Detection of Atherosclerotic Inflammation by 68Ga-DOTATATE PET Compared to [18F]FDG PET Imaging

Abstract

Background: Inflammation drives atherosclerotic plaque rupture. Although inflammation can be measured using fluorine-18-labeled fluorodeoxyglucose positron emission tomography ([18F]FDG PET), [18F]FDG lacks cell specificity, and coronary imaging is unreliable because of myocardial spillover.

Objectives: This study tested the efficacy of gallium-68-labeled DOTATATE (68Ga-DOTATATE), a somatostatin receptor subtype-2 (SST2)-binding PET tracer, for imaging atherosclerotic inflammation.

Methods: We confirmed 68Ga-DOTATATE binding in macrophages and excised carotid plaques. 68Ga-DOTATATE PET imaging was compared to [18F]FDG PET imaging in 42 patients with atherosclerosis.

Results: Target SSTR2 gene expression occurred exclusively in "proinflammatory" M1 macrophages, specific 68Ga-DOTATATE ligand binding to SST2 receptors occurred in CD68-positive macrophage-rich carotid plaque regions, and carotid SSTR2 mRNA was highly correlated with in vivo 68Ga-DOTATATE PET signals (r = 0.89; 95% confidence interval [CI]: 0.28 to 0.99; p = 0.02). 68Ga-DOTATATE mean of maximum tissue-to-blood ratios (mTBRmax) correctly identified culprit versus nonculprit arteries in patients with acute coronary syndrome (median difference: 0.69; interquartile range [IQR]: 0.22 to 1.15; p = 0.008) and transient ischemic attack/stroke (median difference: 0.13; IQR: 0.07 to 0.32; p = 0.003). 68Ga-DOTATATE mTBRmax predicted high-risk coronary computed tomography features (receiver operating characteristics area under the curve [ROC AUC]: 0.86; 95% CI: 0.80 to 0.92; p < 0.0001), and correlated with Framingham risk score (r = 0.53; 95% CI: 0.32 to 0.69; p <0.0001) and [18F]FDG uptake (r = 0.73; 95% CI: 0.64 to 0.81; p < 0.0001). [18F]FDG mTBRmax differentiated culprit from nonculprit carotid lesions (median difference: 0.12; IQR: 0.0 to 0.23; p = 0.008) and high-risk from lower-risk coronary arteries (ROC AUC: 0.76; 95% CI: 0.62 to 0.91; p = 0.002); however, myocardial [18F]FDG spillover rendered coronary [18F]FDG scans uninterpretable in 27 patients (64%). Coronary 68Ga-DOTATATE PET scans were readable in all patients.

Conclusions: We validated 68Ga-DOTATATE PET as a novel marker of atherosclerotic inflammation and confirmed that 68Ga-DOTATATE offers superior coronary imaging, excellent macrophage specificity, and better power to discriminate high-risk versus low-risk coronary lesions than [18F]FDG. (Vascular Inflammation Imaging Using Somatostatin Receptor Positron Emission Tomography [VISION]; NCT02021188).

Keywords: atherosclerosis; inflammation; macrophages; molecular imaging; positron emission tomography; somatostatin receptor.

Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.

Figures

Graphical abstract

Figure 1

The VISION Study Patient (A) and procedure (B) flowcharts. ∗Did not meet study criteria, n = 8; other clinical factors, n = 3; declined/cancelled, n = 49. †Coronary artery PET data excluded in ACS patients with ambiguous culprit arteries (n = 2). ‡Carotid artery PET data excluded in patients with prior carotid surgery (n = 2). §[18F]-FDG PET imaging not completed because of timing of surgery (n = 1). ‖Tissue samples excluded owing to insufficient mRNA extracted for quantitative PCR (n = 2). ¶CT scans not completed (calcium scan, n = 1; coronary angiogram, n = 5; carotid angiogram, n = 2). ACS = acute coronary syndrome; CT = computed tomography; CVD = cardiovascular disease; FDG = fluorodeoxyglucose; PCR = polymerase chain reaction; PET = positron emission tomography; TIA = transient ischemic attack; VISION = Vascular Inflammation imaging using Somatostatin receptor positron emissION tomography.

Figure 2

Target SSRT2 Expression in Proinflammatory Macrophages Heatmap of population-based RNA sequencing data (A) showing high SSTR2 expression in proinflammatory M1 macrophages (n = 4), very low levels of SSTR2 expression in unstimulated M0 macrophages (n = 4), and alternatively activated M2 macrophages (n = 5). For comparison, a heatmap of GLUT1 and GLUT3 shows significant gene expression in all cell types (note, different scales for SSRT and GLUT genes; mean values are log2 fragments per kilobase of transcript for million mapped reads [FPKM+1]). SSTR2 expression in LPS-stimulated macrophages from CVD patients versus age- and sex-matched healthy volunteers (n = 3 for both) using quantitative PCR (B). Photomicrograph shows green fluorescent immunoreactive SST2 staining in macrophages (C), with blue nuclear DAPI-stained ([inset] concentration and isotype-matched IgG negative control). Brightfield photomicrograph shows brown immunoreactive SST2-stained cultured macrophages, with nuclear counterstain (D). Phosphor autoradiographic image shows total binding of 68Ga-DOTATATE (E) in clusters of cultured macrophages ([inset] parallel incubation with 68Ga-DOTATATE and cold competing ligand showing very low levels of nonspecific binding). IgG = immunoglobulin G; LPS = lipopolysaccharide; other abbreviations as in Figure 1.

Figure 3

Coronary PET Inflammation Imaging: ACS Culprit Versus Bystander Lesions X-ray angiography images from a 59-year old man with ACS, showing a culprit first obtuse marginal lesion ([A] hatched oval) and nonculprit (bystander) right coronary artery disease ([E] circle). Identification of a culprit artery was aided by electrocardiographic findings of lateral T-wave inversion. Corresponding CT angiography images (B, F) show stented culprit lesion (*) and native bystander lesion with high-risk plaque morphology ([inset] low attenuation, cross-section of artery with outer wall boundary marked by dotted outline). In both lesions, intense inflammation (arrows) detected by 68Ga-DOTATATE PET (C, G) is reproduced by [18F]FDG PET (D, H). Graphs of culprit versus highest nonculprit coronary 68Ga-DOTATATE TBRmax values in patients (n = 8) with ACS and stented culprit ACS lesions (n = 6) versus stable stented (n = 18) lesions (I). ROC analysis demonstrates good diagnostic accuracy of 68Ga-DOTATATE for culprit coronary lesions (J). Note stable stented lesions are coronary stents that were inserted >3 months prior to PET imaging in all but 1 patient. AUC = area under curve; mTBRmax = mean of maximum tissue-to-blood ratios; ROC = receiver operating characteristic; other abbreviations as in Figures 1 and 2.

Figure 4

Coronary PET Inflammation Imaging: High-Risk CT Features (A) X-ray and (D) CT coronary angiograms of a 67-year-old man with stable angina, showing minor LCx atheroma (hatched oval) with spotty calcification ([inset] *calcium scan) and calcified plaque in the LAD artery. Although 68Ga-DOTATATE PET (B, E) allows unimpeded interpretation of inflammation in the LCx lesion (B, arrow), and lack of signal in the LAD, coronary [18F]FDG imaging is obscured by patchy myocardial tracer uptake (C). Graphs compare 68Ga-DOTATATE (F) with [18F]FDG (G) coronary TBRmax values by CT plaque morphology in coronary segments (68Ga-DOTATATE: NCP or MP, n = 86; normal, n = 45; spotty calcium, n = 30; large calcium, n = 72; LA or PR, n = 11; no high-risk CT, n = 186; [18F]FDG: NCP or MP, n = 43; normal, n = 13; spotty calcium, n = 15; large calcium n = 14; LA or PR, n = 4; no high-risk CT, n = 66), and ROC analysis demonstrating good diagnostic accuracy for high-risk coronary lesions. LA = low attenuation; LAD = left anterior descending; LCx = left circumflex; NCP = noncalcified plaque; MP = mixed plaque; PR = positive remodeling; other abbreviations in Figures 1, 2, and 3.

Figure 5

Carotid PET Inflammation Imaging: TIA/Stroke Views from a 66-year-old man ([top] axial plane) and a 70-year-old man ([bottom] sagittal plane), both of whom had TIAs resulting from right internal carotid artery lesions, shown on CT (A [hatched circle], D [∗]), with intense culprit plaque inflammation (hatched circles/arrows) detected by 68Ga-DOTATATE (B, E) and reproduced by [18F]FDG (C, F). Graphs compare culprit versus nonculprit 68Ga-DOTATATE (G) and [18F]FDG (H) TBRmax values (68Ga-DOTATATE: culprit n = 14; contralateral n = 14; nonculprit [unstable CVD] n = 31; nonculprit [stable CVD] n = 24; contralateral [TIA/stroke] n = 14; diseased stable CVD n = 19; [18F]FDG: culprit n = 13; contralateral n = 13; nonculprit [unstable CVD] n = 31; nonculprit [stable CVD] n = 24; contralateral [TIA/stroke] n = 13; diseased [stable CVD] n = 19). TIA = transient ischemic attack; other abbreviations as in Figures 1, 2, and 3.

Figure 6

Vascular Inflammation Versus Clinical Risk Factors Graphs show correlations of vascular inflammation detected by 68Ga-DOTATATE (A to C) and [18F]FDG (D) versus clinical cardiovascular disease risk factors. (Carotid arteries n = 62; aortas, n = 38; note data from patients not taking statins [n = 4] were excluded to control for this variable). Abbreviations as in Figure 1.

Figure 7

68Ga-DOTATATE Versus [18F]FDG-Defined Inflammation and Other Clinical Factors Graphs show (A) the strong correlations among coronary, carotid, and aortic 68Ga-DOTATATE mTBRmax versus [18F]FDG mTBRmax (n = 123 mean arterial values per tracer); (B) carotid 68Ga-DOTATATE mTBRmax grouped by FRS (<8%, n = 16; 8% to 16%, n = 14; >16%, n = 32); (C) negative correlation of coronary aortic mTBRmax versus calcium score in patients with CAC <400 (n = 19); and (D) carotid 68Ga-DOTATATE TBRmax ROI values in non TIA/stroke patients grouped by statin dosages (n = 20 patients [14 ROIs per artery]; low-dose n = 4; moderate dose, n = 9; high-dose, n = 7). Log transformed CAC values used to account for the non-normal distribution in the general population. Median difference between low-dose vs. high-dose: −0.13; (95% CI: −0.17 to −0.016); p = 0.02; %Δ −8.65. Low dose: simvastatin, 20 mg or lower; high-dose: atorvastatin, 80 mg or equivalent; moderate dose: all other dosages. CAC = coronary artery calcium score; FRS = %10-year Framingham risk score; ROI = region of interest; TIA = transient ischemic attack.

Figure 8

68Ga-DOTATATE Ligand Binding to Macrophage SST2 in Carotid Plaque In vivo CT angiography views of culprit carotid artery (hatched oval = internal carotid artery) in axial (A) and sagittal (E) views, with corresponding fused 68Ga-DOTATATE PET-CT (B). Ex vivo views of macrographic images of the explanted carotid specimen (I, hatched line signifies location of carotid section); phosphor autoradiographic image shows the total binding of 68Ga-DOTATATE to SST2 receptors in macrophages within a transverse carotid section (C) corresponding to the level shown in clinical images. Adjacent section was incubated with 68Ga-DOTATATE and cold competing ligand (D) showing very low levels of nonspecific binding. Brightfield photomicrographs show brown immunoreactive SST2 staining (G, J, M) of macrophages identified with the panmacrophage marker CD68 (H, K, N), colocalized SST2(brown), and CD68 (blue) staining in the same section (L); Movat’s pentachrome stain (F).

Figure 9

Carotid SSTR2/CD68 mRNA Versus In Vivo 68Ga-DOTATATE Activity Graphs show correlations of SSTR2 versus CD68 mRNA within ex vivo carotid plaques measured by quantitative PCR (A); SSTR2(B) and CD68(C) mRNA versus corresponding in vivo 68Ga-DOTATATE TBRmax values measured from clinical images (n = 6). Representative photomicrograph shows red SST2 and green CD68 fluorescent immunoreactive staining of macrophages within carotid plaque (D), with blue nuclear DAPI staining. Note presence of both double positive (+) and double negative (−) staining indicating high cell specificity (E).

Central Illustration

Comparison Between 68Ga-DOTATATE and [18F]FDG Coronary PET Inflammation Imaging Images from a 57-year old man with acute coronary syndrome who presented with deep anterolateral T-wave inversion (arrow) on electrocardiogram (A) and serum troponin-I concentration elevated at 4,650 ng/l (NR: <17 ng/l). Culprit left anterior descending artery stenosis (dashed oval) was identified by X-ray angiography (B). After the patient underwent percutaneous coronary stenting (C), residual coronary plaque (*inset) with high-risk morphology (low attenuation and spotty calcification) is evident on CT angiography (D, E). Use of 68Ga-DOTATATE PET (F, H, I) clearly detected intense inflammation in this high-risk atherosclerotic plaque/distal portion of the stented culprit lesion (arrow) and recently infarcted myocardium (*). In contrast, using [18F]FDG PET (G, J), myocardial spillover completely obscures the coronary arteries. CT = computed tomography; [18F]FDG = fluorine-18-labeled fluorodeoxyglucose; 68Ga-DOTATATE = gallium-68-labeled DOTATATE; PET = positron emission tomography.