Intracoronary Radiation Before Stent Implantation Inhibits Neointima Formation in Stented Porcine Coronary Arteries
Background Stent implantation has been shown to reduce restenosis by establishing a larger lumen but not by reducing neointima formation. We have previously shown that ionizing radiation reduced neointima formation after balloon injury in a swine model of restenosis. The purpose of this study was to determine whether endovascular irradiation of the coronary artery before stent implantation would affect neointima formation.
Methods and Results Nine normolipemic pigs underwent coronary angiography, and segments of the left anterior descending and left circumflex arteries were chosen as targets for stenting. A high-activity 192Ir source was used to deliver 14 Gy by random assignment to one of the vessels. After this, 3.5-mm tantalum stents were implanted in both arteries. Three additional pigs were treated with a 90Sr/Y source (a pure β-emitter) delivering 14 Gy to five segments of coronary vessels that were stented immediately after irradiation. Stent-to-artery ratio was similar in the radiated and the control arteries. Animals received aspirin 325 mg daily and were killed at 28 days. The intimal area was significantly reduced in the irradiated stented arteries compared with control arteries treated with stent only (1.98 mm2 with 192Ir and 2.53 mm2 with 90Sr/Y versus 3.82 mm2 in the control stented arteries, P<.005).
Conclusions Endovascular radiation before coronary stenting reduces neointima formation and may further reduce the restenosis rate after stent implantation.
Intracoronary metallic stents are being used with increasing frequency for prevention of restenosis.1 2 3 4 The presumed mechanism by which stents reduce restenosis is the achievement of a larger initial lumen size and reduction of both the acute and chronic recoil phenomena after balloon dilatation.5 Clinical and experimental data suggest that restenosis results partly from recoil and remodeling but also from migration, proliferation, and matrix deposition by smooth muscle cells after vascular injury.6 7 The only modest effect of stenting on restenosis rate is due to the more pronounced neointimal formation with stents compared with that found in balloon angioplasty. This has been demonstrated in animal models8 and more recently in a clinical study using intravascular ultrasound.9 A potent antiproliferative therapy in conjunction with stenting might therefore provide a means for effective reduction of restenosis.
We have previously demonstrated the efficacy of endovascular low-dose γ irradiation with 192Ir to inhibit neointima formation in response to balloon overstretch injury in pig coronary arteries.10 This work has been corroborated by two additional studies.11 12 13 Similar results were obtained in the same model using a 90Sr/Y source.14 The only clinical study of radiation after balloon dilatation, in which restenotic stented lesions were ballooned and treated with 192Ir, was carried out in peripheral arteries with a very low restenosis rate reported at 5 years of follow-up.15 Recently, low-dose/low-rate radioactive endovascular stents with either β or γ emission energy have been implanted in rabbit iliac arteries and shown to significantly reduce neointimal hyperplasia.16 17
The purpose of this study was to investigate whether delivering endovascular radiation (192Ir or 90Sr/Y) to the coronary artery before stent implantation would affect neointima formation.
All experiments and animal care conformed to National Institutes of Health and American Heart Association guidelines for the care and use of animals and were approved by the Emory University Institutional Animal Care and Use Committee.
Nine normolipemic pigs (Sus scrofa, 21.2 to 30.1 kg) were given aspirin (325 mg) 1 day before and the day of the procedure, sedated, and anesthetized as previously described.10 After placement of an 8F introducer sheath in the right femoral artery by surgical cutdown, each animal received a single dose of heparin (200 U/kg) and bretylium tosylate (2.5 mg/kg). Under fluoroscopic guidance, an 8F hockey-stick guiding catheter was positioned in the left coronary ostium. After the intracoronary administration of nitroglycerin (200 μg), coronary angiography was performed, and segments of the left anterior descending or the left circumflex arteries were chosen as targets for stent implantation. One artery was then randomly assigned to receive radiation treatment. Over a flexible 0.014-in. wire, a 4F perfusion delivery catheter (USCI Corp) was introduced to the chosen site of the assigned artery, the guide wire was withdrawn, and a 3-cm ribbon of 192Ir was positioned at the assigned segments of the left anterior descending or left circumflex arteries by use of cinefluoroscopic visualization. The isotope was left in place within the delivery catheter for a period sufficient to deliver 14 Gy to a depth of 2 mm (28 to 38 minutes, depending on source activity). Then 3.5-mm tantalum stents (Cordis Corp) were implanted and apposed well to the artery wall by use of high-pressure (14-atm) balloon inflation at the irradiated site and in the control, nonirradiated artery. After the completion of stent implantation, additional nitroglycerin (200 μg) was administered to limit coronary spasm. Repeat angiography was then performed to assess vessel patency.
Three additional pigs received radiation treatment with 90Sr/Y (16 mCi) to a total of five coronary arteries with a 4.5F delivery catheter (Novoste Corp). The catheter was positioned over a flexible 0.014-in. guide wire, the wire was removed, and a train 2.5 cm long with five seeds of 90Sr/Y was positioned at the targeted site within the delivery catheter. It was left in place for a period sufficient to deliver the assigned dose (14 Gy) to a depth of 2 mm (196 seconds). After irradiation, the delivery catheter was removed, and 3.5-mm tantalum stents (Cordis Corp) were implanted in the same manner as for the nine pigs treated with 192Ir. At the end of all procedures, the femoral cutdown was repaired, nitroglycerin ointment (1 in.) was administered topically, and the animals were returned to routine care. They received aspirin 325 mg daily until specimen harvest at 28 days.
For 192Ir, the treatment time was determined in standard fashion by entering the activity of the 3-cm 192Ir ribbon, as supplied by the manufacturer (Medi-Physics Inc), into a commercial radiation treatment planning system (CMS Modulex). The dose distribution and the dose rate at the prescription point were then calculated by the system by use of standard brachytherapy dose algorithms. The dose distribution and dose rate around the 2.5-cm 90Sr/Y line source was calculated by the Monte Carlo electron transport code ITS.18 The β-energy spectrum of 90Sr/Y was obtained from Cross et al.19 The activity of each seed and of the total source train was determined by the manufacturer with an NIST-traceable standard. The dose rate at the prescription point of the 192Ir source was calculated with both ITS and the CMS Modulex planning system. These were found to agree within 5%. There was no mechanism for centering of the catheter within the arterial lumen, nor was any attempt made to account for curvature of the artery with either source.
Four weeks after the stent implantation, the animals were heparinized, a lethal dose of barbiturate was given, the chest was opened, and the heart was rapidly excised. The coronary system was perfusion-fixed at 100 to 110 mm Hg driving pressure with 10% formaldehyde for 15 minutes, and the heart was stored overnight in the same fixative. The stented arteries were embedded in methyl methacrylate and sectioned with the stents in place with a low-speed saw. Serial sections spanning the injury site were glued to acrylic slides, ground to 50 μm thick, and stained with toluidine blue. Each segment (three to five per stent, one from the midportion and at least one each in the proximal and distal regions) was analyzed by histopathological and morphometric techniques. The histological features were measured with a computerized IBM-based system (Bioscan 2, Thomas Optical Measurement System, Inc). Sections were magnified at ×26, digitized, and stored in a frame-grabber board. The maximal intimal thickness was determined in each section by a radial line drawn from the lumen border to the internal lamina at the point of greatest tissue growth, just adjacent to the stent wire. Area measurements were obtained by tracing the lumen perimeter (luminal area, mm2), neointima perimeter (intimal area, mm2, defined by the borders of the internal elastic lamina, lumen, media, and external elastic lamina or stent wires), and vessel perimeter (to determine vessel area, mm2). Sections were also evaluated for the presence of intraluminal thrombus and inflammatory cell infiltrate.
Data are presented as mean±SD. Data were analyzed by the two-tailed paired Student’s t test to compare group means for 192Ir-treated versus control stented arteries in the same animal. Arteries treated with 90Sr/Y were compared with the same controls by unpaired t tests. Significance was established at the 95% confidence level (P<.05).
Twelve animals underwent interventions on 23 coronary arteries. There were no significant differences in body weight between groups at the time of balloon injury. Angiographic arterial diameter was similar between groups (2.67±0.15 mm), and stent artery ratio was similar in the irradiated arteries (1.52±0.55) and controls (1.50±.033). The distribution of left anterior descending and left circumflex arteries was similar among the treated and the control stented arteries. In two animals, the distal RCA was irradiated and stented with the 90Sr/Y source (Table 1⇓).
Histological and Morphometric Analysis
All arterial stented segments were examined. In both control and irradiated arteries, the stents were well embedded into the vascular wall, resulting in thinning of the media. There was overall similarity between groups in the degree of injury to the media, including occasional instances of medial fracture and stent penetration into the adventitia. Occasionally there was evidence of hemorrhage into the perivascular space. Control stented arteries showed substantial neointima, consisting of either round, stellate, or spindle-shaped cells in a loose extracellular matrix (Fig 1⇓). Many segments showed inflammatory infiltrates surrounding the stent wire. The inflammatory reaction in the treated arteries was minimal compared with the control arteries. These rarely included foreign-body giant cells (histiocytes) in the neointima. Irradiated stented arteries exhibited modest neointima formation compared with controls (Fig 2⇓), although there was some variability in thickness within the treatment group (Fig 3⇓). A certain degree of eccentricity in the neointimal proliferation was noted. In all samples, there appeared to be complete coverage of the luminal surface by a monolayer of endothelial cell–like cells.
The luminal area was significantly increased and the intimal area was significantly reduced in both radiation treatment groups compared with control, whereas the vessel perimeter and the vessel area were unchanged by the radiation treatment. There were no significant morphological or morphometric differences between arteries treated with the β- or γ-emitting isotopes (Table 2⇓).
Restenosis after stent implantation in patients with coronary artery disease is caused primarily by an intimal hyperplastic response. Stenting induces comparatively more neointimal thickening than balloon angioplasty.9 This study demonstrates the first successful use of intracoronary low-dose radiation treatment as an adjunct to stent implantation and shows the efficacy of this approach in reducing neointimal hyperplasia.
The histological and histomorphometric results in this study using 90Sr/Y were comparable to those for 192Ir. However, pure β-emitters like 90Sr/Y have a distinct advantage over the use of energetic γ-emitters like 192Ir in that β-particles have a limited penetration in tissue and deliver significantly less dose beyond the prescription point than do γ-emitters.19 20 This is important in limiting the whole-body exposure to the patient and the operator. In addition, the treatment with 90Sr/Y took only 190 seconds compared with 32 minutes with 192Ir, thus reducing the length of time of intracoronary catheterization.
The use of radioactive stents, or a stent coated with a radioactive isotope, has been proposed by two separate groups.16 17 Impregnating the stent with a β-emitting isotope would seem to be the more desirable approach of the two, but concerns regarding (1) potential leaching of the radioactive material from the metallic stent, (2) possible thrombosis on the stent wire due to delayed reendothelialization of the stent struts, and (3) continued delivery of dose by a permanently implanted stent beyond the period of time required to inhibit restenosis need to be addressed.
Although radioactive stents have the potential advantage of application of the radioisotope close to the proliferating tissue, the practical issues of active shelf life and the logistical problem of having available the necessary activity of a rapidly decaying source may be problematic. The ease and safety of a catheter-based delivery system with an isotope whose half-life is 28 years make the 90Sr/Y source used in this study an attractive alternative.
The high stent-to-artery ratio (1.5:1) used in this study induced a profound intimal response and provided a rigorous model in which to test our hypothesis. Unlike the balloon response, which appears to be primarily proliferative, the stent tissue has a more heterogeneous origin, including substantial thrombosis and inflammation as well as cellular replication. Despite this, localized irradiation produced marked diminution of neointima formation and inflammatory response. The mechanism by which radiation inhibits the development of neointima is unknown but may be the arrest of mitosis and therefore inhibition of smooth muscle cell growth.
Endovascular low-dose ionizing radiation before stent implantation significantly reduced the development of intimal hyperplasia after stent implantation. The beneficial effect was seen 30 days after stent implantation and was not associated with any adverse reaction to the stented segment or to the adjacent segments. Histological and histomorphometric results are similar between γ (192Ir) and β (90Sr/Y) -irradiated stented arteries.
- Received May 8, 1995.
- Revision received July 12, 1995.
- Accepted July 20, 1995.
- Copyright © 1995 by American Heart Association
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