Imaging of chemokine receptor CXCR4 expression in culprit and nonculprit coronary atherosclerotic plaque using motion-corrected [⁶⁸Ga]pentixafor PET/CT

The C-X-C chemokine receptor 4 (CXCR4) is a transmembrane G-protein-coupled receptor that plays a pivotal role in recruitment of immune and progenitor cells to injured and inflamed tissue via interaction with its ligand CXCL12 [1, 2]. In atherosclerosis, the CXCL12/CXCR4 axis exerts atherogenic, prothrombotic, and plaque-destabilizing effects [3, 4]. CXCR4 is highly expressed by monocytes, differentiated macrophages and lymphocytes migrating into arterial lesions [4‐6], and also by platelets [7]. In porcine models of coronary injury, CXCR4⁺ leucocytes have been shown to enter the injured tissue [6]. Furthermore, CXCR4 is also expressed by different cell types including smooth muscle cell progenitors and endothelial progenitor cells which contribute to plaque evolution [4]. Accordingly, CXCR4 may be a useful target for noninvasive molecular imaging, e.g. to determine the degree of inflammation, the likelihood of lesion progression or repair, and the effect of potential targeted therapies in injured atherosclerotic plaques.

To this end, combined PET/CT has emerged as a well-characterized imaging technique to assess plaque biology via mechanisms such as increased metabolism and microcalcification [8‐10]. Recently, a promising CXCR4-specific ligand, [⁶⁸Ga]pentixafor [11], has been introduced for clinical molecular imaging of CXCR4 expression. [⁶⁸Ga]Pentixafor identifies myocardial inflammation early after acute myocardial infarction [12, 13], and first both experimental and clinical studies support its use to determine vessel wall CXCR4 expression [14‐16]. However, the use of PET/CT for imaging coronary vessels is complicated by the small target size and by blurring from respiratory and cardiac motion. Techniques for motion correction have been proposed to overcome these challenges [17], but have only recently become available for routine clinical application.

Supported by histological verification of the molecular target in plaque specimens, we speculated that CXCR4-targeted PET/CT combined with novel motion-correction techniques may enable in vivo detection of CXCR4-expressing cells in coronary atherosclerotic plaque in the clinical setting. We tested this specific hypothesis in a retrospective analysis of patients who had undergone PET early after reperfusion for acute myocardial infarction, where a spectrum of coronary plaques ranging from culprit injured to nonculprit lesions are readily available as imaging targets.

This study supports the feasibility of targeted PET for visualization of CXCR4 expression in coronary artery lesions. It further establishes [⁶⁸Ga]pentixafor as a molecular imaging marker of CXCR4 expression after vessel wall injury. Clinical PET/CT, combined with motion-correction techniques, identified upregulated CXCR4 signal in culprit coronary lesions early after myocardial infarction and stent-based reperfusion. However, it also identified a CXCR4 signal in subgroups of stented nonculprit lesions, and in nonstented nonculprit lesions, suggesting that both native plaque inflammation and stenting-induced vascular injury can be detected and add up to the signal from culprit lesions. These clinical data are supported by work-up of tissue samples ex vivo, which confirmed CXCR4 expression in advanced atherosclerotic plaque and localized it mainly to inflammatory cells.

The CXCL12/CXCR4 axis plays a crucial role in leucocyte trafficking throughout the body [4]. Monocytes and macrophages express CXCR4 and are recruited via gradients of CXCL12 released at sites of tissue injury [26, 27]. The CXCR4/CXCL12 axis also participates in the rapid release of neutrophils from bone marrow during inflammation [28]. Marked infiltration of culprit atherosclerotic lesions by neutrophils in acute coronary syndromes has been reported [29]. Moreover, previous studies have revealed progressive accumulation of CXCR4⁺ cells during plaque progression, with the highest expression in unstable plaque [30]. In addition, vascular injury increases CXCL12 expression in plasma and vessel wall [8, 27], mediated by apoptosis of intimal cells [31]. Importantly, CXCR4 is also expressed by endothelial and smooth muscle cells in the arterial wall [4], indicating that other cell types make a relevant contribution to the in vivo CXCR4 signal [32]. Although the vast majority of CXCR4⁺ cells in the immunofluorescence analyses in this study were CD68⁺ macrophages, we also observed CD68⁻ CXCR4⁺ cells which may represent other CXCR4⁺ inflammatory cells, but also other cells, e.g. vascular smooth muscle cells. However, their precise nature could not be determined using the applied methodology.

Analysis of various subgroups of coronary lesions in the present study suggests that the increased CXCR4 signal in culprit coronary lesions represents a mixture of both inflammation of a vulnerable plaque that led to rupture and subsequent infarction, as well as inflammation in response to later stent-based reperfusion-related vessel wall injury. Consistently, stented nonculprit lesions also demonstrated upregulated CXCR4 signal, albeit significantly lower than in culprit stented lesions. Intriguingly, the motion-corrected imaging approach also identified foci of CXCR4 upregulation in nonstented nonculprit coronary lesions. This signal was again significantly lower than in culprit stented lesions and, given the detection limits of PET/CT, probably represents only the most intensely CXCR4⁺ native coronary regions. However, the findings are consistent with ex vivo demonstration of an increasing number of CXCR4⁺ cells within the vascular wall as atherosclerotic lesions progress, and they are also consistent with prior intravascular ultrasound observations of several high-risk plaque ruptures at sites other than the culprit lesion in acute coronary syndromes [33, 34].

Indeed, recent preclinical and clinical studies have demonstrated that elevated CXCR4 expression can be detected using [⁶⁸Ga]pentixafor PET in most parts of the arterial tree in both histologically stable and unstable plaque [14, 15]. Interestingly, the CXCR4 PET signal in injured coronary plaque tended to increase with time after reperfusion in the early time-frame covered in this study, and it differed from the temporarily elevated CXCR4 signal in damaged myocardium, spleen and bone marrow, which peaks within 3–5 days and then declines as a function of the postinfarct myocardial and systemic inflammatory response [12]. Along with the detectable CXCR4 signal in stented nonculprit lesions, this observation might be a result of differences in the evolution of inflammatory processes in the myocardium and plaque over time, but also suggests a relevant contribution of stent-induced injury to the signal. Future studies including patients before stent-based reperfusion are needed to determine the relative contribution of native plaque inflammation versus stent-induced injury to the measured composite CXCR4 signal. The current study provides proof-of-principle that CXCR4-targeted imaging of CXCR4⁺ coronary plaque is feasible, and it thereby lays the foundation for such future projects. Speculatively, [⁶⁸Ga]pentixafor coronary PET/CT may even be useful for guiding antiinflammatory treatment or even treatment targeting CXCR4. This has been suggested, for example, to reduce neointimal lesion size, smooth muscle progenitor cell mobilization and neointimal proliferation after experimental arterial injury [35], and noninvasive detection of the presence of the target may be helpful for clinical translation of such interventions.

Besides the novel molecular target, our study also included motion correction as a technical innovation in PET that is highly relevant for noninvasive coronary artery imaging. The detection of small lesions on PET is generally complicated by three-dimensional image blurring introduced by the finite spatial resolution of the imaging system and by image sampling on a voxel grid that does not match the actual contours of the tracer distribution [36]. Because PET acquisition is not fast enough to enable breath-hold imaging, respiratory motion along with intrinsic cardiac motion causes further blurring. Together with the limited spatial resolution, this leads to deviation of the apparent signal intensity from true values as a result of partial volume effects. Motion correction can significantly reduce blurring by accounting for both respiration-induced and cardiac movement [20]. Of note, the detectability of culprit coronary lesions, which provided the strongest signal, was still low when using ungated images, but increased significantly when using motion correction. Dual correction of cardiac and respiratory motion yielded the best results. Relatively small differences in SUV_max between different gating approaches led to relatively high differences in detection rates. This may be explained by the fact that blood-pool signal is usually relatively high using [⁶⁸Ga]pentixafor PET (SUV_mean 1.8, IQR 1.7–2.0) and the lesion to blood-pool signal ratio may be relatively low. Therefore, relatively small increases in signal intensity may lead to markedly higher detection rates (e.g. the mean of differences for dual-gated images compared to ungated images was 0.28). It is also important to keep in mind that localization of culprit lesions was guided by CT (i.e. stents), and not by the PET signal itself. Therefore, knowing precisely which lesion to assess may allow a lesion with even a small increase in tracer intensity to be judged as visually [⁶⁸Ga]pentixafor-positive when the signal exceeds that of the blood pool.

In addition to gating, we also applied point-spread function modelling and time-of-flight reconstruction techniques (Ultra High-Definition, UHD) which have been shown to considerably improve signal intensity for PET analysis [37]. This study established and evaluated a sophisticated dual-gated imaging approach for coronary artery imaging on UHD PET. By contrast, list-mode data-driven gating did not significantly improve lesion detectability. This approach is based directly on frame-by-frame analysis of PET emission data, without the use of external trigger signals [23, 24]. It requires clear contours of the organ to be tracked, such as the myocardium in case of ¹⁸F-FDG imaging, but such myocardial contours are not visible in [⁶⁸Ga]pentixafor PET images owing to lack of uptake in normal myocardium, which on the other hand is an advantage for delineation of coronary uptake without spillover.

Some limitations of the present study should be acknowledged. Obviously, not all lesions observed on PET in vivo can be verified by histology (which applies to all clinical atherosclerosis studies). However, there are recent clinical studies evaluating the usefulness of pentixafor for CXCR4 imaging in plaque [14, 38], and also studies which have demonstrated that the arterial wall uptake can be blocked, is specific, reproducible, and correlates with the presence of CXCR4⁺ cells [16, 38]. In addition, the ex vivo data presented in this work confirm the presence of CXCR4⁺ cells in the coronary wall, and demonstrate an increasing number of these cells with advancing pathology of lesions, in line with the observed in vivo PET data. Second, the signal might have been enhanced by attenuation correction related to the proximity of a metallic stent. However, the signal seen in stented culprit lesions was also seen on images without -attenuation correction, and was significantly higher than in stented nonculprit lesions, supporting the basic hypothesis of this work, and confirming that the stent signal was not caused by attenuation correction. The precise cell population contributing to the in vivo [⁶⁸Ga]pentixafor signal could not be identified. However, ex vivo data showed upregulated CXCR4 expression in atherosclerotic coronary arteries and/or in carotid arteries from symptomatic subjects. This finding was confirmed by mRNA expression, protein expression and immunofluorescence. Moreover, uptake of [⁶⁸Ga]pentixafor was validated by ex vivo autoradiography in carotid arteries, and immunofluorescence dual-staining of CD68 confirmed CXCR4 expression by leucocytes. In addition, two recent experimental studies investigating carotid arteries [15, 16] have shown that CXCR4 expression in injured plaque is predominantly found on leucocytes, supporting our conclusions.