Prevention of coronary restenosis with radiation therapy: a review

Dexamethasone-eluting vascular stents

Percutaneous transluminal angioplasty (PTA) with stenting is widely used in the treatment of vascular disorders, but restenosis remains a significant problem. Drug-eluting stents (DES) have been developed as an attempt to reduce the intimal response leading to restenosis. Drugs used in DES include mainly immunosuppressive and anti-proliferative compounds. Glucocorticoids are also an interesting possibility for those purposes because they have anti-proliferative effects in vascular smooth muscle cells and down-regulate the production of cytokines and growth factors driving inflammation and fibrosis. In this MiniReview, feasibility and safety of drug-eluting metal and biodegradable vascular stents are discussed with special emphasis on dexamethasone-eluting stents.

Approaches for prevention of restenosis

Coronary artery disease is characterized by a narrowing (stenosis) of the arteries that supply blood to the tissue of the heart. Continued restriction of blood flow manifests itself as angina and ultimately myocardial infarction (heart attack) for the patient. Heart bypass was once the only treatment for this condition, but over the years percutaneous coronary intervention (PCI) has become an increasingly attractive alternative to medical therapy and surgical revascularization for the treatment of coronary artery disease. A vascular stent is a medical device designed to serve as a temporary or permanent internal scaffold, to maintain or increase the lumen of a blood vessel. Metallic coronary stents were first introduced to prevent arterial dissections and to eliminate vessel recoil and intimal hyperplasia associated with PCI. Further advancement in the treatment of coronary artery disease is the development of drug-eluting stents that dramatically reduce the incidence of in-stent restenosis to less than 5%. Local drug delivery offers the advantages of allowing a relatively high local concentration of drug at the treatment site while minimizing systemic toxic effect. This review describes approaches for prevention of restenosis. It focuses on drugs for prevention of restenosis, bare metal stents, and drug-eluting stents. It also describes recent advances in bioresorbable stents. One of the chapters is dedicated to our novel composite bioresorbable drug-eluting fibers, designed to be used as basic elements in drug-eluting stents.

Analysis of arterial intimal hyperplasia: review and hypothesis

BACKGROUND

Despite a prodigious investment of funds, we cannot treat or prevent arteriosclerosis and restenosis, particularly its major pathology, arterial intimal hyperplasia. A cornerstone question lies behind all approaches to the disease: what causes the pathology?

HYPOTHESIS

I argue that the question itself is misplaced because it implies that intimal hyperplasia is a novel pathological phenomenon caused by new mechanisms. A simple inquiry into arterial morphology shows the opposite is true. The normal multi-layer cellular organization of the tunica intima is identical to that of diseased hyperplasia; it is the standard arterial system design in all placentals at least as large as rabbits, including humans. Formed initially as one-layer endothelium lining, this phenotype can either be maintained or differentiate into a normal multi-layer cellular lining, so striking in its resemblance to diseased hyperplasia that we have to name it "benign intimal hyperplasia". However, normal or "benign" intimal hyperplasia, although microscopically identical to pathology, is a controllable phenotype that rarely compromises blood supply. It is remarkable that each human heart has coronary arteries in which a single-layer endothelium differentiates early in life to form a multi-layer intimal hyperplasia and then continues to self-renew in a controlled manner throughout life, relatively rarely compromising the blood supply to the heart, causing complications requiring intervention only in a small fraction of the population, while all humans are carriers of benign hyperplasia. Unfortunately, this fundamental fact has not been widely appreciated in arteriosclerosis research and medical education, which continue to operate on the assumption that the normal arterial intima is always an "ideal" single-layer endothelium. As a result, the disease is perceived and studied as a new pathological event caused by new mechanisms. The discovery that normal coronary arteries are morphologically indistinguishable from deadly coronary arteriosclerosis continues to elicit surprise.

CONCLUSION

Two questions should inform the priorities of our research: (1) what controls switch the single cell-layer intimal phenotype into normal hyperplasia? (2) how is normal (benign) hyperplasia maintained? We would be hard-pressed to gain practical insights without scrutinizing our premises.

Cobalt-60 gamma radiation increased the nitric oxide generation in cultured rat vascular smooth muscle cells

Radiation is a promising and new treatment for restenosis following angioplasty. Nitric oxide has been proposed as a potential "anti-restenotic" molecule. We radiated the cultured rat vascular smooth muscle cells with Cobalt-60 gamma radiation at doses of 14 and 25Gy and observed nitrite production, cGMP content, L-arginine uptake, inducible nitric oxide synthase (iNOS) activity, and the gene expression of iNOS. Results showed that radiation at doses of 14 and 25Gy increased cGMP content by 92.4% and 86.4%, respectively. Radiation at the dose of 25Gy increased the iNOS activity and nitrite content, but radiation at the dose of 14Gy had no significant effect on iNOS activity and NO production. Both doses of radiation significantly decreased the L-arginine transport. Radiation at the doses of 14 and 25Gy increased iNOS gene expression significantly, which was consistent with the effect of radiation on iNOS activity. In conclusion, radiation induces the NO generation by up-regulating the iNOS activity.

Drug-Eluting Stents

Advances in catheter and stent design have made stent implantation the standard coronary angioplasty procedure. Unfortunately, in-stent restenosis continues to plague this procedure, with the optimum binary restenosis rates reaching ~10% to 20%. In the past few years, it has become clear that in-stent restenosis is largely due to the migration and proliferation of vascular smooth muscle cells to form a neointima. To address this issue, stents coated with drug-delivery vehicles have been developed to deliver antiproliferative therapeutics. Two drugs, rapamycin and taxol, have been the lead compounds for testing the idea of a drug-eluting stent. These drugs have been successful largely because of the solid mechanistic understanding of their effects and extensive preclinical examination. The result of these years of work is that the rapamycin-coated stent entered the US market in April of 2003, and the taxol-coated stent appears poised to follow soon.

Vessel size measurements in angiograms: manual measurements

Vessel size measurement is perhaps the most often performed quantitative analysis in diagnostic and interventional angiography. Although automated vessel sizing techniques are generally considered to have good accuracy and precision, we have observed that clinicians rarely use these techniques in standard clinical practice, choosing to indicate the edges of vessels and catheters to determine sizes and calibrate magnifications, i.e., manual measurements. Thus, we undertook an investigation of the accuracy and precision of vessel sizes calculated from manually indicated edges of vessels. Manual measurements were performed by three neuroradiologists and three physicists. Vessel sizes ranged from 0.1-3.0 mm in simulation studies and 0.3-6.4 mm in phantom studies. Simulation resolution functions had full-widths-at-half-maximum (FWHM) ranging from 0.0 to 0.5 mm. Phantom studies were performed with 4.5 in., 6 in., 9 in., and 12 in. image intensifier modes, magnification factor = 1, with and without zooming. The accuracy and reproducibility of the measurements ranged from 0.1 to 0.2 mm, depending on vessel size, resolution, and pixel size, and zoom. These results indicate that manual measurements may have accuracies comparable to automated techniques for vessels with sizes greater than 1 mm, but that automated techniques which take into account the resolution function should be used for vessels with sizes smaller than 1 mm.