Aug 6, 2011 - Quantification of Brain Function and Neurotransmission System In Vivo by ..... School of Biosciences, Brain Repair Group, Cardiff University.
Chapter 9 Quantification of Brain Function and Neurotransmission System In Vivo by Positron Emission Tomography: A Review of Technical Aspects and Practical Considerations in Preclinical Research Nadja Van Camp, Yann Bramoullé, and Philippe Hantraye Abstract Unlike many other imaging techniques, positron emission tomography, PET, necessitates gathering a broad array of competences: biologists/physicians have to interact with physicists, chemists and mathematicians to acquire and analyze PET data. The ensemble of a PET imaging experiment, from the creation of the isotope to the interpretation of the imaging data, requires a unique combination of high-tech equipment to be installed preferentially on the same site, such as: a cyclotron, a radiochemistry laboratory, a PET camera, an animal facility and a data treatment and storage facility. In the material and methods chapter for each process, we discuss the requirements of the equipment and the usual procedures, from the creation of the isotope to the modelling of the PET data. Depending on the animal model (mouse, rat, non-human primate, …) or the isotope (11C, 18F, …) used, the challenges and requisites for setting up a PET imaging experiment will be different. The notes section discusses some important considerations on animal handling in PET imaging and the basic experimental set-up to evaluate the characteristics of a radiotracer. This chapter is concluded with some practical examples related to Parkinson disease and neuroin flammation. Key words: Positron emission tomography, PET, Radiochemistry, Radiotracer, Brain imaging, Macaque, Rodents
1. Introduction Preclinical and clinical researches in neurodegenerative diseases have focused recently on the development of interventions that aimed at either halting or slowing down the neurodegenerative processes involved in these disorders. These new therapeutic approaches have stimulated the search for objective markers that Emma L. Lane and Stephen B. Dunnett (eds.), Animal Models of Movement Disorders: Volume I, Neuromethods, vol. 61, DOI 10.1007/978-1-61779-298-4_9, © Springer Science+Business Media, LLC 2011
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would help to assess (ideally to quantify) changes in disease progression potentially triggered by these innovative treatments. In this respect, the contribution of positron emission tomography (PET) imaging, a nuclear imaging technique that provides a versatile means of examining a large variety of molecular/cellular processes involved in normal and pathological cerebral function, has been essential. Relying on the use of radiolabelled molecules (radiotracers) that after systemic administration diffuse freely in the body to selectively bind to specific biological targets (neuroreceptors, enzymes, false transmitters, transporters, intracellular molecules, etc. …), the technique is currently used in various aspects of neuroscience, neuropharmacology and the validation of new therapies (including drug candidates) by academic research institutions, clinical units or pharmaceutical companies. The enthusiasm around the use of this technique is probably due to the fact that PET bears several advantages over other imaging techniques: in addition to its great versatility, PET imaging benefits from a remarkable sensitivity enabling the detection of nanomolar concentrations of a radiotracer. Nevertheless, the technique also requires heavy infrastructures to be brought into play, such as cyclotron facilities, radiochemistry laboratories equipped with shielded hot cells and automated chemical units for radioligand preparation, PET tomographs and a complete multidisciplinary array of human competences, at least including chemists, physicists, biologists, physicians and image processing specialists. Despite the fact that PET carries radio-security issues both for the subject and the experimenter limiting its ease of use and increasing the cost of the technique, there are currently hundreds of radiolabelled compounds of pharmacological and clinical interest ready to be used on the shelf. This makes PET the most “translational” imaging method in neurosciences with numerous successful cases of radiotracer translation from preclinical validation studies to clinical research applications. For many years reserved for non-human primate use because of the relatively low intrinsic spatial resolution of the available PET detectors, recent technological advances in the physics of PET and the design of dedicated high-resolution microbe tomographs for rodent use have given scientists access to a vast array of new applications. What is now referred to as “in vivo molecular imaging” paves the way for a myriad of new applications of the technique not only in biology and preclinical studies, but more importantly for translational applications in medicine and industry. The execution of PET imaging experiments, from the production of the radioisotope to the interpretation of the biological data, requires the teamwork and coordination of scientists with different but complementary skills in order to orchestrate the succession of protocols applied on a unique combination of the high-tech equipment.
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This chapter deals, thus, with the general principles and instrumentation issues of PET imaging as well as with the general methods to produce a radiotracer to set up an imaging experiment and extract biologically relevant measures (receptor density, receptor affinity, enzymatic reaction rate, etc.) from the raw radioactivity events acquired in list mode by the PET scanner.
2. Materials 2.1. Radiochemistry 2.1.1. Properties of a Radiotracer
A major challenge in the production of radiotracers for brain imaging studies is the ability of the radioligand to pass the blood brain barrier (BBB), but not its radiometabolites, which is related to different factors. First, the lipophilicity of the radiotracer should be moderate: it should be sufficiently high to ensure a good cerebral uptake, but not too high to ensure a low non-specific binding. A high lipophilicity also favours binding to blood proteins and, thus, reduces the fraction of radiotracer freely available in the plasma (1). The most commonly used index of compound lipophilicity is logP, where P is the n-octanol/water partition coefficient of the unionized species. At a pH of 7.4, logP should be between +0.5 and +3.0 and optimally equally to +2.0. To ensure a negligible brain uptake of the radioactive metabolites, radiometabolites should preferentially be hydrophilic. Second, passive entry into the brain is promoted by low molecular weight (500 Da), a small crosssectional area (
