radiosynthesis-quality-control-small-animal-positron-emission

Radiosynthesis, Quality Control, and Small Animal Positron Emission Tomography Imaging of <sup>68</sup>Ga-Labelled Nano Molecules

Small animal Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) imaging techniques are crucial in preclinical ...

University Hospital RWTH Aachen

Maastricht University Medical Center

Maastricht University

In vivo pH imaging has been a field of interest for molecular imaging for many years. This is especially important for determining tumor acidity, an important driving force of tumor invasion and metastasis formation, but also in the process of apoptosis.

Radioiodinated phenylalkyl malonic acid derivatives as pH-sensitive SPECT tracers.

Noninvasive molecular imaging, using positron emission tomography (PET), is an important technique to visualize metabolic processes in vivo. It also allows to visualize the process of apoptosis, by using radiolabeled compounds such as Annexin V, that bind to extracellular phosphatidylserine (PS). This chapter describes the radiosynthesis of (68)Ga-labeled Annexin V and how to noninvasively image apoptosis in vivo.

In Vivo Apoptosis Imaging Using Site-Specifically (68)Ga-Labeled Annexin V.

Atherosclerotic events are usually acute and often strike otherwise asymptomatic patients. Although multiple clinical risk factors have been associated with atherosclerosis, as of yet no further individual prediction can be made as to who will suffer from its consequences based on biomarker analysis or traditional imaging methods like CT, MRI or angiography. Previously, non-invasive imaging with F-fluorodeoxyglucose (F-FDG) PET was shown to potentially fill this niche as it offers high sensitive detection of metabolic processes associated with inflammatory changes in atherosclerotic plaques. However, F-FDG PET imaging of arterial vessels suffers from non-specificity and has still to be proven to reliably identify vulnerable plaques, carrying a high risk of rupture. Therefore, it may be regarded only as a secondary marker for monitoring treatment effects and it does not offer alternative treatment options or direct insight in treatment mechanisms. In this review, an overview is given of the current status and the potential of PET imaging of inflammation and angiogenesis in atherosclerosis in general and special emphasis is given to imaging of α7 nicotinic acetylcholine receptors (α7 nAChRs). Due to the gaps that still exist in our understanding of atherogenesis and the limitations of the available PET tracers, the search continues for a more specific radioligand, able to differentiate between stable atherosclerosis and plaques prone to rupture. The potential role of the α7 nAChR as imaging marker for plaque vulnerability is explored. Today, strong evidence exists that nAChRs are involved in the atherosclerotic disease process. They are suggested to mediate the deleterious effects of the major tobacco component, nicotine, a nAChR agonist. Mainly based on in vitro data, α7 nAChR stimulation might increase plaque burden via increased neovascularization. However, in animal studies, α7 nAChR manipulation appears to reduce plaque size due to its inhibitory effects on inflammatory cells. Thus, reliable identification of α7 nAChRs by in vivo imaging is crucial to investigate the exact role of α7 nAChR in atherosclerosis before any therapeutic approach in the human setting can be justified. In this review, we discuss the first experience with α7 nAChR PET tracers and developmental considerations regarding the "optimal" PET tracer to image vascular nAChRs.

Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis.

Brown adipose tissue (BAT) is present in human adults and the current gold standard to visualize and quantify BAT is [F]FDG PET-CT. However, this method fails to detect BAT under insulin-resistant conditions associated with ageing and weight gain, such as type 2 diabetes. The aim of this study was to develop a novel triglyceride-based tracer for BAT. For this purpose we designed a dual-modal fluorescent/PET fatty acid tracer based on commercially available BODIPY-FL-C, which can be esterified to its correspondent triglyceride, radiolabeled and incorporated into pre-synthesized chylomicron-like particles. BODIPY-FL-C was coupled to 1,2-diolein with a subsequent radiolabeling step resulting in [F]BODIPY-C-triglyceride that was incorporated into chylomicron-like particles. Various quality control steps using fluorescent and radioactive methods were conducted before BAT visualization was tested in mice. Triglyceride synthesis, radiolabeling and subsequent incorporation into chylomicron-like particles was carried out in decent yields. This radiotracer appeared able to visualize BAT in vivo, and the uptake of the radiotracer was stimulated by cold exposure. The here reported method can be used to incorporate radiolabeled triglycerides into pre-synthesized chylomicron-like particles. Our approach is feasible to visualize and quantify the uptake of triglyceride-derived fatty acids by BAT.

[F]BODIPY-triglyceride-containing chylomicron-like particles as an imaging agent for brown adipose tissue in vivo.

Reliable quantification of radioactivity in nuclear medicine is becoming increasingly important in various therapeutic applications requiring a high accuracy of nuclear medicine measuring equipment, such as radionuclide calibrators. In this study the accuracy of four different radionuclide calibrators was assessed for Tc, In, Ga and F for measurement geometries clinically used.

A comparison of four radionuclide dose calibrators using various radionuclides and measurement geometries clinically used in nuclear medicine.

Brown adipose tissue (BAT) is considered as a potential target for combating obesity in humans where active BAT metabolizes glucose and fatty acids as fuel resulting in heat production. Prospective studies in humans have been set up to further study the presence and metabolic activity of BAT mostly using Positron Emission Tomography (PET) imaging in cold-stimulated conditions with the radiolabeled glucose derivative [18F]FDG. However, radiotracers beyond [18F]FDG have been proposed to investigate BAT activity, targeting various aspects of BAT metabolism. It remains questionable which tracer is best suited to detect metabolic BAT activity and to what extent those results correlate with ex vivo metabolic BAT activity.

Characterization of BAT activity in rats using invasive and non-invasive techniques.

In vivo imaging of glucose analogue 2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) via positron emission tomography (PET) is the current gold standard to visualize and assess brown adipose tissue (BAT) activity. However, glucose metabolism is only a part of the metabolic activity of BAT. [F]FDG-PET has been shown in clinical trials to often fail to visualize BAT under insulin-resistant conditions associated with aging and weight gain. We employed a novel developed triglyceride-based tracer to visualize BATs metabolic activity under different temperature conditions as well as under diabetic and obese conditions in preclinical models.

Brown adipose tissue uptake of triglyceride-rich lipoprotein-derived fatty acids in diabetic or obese mice under different temperature conditions.

Personalized molecular radiotherapy based on theragnostics requires accurate quantification of the amount of radiopharmaceutical activity administered to patients both in diagnostic and therapeutic applications. This international multi-center study aims to investigate the clinical measurement accuracy of radionuclide calibrators for 7 radionuclides used in theragnostics: Tc, In, I, I, I, Lu, and Y.

An international multi-center investigation on the accuracy of radionuclide calibrators in nuclear medicine theragnostics.

Vitamin K antagonists (VKA) and non-vitamin K oral antagonist anticoagulants (NOAC) are used in the clinic to reduce risk of thrombosis. However, they also exhibit vascular off-target effects. The aim of this study is to compare VKA and NOAC on atherosclerosis progression and calcification in an experimental setup.

Off-target effects of oral anticoagulants - vascular effects of vitamin K antagonist and non-vitamin K antagonist oral anticoagulant dabigatran etexilate.

Angiogenesis is an active process, regulating new vessel growth, and is crucial for the survival and growth of tumours next to other complex factors in the tumour microenvironment. We present possible molecular imaging approaches for tumour vascularisation and vitality, focusing on radiopharmaceuticals (tracers). Molecular imaging in general has become an integrated part of cancer therapy, by bringing relevant insights on tumour angiogenic status. After a structured PubMed search, the resulting publication list was screened for oncology related publications in animals and humans, disregarding any cardiovascular findings. The tracers identified can be subdivided into direct targeting of angiogenesis (i.e., vascular endothelial growth factor, laminin, and fibronectin) and indirect targeting (i.e., glucose metabolism, hypoxia, and matrix metallo-proteases, PSMA). Presenting pre-clinical and clinical data of most tracers proposed in the literature, the indirect targeting agents are not 1:1 correlated with angiogenesis factors but do have a strong prognostic power in a clinical setting, while direct targeting agents show most potential and specificity for assessing tumour vascularisation and vitality. Within the direct agents, the combination of multiple targeting tracers into one agent (multimers) seems most promising. This review demonstrates the present clinical applicability of indirect agents, but also the need for more extensive research in the field of direct targeting of angiogenesis in oncology. Although there is currently no direct tracer that can be singled out, the RGD tracer family seems to show the highest potential therefore we expect one of them to enter the clinical routine.

Article	Year
Radioiodinated phenylalkyl malonic acid derivatives as pH-sensitive SPECT tracers. PloS one\| PubMed ID: 22719886	2012
In Vivo Apoptosis Imaging Using Site-Specifically (68)Ga-Labeled Annexin V. Methods in molecular biology (Clifton, N.J.)\| PubMed ID: 27108428	2016
Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis. Methods (San Diego, Calif.)\| PubMed ID: 28602809	2017
[F]BODIPY-triglyceride-containing chylomicron-like particles as an imaging agent for brown adipose tissue in vivo. Scientific reports\| PubMed ID: 30804455	2019
A comparison of four radionuclide dose calibrators using various radionuclides and measurement geometries clinically used in nuclear medicine. Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)\| PubMed ID: 31000075	2019
Characterization of BAT activity in rats using invasive and non-invasive techniques. PloS one\| PubMed ID: 31091250	2019
Brown adipose tissue uptake of triglyceride-rich lipoprotein-derived fatty acids in diabetic or obese mice under different temperature conditions. EJNMMI research\| PubMed ID: 33085016	2020
An international multi-center investigation on the accuracy of radionuclide calibrators in nuclear medicine theragnostics. EJNMMI physics\| PubMed ID: 33226485	2020
Off-target effects of oral anticoagulants - vascular effects of vitamin K antagonist and non-vitamin K antagonist oral anticoagulant dabigatran etexilate. Journal of thrombosis and haemostasis : JTH\| PubMed ID: 33687782	2021
Molecular Imaging of Angiogenesis in Oncology: Current Preclinical and Clinical Status. International journal of molecular sciences\| PubMed ID: 34073992	2021

Matthias Bauwens

Biography

JoVE Journal Publications

Other Publications