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.

Conflict of interest statement

Funding Support and Author Disclosures This work was funded by grants to Dr Tarkin from the Wellcome Trust (Clinical Research Career Development Fellowship 211100/Z/18/Z), the National Institute for Health Research (NIHR) Imperial Biomedical Research Centre (BRC); and the British Heart Foundation (BHF) (Clinical Research Training Fellowship for Dr Ćorović [FS/CRTF/20/24035]). This work was additionally supported by the Cambridge BHF Centre of Research Excellence (18/1/34212) and the Cancer Research UK Cambridge Centre (A25177). For the purpose of open access, the lead author has applied a CC BY public copyright license to any Author Accepted Manuscript. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR, or the Department of Health and Social Care. Dr Nus; authors Imaz and Lambert; Dr Frontini (FS/18/53/33863); Dr Davenport (TG/18/4/33770); and Drs Huang, Mallat, Dweck, Newby, and Bennett are supported by the BHF. Author Zulcinski is supported by the European Union’s Horizon 2020 Research and Innovation Programme (Marie Skłodowska-Curie grant agreement no. 813545). Drs Jayne, Rassl, and Graves are supported by the NIHR Cambridge BRC. Dr Fayad is supported by the National Institutes of Health/National Heart, Lung, and Blood Institute (R01HL135878). Dr Reynolds is supported by the Wellcome Trust. Dr Morgan is supported by the Medical Research Council (MRC) (MR/N011775/1), the NIHR Leeds BRC, the NIHR Leeds Medtech, and In Vitro Diagnostics Co-operative as well as an NIHR Senior Investigator award. Dr Aboagye acknowledges support from Imperial Experimental Cancer Research Centre and MRC (MR/J007986/1, MR/N020782/1); and is an inventor on the patent that developed the (18)F-FET-βAG-TOCA radiotracer. Dr Peters is supported by a UK Research and Innovation Fellowship at Health Data Research UK (MR/S004068/2). Dr Rudd is partly supported by the NIHR Cambridge BRC, the BHF, the Higher Education Funding Council for England, the Engineering and Physical Sciences Research Council, and the Wellcome Trust. Drs Gopalan, Maughan, Pericleous, Barwick, Aboagye, Peters, and Mason acknowledge support from the NIHR Imperial BRC. The authors have reported that they have no relationships relevant to the contents of this paper to disclose.

Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.

Figures

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 TBRmax. (E) Receiver-operating characteristic analyses of SST2 PET mean TBRmax and mdsTBRmax 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; TBRmax = 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 TBRmax. (D) Receiver-operating characteristic analysis of SST2 PET mean TBRmax 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 TBRmax, 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 (TBRmax) 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 TBRmax 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.