Recent Advances in Visualizing Vulnerable Plaque: Focus on Noninvasive Molecular Imaging

Molecular imaging research in atherosclerosis: A 23-year scientometric and visual analysis

Background: Cardiovascular and cerebrovascular diseases are major global health problems, and the main cause is atherosclerosis. Recently, molecular imaging has been widely employed in the diagnosis and therapeutic applications of a variety of diseases, including atherosclerosis. Substantive facts have announced that molecular imaging has broad prospects in the early diagnosis and targeted treatment of atherosclerosis. Objective: We conducted a scientometric analysis of the scientific publications over the past 23 years on molecular imaging research in atherosclerosis, so as to identify the key progress, hotspots, and emerging trends. Methods: Original research and reviews regarding molecular imaging in atherosclerosis were retrieved from the Web of Science Core Collection database. Microsoft Excel 2021 was used to analyze the main findings. CiteSpace, VOSviewer, and a scientometric online platform were used to perform visualization analysis of the co-citation of journals and references, co-occurrence of keywords, and collaboration between countries/regions, institutions, and authors. Results: A total of 1755 publications were finally included, which were published by 795 authors in 443 institutions from 59 countries/regions. The United States was the top country in terms of the number and centrality of publications in this domain, with 810 papers and a centrality of 0.38, and Harvard University published the largest number of articles (182). Fayad, ZA was the most productive author, with 73 papers, while LIBBY P had the most co-citations (493). CIRCULATION was the top co-cited journal with a frequency of 1,411, followed by ARTERIOSCL THROM VAS (1,128). The co-citation references analysis identified eight clusters with a well-structured network (Q = 0.6439) and highly convincing clustering (S = 0.8865). All the studies calculated by keyword co-occurrence were divided into five clusters: "nanoparticle," "magnetic resonance imaging," "inflammation," "positron emission tomography," and "ultrasonography". Hot topics mainly focused on cardiovascular disease, contrast media, macrophage, vulnerable plaque, and microbubbles. Sodium fluoride ⁃PET, targeted drug delivery, OCT, photoacoustic imaging, ROS, and oxidative stress were identified as the potential trends. Conclusion: Molecular imaging research in atherosclerosis has attracted extensive attention in academia, while the challenges of clinical transformation faced in this field have been described in this review. The findings of the present research can inform funding agencies and researchers toward future directions.

Cellular Uptake of Plain and SPION-Modified Microbubbles for Potential Use in Molecular Imaging

Introduction

Both diagnostic ultrasound (US) and magnetic resonance imaging (MRI) accuracy can be improved by using contrast enhancement. For US gas-filled microbubbles (MBs) or silica nanoparticles (SiNPs), and for MRI superparamagnetic or paramagnetic agents, contribute to this. However, interactions of MBs with the vascular wall and cells are not fully known for all contrast media.

Methods

We studied the in vitro interactions between three types of non-targeted air-filled MBs with a polyvinyl-alcohol shell and murine macrophages or endothelial cells. The three MB types were plain MBs and two types that were labelled (internally and externally) with superparamagnetic iron oxide nanoparticles (SPIONs) for US/MRI bimodality. Cells were incubated with MBs and imaged by microscopy to evaluate uptake and adhesion. Interactions were quantified and the MB internalization was confirmed by fluorescence quenching of non-internalized MBs.

Results

Macrophages internalized each MB type within different time frames: plain MBs 6 h, externally labelled MBs 25 min and internally labelled MBs 2 h. An average of 0.14 externally labelled MBs per cell were internalized after 30 min and 1.34 after 2 h; which was 113% more MBs than the number of internalized internally labelled MBs. The macrophages engulfed these three differently modified new MBs at various rate, whereas endothelial cells did not engulf MBs.

Conclusions

Polyvinyl-alcohol MBs are not taken up by endothelial cells. The MB uptake by macrophages is promoted by SPION labelling, in particular external such, which may be important for macrophage targeting.

Molecular imaging of fibrosis using a novel collagen-binding peptide labelled with 99mTc on SPECT/CT

Fibrosis, closely related to chronic various diseases, is a pathological process characterised by the accumulation of collagen (largely collagen type I). Non-invasive methods are necessary for the diagnosis and follow-up of fibrosis. This study aimed to develop a collagen-targeted probe for the molecular imaging of fibrosis. We identified CPKESCNLFVLKD (CBP1495) as an original collagen-binding peptide using isothermal titration calorimetry and enzyme-linked immunosorbent assay. CBP1495 effectively bound to collagen type I (K _d = 861 nM) and (GPO)₉ (K _d = 633 nM), a collagen mimetic peptide. Western blot and histochemistry validated CBP1495 targeting collagen in vitro and ex vivo. (Gly-(D)-Ala-Gly-Gly) was introduced to CBP1495 for coupling ^99mTc. Labelling efficiency of ^99mTc-CBP1495 was 95.06 ± 1.08 %. The physico-chemical properties, tracer kinetics and biodistribution of ^99mTc-CBP1495 were carried out, and showed that the peptide stably chelated ^99mTc in vitro and in vivo. SPECT/CT imaging with ^99mTc-CBP1495 was performed in rat fibrosis models, and revealed that ^99mTc-CBP1495 significantly accumulated in fibrotic lungs or livers of rats. Finally, ^99mTc-CBP1495 uptake and hydroxyproline (Hyp), a specific amino acid of collagen, were quantitatively analysed. The results demonstrated that ^99mTc-CBP1495 uptake was positvely correlated with Hyp content in lungs (P < 0.0001, r ² = 0.8266) or livers (P < 0.0001, r ² = 0.7581). Therefore, CBP1495 is a novel collagen-binding peptide, and ^99mTc-labelled CBP1495 may be a promising radiotracer for the molecular imaging of fibrosis.

Recent Advances of Radionuclide-Based Molecular Imaging of Atherosclerosis

Atherosclerosis is a systemic disease characterized by the development of multifocal plaque lesions within vessel walls and extending into the vascular lumen. The disease takes decades to develop symptomatic lesions, affording opportunities for accurate detection of plaque progression, analysis of risk factors responsible for clinical events, and planning personalized treatment. Of the available molecular imaging modalities, radionuclidebased imaging strategies have been favored due to their sensitivity, quantitative detection and pathways for translational research. This review summarizes recent advances of radiolabeled small molecules, peptides, antibodies and nanoparticles for atherosclerotic plaque imaging during disease progression.

Targeting of vascular cell adhesion molecule-1 by 18F-labelled nanobodies for PET/CT imaging of inflamed atherosclerotic plaques

AIMS

Positron emission tomography-computed tomography (PET-CT) is a highly sensitive clinical molecular imaging modality to study atherosclerotic plaque biology. Therefore, we sought to develop a new PET tracer, targeting vascular cell adhesion molecule (VCAM)-1 and validate it in a murine atherosclerotic model as a potential agent to detect atherosclerotic plaque inflammation.

METHODS AND RESULTS

The anti-VCAM-1 nanobody (Nb) (cAbVCAM-1-5) was radiolabelled with Fluorine-18 ((18)F), with a radiochemical purity of >98%. In vitro cell-binding studies showed specific binding of the tracer to VCAM-1 expressing cells. In vivo PET/CT imaging of ApoE(-/-) mice fed a Western diet or control mice was performed at 2h30 post-injection of [(18)F]-FB-cAbVCAM-1-5 or (18)F-control Nb. Additionally, plaque uptake in different aorta segments was evaluated ex vivo based on extent of atherosclerosis. Atherosclerotic lesions in the aortic arch of ApoE(-/-) mice, injected with [(18)F]-FB-anti-VCAM-1 Nb, were successfully identified using PET/CT imaging, while background signal was observed in the control groups. These results were confirmed by ex vivo analyses where uptake of [(18)F]-FB-cAbVCAM-1-5 in atherosclerotic lesions was significantly higher compared with control groups. Moreover, uptake increased with the increasing extent of atherosclerosis (Score 0: 0.68 ± 0.10, Score 1: 1.18 ± 0.36, Score 2: 1.49 ± 0.37, Score 3: 1.48 ± 0.38%ID/g, Spearman's r(2) = 0.675, P < 0.0001). High lesion-to-heart, lesion-to-blood, and lesion-to-control vessel ratios were obtained (12.4 ± 0.4, 3.3 ± 0.4, and 3.1 ± 0.6, respectively).

CONCLUSION

The [(18)F]-FB-anti-VCAM-1 Nb, cross-reactive for both mouse and human VCAM-1, allows non-invasive PET/CT imaging of VCAM-1 expression in atherosclerotic plaques in a murine model and may represent an attractive tool for imaging vulnerable atherosclerotic plaques in patients.