Targeting Chemokine Receptor CXCR4 and Translocator Protein for Characterization of High-Risk Plaque in Carotid Stenosis Ex Vivo

Feasibility of in vivo Imaging of Fibroblast Activation Protein in Human Arterial Walls

Increased expression of fibroblast activating protein (FAP) in fibrous caps may contribute to progression of atherosclerotic plaques. Methods: Forty-one patients who underwent gallium-68-conjugated quinoline-based FAP inhibitor (⁶⁸Ga-FAPI-04) PET/CT for non-cardiovascular indications were retrospectively analyzed. Correlations were assessed between the uptake of ⁶⁸Ga-FAPI-04 in large arterial walls (SUV_max and target-to-background ratio, TBR) and degree of calcification and cardiovascular risk factors. Results: Focal arterial uptake of ⁶⁸Ga-FAPI-04 or calcification was detected in 1,177 arterial segments in all 41 patients. TBR was negatively correlated with the degree of calcification (HU) (r = -0.27, P < 0.01). Mean TBR in higher-risk patients was greater than lower-risk patients (2.2 ± 0.3 vs. 1.8 ± 0.3, P < 0.01). Immunohistochemical labeling of carotid plaques exhibited prominent FAP expression in a thin fibrous cap and moderate FAP expression in a thick cap. Conclusion: ⁶⁸Ga-FAPI-04 PET/CT might have potential for imaging fibroblastic activation in the arterial wall.

Distinct systemic cytokine networks in symptomatic and asymptomatic carotid stenosis

Inflammatory processes are crucial in atherosclerosis and atherothrombosis. This study aimed to identify a cytokine-pattern that is associated with plaque-vulnerability or symptomatic state in comprehensively investigated patients with symptomatic (sCS) and asymptomatic carotid stenosis (aCS). Twenty-two patients with sCS and twenty-four patients with aCS undergoing carotid endarterectomy (CEA) were considered. A cytokine-panel was measured in plasma-specimens prior to surgery and at a 90 day follow-up. Doppler-ultrasound detecting microembolic signals (MES) in the ipsilateral middle cerebral artery was performed. Carotid plaques were analysed regarding histopathological criteria of plaque-vulnerability and presence of chemokine receptor CXCR4. Correction for multiple comparisons and logistic regression analysis adjusting for vascular risk factors, grade of stenosis, antithrombotic and statin pretreatment were applied. In sCS-patients higher plasma-levels of Fractalkine (CX₃CL1), IFN-α2, IL-1β, IL-2, IL-3, IL-7 were found compared to aCS-patients. CXCR4-expression on inflammatory cells was more evident in sCS- compared to aCS-plaques and was associated with vulnerability-criteria. In contrast, plasma-cytokine-levels were not related to CXCR4-expression or other vulnerability-criteria or MES. However, in both groups distinct inter-cytokine correlation patterns, which persisted at follow-up and were more pronounced in the sCS-group could be detected. In conclusion, we identified a distinct cytokine/chemokine-network in sCS-patients with elevated and closely correlated mediators of diverse functions.

Nonstenotic Carotid Plaque in Embolic Stroke of Undetermined Source: Interplay of Arterial and Atrial Disease

BACKGROUND AND PURPOSE

Approximately one-sixth of all ischemic strokes are attributable to embolic stroke of undetermined source (ESUS). Recent analyses suggest that atrial cardiopathy and nonstenotic carotid plaque (nsCP) may represent 2 distinct underlying causes in patients with ESUS, although both diseases share common risk factors and are pathophysiologically intertwined. In this study, we, therefore, aimed to search for associations between nsCP and markers of atrial remodeling and function in patients with embolic stroke.

METHODS

Sixty-eight patients with ESUS or atrial fibrillation (AF)-related stroke proven by imaging who underwent comprehensive echocardiographic studies, including measurements of left atrial function and remodeling, were considered. Patients with ESUS underwent a follow-up of at least 1 year after index stroke. For 20 patients with ESUS, NT-proBNP (N-terminal pro-B-type natriuretic peptide) values were available. Presence of nsCP was evaluated considering Duplex sonography and computed tomography angiography and was further categorized in possibly or probably symptomatic nsCP.

RESULTS

ESUS patients with nsCP tended to have higher values of septal and lateral total atrial conduction times (P=0.071 and P=0.072, respectively), left atrial volume index (P=0.077), and revealed significantly higher strain rates during early diastole (P=0.013) as well as higher NT-proBNP values (P=0.010) than ESUS patients without nsCP. Moreover, septal total atrial conduction time was significantly longer in ESUS patients with possibly symptomatic nsCP compared with those without (P=0.015). Comparison of ESUS with AF patients revealed significantly higher proportions of nsCP (P=0.010), possibly symptomatic nsCP (P=0.037), and probably symptomatic nsCP (P=0.036) in patients with atrial fibrillation-related stroke. In the regression analysis adjusted for vascular risk factors probably symptomatic nsCP remained significantly associated with AF (P=0.048, odds ratio: 4.46 [95% CI, 1.02-19.56]).

CONCLUSIONS

Presence of nsCP is associated with AF and markers of left atrial disease in patients with embolic stroke. Therefore, a thorough evaluation regarding atrial cardiopathy and AF in patients with ESUS should not be restricted if nsCP are found, even if high-risk plaque characteristics are evident.

A new class of PentixaFor- and PentixaTher-based theranostic agents with enhanced CXCR4-targeting efficiency

Non-invasive PET imaging of CXCR4 expression in cancer and inflammation as well as CXCR4-targeted radioligand therapy (RLT) have recently found their way into clinical research by the development of the theranostic agents [⁶⁸Ga]PentixaFor (cyclo(D-Tyr¹-D-[NMe]Orn²(AMBS-[⁶⁸Ga]DOTA)-Arg³-Nal⁴-Gly⁵) = [⁶⁸Ga]DOTA-AMBS-CPCR4) and [¹⁷⁷Lu/⁹⁰Y]PentixaTher (cyclo(D-3-iodo-Tyr¹-D-[NMe]Orn²(AMBS-[¹⁷⁷Lu/⁹⁰Y]DOTA)-Arg³-Nal⁴-Gly⁵) = [¹⁷⁷Lu/⁹⁰Y]DOTA-AMBS-iodoCPCR4). Although convincing clinical results have already been obtained with both agents, this study was designed to further investigate the required structural elements for improved ligand-receptor interaction for both peptide cores (CPCR4 and iodoCPCR4). To this aim, a series of DOTA-conjugated CPCR4- and iodoCPCR4-based ligands with new linker structures, replacing the AMBA-linker in PentixaFor and PentixaTher, were synthesized and evaluated. Methods: The in vitro investigation of the novel compounds alongside with the reference peptides PentixaFor and PentixaTher encompassed the determination of hCXCR4 and mCXCR4 affinity (IC₅₀) of the respective ^natGa-, ^natLu-, ^natY- and ^natBi-complexes in Jurkat and Eμ-myc 1080 cells using [¹²⁵I]FC-131 and [¹²⁵I]CPCR4.3 as radioligands, respectively, as well as the evaluation of the internalization and externalization kinetics of selected ⁶⁸Ga- and ¹⁷⁷Lu-labeled compounds in hCXCR4-transfected Chem-1 cells. Comparative small animal PET imaging studies (1h p.i.) as well as in vivo biodistribution studies (1, 6 and 48h p.i.) were performed in Daudi (human B cell lymphoma) xenograft bearing CB17 SCID mice. Results: Based on the affinity data and cellular uptake studies, [⁶⁸Ga/¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 and [⁶⁸Ga/¹⁷⁷Lu]DOTA-r-a-ABA-iodoCPCR4 (with r-a-ABA = D-Arg-D-Ala-4-aminobenzoyl-) were selected for further evaluation. Both analogs show app. 10-fold enhanced hCXCR4 affinity compared to the respective references [⁶⁸Ga]PentixaFor and [¹⁷⁷Lu]PentixaTher, four times higher cellular uptake in hCXCR4 expressing cells and improved cellular retention. Unfortunately, the improved in vitro binding and uptake characteristics of [⁶⁸Ga]DOTA-r-a-ABA-CPCR4 and -iodoCPCR4 could not be recapitulated in initial PET imaging studies; both compounds showed similar uptake in the Daudi xenografts as [⁶⁸Ga]PentixaFor, alongside with higher background accumulation, especially in the kidneys. However, the subsequent biodistribution studies performed for the corresponding ¹⁷⁷Lu-labeled analogs revealed a clear superiority of [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 and [¹⁷⁷Lu]DOTA-r-a-ABA-iodoCPCR4 over [¹⁷⁷Lu]PentixaTher with respect to tumor uptake (18.3±3.7 and 17.2±2.0 %iD/g, respectively, at 1h p.i. vs 12.4±3.7%iD/g for [¹⁷⁷Lu]PentixaTher) as well as activity retention in tumor up to 48h. Especially for [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 with its low background accumulation, tumor/organ ratios at 48h were 2- to 4-fold higher than those obtained for [¹⁷⁷Lu]PentixaTher (except for kidney). Conclusions: The in-depth evaluation of a series of novel CPCR4- and iodoCPCR4 analogs with modified linker structure has yielded reliable structure-activity relationships. It was generally observed that a) AMBA-by-ABA-substitution leads to enhanced ligand internalization, b) the extension of the ABA-linker by two additional amino acids (DOTA-Xaa₂-Xaa₁-ABA-) provides sufficient linker length to minimize the interaction of the [M³⁺]DOTA-chelate with the receptor, and that c) introduction of a cationic side chain (Xaa₂) greatly enhances receptor affinity of the constructs, obliterating the necessity for Tyr¹-iodination of the pentapeptide core to maintain high receptor affinity (such as in [¹⁷⁷Lu]PentixaTher). As a result, [¹⁷⁷Lu]DOTA-r-a-ABA-CPCR4 has emerged from this study as a powerful second-generation therapeutic CXCR4 ligand with greatly improved targeting efficiency and tumor retention and will be further evaluated in preclinical and clinical CXCR4-targeted dosimetry and RLT studies.

Diagnostic and Therapeutic Potential of TSPO Studies Regarding Neurodegenerative Diseases, Psychiatric Disorders, Alcohol Use Disorders, Traumatic Brain Injury, and Stroke: An Update

Neuroinflammation and cell death are among the common symptoms of many central nervous system diseases and injuries. Neuroinflammation and programmed cell death of the various cell types in the brain appear to be part of these disorders, and characteristic for each cell type, including neurons and glia cells. Concerning the effects of 18-kDa translocator protein (TSPO) on glial activation, as well as being associated with neuronal cell death, as a response mechanism to oxidative stress, the changes of its expression assayed with the aid of TSPO-specific positron emission tomography (PET) tracers' uptake could also offer evidence for following the pathogenesis of these disorders. This could potentially increase the number of diagnostic tests to accurately establish the stadium and development of the disease in question. Nonetheless, the differences in results regarding TSPO PET signals of first and second generations of tracers measured in patients with neurological disorders versus healthy controls indicate that we still have to understand more regarding TSPO characteristics. Expanding on investigations regarding the neuroprotective and healing effects of TSPO ligands could also contribute to a better understanding of the therapeutic potential of TSPO activity for brain damage due to brain injury and disease. Studies so far have directed attention to the effects on neurons and glia, and processes, such as death, inflammation, and regeneration. It is definitely worthwhile to drive such studies forward. From recent research it also appears that TSPO ligands, such as PK11195, Etifoxine, Emapunil, and 2-Cl-MGV-1, demonstrate the potential of targeting TSPO for treatments of brain diseases and disorders.

Imaging Inflammation in Atherosclerosis with CXCR4-Directed 68Ga-Pentixafor PET/CT: Correlation with 18F-FDG PET/CT

C-X-C motif chemokine receptor 4 (CXCR4) is expressed on the surface of various cell types involved in atherosclerosis, with a particularly rich receptor expression on macrophages and T cells. First pilot studies with ⁶⁸Ga-pentixafor, a novel CXCR4-directed PET tracer, have shown promise to noninvasively image inflammation within atherosclerotic plaques. The aim of this retrospective study was to investigate the performance of ⁶⁸Ga-pentixafor PET/CT for imaging atherosclerosis in comparison to ¹⁸F-FDG PET/CT. Methods: Ninety-two patients (37 women and 55 men; mean age, 62 ± 10 y) underwent ⁶⁸Ga-pentixafor and ¹⁸F-FDG PET/CT for staging of oncologic diseases. In these subjects, lesions in the walls of large arteries were identified using morphologic and PET criteria for atherosclerosis (n = 652). Tracer uptake was measured and adjusted for vascular lumen (background) signal by calculation of target-to-background ratios (TBRs) by 2 investigators masked to the other PET scan. On a lesion-to-lesion and patient basis, the TBRs of both PET tracers were compared and additionally correlated to the degree of arterial calcification as quantified in CT. Results: On a lesion-to-lesion basis, ⁶⁸Ga-pentixafor and ¹⁸F-FDG uptake showed a weak correlation (r = 0.28; P < 0.01). ⁶⁸Ga-pentixafor PET identified more lesions (n = 290; TBR ≥ 1.6, P < 0.01) and demonstrated higher uptake than ¹⁸F-FDG PET (1.8 ± 0.5 vs. 1.4 ± 0.4; P < 0.01). The degree of plaque calcification correlated negatively with both ⁶⁸Ga-pentixafor and ¹⁸F-FDG uptake (r = -0.38 vs. -0.31, both P < 0.00001). Conclusion: CXCR4-directed imaging of the arterial wall with ⁶⁸Ga-pentixafor PET/CT identified more lesions than ¹⁸F-FDG PET/CT, with only a weak correlation between tracers. Further studies to elucidate the underlying biologic mechanisms and sources of CXCR4 positivity, and to investigate the clinical utility of chemokine receptor-directed imaging of atherosclerosis, are highly warranted.