Feasibility of ¹¹C-Acetate PET/CT for Imaging of Fatty Acid Synthesis in the Atherosclerotic Vessel Wall

DISCUSSION

To our knowledge, this is the first study that reports on the association of ¹¹C-acetate uptake with arterial wall alterations in large arteries. Our results indicate the feasibility of using ¹¹C-acetate PET for in vivo imaging of FAS activity in arterial wall lesions. Although mainly used for the evaluation of both malignant tumors and myocardial oxidative metabolism to date, our results suggest that ¹¹C-acetate may be able to depict active fatty acid synthesis in the atherosclerotic vessel wall.

¹¹C-acetate uptake was observed in both calcified and noncalcified vessel wall alterations, and the distribution of tracer accumulation was consistent with established atherosclerotic topography—that is, most active lesions were noted in the abdominal and thoracic aorta (15,27), as was also true for calcifications representing a well-known marker of atherosclerosis. Furthermore, arterial tracer uptake correlated significantly with established atherogenic risk factors such as age (P < 0.01) and male sex (P = 0.03). Radiotracer uptake was not significantly associated with other atherogenic risk factors such as hypertension or a history of cardiovascular events in our study, as is possibly explained by the limited statistical power due to the relatively small number of patients included. In contrast, the observed significant correlations become even more compelling, considering that the study population was not large enough to find significant correlations between risk factors and CP, presenting an established marker of atherosclerosis. Therefore, ¹¹C-acetate uptake demonstrates stronger correlations with at least some determinants of cardiovascular risk than arterial calcification. The use of ¹¹C-acetate as a novel PET tracer for imaging of arterial wall lesions may hold potential to provide information about intralesional fatty acid synthesis in both formation and evolution of atherosclerotic lesions.

With regard to the colocalization of tracer accumulation and calcification sites, the findings of this study add novel observations about ¹¹C-acetate, compared with other tracers used for characterization of atherosclerotic lesions. Various studies reported colocalization in about 2%–14% of sites, using ¹⁸F-FDG as a marker of macrophage activity (27–29). When ¹⁸F-sodium fluoride is used as an indicator of active mineral deposition, uptake has been shown to be coincident with calcification in up to 88% of sites (15). In the present study, no colocalization of ¹¹C-acetate accumulation with calcification was observed in 70.9% of lesions with tracer uptake, suggesting that increased fatty acid synthesis is of particular pathophysiologic importance in noncalcified vessel wall alterations, which are usually assumed to represent early stages of disease (3). Consequently, no coincident radiotracer accumulation was found in most of the CPs (86.7%). Interestingly, in as much as 29.1% of lesions with increased tracer uptake, concordant calcification was found in at least some part of the CP, possibly indicating that fatty acid synthesis may also be observed in CP. Another hypothesis for radiotracer accumulation in calcified lesions might be nonspecific tracer binding to arterial calcifications, as observed by Laitinen et al. for ¹⁸F-FDG (30). However, it is currently not known whether—and to what extent—nonspecific mechanisms are involved in the vascular accumulation of tracers other than ¹⁸F-FDG.

With regard to the intensity of ¹¹C-acetate accumulation in arterial lesions, the measured TBRs were comparable with those for other established tracers for plaque imaging. In a study by Rominger et al., mean TBR for vascular ¹⁸F-FDG uptake in the abdominal aorta was 1.57 ± 0.35 (11). In another study using ¹⁸F-sodium fluoride for visualization of mineral deposition in plaque, mean TBR was 2.3 ± 0.7 (29). In the present study, the mean intensity of tracer uptake as determined by TBR was 2.5 ± 1.0, underlining the excellent vascular contrast for ¹¹C-acetate, which facilitates evaluation of large arteries. The highest intensity of ¹¹C-acetate uptake was found in the abdominal aorta, which was true for both TBR and SUV_max. This is in line with previous studies using other tracers such as ¹⁸F-FDG—studies that observed the highest TBRs in the aortic wall (11), although some have reported higher TBR values for the carotid arteries as compared with aortic territories (31). However, the intensity of arterial tracer uptake may be associated with plaque-specific factors including size or inflammatory state rather than with the localization of an artery within the arterial tree.

Both plaque composition and intraplaque distribution of biologic markers vary between intact and disrupted plaques. For instance, concentrations of fatty acids are increased at the edge of disrupted plaques, compared with the center (5). Interestingly, the extent of inflammation as determined by macrophage concentrations particularly at the plaque shoulders appears to correlate with degree of vulnerability (32). It would be highly desirable to obtain comparative data from the relative distribution of ¹⁸F-FDG, ¹⁸F-fluoride, and ¹¹C-acetate uptake within atheroma to assess the relative contribution of these tracers for the functional evaluation of different pathophysiologic processes in atherosclerotic plaque. Autoradiographic studies and histologic evaluation might lead to a better understanding of the involved mechanisms of intralesional tracer accumulation.

Fatty acid synthesis is an energy-consuming process creating fatty acids through action of the multifunctional enzyme FAS. First, ¹¹C-acetate is converted into ¹¹C-acetyl-CoA by acetyl-CoA synthetase inside the cell (33). Once ¹¹C-acetyl-CoA is formed, it may be used in de novo lipogenesis. After priming with acetyl-CoA, FAS synthesizes palmitate and other saturated fatty acids using malonyl-CoA as substrate, whereby the latter is formed by decarboxylating acetyl-coA (19). The main classes of lipids in atherosclerotic lesions—that is, cholesteryl esters, triglycerides, and phospholipids—all contain fatty acids (16). ¹¹C-acetate PET may be used to in vivo visualize the first biochemical reactions of fatty acid synthesis in plaque. On the one hand, these fatty acids are crucial for macrophage differentiation and foam cell formation (21). On the other hand, fatty acids are involved in intralesional lipid homeostasis and macrophage inflammatory responses through action of peroxisome proliferator-activated receptors and liver X receptors (3). Therefore, ¹¹C-acetate depicts molecular processes in a manner different from ¹⁸F-FDG or choline compounds, which may be used to provide information about macrophage-mediated inflammation or cell membrane proliferation in atherosclerotic lesions (34,35). The increasing number of tracers available for the characterization of pathophysiologic processes in atherosclerotic plaque—and their complementary use—may eventually lead to a more thorough understanding of the complex interactions in the development and evolution of plaque.

Recently, Schneider et al. demonstrated that FAS deficiency in macrophages decreases diet-induced atherosclerosis by inactivating FAS in macrophages of apolipoprotein E–deficient mice. In these knock-out mice, compared with wild-type mice, the extent of atherosclerosis was decreased 20%–40% in different aortic regions, underlining the crucial role of FAS in the development of atherosclerotic plaque (22). In another work, Ecker et al. demonstrated that induction of fatty acid synthesis is a key requirement for the development of phagocytic capacity in human monocytes and that suppression of fatty acid synthesis prevents uptake of lipoproteins such as enzymatically modified low-density lipoprotein (21). These findings demonstrate that inhibition of FAS might decrease foam cell development. Therefore, ¹¹C-acetate PET has the potential to provide new insights into the biology of fatty acid synthesis and macrophage differentiation in atherosclerotic plaque.

Fatty acid synthesis is dramatically increased in malignant cells (36). Because of the strong expression of FAS in these cells, considerable interest has been shown in the discovery and development of pharmacologic agents that block FAS activity, such as C75 or cerulenin (36,37). If suitable clinical FAS inhibitors become available, they may also prevent de novo fatty acid synthesis in atherosclerotic plaque (22). The incorporation of ¹¹C-acetate PET into preclinical models and clinical therapy monitoring studies could provide a promising technology to image fatty acid synthesis or the inhibition of FAS in plaque and could therefore represent a unique opportunity for in vivo target validation.

The present study had limitations. First, PET/CT was performed in oncologic patients, and therefore its findings might not be perfectly generalizable to other patient populations. However, factors that might influence tracer uptake in the arterial wall were carefully excluded. Second, a relatively small number of patients (n = 36) was included in this pilot study, influencing the statistical power to detect significant associations with atherogenic risk factors, because it has been shown that large patient cohorts are needed to reproduce known significant correlations in PET/CT of atherosclerotic plaque (29). However, such correlations have been observed in the present study for both age and sex, indicating an association of ¹¹C-acetate uptake with cardiovascular risk. Additionally, dynamic imaging might provide further information about the nature of tracer uptake in vessel wall alterations. Finally, as an intrinsic limitation of ¹¹C-acetate, assessment of relevant arteries such as the right coronary artery using ¹¹C-acetate is impaired because of high uptake of this tracer in the liver. Additionally, the clinical use of ¹¹C-acetate is limited by its short half-life, but synthesis of fluorinated ¹⁸F-acetate is possible and might overcome that limitation (23). The results of this study need to be prospectively evaluated in a larger patient cohort to further explore the experimental and clinical potential of ¹¹C-acetate PET for imaging of fatty acid synthesis in arterial lesions, particularly for therapy-monitoring studies.