Catherine Ghezzi

Nanobodies Targeting Mouse/Human VCAM1 for the Nuclear Imaging of Atherosclerotic Lesions

Rationale: A noninvasive tool allowing the detection of vulnerable atherosclerotic plaques is highly needed. By combining nanomolar affinities and fast blood clearance, nanobodies represent potential radiotracers for cardiovascular molecular imaging. Vascular cell adhesion molecule-1 (VCAM1) constitutes a relevant target for molecular imaging of atherosclerotic lesions. Objective: We aimed to generate, radiolabel, and evaluate anti-VCAM1 nanobodies for noninvasive detection of atherosclerotic lesions. Methods and Results: Ten anti-VCAM1 nanobodies were generated, radiolabeled with technetium-99m, and screened in vitro on mouse and human recombinant VCAM1 proteins and endothelial cells and in vivo in apolipoprotein E–deficient (ApoE−/−) mice. A nontargeting control nanobody was used in all experiments to demonstrate specificity. All nanobodies displayed nanomolar affinities for murine VCAM1. Flow cytometry analyses using human human umbilical vein endothelial cells indicated murine and human VCAM1 cross-reactivity for 6 of 10 nanobodies. The lead compound cAbVCAM1-5 was cross-reactive for human VCAM1 and exhibited high lesion-to-control (4.95±0.85), lesion-to-heart (8.30±1.11), and lesion-to-blood ratios (4.32±0.48) (P<0.05 versus control C57Bl/6J mice). Aortic arch atherosclerotic lesions of ApoE−/− mice were successfully identified by single-photon emission computed tomography imaging. 99mTc-cAbVCAM1-5 binding specificity was demonstrated by in vivo competition experiments. Autoradiography and immunohistochemistry further confirmed cAbVCAM1-5 uptake in VCAM1-positive lesions. Conclusions: The 99mTc-labeled, anti-VCAM1 nanobody cAbVCAM1-5 allowed noninvasive detection of VCAM1 expression and displayed mouse and human cross-reactivity. Therefore, this study demonstrates the potential of nanobodies as a new class of radiotracers for cardiovascular applications. The nanobody technology might evolve into an important research tool for targeted imaging of atherosclerotic lesions and has the potential for fast clinical translation.

[123I]-6-deoxy-6-iodo-d-glucose (6DIG): A potential tracer of glucose transport

A glucose analogue labelled with iodine-123 in position 6 has been synthesized: [123I]-6-deoxy-6-iodo-D-glucose (6DIG). The aim of this study was to examine its biological behaviour in order to assess whether it could be used to evaluate glucose transport with SPECT. To establish whether 6DIG enters the cells using the glucose transporter, four biological models have been used: human erythrocytes in suspension, neonatal rat cardiomyocytes in culture, isolated perfused rat hearts, and biodistribution in mice. 6DIG competed with D-glucose to enter the cells and its entry was increased by insulin and inhibited in the presence of cytochalasin B. The biological behaviour of 6DIG was similar to that of 3-O-methyl-D-glucose. 6DIG is a tracer of glucose transport which is very promising for clinical studies.

Pre-clinical and clinical evaluation of nuclear tracers for the molecular imaging of vulnerable atherosclerosis: an overview

Cardiovascular diseases (CVD) are the leading cause of mortality worldwide. Despite major advances in the treatment of CVD, a high proportion of CVD victims die suddenly while being apparently healthy, the great majority of these accidents being due to the rupture or erosion of a vulnerable coronary atherosclerotic plaque. A non-invasive imaging methodology allowing the early detection of vulnerable atherosclerotic plaques in selected individuals prior to the occurrence of any symptom would therefore be of great public health benefit. Nuclear imaging could allow the identification of vulnerable patients by non-invasive in vivo scintigraphic imaging following administration of a radiolabeled tracer. The purpose of this review is to provide an overview of radiotracers that have been recently evaluated for the detection of vulnerable plaques together with the biological rationale that initiated their development. Radiotracers targeted at the inflammatory process seem particularly relevant and promising. Recently, macrophage targeting allowed the experimental in vivo detection of atherosclerosis using either SPECT or PET. A few tracers have also been evaluated clinically. Targeting of apoptosis and macrophage metabolism both allowed the imaging of vulnerable plaques in carotid vessels of patients. However, nuclear imaging of vulnerable plaques at the level of coronary arteries remains challenging, mostly because of their small size and their vicinity with unbound circulating tracer. The experimental and pilot clinical studies reviewed in the present paper represent a fundamental step prior to the evaluation of the efficacy of any selected tracer for the early, non-invasive detection of vulnerable patients.

Experimental models, protocols, and reference values for evaluation of iodinated analogues of glucose.

For an iodinated analogue of glucose to be useful for evaluating glucose uptake using single-photon emission computed tomography (SPECT), it must enter the cell via the same transporter as glucose and accumulate within the cell without being degraded. The biological behavior of the iodinated tracer must therefore be similar to that of 2-deoxy-D(-)[1-14C]-glucose (2-DG). In the present study, four experimental models (biodistribution in mouse, isolated rat heart, human erythrocytes in suspension and cultured neonatal rat cardiomyocytes) have been chosen and protocols have been set up which allow for the examination of small quantities of iodinated analogues of glucose. The uptakes of 2-DG and of L(-)[1-14C]-glucose have been measured in these models to establish reference values which will be compared with uptake values for iodinated analogues of glucose.

Early pre-diabetic state alters adaptation of myocardial glucose metabolism during ischemia in rats.

Pre-diabetic subjects with high insulin secretory capacity have double risk of cardiovascular disease compared with subjects who do not develop insulin-resistance. It is well established that the ability of the myocardium to increase its glycolytic ATP production plays a crucial role in determining cell survival under conditions of ischemia. Up to now, whether the pre-diabetic state reduces the tolerance of the heart to ischemia by affecting its ability to increase its energy production through glycolysis remains unknown. The aim of the present study was to assess whether insulin resistance affects the ability of the myocardium to increase glycolysis under ischemic conditions. Male Wistar rats were fed for 8 weeks a fructose-enriched (33%) diet to induce a pre-diabetic state. Hearts were isolated and subjected to ex-vivo low-flow (2%) ischemia for 30 min. The fructose diet increased sarcolemmal GLUT4 localisation in myocardial cells under basal conditions compared with controls. This effect was not accompanied by increased glucose utilisation. Ischemia induced the translocation of GLUT4 to the plasma membrane in controls but did not significantly modify the distribution of these transporters in pre-diabetic hearts. Glycolytic flux under ischemic conditions was significantly lower in fructose-fed rat hearts compared with controls. The reduction of glycolytic flux during ischemia in fructose-fed rat hearts was not due to metabolic inhibition downstream hexokinase II since no cardiac accumulation of glucose-6-phosphate was detected. In conclusion, our results suggest that the pre-diabetic state reduces the tolerance of the myocardium to ischemia by decreasing glycolytic flux adaptation.

Assessing Low Levels of Mechanical Stress in Aortic Atherosclerotic Lesions From Apolipoprotein E−/− Mice—Brief Report

Objective—Despite the fact that mechanical stresses are well recognized as key determinants for atherosclerotic plaque rupture, very little is known about stress amplitude and distribution in atherosclerotic lesions, even in the standard apolipoprotein E (apoE)−/− mouse model of atherosclerosis. Our objectives were to combine immunohistology, atomic force microscopy measurements, and finite element computational analysis for the accurate quantification of stress amplitude and distribution in apoE−/− mouse aortic atherosclerotic lesions. Methods and Results—Residual stresses and strains were released by radially cutting aortic arch segments from 7- to 30-week-old pathological apoE−/− (n=25) and healthy control mice (n=20). Immunohistology, atomic force microscopy, and biomechanical modeling taking into account regional residual stresses and strains were performed. Maximum stress values were observed in the normal arterial wall (276±71 kPa), whereas low values (<20 kPa) were observed in all plaque areas. Stress distribution was not correlated to macrophage infiltration. Conclusion—Low mechanical stress amplitude was observed in apoE−/− mouse aortic atherosclerotic lesions. This original study provides a basis for further investigations aimed at determining whether low stress levels are responsible for the apparently higher stability of murine aortic atherosclerotic lesions.

Cellular Uptake Mechanisms of 99mTcN-NOET in Cardiomyocytes From Newborn Rats Calcium Channel Interaction

BACKGROUND Bis[N-ethoxy,N-ethyl(dithiocarbamato)]nitrido Tc (V) (TcN-NOET) is a new technetium complex proposed as a tracer of myocardial perfusion. However, its cellular uptake mechanisms are unknown, although membrane localization on rat heart preparations and preferential binding to polymorphonuclear neutrophils (PMNs) have been reported. Because of the central role of calcium in PMN actions, a relationship was hypothesized between this ion flux and TcN-NOET cellular uptake. METHODS AND RESULTS The mechanisms of cellular uptake of TcN-NOET were investigated in newborn rat cardiomyocytes by study of the effect of calcium channel modulators on tracer binding. Nifedipine had no effect on tracer uptake at 1 minute. However, verapamil 0.1 micromol/L and diltiazem 0.5 micromol/L induced a 40% decrease in uptake. Conversely, Bay K 8644 0.25 micromol/L increased TcN-NOET uptake by 73%. Alterations in other membrane ion transports failed to modify tracer uptake, indicating the specificity of the relationship between TcN-NOET uptake and calcium channels. Kinetic studies indicated that cellular net accumulation of the tracer was slow (t1/2=28.5 minutes) and retention was prolonged (84% of initial activity retained after 120 minutes of washout). The energy dependence of TcN-NOET uptake was investigated after 60 minutes of metabolic inhibition by iodoacetic acid plus rotenone. The ATP decrease was not associated with reduction in tracer uptake at 1 minute (114.9+/-21.9% of control, P=NS). CONCLUSIONS The decrease in uptake observed with verapamil and diltiazem, the increase with Bay K 8644, and the lack of effect with nifedipine suggest that TcN-NOET binds to L-type calcium channels in the open configuration, without entering cardiomyocytes. The kinetics of TcN-NOET accumulation and retention are slow, and the mechanism for cellular uptake is not energy-dependent. From a clinical point of view, the effect of concurrent treatment by calcium inhibitors on myocardial binding of TcN-NOET should be taken into account.

Multiscale investigation of USPIO nanoparticles in atherosclerotic plaques and their catabolism and storage in vivo

The storage and catabolism of Ultrasmall SuperParamagnetic Iron Oxide (USPIO) nanoparticles were analyzed through a multiscale approach combining Two Photon Laser Scanning Microscopy (TPLSM) and High-Resolution Transmission Electron Microscopy (HRTEM) at different times after intravenous injection in an atherosclerotic ApoE(-/-) mouse model. The atherosclerotic plaque features and the USPIO heterogeneous biodistribution were revealed down from organs scale to subcellular level. The biotransformation of the nanoparticle iron oxide (maghemite) core into ferritin, the non-toxic form of iron storage, was demonstrated for the first time ex vivo in atherosclerotic plaques as well as in spleen, the iron storage organ. These results rely on an innovative spatial and structural investigation of USPIOs catabolism in cellular phagolysosomes. This study showed that these nanoparticles were stored as non-toxic iron compounds: maghemite oxide or ferritin, which is promising for MRI detection of atherosclerotic plaques in clinics using these USPIOs. From the Clinical Editor: Advance in nanotechnology has brought new contrast agents for clinical imaging. In this article, the authors investigated the use and biotransformation of Ultrasmall Super-paramagnetic Iron Oxide (USPIO) nanoparticles for analysis of atherosclerotic plagues in Two Photon Laser Scanning Microscopy (TPLSM) and High-Resolution Transmission Electron Microscopy (HRTEM). The biophysical data generated from this study could enable the possible use of these nanoparticles for the benefits of clinical patients.

Biodistribution, Stability, and Blood Distribution of the Cell Penetrating Peptide Maurocalcine in Mice

Maurocalcine (MCa) is the first natural cell penetrating peptide to be discovered in animal venom. In addition to the fact that it represents a potent vector for the cell penetration of structurally diverse therapeutic compounds, MCa also displays several distinguishing features that make it a potential peptide of choice for clinical and biotechnological applications. The aim of the present study was to gain new information about the properties of MCa in vivo in order to delineate the future potential applications of this vector. For this purpose, two analogues of this peptide with (Tyr-MCa) and without (Lin-Tyr-MCa) disulfide bridges were synthesized, radiolabeled with 125I, and their in vitro stabilities were first evaluated in mouse blood. The results indicated that 125I-Tyr-MCa was stable in vitro and that the disulfide bridges conferred a competitive advantage for the stability of peptide. Following in vivo injection in mice, 125I-Tyr-MCa targeted peripheral organs with interesting quantitative differences and the main route of peptide elimination was renal.

Periaortic Brown Adipose Tissue as a Major Determinant of [18F]-Fluorodeoxyglucose Vascular Uptake in Atherosclerosis-Prone, ApoE−/− Mice

Background [18F]-fluorodeoxyglucose (FDG) has been suggested for the clinical and experimental imaging of inflammatory atherosclerotic lesions. Significant FDG uptake in brown adipose tissue (BAT) has been observed both in humans and mice. The objective of the present study was to investigate the influence of periaortic BAT on apolipoprotein E-deficient (apoE−/−) mouse atherosclerotic lesion imaging with FDG. Methods ApoE−/− mice (36±2 weeks-old) were injected with FDG (12±2 MBq). Control animals (Group A, n = 7) were injected conscious and kept awake at room temperature (24°C) throughout the accumulation period. In order to minimize tracer activity in periaortic BAT, Group B (n = 7) and C (n = 6) animals were injected under anaesthesia at 37°C and Group C animals were additionally pre-treated with propranolol. PET/CT acquisitions were performed prior to animal euthanasia and ex vivo analysis of FDG biodistribution. Results Autoradiographic imaging indicated higher FDG uptake in atherosclerotic lesions than in the normal aortic wall (all groups, P<0.05) and the blood (all groups, P<0.01) which correlated with macrophage infiltration (R = 0.47; P<0.001). However, periaortic BAT uptake was either significantly higher (Group A, P<0.05) or similar (Group B and C, P = NS) to that observed in atherosclerotic lesions and was shown to correlate with in vivo quantified aortic FDG activity. Conclusion Periaortic BAT FDG uptake was identified as a confounding factor while using FDG for the non-invasive imaging of mouse atherosclerotic lesions.