[68Ga]Pentixafor PET/MR imaging of chemokine receptor 4 expression in the human carotid artery

Abstract

Purpose: Type 4 chemokine receptor (CXCR4) plays an important role in immune cell migration during the atherosclerosis progression. We aimed to evaluate [⁶⁸Ga]Pentixafor positron emission tomography (PET) in combination magnetic resonance imaging (MRI) for in vivo quantification of CXCR4 expression in carotid plaques.

Methods: Seventy-two patients with lymphoma were prospectively scheduled for whole body [⁶⁸Ga]Pentixafor PET/MRI with an additional T2-weighted carotid sequence. Volumes of interest (VOIs) were drawn along the carotid bifurcation regions, and the maximum tissue-to-blood ratios (TBR) of [⁶⁸Ga]Pentixafor uptake were calculated. Lesions were categorized into non-eccentric (n = 27), mild eccentric (n = 67), moderately (n = 41) and severely (n = 19) eccentric carotid atherosclerosis. A different cohort of symptomatic patients (n = 10) with carotid stenosis scheduled for thrombendarterectomy (TEA) was separately imaged with 3T MRI with dedicated plaque sequences (time of flight, T1-, and T2-weighted). MRI findings were correlated with histochemical assessment of intact carotid plaques.

Results: At hybrid PET/MRI, we observed significantly increased [⁶⁸Ga]Pentixafor uptake in mildly (mean TBR_max = 1.57 ± 0.27, mean SUV_max = 2.51 ± 0.39), moderately (mean TBR_max = 1.64 ± 0.37, mean SUV_max = 2.61 ± 0.55) and severely eccentric carotids (mean TBR_max = 1.55 ± 0.26, mean SUV_max = 2.40 ± 0.44) as compared to non-eccentric carotids (mean TBR_max = 1.29 ± 0.21, mean SUV_max = 1.77 ± 0.42) (p ≤ 0.05). Histological findings from TEA confirmed that prominent CXCR4 expression was localized within inflamed atheromas and preatheromas. Co-localization of cellular CXCR4 and CD68 expression in the plaque was observed by immunofluorescence staining.

Conclusions: In vivo evaluation of CXCR4 expression in carotid atherosclerotic lesions is feasible using [⁶⁸Ga]Pentixafor PET/MRI. In atherosclerotic plaque tissue, CXCR4 expression might be used as a surrogate marker for inflammatory atherosclerosis.

Keywords: Atherosclerosis; CXCR4; Carotid artery; PET/MRI; [68Ga]Pentixafor.

Fig. 1

Study flowchart. MALT, mucosa-associated lymphoid tissue; SUV, standardized uptake value; TBR, target to background ratio; TOF, time of flight

Fig. 2

Example transaxial [⁶⁸Ga]Pentixafor PET/MRI images of carotid lesions within different groups. Focal uptake was observed in a mildly atherosclerotic carotid artery showing a slightly eccentric thickening (Group 1) and in a moderately (Group 3) and severely (Group 4) atherosclerotic carotid showing significant eccentric thickening, increased tracer uptake was absent at non-significantly eccentric control carotid (Group 1). Arrows indicate the arterial regions of interest

Fig. 3

a and b [⁶⁸Ga]Pentixafor uptake ratios (mean of TBR_max and SUV_max) of categorized atherosclerotic lesions. In group 1, non-obstructive carotids (N = 27), uptake was significantly lower (*p < 0.05) compared to group 2 (mildly eccentric carotid atherosclerosis, N = 67), group 3 (moderately eccentric carotid atherosclerosis, N = 41) and group 4 (severely eccentric carotid atherosclerosis, N = 19). There was no significant difference between other groups. c Linear relationship between TBR_max and SUV_max in all lesions (Pearson’s r = 0.72, *p < 0.01). d The percentages (r) of CXCR4+ plaque area/ total plaque area in cross-sections were assessed in categorized carotid lesions as 9.08 ± 0.74% for fibroatheroma (N = 2), 16.95 ± 2.01% for preatheroma (N = 3) and 28.58 ± 3.83% for inflamed atheroma (N = 5). * Significance level of p ≤ 0.05

Fig. 4

Multi-sequenced MRI of the internal carotid arteries, including time-of-flight (TOF), T1-, and T2-weighted protocols. Corresponding histology (collagen Masson’s Trichrome stain) and immunohistochemistry (CD68 and CXCR4) examination of excised carotid plaques were compared with MRI. Arrows indicate the carotid plaque regions of interests in cross-section views, which were correlated with immunohistochemistry results. Relative lower CD68 expression and CXCR4 expression was observed within fibrous tissue (first column) and increased CXCR4 expression along with increased infiltrated monocytes/macrophages, as indicated by CD68, was detected in preatheroma (second column) during early plaque formation, particularly prominent in atheroma with thin fibrotic cap (third column)

Fig. 5

Representative immunofluorescence staining of an excised carotid plaque. CXCR4 expression (green) in excised carotid plaque, CD3 staining (left red) for expression of T cells, CD68 staining (left red) for expression of macrophages and nuclear counterstaining with DAPI (blue), 4′,6-diamidino-2-phenylindole. CXCR4 expression was partly co-localized with CD3-positive cells and mostly co-localized with CD68-positive cells