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J Am Coll Cardiol, 2006; 48:1825-1831, doi:10.1016/j.jacc.2006.03.069
(Published online 16 October 2006). © 2006 by the American College of Cardiology Foundation |
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* Department of Medicine/Cardiovascular Research Institute, Division of Cardio-Vascular Medicine
Department of Radiology, Kurume University School of Medicine, Kurume, Japan
Manuscript received January 10, 2006; revised manuscript received March 3, 2006, accepted March 6, 2006.
* Reprint requests and correspondence: Dr. Hisashi Kai, Department of Medicine/Cardiovascular Research Institute, Division of Cardio-Vascular Medicine, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan. (Email: naikai{at}med.kurume-u.ac.jp).
| Abstract |
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BACKGROUND: Inflammation plays a key role in progression and destabilization of atherosclerotic plaque. 18 F-fluorodeoxyglucose PET is a promising tool for visualizing inflammation of atherosclerotic plaque. Antiinflammatory action is one of the pleiotropic effects of statins.
METHODS: Forty-three consecutive subjects, who underwent 18 FDG-PET for cancer screening and had 18 FDG uptakes in the thoracic aorta and/or the carotid arteries, were randomized to either statin group receiving simvastatin (n = 21) or diet group receiving dietary management only (n = 22). The maximum standardized uptake values (SUVs) were measured in individual plaques, and were averaged for analysis of the subjectwise results. The responses were assessed after 3-month treatments.
RESULTS: Positron emission tomography revealed 117 and 123 18 FDG-positive plaques in the statin and diet groups, respectively. Simvastatin, but not diet alone, attenuated plaque 18 FDG uptakes and decreased the SUVs (p < 0.01). Simvastatin reduced low-density lipoprotein cholesterol (LDL-C) by 30% (p < 0.01) and increased high-density lipoprotein cholesterol (HDL-C) by 15% (p < 0.01), whereas LDL-C and HDL-C levels were not changed in the diet group. In the statin group, the decrease in the SUV was well correlated with the HDL-C elevation (p < 0.01) but not with the LDL-C reduction.
CONCLUSIONS: 18 F-fluorodeoxyglucose PET visualized plaque inflammation and simvastatin attenuated it. The LDL-Cindependent effects of simvastatin may participate in the beneficial effect. 18 F-fluorodeoxyglucose PET has a potential for visually monitoring plaque inflammation and the therapeutic effectiveness of statins.
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Lipid-lowering therapy with statins significantly decreases cardiovascular morbidity and mortality in primary and secondary prevention (7,8 ). Statins exert their benefits through the inhibition of de novo cholesterol synthesis, resulting in significant reductions in plasma low-density lipoprotein cholesterol (LDL-C) levels. It remains controversial whether LDL-C lowering is the only mechanism for the observed beneficial effects. Many LDL-Cindependent pleiotropic effects have been postulated. One of them is attenuation of inflammation because statins have been shown to decrease systemic inflammatory markers (9,10 ). However, no visual evidence that statins attenuate plaque inflammation has been documented noninvasively in clinical practice. 18 F-fluorodeoxyglucose PET may provide noninvasive longitudinal evaluation of plaque inflammation after statin therapy. Accordingly, in the present study, we investigated by serial 18 FDG-PET imaging whether simvastatin reduced inflammation of the plaques.
| Methods |
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Treatment. The study subjects were randomized to either diet group receiving dietary management (n = 22) or statin group receiving simvastatin in addition to dietary management (n = 21). Dietary management was performed according to the Japan Atherosclerosis Society Guidelines for Diagnosis and Treatment of Atherosclerotic Cardiovascular Diseases (11 ). In the statin group, the starting dose of simvastatin was 5 or 10 mg/day. When LDL-C level was more than 130 mg/dl (3.4 mmol/l) after 1 month of treatment, the dose of simvastatin was increased to 20 mg/day, the approved maximum dose in Japan. Consequently, the maintenance dose of simvastatin was 5 mg/day for 2 subjects, 10 mg/day for 14 subjects, and 20 mg/day for 5 subjects.
18 FDG PET imaging. After at least 12 h of fasting, the study subjects received an intravenous administration of 4.2 MBq (0.11 mCi)/kg of 18 FDG. One hour after 18 FDG injection, 3-dimensional whole-body PET imaging was carried out using a PET scanner (Allegro, Philips Medical Systems [Cleveland], Inc., Cleveland, Ohio), which uses gadolinium oxyorthosilicate as the detector material. Contrast-enhanced computed tomographic (CT) images were also taken from the skull base to the diaphragm using Light Speed Ultra 16 (GE Healthcare, Milwaukee, Wisconsin). The co-registration of PET and CT images (software image fusion) was performed for review on a workstation (Sun Microsystems, Inc., Santa Clara, California) as described previously (6 ).
The 18
FDG-PET images were visually evaluated for the presence of abnormal 18
FDG uptakes in the aorta and the carotid arteries on the basis of the agreement of 3 radiologists specializing in nuclear medicine blinded to other clinical information and treatment assignments. Arterial uptake of 18
FDG was validated when co-registration of PET and CT showed that the accumulation overlapped on the vascular wall. The intensity of 18
FDG uptake was quantified by determining the standardized uptake value (SUV) corrected for lean body mass. A region of interest was placed on the transaxial image to totally surround the most intense area of the 18
FDG uptake, and the SUV was calculated by using the maximum pixel activity value within the region of interest. Special attention was given to match PET images of the same patient at baseline and at follow-up by measuring the distance from the carotid bifurcation for the carotid lesions or the distance from the top of the aortic arch for the aortic lesions, assessed by co-registered CT images. Two radiologists measured the SUV, and the measurements were averaged in each plaque. The intra- and interobserver variabilities of SUV measurements were <5%. If multiple abnormal areas of 18
FDG uptake were found in 1 vessel segment, and if they were each clearly distinguishable from one another, they were recorded separately. In individual plaques, the changes in the SUV (
SUV) were calculated by subtracting the SUV at baseline from the SUV after 3-month treatment. The subjectwise SUV and
SUV were calculated by averaging the plaquewise SUVs and
SUVs, respectively, in each subject.
Lipids and inflammatory markers. On the day of PET study, after overnight fasting, peripheral blood was drawn from the antecubital vein for the measurements of LDL-C, high-density lipoprotein cholesterol (HDL-C), triglycerides, glucose, hemoglobin A1c, and high-sensitivity C-reactive protein (hsCRP). They were measured at a commercial laboratory (SRL, Fukuoka, Japan).
Statistical analysis. Data are described as mean values ± SD. Paired and unpaired t tests were performed for comparisons between the baseline and follow-up and between the 2 groups, respectively. Pearson correlation coefficient was used for correlation analysis. A value of p < 0.05 was considered to be statistically significant.
| Results |
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| Discussion |
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Noninvasive identification of inflammatory plaque has been challenging. With the recent advance in imaging technologies, we can get the morphology of plaques and some information about plaque stability (12,13 ) but cannot see the inflammation directly. Because 18 FDG-PET can visualize tissue glucose metabolism with high sensitivity and we can quantify 18 FDG uptake in the region of interest, we used this metabolic imaging technique for the detection and monitoring of plaque inflammation. Although PET has limited spatial resolution, we showed that co-registration with CT can localize the 18 FDG uptake to individual atherosclerotic vessel walls (Fig. 1 ). This is consistent with previous studies showing that the co-registered 18 FDG-PET with CT (5,6 ) or high-resolution magnetic resonance imaging (MRI) (14 ) improved the accuracy of the diagnosis of plaque inflammation. The 18 FDG uptake has been attributed to infiltrating inflammatory cells (i.e., macrophages and lymphocytes) and subendothelial proliferation of macrophages and smooth muscle cells within atherosclerotic lesions (3,1517 ). In an experimental rabbit model, 18 FDG accumulation corresponded to cellular infiltration in atherosclerotic lesions (18 ). Furthermore, it has been shown that 18 FDG is accumulated in the macrophage-rich area of the plaques, which were endoatherectomized from the PET-positive carotid lesions (6 ). These observations raised a possibility that serial 18 FDG-PET imaging is capable of surveying inflammatory activity within the plaque.
Blood pool activity may be a possible source of the vascular 18 FDG accumulation. However, it was unlikely in the present study. First, the co-registered image with CT clearly demonstrated that the uptakes were located in the vessel wall (Fig. 1 ). Second, vascular 18 FDG uptakes were not uniformly distributed along the course of the arteries (Fig. 2 ). In addition, nonvisualization of the large companion veins of the 18 FDG-labeled arteries was considered further indirect evidence against the blood pool hypothesis. Thus, it is unlikely that blood pooling activity is the major source of the vascular 18 FDG accumulation seen in the present study. Taken together, it is likely that the arterial 18 FDG uptakes are present in the vessel wall but not in blood pooling.
For quantitative analysis, we evaluated serial changes in the SUV within the plaque. The SUV is a quantified parameter of inflammation. So far, there has been no available information regarding the SUV levels of atherosclerotic plaques. In the present study, the plaque SUV at baseline was 1.69 ± 0.27 (ranging from 1.20 to 2.36) (Fig. 3 A). The observed SUV levels were comparable to those seen in small gastric cancers or small metastatic thyroid tumors (19,20 ). Moreover, a very recent study has shown that given the SUV cut-off point of 1.30, 18 FDG-PET co-registered with enhanced CT has a sensitivity of 90.9% and a specificity of 88.8% for the diagnosis of active vessel inflammation in patients with aortitis syndrome (21 ). It was possible that high plasma glucose impaired FDG uptake in diabetic patients. Although 7 diabetic subjects were included in the present study, they were well controlled on dietary therapy with or without antidiabetics. In the statin and diet groups, there were no differences in the baseline SUV and the magnitude of the SUV changes between diabetic and nondiabetic subjects (data not shown), suggesting that the presence of diabetes had no impact on the plaque FDG uptake in this study. Taken together, it is indicated that the observed 18 FDG uptake in the plaques indicates vessel inflammation. In our study, the intra- and interobserver variability was <5%, and repeated studies with the 3-month interval gave similar SUVs in the diet group. Thus, the SUV measurements had good reproducibility. Accordingly, the present study has indicated that atherosclerosis is an inflammatory process and that the individual plaque activities can be quantified by 18 FDG-PET in humans.
The most important finding of the present study is that 18 FDG-PET metabolic imaging can clearly visualize anti-inflammatory effects of simvastatin on the plaque (Fig. 2 ). Recent studies using high-resolution MRI demonstrated that it took more than 12 months for simvastatin to regress atherosclerotic plaques (12,13 ). Thus, although we did not examine the morphologic changes in this study, it is likely that in spite of the attenuation of plaque inflammation, the regression of the plaque might not have yet occurred in our patients with 3 months of simvastatin treatment. The observed anti-inflammatory effect of simvastatin may be related to the reduction of inflammatory cell infiltration and plaque stabilization. Crisby et al. (22 ) have demonstrated that patients after 3-month statin treatment had less lipid and oxidized LDL-C and fewer macrophages and T cells in the endatherectomized carotid samples compared with patients without statin. In the MIRACL (Myocardial Ischemia Reduction With Aggressive Cholesterol Lowering) (23 ) and PROVE-IT (Pravastatin or Atorvastatin Evaluation and Infection Therapy) (24 ) trials, significant benefits (i.e., the decreased incidence of unstable angina) occurred within the first month of treatment. Thus, our present study may support the hypothesis that the attenuation of plaque inflammation, rather than the anatomic regression, plays a role in the mechanism underlying the early beneficial effects of statins seen in clinical practice. In the present study, the effects of simvastatin on the serum CRP level, a systemic inflammatory marker, were not significant. One possible reason was the small size of the study. A second was the wide variability of the hsCRP levels in the present study. Another possibility is that the dose of simvastatin in this study (5 to 20 mg/day) was too small to reduce the systemic inflammatory marker. Future investigation with a larger study size is necessary to address this issue.
There were several possible mechanisms whereby simvastatin attenuated plaque inflammation. Simvastatin treatment reduced LDL-C by 30% (Table 2 , Fig. 4 ). However, the reduction in the SUV was not correlated with the extent of LDL-C-lowering in the statin group (Fig. 5 ), suggesting that the LDL-Cdependent effect was the minor mechanism of the observed benefits. The reduction in plaque inflammation was correlated with the increase in HDL-C in the statin group. Thus, the HDL-C increase may be important in the mechanism of attenuation of plaque inflammation by simvastatin. However, it is also possible that pleiotropic effects other than HDL-Cdependent mechanism mediate the antiinflammatory effect of simvastatin on the plaque.
Study limitations. First, the small study size limits our interpretation and discussion. Second, longer drug administration and observation periods might provide us abundantly clear evidence of the anti-inflammatory effects of simvastatin on the plaque. Third, the control group should have received lipid-lowering drugs (i.e., resins) to reduce the LDL-C to the same level of the statin group. However, the baseline LDL-C levels were not so high to warrant using other lipid-lowering drugs other than statins. Thus, we created the diet group as control. Next, several patients having aspirin and angiotensin II receptor blocker/angiotensin-converting enzyme inhibitor were included in the present study. We do not deny the possibility that anti-inflammatory properties of these agents may have enhanced the effects of simvastatin on the plaques. Finally, a future prospective study with a larger number of patients is needed to address whether the 18 FDG uptake is a predictor of cardiovascular events. Future technical innovations (e.g., the development of macrophage-specific PET tracers) would provide more specific information for detecting vulnerable plaques.
Conclusions. 18 F-fluorodeoxyglucose PET visualized inflammation of atherosclerotic plaques and that simvastatin attenuated it. The anti-inflammatory effect of simvastatin on the plaques may be one of the pleiotropic effects independent of LDL-Clowering effects.
| Footnotes |
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