QuaDDS-QP2 (SCORE) was a randomised multicentre trial that was terminated prematurely after an interim analysis showed a dramatically increased predisposition for subacute and delayed stent thrombosis (9.4%) in the QP2-stent group as compared with the uncoated stent group (0%). It is still premature to comment on the safety profile of stents coated with potent antimitotic agents such as sirolimus and paclitaxel. These agents inhibit smooth muscle cell proliferation, and therefore have a mechanism of action similar to that of radioactive stents. Synthetic polymers are often used as carriers for these agents, and polymer biocompatibility remains a concern, as polymers often induce an exaggerated inflammatory reaction. Chronic, low-grade inflammation, poor wound healing responses with incomplete endothelialisation, and intra-intimal haemorrhage have been noted in porcine coronary arteries treated with paclitaxel-coated stents.[37] Accelerated atherosclerosis immediately proximal and distal to QP2-coated stents has also been reported in humans although preliminary trial data suggest that this may not be a major issue.[38],[39] Delayed stent thrombosis has also been described with QP2-coated stents.[40] Concern regarding this phenomenon has prompted many clinical trials investigating stents eluting antimitotic agents to treat enrolled patients with oral antithrombotic agents for up to 6 months after implantation of these stents. Lack of a long-term effect on restenosis as described with radioactive stents may also become apparent in the future.[41] It has been postulated that the prevention of restenosis in recent clinical trials of drug-eluting stents represents a near-absent or incomplete phase of intimal healing.[42] Up to this point, the negative findings of drug-eluting stents in 90 and 180-day animal studies-at a time when healing is complete-may correspond to a reasonable approximation of 2 to 3 years in humans. However, there is still paucity of long-term trial data (is this safety data or efficacy data?)(>2 years). Continued long-term follow-up of patients with drug-eluting stents for MACE and angiographic restenosis is therefore imperative. At best, drug-eluting stents may have solved the in-stents restenosis problems; at worst, they may lead to adverse long-term late thrombosis and restenosis. The added cost of these stents may, at least initially, limit their use to patients at high risk of in-stent stenosis and this is an issue yet to be addressed. In spite of all these concerns, the CypherTM sirolimus-eluting stent (Cordis/Johnson & Johnson) has been approved by the US FDA for the treatment of restenosis, making it the first drug-eluting stent on the US market. The stents will cost $3195 regardless of their length or diameter. Although many other potential problems with these stents may be foreseen, the small numbers of patients enrolled and the short follow-up periods of the clinical trials evaluating drug-eluting stents remain the most important limitations.
Copyright 2003 - Journal of Postgraduate Medicine. Online full-text also available at http://www.jpgmonline.com/ |
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