Somatostatin Receptor PET/MR Imaging of Inflammation in Patients With Large Vessel Vasculitis and Atherosclerosis

Abstract

Background: Assessing inflammatory disease activity in large vessel vasculitis (LVV) can be challenging by conventional measures.

Objectives: We aimed to investigate somatostatin receptor 2 (SST₂) as a novel inflammation-specific molecular imaging target in LVV.

Methods: In a prospective, observational cohort study, in vivo arterial SST₂ expression was assessed by positron emission tomography/magnetic resonance imaging (PET/MRI) using ⁶⁸Ga-DOTATATE and ¹⁸F-FET-βAG-TOCA. Ex vivo mapping of the imaging target was performed using immunofluorescence microscopy; imaging mass cytometry; and bulk, single-cell, and single-nucleus RNA sequencing.

Results: Sixty-one participants (LVV: n = 27; recent atherosclerotic myocardial infarction of ≤2 weeks: n = 25; control subjects with an oncologic indication for imaging: n = 9) were included. Index vessel SST₂ maximum tissue-to-blood ratio was 61.8% (P < 0.0001) higher in active/grumbling LVV than inactive LVV and 34.6% (P = 0.0002) higher than myocardial infarction, with good diagnostic accuracy (area under the curve: ≥0.86; P < 0.001 for both). Arterial SST₂ signal was not elevated in any of the control subjects. SST₂ PET/MRI was generally consistent with ¹⁸F-fluorodeoxyglucose PET/computed tomography imaging in LVV patients with contemporaneous clinical scans but with very low background signal in the brain and heart, allowing for unimpeded assessment of nearby coronary, myocardial, and intracranial artery involvement. Clinically effective treatment for LVV was associated with a 0.49 ± 0.24 (standard error of the mean [SEM]) (P = 0.04; 22.3%) reduction in the SST₂ maximum tissue-to-blood ratio after 9.3 ± 3.2 months. SST₂ expression was localized to macrophages, pericytes, and perivascular adipocytes in vasculitis specimens, with specific receptor binding confirmed by autoradiography. SSTR2-expressing macrophages coexpressed proinflammatory markers.

Conclusions: SST₂ PET/MRI holds major promise for diagnosis and therapeutic monitoring in LVV. (PET Imaging of Giant Cell and Takayasu Arteritis [PITA], NCT04071691; Residual Inflammation and Plaque Progression Long-Term Evaluation [RIPPLE], NCT04073810).

Keywords: Takayasu arteritis; atherosclerosis; giant cell arteritis; inflammation; molecular imaging; somatostatin receptor.

Graphical abstract

Figure 1

Patient Cohorts and Tissue Samples Flowchart summarizing the patient cohorts and arterial samples included in the study. Participants with active and inactive LVV were enrolled from 2 hospital sites. PET/MRI scans from patients with LVV were compared to those of patients with recent MI enrolled in a parallel study and control subjects from a prior oncology trial. Sources of arterial samples and RNAseq data from patients with GCA, carotid atherosclerosis, and nondiseased control arteries used to evaluate target expression of somatostatin receptor 2 in LVV are as shown. DOTATATE = DOTA-(Tyr3)-octreotate; FET = fluoroethyltriazole; GCA = giant cell arteritis; LVV = large vessel vasculitis; MI = myocardial infarction; MRI = magnetic resonance imaging; PET = positron emission tomography; RNAseq = RNA sequencing.

Figure 2

Clinical LVV Activity Aortic SST₂ PET/MRI signals (white arrows) in TAK patients grouped by clinical disease activity: (A) active disease (treatment naive at baseline imaging) with arterial thickening (asterisk), (B) grumbling disease with left subclavian occlusion (asterisk) and brachiocephalic stenosis, and (C) inactive disease with no aortic thickening. Contemporaneous ¹⁸F-fluorodeoxyglucose (FDG) PET images (black arrows). (D) Quantitative comparisons of mean and most diseased segment SST₂ PET TBR_max. (E) Receiver-operating characteristic analyses of SST₂ PET mean TBR_max and mdsTBR_max for differentiating active/grumbling from inactive LVV. Image scale bars indicate standardized uptake value; error bars inD indicate median (IQR). ¹⁸F-FDG = ¹⁸F-fluorodeoxyglucose; AUC = area under the curve; CT = computed tomography; F = female; m = mean; mds = most diseased segment; MR = magnetic resonance; sens = sensitivity; spec = specificity; SST₂ = somatostatin receptor 2; TAK = Takayasu arteritis; TBR_max = maximum tissue-to-blood ratio; other abbreviations as in Figure 1.

Figure 3

Vasculitis vs Atherosclerotic Inflammation Aortic SST₂ PET/MRI signals (white arrows) in patients with (A) active GCA (treatment naive at baseline imaging) and aortic thickening (asterisk) and (B) recent MI with aortic atherosclerosis (asterisk) and inferior infarction (arrowhead; note the infarct-related myocardial PET uptake). Contemporaneous ¹⁸F-FDG PET images in A show aortic uptake (black arrows). (C) Quantitative comparison of aortic mean SST₂ PET TBR_max. (D) Receiver-operating characteristic analysis of SST₂ PET mean TBR_max for differentiating active/grumbling from inactive LVV. Image scale bars indicate SUV; error bars in C indicate median (IQR). M = male; other abbreviations as in Figures 1 and 2.

Figure 4

Varied Patterns of LVV Involvement SST₂ PET/MRI signals (white arrows) in (A) the aortic root and left main coronary artery with adjacent periaortic thickening (asterisk) and ¹⁸F-FDG PET uptake (black arrow) in a patient with active TAK, (B) the basal inferolateral left ventricular myocardium in a patient with inactive TAK and subclinical myocarditis confirmed by mid-wall late gadolinium enhancement (asterisk) and increased T₂-edema signal (asterisk), (C) the thickened intracranial portion of the right internal carotid artery (asterisk) in a patient with TAK and previous stroke in whom ¹⁸F-FDG failed to detect this abnormality, and (D) the right glenohumeral joint in a patient with GCA and polymyalgia rheumatica symptoms. Note that there is a reduction in PET signal intensity following immunosuppressive treatment in C and D. Image scale bars indicate SUV. LGE = late gadolinium enhancement; other abbreviations as in Figures 1 and 2.

Figure 5

Repeat Imaging Baseline and follow-up images from patients with (A) newly diagnosed active GCA (treatment naive at baseline imaging) and (B) active GCA who, of their own volition, remained off treatment during the study because of side effects from prednisolone and methotrexate (subsequently well controlled with tocilizumab). SST₂ PET/MRI shows resolution in aortic inflammation (white arrows) after treatment and no change with lack of treatment. Contemporaneous (pretreatment) ¹⁸F-FDG PET images showing similar aortic uptake (black arrows) to SST₂ PET. (C to E) Graphs showing changes in baseline vs follow-up clinical disease activity grading, index vessel mean SST₂ TBR_max, CRP, and ESR in (C) patients with any escalation in treatment, (D) those with no treatment change, and (E) those who received tocilizumab as part of their therapy regime. Note that the patients in E are a subset of those in C. Further clinical data for each patient who underwent repeat imaging are provided in Supplemental Table 4. Image scale bars indicate SUV. BL = baseline; CRP = C-reactive protein; ESR = erythrocyte sedimentation rate; FU = follow-up; other abbreviations as in Figures 1, 2, and 3.

Figure 6

RNA Sequencing Plots of (A) bulk, (B) single-cell, and (C, D) single-nucleus RNAseq data from temporal arteries and a carotid atherosclerotic specimen, showing SSTR2 expression localized to populations of macrophages and pericytes. Gene-level expression data displayed as normalized counts per million in A confirm that SSTR2 is the dominant somatostatin receptor subtype expressed in temporal arteritis. In B, labels 1 to 8 indicate data from individual patients. GCA = giant cell arteritis; VSMC = vascular smooth muscle cell.

Figure 7

SST₂ Staining and Autoradiography in Temporal Arteritis (A, B) Histologic images from patients with temporal arteritis showing immunofluorescence SST₂ costaining in CD68⁺ macrophages (arrow), as well as cells with the morphologic appearance of pericytes (arrowhead), with corresponding hematoxylin and eosin slides. (C) Control artery shows no SST₂ staining. (D) Autoradiographic images and (E) quantitative autoradiographic data confirm higher specific binding of ⁶⁸Ga-DOTATATE to SST₂ receptors in temporal arteritis specimens than control arteries. For A and B, patients had received prednisolone 40 mg for 20 days and 10 days, respectively, before undergoing temporal artery biopsy. Abbreviations as in Figures 1, 2, and 3.

Figure 8

Localization of SST₂ in Temporal Arteritis Using Imaging Mass Cytometry (A, B) Histologic images from patients with temporal arteritis performed using imaging mass cytometry showing costaining of SST₂ with clusters of macrophages (CD68⁺/CD80⁺/CD206⁺, arrows) and pericytes (αSMA⁺/NG2⁺, dashed arrows) around neovessels in the periadventitia as well as periadventitial cells with adipocyte morphology (asterisk), with related hematoxylin and eosin slides shown. (C) In contrast, there is minimal SST₂ staining in the control artery and perivascular tissue. Sections in A are from the same patient as in Figure 6B. M = male.

Central Illustration

SST₂ PET/MRI in LVV: In Vivo Imaging and Ex Vivo Target Mapping Patients with large vessel vasculitis (LVV) and recent atherosclerotic myocardial infarction (MI) underwent somatostatin receptor 2 (SST₂) positron emission tomography/magnetic resonance imaging (PET/MRI) in a prospective observational cohort study. In parallel, ex vivo mapping of the imaging target was performed using RNA sequencing, histology, and autoradiography. The research methods and main study findings are summarized. Arterial SST₂ signal (arrow) measured by the maximum tissue-to-background ratio (TBR_max) using PET/MRI accurately differentiated patients with active/grumbling LVV from those with inactive LVV and recent MI, as well as control subjects. There was also a strong correlation between SST₂ mean TBR_max and para-aortic thickening in LVV patients. SST₂ expression was identified in macrophages (dashed arrow), pericytes, and perivascular adipocytes with arterial specimens from patients with LVV. DOTATATE = DOTA-(Tyr3)-octreotate; FET = fluoroethyltriazole; IF = immunofluorescence.