Endovascular irradiation from beta-particle-emitting gold stents results in increased neointima formation in a porcine restenosis model.

BACKGROUND: Recent studies have shown that ionizing radiation reduces neointima formation after balloon angioplasty and stent implantation in experimental models of restenosis and first clinical trials. The objective of this study was to determine the dose distribution of a new beta-particle-emitting radioactive gold stent and to evaluate the dose-dependent vascular response in the coronary overstretch pig model. METHODS AND RESULTS: Sixteen Göttinger minipigs underwent placement of 11 nonradioactive and 36 beta-particle-emitting stents with activity levels of 10.4+/-0.6, 14.9+/-2.4, 22.8+/-1.3, 35.8+/-2. 8, and 55.4+/-5.3 microCi of (198)Au. Three months after implantation, the percent area stenosis, neointimal thickness, neointimal area, and vessel injury were analyzed by quantitative histomorphometry. The lifetime radiation doses at a depth of 1 mm were 3.3+/-0.2, 4.7+/-0.5, 7.2+/-0.4, 11.4+/-0.9, and 17.6+/-1.7 Gy for the different activity groups. No dose-response relationship was observed in the radioactive stents with respect to percent area stenosis (P=0.297), mean neointimal thickness (P=0.82), or mean neointimal area (P=0.65). Significantly lower neointima formation and less luminal narrowing was seen in the control group than in the beta-particle-emitting stents (P<0.001). Multilinear regression analysis revealed that only radioactivity made a significant independent contribution to the degree of percent area stenosis (P<0. 001). CONCLUSIONS: Neointima formation in pigs is markedly increased by beta-particle-emitting stents with (198)Au as the radioisotope. This study provides evidence that dosages of 3 to 18 Gy of low-dose-rate beta-particle irradiation via endovascular stents cause pronounced luminal narrowing in the animal model at 3 months.

Coronary stent symmetry and vascular injury determine experimental restenosis.

OBJECTIVE: To assess the impact of stent symmetry on restenosis using the coronary overstretch sheep model. METHODS: Neointimal thickness, injury index, and percentage diameter and area stenosis were calculated by digital morphometry. The standard deviation of the angular burden was used to assess stent symmetry for each section. MATERIALS: 15 healthy Merino sheep (63-75 kg) underwent implantation of 30 slotted tube stents (7 mm). Restenosis was induced by calculated overstretch of the coronary artery. Twenty eight days after implantation, stents were excised and underwent histological examination using quantitative digital morphometry. RESULTS: The severity of vessel injury was positively correlated with neointimal thickness and with percentage diameter and area stenosis (p < 0.001). Mean neointimal thickness and mean vascular injury per cross section were strongly related to the standard deviation of angular burden, with correlation coefficients of 0.6 and 0.8, respectively (p < 0.001). CONCLUSIONS: The well known relation between vascular injury and restenosis was confirmed, and a new relation was discovered between stent asymmetry and restenosis. If these results apply to human coronary arteries, restenosis may also be dependent on the degree of asymmetric stent expansion. These results should influence the development of new stent designs to reduce asymmetric stent expansion, leading to a more homogeneous strain distribution in stented coronary segments.

Focal increases in vascular cell adhesion molecule-1 and intimal macrophages at atherosclerosis-susceptible sites in the rabbit aorta after short-term cholesterol feeding.

We tested the hypotheses that vascular cell adhesion molecule-1 (VCAM-1) expression on endothelium at lesion-prone sites in the rabbit aorta correlates with exposure to plasma cholesterol and that macrophage accumulation is associated with endothelial cells expressing VCAM-1. After rabbits were fed 0.25% cholesterol for 2 weeks, VCAM-1 expression was selectively increased at the distal and lateral portions of the major abdominal branches. In the arch and the celiac, superior mesenteric, and renal artery branches, VCAM-1 expression was positively correlated with the plasma cholesterol integrated over the duration of the experiments. After 2 weeks of cholesterol feeding, more macrophages were present around distal and lateral portions of the intercostal arteries and major abdominal branches relative to nonbranch regions. In the arch and around the intercostals and major abdominal branches, macrophage densities were positively correlated with the integrated plasma cholesterol. VCAM-1 and macrophage levels were correlated in lesion-prone regions. In normocholesterolemic rabbits, 23+/-4% (mean+/-SEM) of the macrophages were directly associated with VCAM-1-positive endothelium. After 2 weeks of 0.25% cholesterol feeding, the association increased to 37+/-4% (P<0.015). Associations were highest around the lateral and distal regions of the major abdominal branches. These results suggest that (1) VCAM-1 expression and intimal macrophage densities are influenced by plasma cholesterol and regional factors such as arterial fluid dynamics and (2) VCAM-1 plays a significant role in the localization of macrophages.

The distribution of intimal white blood cells in the normal rabbit aorta.

Macrophages play an important role in atherogenesis and have been reported within the intima at lesion-prone sites in normocholesterolemic animals as well as infants and children. The objective of this study was to determine the spatial distribution of intimal white blood cells (WBC) in the normal rabbit aorta and the association of intimal WBC with replicating endothelial cells and sites of increased 125I-LDL permeability. Intimal WBC and macrophages were identified en face on whole aortic tissue and on Häutchen preparations based on their morphology, ingestion of exogenous horseradish peroxidase, non-specific esterase activity, and labeling with a monoclonal antibody for rabbit macrophages (RAM11). WBC were primarily located in the lesion-prone flow divider regions of the large abdominal branch arteries. Using [3H]thymidine autoradiography to determine cell proliferation, 4.4% of the WBC and 0.12% of the endothelial cells were labeled on the Häutchen preparations. The distribution of replicating endothelial cells was not localized to the arterial orifices and was not correlated with the distribution of intimal WBC. Intimal WBC were, however, spatially correlated with the distribution of 125I-LDL permeable sites about the celiac artery orifice and were directly associated with 31% of the LDL permeable spots. Moreover, mitotic endothelial cells accounted for only 8% of the total number of LDL permeable sites. The presence of intimal WBC at lesion-prone sites in the normocholesterolemic rabbit suggests that these cells may be important in the initiation of atherosclerotic lesions.

Characterization of sites with elevated LDL permeability at intercostal, celiac, and iliac branches of the normal rabbit aorta.

En face autoradiography of the endothelium was used to quantify the distribution, area, and permeability of sites with enhanced permeability to 125I-low-density lipoprotein (125I-LDL) around the intercostal and celiac arteries and at the iliac bifurcation of normal rabbit aortas. The density of such sites was highest in the upper thoracic aorta and around the celiac and superior mesenteric branches and was lowest in the lower abdominal aorta. Permeable sites occurred more frequently distal to the intercostal branch orifices and both lateral and distal to the orifice at the celiac branch. At the intercostal branch orifices, these sites were larger, with a lower permeability and higher frequency than those away from the branch. At the celiac flow divider, sites of elevated autoradiographic grain density were more permeable and larger than at other locations in the abdominal aorta. Mean regional permeabilities were obtained by weighted area averages of low- and high-permeability sites. Mean regional permeabilities around the intercostal branches were 1.5 times higher than values away from the intercostal branches. Within 0.25 and 1 mm away from the celiac flow divider, mean regional permeability was 3.1 and 1.3 times higher, respectively, than those away from the flow divider. Few sites of elevated permeability were found distal to the aortoiliac bifurcation, and the permeabilities at the medial and lateral walls of the iliac arteries were not different. Mitotic cells were associated with 13 +/- 8% of all sites with elevated permeability to 125I-LDL. The frequency of mitotic endothelial cells was not increased at branch sites, suggesting that mechanisms other than cell replication were responsible for increased LDL permeability in the rabbit. These results suggest that the permeability and frequency of occurrence of sites with elevated permeability around the celiac and intercostal branches may influence the distribution and severity of early lesions in rabbits fed a hypercholesterolemic diet.

Measurement of endothelial permeability to 125I-low density lipoproteins in rabbit arteries by use of en face preparations.

A procedure of en face quantitative autoradiography of the endothelium (Hautchen preparations) was developed to examine regional variations in 125I-low density lipoprotein (125I-LDL) permeability in the arterial wall in vivo. Endothelial preparations from fixed arterial tissue and calibration standards consisting of known concentrations of 125I-albumin were dipped in nuclear emulsion, exposed for 1-3 months, developed, and stained with hematoxylin. Digital image analysis was used to analyze dark-field images of autoradiographs. Background grain densities on cold endothelial preparations were 30-100% higher than on glass, but the variability in grain densities on the two different surfaces was similar. Regression slopes of grain density versus concentration for calibration standards were the same for sections placed on cold tissue or glass. For 1-5-microns-thick calibration standards of the same concentration, the grain density was proportional to the total amount of radioactivity per unit area. The results indicated that errors arising from nonuniformities in preparation thickness were minimal, and permeabilities and intimal concentrations could be determined. Rabbits were killed 10 minutes after injection of 125I-LDL, and endothelial preparations were made. For regions of uniformly low grain density in the rabbit aorta, the 125I-LDL permeability was 1.9 +/- 0.8 x 10(-8) cm/sec, and the effective diffusion coefficient was 5.4 +/- 3.1 x 10(-10) cm2/sec. Errors in the estimated permeability arising from nonuniformities in tissue thickness were the same as the reported experimental variability. Analysis of elevated regions of permeability suggested that 125I-LDL was binding to the extracellular matrix. Approximately 25% of the sites of elevated grain density were associated with mitotic endothelial cells, and such regions had higher permeabilities than sites associated with nonmitotic cells. Around intercostal arteries, sites of highest permeability were distal and lateral to the vessels and occurred where lesions first develop in hypercholesterolemic rabbits.