Drug eluting stents: are human and animal studies comparable?

Animal models of stenting probably predict human responses as the stages of healing are remarkably similar. What is characteristically different is the temporal response to healing, which is substantially prolonged in humans. The prevention of restenosis in recent clinical trials of drug eluting stents may represent a near absent or incomplete phase of intimal healing. Continued long term follow up of patients with drug eluting stents for major adverse cardiac events and angiographic restenosis is therefore imperative.

The thin-cap fibroatheroma: a type of vulnerable plaque: the major precursor lesion to acute coronary syndromes.

While the concept of plaque 'vulnerability' implies a propensity towards thrombosis, the term vulnerable was originally intended to provide a morphologic description consistent with plaques that are prone to rupture. It is now known that the etiology of coronary thrombi is diverse and can arise from entities of plaque erosion or calcified nodules. These findings have prompted the search for more definitive terminology to describe precursor lesions associated with rupture, now referred to as thin-cap fibroatheromas. This review focuses on the thin-cap fibroatheroma, as a specific cause of acute coronary syndromes. To put these issues into current perspective, we need to revisit some of the older literature describing plaque morphology in stable and unstable angina, acute myocardial infarction, and sudden coronary death. The morphology, frequency, and precise location of these thin-cap fibroatheromas are further discussed in detail. Potential mechanisms of fibrous cap thinning are also addressed, in particular emerging data, which suggests the role of cell death "apoptosis" in cap atrophy.

Sudden cardiac death.

The rate of cardiac deaths that are sudden is approximately 50%, and decreases with age. The causes of sudden cardiac death are diverse, and are a function of age. In children and adolescents, coronary anomalies, hypertrophic cardiomyopathy and myocarditis are frequent substrates for lethal arrhythmias; in adults, coronary atherosclerosis and acquired forms of cardiomyopathy are the most common findings at autopsies of sudden cardiac death. This review focuses on coronary causes of sudden cardiac death, especially congenital coronary artery anomalies, which result in sudden death almost exclusively in adults younger than age 35, and coronary thrombosis. The most lethal coronary artery anomaly is the left coronary artery arising from the right sinus of Valsalva; this anomaly often results in fatal arrhythmias, often with exercise. The right coronary artery arising from the left sinus of Valsalva may also be lethal in adolescents and young adults, but, unlike the anomalous left, is more often an incidental finding at autopsy. Approximately 60% of sudden coronary death is caused by coronary thrombosis, the rest die with severe coronary disease in the absence of thrombosis. The two major substrates of coronary thrombosis are plaque rupture and plaque erosion, and are not only different pathologically, but are seen in patients with divergent risk factor profiles. Plaque rupture is the most common cause of fatal coronary thrombus, and is characterized by necrotic core with a thin fibrous cap, infiltrated by macrophages. The factors that result in plaque instability and rupture are largely unknown, and are under intense scrutiny; morphologic studies have identified serum lipid abnormalities as a key risk factor in the development of plaque rupture. Plaque erosion, in contrast to plaque rupture, is seen in younger men and women, is not associated with lipid abnormalities, and does not result from exposure of the lipid core to the lumen. The heterogeneity of the atherosclerotic plaque and the diverse mechanics of plaque progression and thrombosis have only been relatively recently explored, and are largely elucidated by autopsy studies of victims of sudden coronary death.

Pathophysiology of calcium deposition in coronary arteries.

BACKGROUND AND MORPHOLOGIC STUDIES: Because coronary artery calcification correlates highly with plaque burden, it is an excellent disease marker for atherosclerosis. However, it is not a sensitive indicator of disease activity, and does not predict luminal compromise because of compensatory remodeling. In addition, most data do not support the concept that plaque calcification is related to plaque instability. Plaques demonstrating acute rupture usually show mild or moderate calcification, and biophysical models do not predict that calcium should result in an increased propensity to rupture. This review outlines morphologic studies relating calcification to risk factors and coronary plaque morphology.

Pathological analysis of local delivery of paclitaxel via a polymer-coated stent.

BACKGROUND: Paclitaxel can inhibit vascular smooth muscle proliferation in vitro, and early studies suggest that paclitaxel may be useful in preventing restenosis. Early and late intimal growth and local vascular pathological changes associated with paclitaxel delivered via stents have not been fully explored. METHODS AND RESULTS: Localized drug delivery was accomplished with balloon-expandable stainless steel stents coated with a cross-linked biodegradable polymer, chondroitin sulfate and gelatin (CSG), containing various doses of paclitaxel. CSG-coated stents with paclitaxel (42.0, 20.2, 8.6, or 1.5 microgram of paclitaxel per stent), CSG-coated stents without paclitaxel, and uncoated stents (without paclitaxel or CSG) were deployed in the iliac arteries of New Zealand White rabbits, which were killed 28 days after implant. Mean neointimal thickness at stent strut sites was reduced 49% (P<0.0003) and 36% (P<0.007) with stents containing 42.0 and 20.2 microgram of paclitaxel per stent, respectively, versus CSG-coated stents without paclitaxel. However, histological findings suggested incomplete healing in the higher-dose (42.0 and 20.2 microgram) paclitaxel-containing stents consisting of persistent intimal fibrin deposition, intraintimal hemorrhage, and increased intimal and adventitial inflammation. Stents coated with CSG alone (without paclitaxel) had similar neointimal growth as uncoated stents. In a separate group of rabbits killed at 90 days, neointimal growth was no longer suppressed by CSG-coated stents containing 42.0 or 21.0 microgram of paclitaxel CONCLUSIONS: CSG coating appears to be a promising medium for localized drug delivery. Paclitaxel polymer-coated stents reduce neointima formation but are associated with evidence of incomplete healing at 28 days. However, neointimal suppression was not maintained at 90 days.

Late arterial responses (6 and 12 months) after (32)P beta-emitting stent placement: sustained intimal suppression with incomplete healing.

BACKGROUND: Three-month studies of stent-delivered brachytherapy in the rabbit model show reduced neointimal growth. However, intimal healing is delayed, raising the possibility that intimal inhibition is merely delayed rather than prevented. The purpose of this study was to explore the long-term histological changes after placement of beta-emitting radioactive stents in normal rabbit iliac arteries. METHODS AND RESULTS: Three-millimeter beta-emitting (32)P stents (6, 24, and 48 microCi) were placed in normal rabbit iliac arteries with nonradioactive stents as controls. Animals were euthanatized at 6 and 12 months, and histological assessment, morphometry, and analysis of endothelialization were performed. Morphometric measurements demonstrated a >50% reduction in intimal growth and percent lumen stenosis within 24- and 48-microCi stents versus control nonradioactive stents at both 6 and 12 months. However, the 24- and 48-microCi stents also showed delayed healing of the intimal surface, characterized by persistent fibrin thrombus with nonconfluent areas of matrix, incomplete endothelialization, and increased intimal cellular proliferation. Stent edge stenosis was present at 12 months in the 24- and 48-microCi stent groups, characterized by both intimal thickening and negative arterial remodeling. CONCLUSIONS: Inhibition of intimal growth is maintained 6 and 12 months after (32)P beta-emitting stent placement. However, delayed arterial healing, incomplete endothelialization, and edge effects are present.

Coronary atherosclerosis in unheralded sudden coronary death under age 50: histo-pathologic comparison with 'healthy' subjects dying out of hospital.

AIM: sudden coronary death (SCD) in older individuals is generally associated with extensive coronary atherosclerosis, although it may be the first manifestation of ischaemic heart disease. In younger age-groups, SCD may occur in the presence of less severe disease. We sought to (1) examine the extent of coronary atherosclerosis in young victims of SCD compared with age- and sex-matched controls, (2) analyse the composition of atherosclerotic plaques in these patients, (3) identify the predominant mechanism of SCD, and (4) evaluate the possibility of detecting this mechanism on the basis of morphologic plaque features, in particular presence and amount of lipid accumulation and calcific deposits. METHODS AND RESULTS: coronary arteries were obtained at autopsy from 28 victims of SCD under age 50 with no prior clinical manifestation of ischaemic heart disease (IHD) and no myocardial scar formation and from 16 age- and sex-matched subjects dying of noncardiac causes out of hospital. Sections of all available major coronary arteries were cut in 5-mm intervals to yield a total of 1357 histologic sections, which were analysed using digitised planimetry. Victims of SCD had significantly more major coronary arteries per subject with luminal area narrowing > or = 75% than controls (on average, 2.1 vs. 0.2). Plaque area per histologic section was 5.1 +/- 2.1 mm(2) in SCD cases and 2.0 +/- 0.9 mm(2) in controls (P < 0.001). The major constituent of all plaques was fibrous tissue. Lipid core area per section was 0.49 +/- 0.59 mm(2) in SCD cases and 0.004 +/- 0.01 mm(2) in controls (P < 0.001), and calcified plaque area was 0.18 +/- 0.19 mm(2) in SCD cases and 0.02 +/- 0.05 mm(2) in controls (P < 0.001), both defining significant differences between SCD cases and controls. Arterial thrombosis, most often with underlying plaque rupture was the mechanism of SCD in > 80% of the cases. Considering histologic sections with > or = 50 and with > or = 75% area stenosis, plaque rupture was independently predicted by lipid core area. Calcific deposits were a frequent feature of plaque rupture but were only associated with it in univariate analysis. CONCLUSIONS: the extent and severity of coronary atherosclerosis in young victims of SCD as the first manifestation of IHD was substantially greater than in age-and sex-matched controls and comparable with that previously reported in SCD cases with a broader age range. Lipid core and calcified plaque areas provided for excellent separation between the two groups, which may have implications for identifying persons at increased risk for SCD by non invasive visualisation and assessment of the coronary arteries.

Effect of menopause on plaque morphologic characteristics in coronary atherosclerosis.

BACKGROUND: Coronary artery disease in women appears 10 to 15 years later than in men. To test the hypothesis that the effects of estrogen may manifest themselves as histologic differences in coronary plaques, we examined the hearts of premenopausal and postmenopausal women who died suddenly from coronary artery disease. METHODS: We studied 51 cases of sudden coronary death and 47 deaths in women who died from noncoronary causes. Coronary deaths were classified on the basis of histologic features. The number of acute plaque ruptures, healed plaque ruptures, vulnerable plaques, and acute plaque erosions were compared between groups. Postmortem values of serum total cholesterol, HDL cholesterol, and thiocyanate were measured, and menopausal status was confirmed by calculating body mass index. RESULTS: Women older than 50 years of age were much more likely to have a ruptured plaque than were younger, premenopausal women. Plaque rupture was significantly associated with elevated total cholesterol level. In the 51 women who died of coronary disease, the mean number of vulnerable plaques increased significantly as women advanced into the postmenopausal years. CONCLUSIONS: Our data suggest that estrogen has an anti-inflammatory effect on atherosclerotic plaques, resulting in plaque stabilization. Plaque erosion, the major substrate for thrombosis in premenopausal women, does not appear to be inhibited by estrogen. Because plaque progression may result both from repeated rupture and repeated erosion, a better understanding of the effect of estrogen on atherosclerosis may yield insights into the nature of coronary artery disease.

Healed plaque ruptures and sudden coronary death: evidence that subclinical rupture has a role in plaque progression.

BACKGROUND: Subclinical episodes of plaque disruption followed by healing are considered a mechanism of increased plaque burden. Detailed pathological studies of healed ruptures, however, are lacking. METHODS AND RESULTS: We identified acute and healed ruptures from 142 men who died of sudden coronary death and performed morphometric measurements of plaque burden, luminal stenosis, and smooth muscle cell phenotype. Healed ruptures were found in 61% of hearts and were associated with healed myocardial infarction, increased heart weight, dyslipidemia, and diabetes. Multiple healed rupture sites with layering were frequently found in segments with acute and healed rupture; the percent area luminal narrowing increased with increased numbers of healed sites of previous rupture. The underlying percent luminal narrowing for acute ruptures (mean 79+/-15%) exceeded that for healed ruptures (mean 66+/-14%, P:=0.0001), and the area within the internal elastic lamina was significantly less in healed ruptures than in acute ruptures, when segments were grouped by distance from the ostium. Healed ruptures favored the accumulation of immature smooth muscle cells at repair sites, with a cellular proliferation index of 0.40+/-0.09%, significantly higher than the index at the sites of rupture (P:=0.008). CONCLUSIONS: These data provide evidence that silent plaque rupture is a form of wound healing that results in increased percent stenosis. Healed ruptures occur in arteries with less cross-sectional area luminal narrowing than acute ruptures and are a frequent finding in men who die suddenly with severe coronary atherosclerosis.

Local arterial responses to 32P beta-emitting stents.

PURPOSE: 32P beta-emitting stents reduce neointimal growth in rabbit iliac arteries for at least 12 months after deployment but are associated with incomplete healing. The aim of this study was to quantitate arterial cellularity, with emphasis on the inflammatory response following radioactive stenting. METHODS: 32P beta-emitting stents were placed in rabbit iliac arteries and analyzed at 3 months (6 and 24 microCi), and 6 and 12 months (6, 24, and 48 microCi). Arterial cellular proliferation and cell densities of smooth muscle cells (SMC), mononuclear cells (macrophages and lymphocytes), and neutrophils (PMN) were determined. RESULTS: Total intimal cell density was greatest in control stents at all three time points, composed mostly of SMCs. SMC density associated with radioactive stents increased from 3 to 12 months but was significantly less than control nonradioactive stents. There was a 4-fold increase in cellular proliferation in the 24 and 48 microCi group vs. control stents. In the media, SMC density of radioactive stent groups was significantly reduced vs. control stents at all three time points, for all three activities. At 3, 6 and 12 months, there was a dose-dependent increase in intimal inflammatory cell density, which consisted mostly of macrophages. For 6-microCi stents inflammation peaked at 3 months and decreased thereafter. Inflammation for 24-microCi stents peaked at 6 months and then decreased at 12 months. Inflammation associated with 48-microCi stents remained high at 6 and 12 months. Focal atherosclerotic change was seen in 11% of stents in the 24-microCi group, and 37% and 50% in the 48-microCi group at 6 and 12 months, respectively. CONCLUSION: Intimal SMC density remains suppressed out to 12 months after placement of 32P beta-emitting stents. However, inflammation and cell proliferation remain increased and may potentially result in greater neointimal formation over time.

Pathologic validation of a new method to quantify coronary calcific deposits in vivo using intravascular ultrasound.

Current methods of calcium quantification by intravascular ultrasound (IVUS) measure the arc of calcium using the cross-sectional image at the lesion and at the reference site while neglecting calcium elsewhere. Calcium at these sites may not adequately represent the extent of total epicardial coronary calcium. We devised a new method to quantify calcium as a percentage of the coronary luminal surface. This study examines whether this new method accurately reflects coronary calcium determined by histology. Seventeen postmortem coronary arteries were pressure-fixed and imaged by IVUS using a motorized pullback device. Total plaque-luminal circumferential length and calcified plaque-luminal circumferential length were measured from serial cross-sectional IVUS images every 1 mm. With use of Simpson's method, the total plaque and calcified plaque surface area was then calculated. Histologic sections were stained with hematoxylin-eosin and Movat pentachrome at 3-mm intervals. Calcium was independently quantified by planimetry under light microscopy. Histologic analysis (n = 253 sections) revealed a wide range of calcium (0 to 47 mm2; mean 12 +/- 16 mm3). The IVUS-derived calcified plaque surface area was 17 +/- 23 mm2), which represented 3.1 +/- 4.1% (range 0% to 13.9%) of the total plaque surface area. The histologic and IVUS quantification of calcium by this method was strongly related (r = 0.84, p <0.0001), which was an improvement over current 2-dimensional measures of calcium arc (r = 0.41, p = 0.18). Calculation of calcified plaque surface area from sequential IVUS images appears to accurately reflect the degree of total coronary calcification.

Neointimal responses 3 months after (32)P beta-emitting stent placement.

PURPOSE: Studies have shown a potential benefit of brachytherapy in preventing restenosis. However, the effects of intravascular radiation on arterial healing have not been well-established. The purpose of this study was to explore the histologic changes following placement of beta-emitting radioactive stents in arteries focusing on intimal responses and endothelialization. METHODS AND MATERIALS: 3.0-mm beta-emitting (32)P stents (6-microCi and 24-microCi) were placed in rabbit iliac arteries with nonradioactive stents serving as controls. Animals were euthanized at 3 months and histologic assessment, morphometry, and analysis of endothelialization were performed. RESULTS: The lumen areas of 24-microCi stents (4.24 +/- 0.22 mm(2), p < 0.0007) and 6-microCi stents (4.23 +/- 0.49 mm(2), p < 0.01) were larger than control stents (3.64 +/- 0.44 mm(2)). The mean lumen percent stenosis was 11. 4 +/- 3.0% in the 24-microCi stents (p < 0.007 vs. 6-microCi stents and p < 0.0001 vs. control stents), 18.7 +/- 6.4% in the 6-microCi stents (p < 0.02 vs. control stents), and 25.0 +/- 4.9% in control stents. Neointimal area was least in the 24-microCi stent (54.2% smaller than controls and 42.7% smaller than 6-microCi); the neointimal area of the 6-microCi stents was 20.0% less than controls. The control stent neointima consisted of smooth muscle cells in a proteoglycan and collagen matrix. In contrast, the intima of radioactive stents showed persistent fibrin thrombus with nonconfluent areas of matrix. Actin-positive intimal cell density was reduced with radioactive stenting, but intimal cell proliferation was increased. Evans blue staining, an indicator of increased endothelial permeability, was present on 86 +/- 9% of the stented segment of 6-microCi stents vs. 10 +/- 11% in controls (p < 0.0001). Scanning electron microscopy demonstrated endothelialization of 97 +/- 8% of the intimal surface of control stents; in contrast, the midportion of the 6-microCi stents remained nonendothelialized, and only 33 +/- 15% (p < 0.0001) of the entire stent surface was endothelialized. CONCLUSIONS: (32)P beta-emitting stents reduce neointimal growth, but healing is incomplete with poor endothelialization at 3 months. Longer-term studies with complete arterial healing are needed to determine whether there is sustained neointimal inhibition by stent-delivered brachytherapy.

Localization of apoptotic macrophages at the site of plaque rupture in sudden coronary death.

Although apoptosis is a well-recognized phenomenon in chronic atherosclerotic disease, its role in sudden coronary death, in particular, acute plaque rupture is unknown. Culprit lesions from 40 cases of sudden coronary death were evaluated. Cases were divided into two mechanisms of death: ruptured plaques with acute thrombosis (n = 25) and stable plaques with and without healed myocardial infarction (n = 15). Apoptotic cells were identified by staining of fragmented DNA and confirmed in select cases by gold conjugate labeling combined with ultrastructural analysis. Additional studies were performed to examine the expression and activation of two inducers of apoptosis, caspases-1 and -3. Ruptured plaques showed extensive macrophage infiltration of the fibrous cap, in particular at rupture sites contrary to stable lesions, which contained fewer inflammatory cells. Among the culprit lesions, the overall incidence of apoptosis in fibrous caps was significantly greater in ruptured plaques (P < 0.001) and was predominantly localized to the CD68-positive macrophages. Furthermore, apoptosis at plaque rupture sites was more frequent than in areas of intact fibrous cap (P = 0. 028). Plaque rupture sites demonstrated a strong immunoreactivity to caspase-1 within the apoptotic macrophages; staining for caspase-3 was weak. Immunoblot analysis of ruptured plaques demonstrated caspase-1 up-regulation and the presence of its active p20 subunit whereas stable lesions showed only the precursor; nonatherosclerotic control segments were negative for both precursor and active enzyme. These findings demonstrate extensive apoptosis of macrophages limited to the site of plaque rupture. The proteolytic cleavage of caspase-1 in ruptured plaques suggests activation of this apoptotic precursor. Whether macrophage apoptosis is essential to acute plaque rupture or is a response to the rupture itself remains to be determined.