Author + information
- Received December 14, 2015
- Revision received January 13, 2016
- Accepted January 13, 2016
- Published online August 1, 2016.
- Carlos Pérez-Medina, PhDa,
- Tina Binderup, PhDb,
- Mark E. Lobatto, MD, PhDa,c,
- Jun Tang, PhDd,
- Claudia Calcagno, MD, PhDa,
- Luuk Giesen, BSca,
- Chang Ho Wessel, MDa,
- Julia Witjes, MDa,
- Seigo Ishino, PhDa,
- Samantha Baxter, MHSa,
- Yiming Zhao, PhDa,
- Sarayu Ramachandran, PhDa,
- Mootaz Eldib, MSca,
- Brenda L. Sánchez-Gaytán, PhDa,
- Philip M. Robson, PhDa,
- Jason Bini, PhDe,
- Juan F. Granada, MDf,
- Kenneth M. Fish, PhDg,
- Erik S.G. Stroes, MD, PhDc,
- Raphaël Duivenvoorden, MD, PhDc,
- Sotirios Tsimikas, MDh,
- Jason S. Lewis, PhDd,
- Thomas Reiner, PhDd,
- Valentín Fuster, MD, PhDi,
- Andreas Kjær, MD, PhDj,
- Edward A. Fisher, MD, PhDk,
- Zahi A. Fayad, PhDa and
- Willem J.M. Mulder, PhDa,l,∗ ()
- aTranslational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, New York
- bClinical Physiology, Nuclear Medicine, PET and Cluster for Molecular Imaging, University of Copenhagen, Copenhagen, Denmark
- cDepartment of Vascular Medicine, Academic Medical Center, Amsterdam, the Netherlands
- dDepartment of Radiology, Memorial Sloan Kettering Cancer Center, New York, New York
- eSchool of Engineering & Applied Science, Yale University, New Haven, Connecticut
- fCRF Skirball Center for Innovation, The Cardiovascular Research Foundation, Orangeburg, New York
- gCardiovascular Research Center, Icahn School of Medicine at Mount Sinai, New York, New York
- hDivision of Cardiovascular Diseases, Department of Medicine, University of California San Diego, La Jolla, California
- iZena and Michael A. Wiener Cardiovascular Institute, Icahn School of Medicine at Mount Sinai, New York, New York
- jClinical Physiology, Nuclear Medicine and PET, University of Copenhagen, Copenhagen, Denmark
- kLeon H. Charney Division of Cardiology and Marc and Ruti Bell Program in Vascular Biology, New York University School of Medicine, New York, New York
- lDepartment of Medical Biochemistry, Academic Medical Center, Amsterdam, the Netherlands
- ↵∗Reprint requests and correspondence:
Dr. Willem J.M. Mulder, Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, One Gustave L. Levy Place, Box 1234, New York, New York 10029.
Objectives The goal of this study was to develop and validate a noninvasive imaging tool to visualize the in vivo behavior of high-density lipoprotein (HDL) by using positron emission tomography (PET), with an emphasis on its plaque-targeting abilities.
Background HDL is a natural nanoparticle that interacts with atherosclerotic plaque macrophages to facilitate reverse cholesterol transport. HDL-cholesterol concentration in blood is inversely associated with risk of coronary heart disease and remains one of the strongest independent predictors of incident cardiovascular events.
Methods Discoidal HDL nanoparticles were prepared by reconstitution of its components apolipoprotein A-I (apo A-I) and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine. For radiolabeling with zirconium-89 (89Zr), the chelator deferoxamine B was introduced by conjugation to apo A-I or as a phospholipid-chelator (1,2-distearoyl-sn-glycero-3-phosphoethanolamine–deferoxamine B). Biodistribution and plaque targeting of radiolabeled HDL were studied in established murine, rabbit, and porcine atherosclerosis models by using PET combined with computed tomography (PET/CT) imaging or PET combined with magnetic resonance imaging. Ex vivo validation was conducted by radioactivity counting, autoradiography, and near-infrared fluorescence imaging. Flow cytometric assessment of cellular specificity in different tissues was performed in the murine model.
Results We observed distinct pharmacokinetic profiles for the two 89Zr-HDL nanoparticles. Both apo A-I- and phospholipid-labeled HDL mainly accumulated in the kidneys, liver, and spleen, with some marked quantitative differences in radioactivity uptake values. Radioactivity concentrations in rabbit atherosclerotic aortas were 3- to 4-fold higher than in control animals at 5 days’ post-injection for both 89Zr-HDL nanoparticles. In the porcine model, increased accumulation of radioactivity was observed in lesions by using in vivo PET imaging. Irrespective of the radiolabel’s location, HDL nanoparticles were able to preferentially target plaque macrophages and monocytes.
Conclusions 89Zr labeling of HDL allows study of its in vivo behavior by using noninvasive PET imaging, including visualization of its accumulation in advanced atherosclerotic lesions. The different labeling strategies provide insight on the pharmacokinetics and biodistribution of HDL’s main components (i.e., phospholipids, apo A-I).
Atherosclerosis is a systemic inflammatory disorder that underlies cardiovascular disease (1). Lipid deposition and immune cell infiltration drive development of atherosclerotic plaque (2), which can eventually rupture (3), potentially causing atherothrombosis and ensuing acute coronary events (4).
High-density lipoprotein (HDL), a natural nanoparticle mainly composed of phospholipids, cholesterol and cholesteryl esters, and apolipoprotein A-I (apo A-I), is involved in the process of reverse cholesterol transport (5). HDL and apo A-I have demonstrated atheroprotective properties (6–8). The principal mechanism whereby HDL exerts this protective effect is generally ascribed to promoting cholesterol efflux from macrophages in plaques and transporting it to the liver for excretion, although other antiatherogenic properties have been reported (9). Cholesterol efflux is mediated by several membrane receptors abundantly expressed on macrophages to which apo A-I and HDL bind (10,11).
The study of HDL can be modernized by the integration of noninvasive imaging. Capitalizing on its properties, our group pioneered the use of HDL magnetic resonance imaging (MRI) (12,13). However, to noninvasively study the pharmacokinetics, trafficking, and metabolism of HDL, a quantitative and highly sensitive modality is warranted. Recent advances in labeling technology now allow positron emission tomography (PET) radiotracer imaging at unprecedented high-sensitivity down to the picomolar range, at limitless tissue penetration, in a so-called “hot spot” fashion (14). Tracing of HDL requires long-lived radioisotopes because, upon intravenous administration, it circulates for extended periods of time. Recently, we have developed modular zirconium-89 (89Zr)-labeling methods for lipid-based nanoparticles (15,16). The physical half-life of 89Zr (78.4 h) makes it an ideal radiolabel for such long-circulating materials (15) and provides an opportunity to study the in vivo behavior of the HDL nanoparticles.
Through extensive studies involving a unique arrangement combining PET/computed tomography (CT) and PET/MRI in murine, rabbit, and porcine atherosclerosis models, the present paper illustrates that 89Zr labeling is a valuable tool to noninvasively assess the pharmacokinetics and distribution of the main components of HDL.
A detailed description of the animal models and experimental procedures can be found in the Online Appendix. All animal experiments were performed in accordance with protocols approved by the Institutional Animal Care and Use Committees of Mount Sinai, Memorial Sloan Kettering Cancer Center, and/or CRF Skirball Center for Innovation. All experiments adhered to National Institutes of Health guidelines for animal welfare.
Radiolabeling of HDL nanoparticles
The composition, dynamic light scattering–measured size, and size exclusion retention time for all HDL nanoparticles used in this study are summarized in Online Table 1. To incorporate the radioactive label (89Zr), we either conjugated deferoxamine B to apo A-I via its attachment to lysine residues, or included the phospholipid chelator 1,2-distearoyl-sn-glycero-3-phosphoethanolamine–deferoxamine B in the formulation of the particles. Reaction of these deferoxamine B–bearing nanoparticles with 89Zr-oxalate generated 89Zr-apo A-I–labeled HDL (89Zr-AI-HDL) or 89Zr-phospholipid-labeled HDL (89Zr-PL-HDL) (Figure 1A), at high radiochemical yields (96 ± 2% [n = 5] and 81 ± 10% [n = 7], respectively), and radiochemical purities of >98%. Radiolabeling did not impact the size of HDL.
Cellular targets of HDL nanoparticles
In addition, fluorescent, nonradioactive analogues of the 2 89Zr-labeled HDL nanoparticles were prepared to investigate their cellular targets in different tissues by using flow cytometry (Figure 1B, Online Figure 1). These particles were prepared in a manner identical to that of their radioactive counterparts but contained the dye DiR (Online Table 1) and nonradioactive Zr. In Apoe–/– mice, flow cytometric analyses revealed little targeting differences between the 2 Zr-labeled nanoparticles in blood, spleen, and aorta at 24 h post-injection (Figure 1C). In atherosclerotic aortas, macrophages and monocytes were preferentially targeted over neutrophils (4.5- and 3.1-fold greater DiR uptake, respectively) and lineage-positive cells (290- and 220-fold greater DiR uptake), whereas mostly macrophages were targeted in the spleen. In blood, dendritic cells and monocytes were targeted to nearly the same extent.
Pharmacokinetics and biodistribution of 89Zr-HDL nanoparticles in mice
Blood radioactivity clearance in Apoe–/– and wild-type mice was investigated (Figure 2A). The weighted half-lives for 89Zr-AI-HDL–associated radioactivity in blood were 1.4 and 2.0 h in atherosclerotic and wild-type mice, respectively, whereas for 89Zr-PL-HDL, these values were 2.8 and 1.1 h. Radioactivity distribution at 24 and 48 h was also measured for selected tissues. Figure 2B illustrates a comparison of radioactivity uptake values for both nanotracers at 24 h post-injection, whose target organs were the same. For both 89Zr-AI-HDL and 89Zr-PL-HDL, kidneys were the main radioactivity accumulation site, although with marked quantitative differences. No statistically significant differences were found in tissue uptakes for 89Zr-AI-HDL between diseased and control animals. However, when whole organ radioactivity accumulation was considered, uptake in the kidneys, liver, and spleen was significantly higher for atherosclerotic mice (Online Figure 2A). For 89Zr-PL-HDL, kidney and liver uptakes were significantly higher for control animals. At 48 h post-injection (Online Figures 2B and 2C), kidneys were also the main activity accumulation site for both nanoparticles. For 89Zr-AI-HDL, uptakes were as high as 80 %ID/g (i.e., the percent injected dose per gram).
Micro-PET/CT imaging studies in mice
Noninvasive visualization of 89Zr-HDL biodistribution in mice was investigated at 24 h (Figure 2C) and 48 h (Online Figure 2B) after intravenous administration. For 89Zr-AI-HDL, images were clearly dominated by strong signal from the kidneys, whereas for 89Zr-PL-HDL, liver and kidneys were the preferred accumulation sites. PET-derived uptake values were in good agreement with the ex vivo results (Online Figure 2D).
Plaque targeting of 89Zr-HDL nanoparticles in mice
Increased radioactivity accumulation in whole aortas of Apoe–/– mice was found for both nanoparticles at 24 h. For 89Zr-AI-HDL, the difference was significantly higher when only the aortic root was considered (Figure 3A). At 48 h, a significantly higher accumulation in atherosclerotic aortas was found for 89Zr-PL-HDL only (0.019 ± 0.04 %ID [n = 6] vs. 0.013 ± 0.03 %ID [n = 4] for control animals; p = 0.02). In autoradiographic analysis, higher deposition of radioactivity was observed in atherosclerotic aortas (Figure 3B), especially in aortic roots. Increased plaque accumulation of the fluorescent analogues of 89Zr-HDL nanoparticles (DiR@Zr-AI-HDL and DiR@Zr-PL-HDL) was also observed in atherosclerotic aortas by using near-infrared fluorescence (NIRF) imaging. Strong signals were observed originating from aortic roots of diseased animals at 24 and 48 h post-injection. Good correlations were found for both 89Zr-AI-HDL (ρ = 0.80) and 89Zr-PL-HDL (ρ = 0.72) between total radioactivity accumulation and total radiant efficiency of their fluorescent analogues at 24 h post-injection (Figure 3C).
Pharmacokinetics and biodistribution of 89Zr-HDL nanoparticles in rabbits
Similarly to the murine model, radioactivity clearance was slower for 89Zr-AI-HDL (Figure 4A), whose half-lives were 1.13 and 1.05 days in atherosclerotic and control animals, respectively. For 89Zr-PL-HDL, clearance was slower in animals with atherosclerosis (0.44 vs. 0.34 day for control animals). Radioactivity distribution in selected tissues was measured at 5 days’ post-injection (Figure 4B). Kidneys exhibited the highest uptake values for both nanoparticles, although they were 3- to 4-fold higher for 89Zr-AI-HDL. Spleen and liver uptakes were approximately 0.1 %ID/g for the 2 nanoparticles.
Imaging studies in rabbits
The 89Zr-HDL nanoparticles were evaluated in rabbits by using in vivo PET imaging on a clinical PET/CT scanner. Representative PET/CT fusion images of the abdominal region at 3 different time points can be seen in Figure 4C for both nanoparticles. In addition, a unique clinical PET/MRI system was used to investigate the in vivo behavior of the nanoparticles in the same animals and time points for direct comparison. Figure 5A displays representative PET/MRI fusion images of the same animals shown in Figure 4C. The PET images obtained with both scanners were almost identical (Online Video 1). More importantly, there was a strong correlation between the standardized uptake values (SUVs) measured from both systems (Figure 5B) and also excellent agreement between the values (Figure 5C), with an intraclass correlation coefficient of 0.99 (95% confidence interval: 0.98 to 0.99).
Time-activity curves based on data obtained from PET/CT and PET/MRI image analysis are shown in Online Figures 3 to 6. Initially, within the first hour after intravenous administration, images were dominated by a strong blood pool signal for both 89Zr-AI-HDL and 89Zr-PL-HDL (Figures 4C and 5A), as well as high signal from the liver, spleen, and kidneys. Blood radioactivity half-lives derived from PET/CT SUVs measured in the cardiac chambers were shorter than those obtained by blood sampling for both 89Zr-AI-HDL (0.55 and 0.56 day for diseased animals and control animals, respectively) and 89Zr-PL-HDL (0.27 and 0.31 day for diseased animals and control animals). At later time points, 89Zr-AI-HDL images showed very high radioactivity accumulation in the kidneys, which was significantly higher in atherosclerotic animals at all time points investigated (Online Figures 3A and 4A). For 89Zr-PL-HDL, strong gallbladder signals, with SUVs as high as 20 g/ml (Figure 4C), were observed at 1 and 2 days’ post-injection, which was accompanied by high radioactivity accumulation in the stomach and intestines. Intense signals from the kidneys were also observed at all time points for 89Zr-PL-HDL. In both cases, bone radioactivity accumulation was also detected over the 5-day period (SUVs approximately 5 g/ml). Ex vivo analysis proved that this finding was mostly due to bone marrow uptake, as opposed to mineral bone. Bone marrow uptake was higher in animals with atherosclerosis for 89Zr-AI-HDL (0.17 ± 0.03 %ID/g vs. 0.13 ± 0.01 %ID/g for control animals); for 89Zr-PL-HDL, values were similar (0.039 ± 0.010 %ID/g and 0.050 ± 0.002 %ID/g for rabbits with atherosclerosis and control animals). PET-measured SUVs were in good agreement with SUVs determined according to ex vivo gamma counting (Online Figure 7), although they were typically lower.
Increased aortic radioactivity accumulation was found at day 5 post-injection for both probes in atherosclerotic aortas compared with control aortas according to PET/CT imaging (Figure 6A). The difference was statistically significant for 89Zr-PL-HDL (p = 0.03). For 89Zr-AI-HDL at 1, 2, and 3 days’ post-injection, the high blood pool signal dominated the measurements, as evidenced by the similar blood and aorta ratios between SUVs in atherosclerotic and control rabbits (Online Figure 8A). We found a very similar increased accumulation in atherosclerotic aortas for either 89Zr-AI-HDL (0.40 ± 0.08 g/ml vs. 0.34 ± 0.10 g/ml) or 89Zr-PL-HDL (0.24 ± 0.13 g/ml vs. 0.15 ± 0.08 g/ml) at day 5 post-injection using PET/MRI. At earlier time points, values were similar for both nanoparticles in animals with atherosclerosis and in the control animals.
Plaque targeting of 89Zr-HDL nanoparticles in rabbits
Radioactivity concentration was significantly higher in atherosclerotic aortas for both 89Zr-AI-HDL (p = 0.03) and 89Zr-PL-HDL (p = 0.03) as determined by gamma counting at 5 days’ post-injection (Figure 6A). Autoradiography of explanted aortas revealed a patchy distribution of radioactivity in atherosclerotic aortas, illustrating preferential accumulation in lesions (Figure 6B). Analysis according to NIRF imaging found increased accumulation of Cy5.5-HDL in atherosclerotic aortas. A good correlation was found between total radiant efficiency and radioactivity concentration (Figure 6C, Online Figure 8B). Interestingly, the correlation between radioactivity concentration and Cy7-albumin NIRF intensity was weak (Online Figure 8C), suggesting that permeability may not be an HDL accumulation determinant.
Oxidized HDL imaging experiments in atherosclerotic rabbits
Blood radioactivity clearance for 89Zr-apo A-I labeled oxidized HDL (89Zr-AI-HDLOx) nanoparticles was significantly faster than for 89Zr-AI-HDL in both animals with atherosclerosis and in the control animals (Online Figure 9A), and its half-life was 3 times as short (0.33 day). Radioactivity distribution, however, showed a similar pattern at day 5 post-injection (Online Figure 9B). 89Zr-AI-HDLOx uptake values were significantly lower in liver and lungs compared with 89Zr-AI-HDL in both atherosclerotic and control animals, and in the spleen compared with 89Zr-AI-HDL in diseased animals. Images from PET/CT (Online Figure 9C) and PET/MRI analysis revealed high accumulation in kidneys at all time points, with SUVs as high as 48 g/ml (1.0 %ID/g). These values were significantly higher than those measured for 89Zr-AI-HDL in control animals and rabbits with atherosclerosis (Online Figure 9D). PET-quantified SUVs were qualitatively similar to the values obtained after the day 5 scan by using ex vivo gamma counting (Online Figure 9D). Interestingly, radioactivity accumulation in atherosclerotic aortas for 89Zr-AI-HDLOx was significantly lower than for nonoxidized 89Zr-AI-HDL (Online Figure 9E).
Imaging studies in pigs
Three pigs were imaged at 48 h after administration of 89Zr-PL-HDL. A whole-body PET maximum intensity projection image obtained on the clinical PET/CT scanner can be seen in Figure 7A. Strong signals were observed in the kidneys, liver, and bone, as well as in the intestines. High radioactivity accumulation in the femoral arteries could be clearly observed in vivo (Figure 7B). Radioactivity concentration in these vessels was higher than in noninjured internal iliac arteries, which served as controls. Radioactivity distribution in selected organs determined according to gamma counting after the 48 h scan is shown in Figure 7C and mirrors the PET imaging observations. The increased signal in the femoral arteries was due to accumulation in lesions, as corroborated by ex vivo autoradiography analysis (Figure 7D).
In the present study, we developed and validated a noninvasive quantifiable PET imaging tool to study the in vivo behavior of HDL in multiple atherosclerosis models. Using nonradioactive fluorescent analogues of the 89Zr-labeled nanoparticles, we found that both preferentially targeted macrophages in the atherosclerotic aortas of Apoe–/– mice, in line with our previous studies (17,18). This finding also suggests that the modifications were well tolerated and did not affect the ability of HDL to interact with plaque macrophages (19).
Radioactivity clearance followed similar patterns in both mice and rabbits, exhibiting longer half-lives for 89Zr-AI-HDL (apolipoprotein-labeled) in healthy control animals and for 89Zr-PL-HDL (phospholipid-labeled) in atherosclerotic animals. 89Zr-AI-HDL’s longer half-life compared with 89Zr-PL-HDL in control animals may be explained by the low net internalization rate and recycling of the protein (20). However, the slower clearance of 89Zr-PL-HDL in animals with atherosclerosis, especially in the Apoe–/– mouse model, may be due to the elevated lipoprotein blood pool concentration and impaired lipid metabolism and clearance.
Modification of apo A-I can affect its blood circulation time as reported for radioiodination (21), which has frequently been used to study the pharmacokinetics and metabolism of the protein and of HDL (22). This labeling strategy is convenient and results in iodination of tyrosine residues as well as faster clearance than the unmodified protein (21), which is most likely due to rapid deiodination by deiodinases (23). Therefore, label stability and kinetics play an important role when designing an imaging tool. Importantly, using our approach, the radioactivity blood half-life of 89Zr-AI-HDL in rabbits is in close agreement with values obtained by radioimmunometric assays using unmodified human apo A-I in the same animal (21), implying that the modifications to incorporate the 89Zr label were well tolerated. Previously reported data do not show this strong corroboration between 125I-labeled HDL (i.e. 125I-HDL/125I-apo A-I) and unmodified HDL, which underestimates the actual blood half-life by a factor of 7 to 8.
Our imaging studies revealed the same patterns in radioactivity distribution for the 3 animal models used; the kidneys were the main site of accumulation for both 89Zr-AI-HDL and 89Zr-PL-HDL (Online Figure 10). It is well established that the kidney is a major catabolism site for apo A-I and certain HDL subclasses (24–26). Quantitative analysis revealed a significantly higher kidney uptake in atherosclerotic rabbits at all time points for 89Zr-AI-HDL. The SUVs showed little variation over time, suggesting a steady-state situation. More importantly, the higher kidney uptakes found for 89Zr-AI-HDL in rabbits with atherosclerosis were accompanied by lower blood radioactivity concentrations; indeed, a higher glomerular filtration rate has been related to lower HDL and apo A-I blood levels in humans without kidney disease (27) and to subclinical cardiovascular disease in nondiabetic subjects (28).
Although 89Zr-AI-HDL showed very high uptake in the kidneys, the radioactivity for 89Zr-PL-HDL was more evenly distributed between the kidneys, liver, and spleen and, at later time points, the digestive system. Initially, very high uptakes were found in the gallbladder (both for control animals and animals with atherosclerosis), suggesting incorporation of the radiolabeled phospholipid into the bile and subsequent excretion. The detection of radioactivity in the stomach can be explained by the presence of the radioactive compound in the cecotropes (nutritious stools resulting from gut bacteria digestion) that rabbits selectively ingest (29).
Ex vivo analysis of rabbit bone samples revealed that most of this uptake originated from the bone marrow and that, for 89Zr-AI-HDL, this uptake was higher in animals with atherosclerosis. This finding may reflect the interaction of 89Zr-HDL nanoparticles with hematopoietic stem and multipotent progenitor cells, which overexpress the ABCG1 transporter involved in HDL-mediated cholesterol efflux (30).
Interestingly, blood radioactivity clearance for oxidized HDL nanoparticles (89Zr-AI-HDLOx) in atherosclerotic animals was significantly faster than its nonoxidized counterpart in both control and diseased animals and was paralleled by a significantly increased kidney radioactivity accumulation. The shorter blood half-life of 89Zr-AI-HDLOx is probably the main reason why its accumulation in the plaque was significantly decreased compared with nonoxidized 89Zr-AI-HDL, but impaired interaction with its receptors could also be partly responsible. These results are in agreement with a recent study using myeloperoxidase-oxidized apo A-I (31).
As a proof of principle, we tested 1 of our 89Zr-labeled nanoparticles, namely 89Zr-PL-HDL, in a porcine model of atherosclerosis. The target organs were the same (mainly kidneys and liver), with SUVs comparable to those observed in the rabbit studies. Atherosclerotic lesions were clearly visible at 48 h post-administration despite the blood pool signal, suggesting that vessel wall inflammation can be imaged by using HDL-PET in severe cases.
In addition to PET/CT imaging, we also performed extensive PET/MRI studies in the rabbit model. Importantly, the PET/MRI data were in excellent agreement with the PET/CT data. Integrative PET/MRI has only recently become available (32) and allows direct vessel (morphology) visualization, without administration of an additional agent to delineate the vasculature. To the best of our knowledge, this study is the first time PET/MRI has been applied to monitor nanoparticle accumulation in the atherosclerotic vessel wall.
HDL, an aggregate of lipids and protein, inherently is a dynamic system, constantly converting into particles of different shapes and sizes by exchanging components with cells and other lipoproteins (33). Since apo A-I is the protein component that dictates HDL's biological function, 89Zr-AI-HDL will primarily allow apolipoprotein tracking. Radioactivity distribution for 89Zr-PL-HDL, on the other hand, will be the result of complex exchange processes, as a fraction of the radiolabeled phospholipids will be exchanged with other plasma proteins (16). A third labeling strategy that we did not develop, i.e., the inclusion of a lipophilic 89Zr label, could shed light on lipid transportation throughout the body. PET imaging of the vessel wall has inherent limitations, mainly related to the limited spatial resolution. Therefore, reliable quantification of vessel wall uptake requires a nearly complete radioactivity clearance from circulation, which takes days for 89Zr-PL-HDL and even longer for 89Zr-AI-HDL.
89Zr labeling allows study of the in vivo behavior of HDL by using PET imaging combined with CT or MRI. This tool allows the noninvasive evaluation of HDL and could be of great value to study its metabolism and trafficking in preclinical and clinical settings. Ultimately, in the context of advanced clinical atherosclerosis, this noninvasive imaging tool is ideal for identifying patients amenable to HDL therapy and subsequent treatment monitoring in a theranostic fashion.
COMPETENCY IN MEDICAL KNOWLEDGE: HDL labeled with the long-lived radioisotope 89Zr is a novel tracer to study its pharmacokinetics and biodistribution with in vivo PET imaging. Our PET imaging technology can help to elucidate certain aspects of the function of HDL under physiological and pathophysiological conditions.
TRANSLATIONAL OUTLOOK: HDL-cholesterol is a strong predictor of cardiovascular risk. As such, pharmaceutical companies are exploring its use as an injectable therapeutic. Using the presented imaging approach, the organ and atherosclerotic plaque accumulation kinetics of HDL can be quantitatively studied, in a noninvasive fashion, and help to identify patients that are amenable to HDL therapy.
The authors thank the Small Animal Imaging Core and the Radiochemistry and Molecular Imaging Probes Core at Memorial Sloan Kettering Cancer Center for their assistance.
For a supplemental video, an expanded Methods section, and supplemental figures and a table, please see the online version of this article.
Financial support was received from the following: the National Institutes of Health (NIH) Program of Excellence in Nanotechnology Award (HHSN368201000045C, to Dr. Fayad); and NIH grants R01 EB009638 (Dr. Fayad), R01 HL118440 (Dr. Mulder), R01 HL125703 (Dr. Mulder), and R01 CA155432 (Dr. Mulder); NWO Vidi 91713324 (Dr. Mulder); NIH HL119828 (Dr. Tsimikas); and the CNIC CardioImage program (for Dr. Pérez-Medina). All other authors have reported that they have no relationships relevant to the contents of this paper to disclose. Prediman K. Shah, MD, served as guest editor for this paper.
- Abbreviations and Acronyms
- zirconium-89 apolipoprotein A-I–labeled high-density lipoprotein
- zirconium-89 phospholipid-labeled high-density lipoprotein
- apo A-I
- apolipoprotein A-I
- computed tomography
- high-density lipoprotein
- high-fat diet
- percent injected dose per gram
- magnetic resonance imaging
- near-infrared fluorescence
- standardized uptake value
- Received December 14, 2015.
- Revision received January 13, 2016.
- Accepted January 13, 2016.
- American College of Cardiology Foundation
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