Detecting Vulnerable Atherosclerotic Plaques by 68Ga-Labeled Divalent Cystine Knot Peptide

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

Early Assessment of Atherosclerotic Lesions and Vulnerable Plaques in vivo by Targeting Apoptotic Macrophages with AV Nanobubbles

Background

The early detection of atherosclerotic lesions is particularly important for risk prediction of acute cardiovascular events. Macrophages apoptosis was significantly associated with the degree of AS lesions and especially contributed to plaque vulnerability. In this research, we mainly sought to explore the feasibility of a home-made AV-nanobubbles (NB_AV) for visualization of apoptotic macrophages and assessment of atherosclerosis (AS) lesions by contrast-enhanced ultrasound (CEUS) imaging.

Methods

NB_AV were prepared by “Optimized Thin-Film Hydration” and “Biotin-Avidin-Biotin” methods. Then, the characterization and echogenicity of NB_AV were measured and analyzed in vitro. The targeting ability of NB_AV to ox-LDL–induced apoptotic macrophages was observed by laser scanning confocal microscope. The ApoE^−/− mice mode fed with high fat diet were observed by high-frequency ultrasound, microanatomy and oil red O staining. CEUS imaging in vivo was performed on AS plaques with NB_AV and NB_Ctrl injection through the tail vein in turn in ApoE^−/− mice. After CEUS imaging, the plaques were confirmed and analyzed by histopathological and immunological assessment.

Results

The prepared NB_AV had a nano-scale size distribution with a low PDI and a negative zeta potential. Moreover, NB_AV showed an excellent stability and exhibited a significantly echogenic signal than saline in vitro. In addition, we found that NB_AV could target apoptotic macrophages induced by ox-LDL. Compared with NB_Ctrl, CEUS imaging of NB_AV showed strong and sustained echo enhancement in plaque area of aortic arch in vivo. Further research showed that NB_AV sensitive plaques presented more significant pathological changes with several vulnerable plaque features and abundant TUNEL-positive area.

Conclusion

NB_AV displayed a sensitive indicator to evaluate apoptotic macrophages, indicating a promising CEUS molecular probe for AS lesions and vulnerable plaques identification.

Identification Markers of Carotid Vulnerable Plaques: An Update

Vulnerable plaques have been a hot topic in the field of stroke and carotid atherosclerosis. Currently, risk stratification and intervention of carotid plaques are guided by the degree of luminal stenosis. Recently, it has been recognized that the vulnerability of plaques may contribute to the risk of stroke. Some classical interventions, such as carotid endarterectomy, significantly reduce the risk of stroke in symptomatic patients with severe carotid stenosis, while for asymptomatic patients, clinically silent plaques with rupture tendency may expose them to the risk of cerebrovascular events. Early identification of vulnerable plaques contributes to lowering the risk of cerebrovascular events. Previously, the identification of vulnerable plaques was commonly based on imaging technologies at the macroscopic level. Recently, some microscopic molecules pertaining to vulnerable plaques have emerged, and could be potential biomarkers or therapeutic targets. This review aimed to update the previous summarization of vulnerable plaques and identify vulnerable plaques at the microscopic and macroscopic levels.

68Ga-Labeled Cystine Knot Peptide Targeting Integrin αvβ6 for Lung Cancer PET Imaging

Integrin α_vβ₆ has been considered as a promising biomarker for lung cancer, and its expression is often related to poor prognosis. An α_vβ₆-binding cystine knot peptide R₀1-MG was previously engineered and validated. Here, we developed a positron emission tomography (PET) probe of R₀1-MG for imaging α_vβ₆-positive lung cancer. Cystine knot peptide R₀1-MG was synthesized through solid-phase peptide synthesis chemistry and radiolabeled with ⁶⁸Ga after being conjugated with 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA). The stability of ⁶⁸Ga-DOTA-R₀1-MG was analyzed in phosphate-buffered saline (PBS) (pH 7.4) and fetal bovine serum (FBS). The cell uptake assay of the probe was evaluated using α_vβ₆-positive (A549 and H1975) and α_vβ₆-negative (H1299) lung cancer cell lines. In addition, small animal PET imaging and biodistribution studies of ⁶⁸Ga-DOTA-R₀1-MG were performed in α_vβ₆-positive and α_vβ₆-negative lung cancer models. Our study showed that ⁶⁸Ga-DOTA-R₀1-MG exhibited excellent stability in PBS and FBS. Small animal PET imaging and biodistribution data revealed that ⁶⁸Ga-DOTA-R₀1-MG displayed rapid and good tumor uptake in animal models with α_vβ₆-positive lung cancer, and the probe was rapidly cleared from the normal tissues, resulting in good tumor-to-normal tissue contrasts. Meanwhile, no obvious tumor uptake of ⁶⁸Ga-DOTA-R₀1-MG was observed in animal models with α_vβ₆-negative lung cancer, demonstrating specific binding of the probe to integrin α_vβ₆. In conclusion, ⁶⁸Ga-DOTA-R₀1-MG has great potential to be a promising PET tracer for imaging α_vβ₆-positive lung cancer.

Omentin-1 Modulates Macrophage Function via Integrin Receptors αvβ3 and αvβ5 and Reverses Plaque Vulnerability in Animal Models of Atherosclerosis

Backgrounds: Omentin-1 is a novel cytokine that is primarily released by the epicardial adipose tissue. Molecular structure analysis revealed that it contained a fibrinogen-like domain. Clinical studies have demonstrated that the expression of omentin-1 is tightly associated with the development of cardiovascular diseases, but the receptor by which omentin-1 modulates macrophage function has not been identified yet. Objective: This study sought to investigate the effect of omentin-1 on already-established atherosclerosis (AS) lesions in both ApoE-/- and Ldlr^-/- mice and further, study its underlying mechanisms. Methods and Results: We investigated the effect of omentin-1 on the plaque phenotype by implanting a minipump in ApoE-/- and Ldlr^-/- mice. In vivo studies showed that the infusion of omentin-1 increased the collagen content and mitigated the formation of the necrotic core in both animal models. Immunohistochemistry and immunofluorescence analysis revealed that omentin-1 suppressed inflammatory cytokines expression, macrophage infiltration, and apoptosis within the plaque. An immunoprecipitation experiment and confocal microscopy analysis confirmed the binding of omentin-1 to the integrin receptors αvβ3 and αvβ5. The cell studies demonstrated that omentin-1 suppressed the apoptosis and inflammatory cytokines expression induced by the oxidized low-density lipoprotein in the macrophage. In addition, omentin-1 promoted the phosphorylation of the integrin-relevant signaling pathway as well as the Akt and AMPK in the macrophage. The addition of the inhibitor of the integrin receptor or interfering with the expression of the integrin subunit αv (ITGAV) both significantly abrogated the bioeffects induced by omentin-1. A flow cytometry analysis indicated that the antibodies against αvβ3 and αvβ5 had a competitive effect on the omentin-1 binding to the cell membrane. Conclusions: The administration of adipokine omentin-1 can inhibit the necrotic cores formation and pro-inflammatory cytokines expression within the AS lesion. The mechanisms may include the suppression of apoptosis and pro-inflammatory cytokines expression in the macrophage by binding to the integrin receptors αvβ3 and αvβ5.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Dual-modality imaging of atherosclerotic plaques using ultrasmall superparamagnetic iron oxide labeled with rhodamine

Aim: The diagnosis of vulnerable atherosclerotic plaques remains challenging. This study labeled ultrasmall superparamagnetic iron oxide with rhodamine (USPIO-R) and evaluated USPIO-R for imaging atherosclerotic plaques. Methods: Apolipoprotein E-deficient mice were fed a high-fat diet and underwent MRI before and after an intravenous injection of USPIO-R. Subsequently, an aortic specimen from the mice was removed and sliced for fluorescence imaging and Prussian blue and immunofluorescent staining. Results: T2 signal loss appeared and persisted in the aortic plaque postinjection, and spontaneous fluorescence from the plaque was observed. The accumulated mechanism of USPIO-R by plaque was the macrophage internalization by Prussian blue and immunofluorescence. Conclusion: USPIO-R is a promising dual-modality probe for diagnosing and monitoring vulnerable atherosclerotic plaques.