The alpha2C-adrenoceptor as a neuropsychiatric drug tar-get - PET studies in human subjects

Positron emission tomography imaging has both academic and applied uses in revealing the distribution and density of different molecular targets in the central nervous system. Following the significant progress made with the dopamine D2 receptor, advances have been made in developing PET tracers to allow analysis of receptor occupancy of many other receptor types as well as evaluating changes in endogenous synaptic transmitter concentrations of transmitters e.g. serotonin and noradrenaline. Noradrenergic receptors are divided into α1-, α2- and β-adrenoceptor subfamilies, in humans each of which is composed of three receptor subtypes. The α2-adrenoceptors have an important presynaptic auto-inhibitory function on noradrenaline release but they also have postsynaptic roles in modulating the release of other neurotransmitters, such as serotonin and dopamine. One of the subtypes, the α2C-adrenoceptor, has been detected at distinct locations in the central nervous system, most notably the dorsal striatum. Several serious neurological conditions causing dementia, Alzheimer’s disease and Parkinson’s disease have been linked to disturbed noradrenergic signaling. Furthermore, altered noradrenergic signaling has also been implicated in conditions like ADHD, depression, anxiety and schizophrenia. In order to benefit future research into these central nervous system disorders as well as being useful in the clinical development of drugs affecting brain noradrenergic neurotransmission, validation work of a novel tracer for positron emission tomography studies in humans was performed. Altogether 85 PET imaging experiments were performed during four separate clinical trials. The repeatability of [11C]ORM-13070 binding was tested in healthy individuals, followed by a study to evaluate the dose-dependent displacement of [11C]ORM-13070 from α2C-adrenoceptors by a competing ligand, and the final two studies examined the sensitivity of [11C]ORM-13070 binding to reflect changes in endogenous noradrenaline levels. The repeatability of [11C]ORM-13070 binding was very high. The binding properties of the tracer allowed for a reliable estimation of α2C-AR occupancy by using the reference tissue ratio method with low test-retest variability. [11C]ORM-13070 was dose-dependently displaced from its specific binding sites by the subtype-nonselective α2-adrenoceptor antagonist atipamezole, and thus it proved suitable for use in clinical drug development of novel α2C-adrenoceptor ligands e.g. to determine the best doses and dosing intervals for clinical trials. Convincing experimental evidence was gained to support the suitability of [11C]ORM-13070 for detecting an increase in endogenous synaptic noradrenaline in the human brain. Tracer binding in the thalamus tended to increase in accordance with reduced activity of noradrenergic projections from the locus coeruleus, although statistical significance was not reached. Thus, the investigation was unable to fully validate [11C]ORM-13070 for the detection of pharmacologically evoked reductions in noradrenaline levels.Siirretty Doriast

Development of novel radiotracers as tools for imaging the human brain

Introduction: Brain imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) can be used to study the processes underlying neurological and psychiatric disorders. In addition, in vivo brain imaging using SPECT or PET may provide new approaches for drug target identification, pre-clinical testing and occupancy studies, and therefore improve drug discovery. The utility of in vivo brain imaging using SPECT or PET relies on the ability of different radiotracers (typically organic compounds labelled with radionuclides) to bind to a wide variety of targets, including receptors, transporters and enzymes. Therefore the development of novel radiotracers for in vivo brain imaging using SPECT of PET is of vital importance. This thesis is focused on the process of developing novel radiotracers as tools for imaging the human brain, where the radiotracer discovery and development pipeline is discussed and each step prior to clinical trials investigated. Radiotracer discovery: Previously, discovery of novel brain radiotracers has largely relied on simplistic screening tools. Improved selection methods at the early stages of radiotracer discovery and an increased understanding of the relationships between in vitro physicochemical and in vivo radiotracer properties are needed. This thesis investigated if high performance liquid chromatography (HPLC) methodologies could provide criteria for lead candidate selection by comparing HPLC measurements with radiotracer properties in humans. In this study, ten molecules, previously used as radiotracers in humans, were analysed to obtain the following measures: partition coefficient (Log P); permeability (Pm); percentage of plasma protein binding (%PPB); and membrane partition coefficient (Km). Relationships between brain entry measurements (Log P, Pm and %PPB) and in vivo brain percentage injected dose (%ID); and Km and specific binding in vivo (BPND) were investigated. Results showed that HPLC measurements of Pm, %PPB and Km were potentially useful in predicting in vivo performance and hence allow evaluation and ranking of compound libraries for the selection of lead radiotracer candidates at early stages of radiotracer discovery. The HPLC tool developed provides information on in vivo non-specific binding and binding potential that is not possible using conventional screening methods. Another important finding reported in this thesis is that Log P should not be relied on as a predictor of brain entry. The HPLC tool developed, together with competition binding assays, was used to characterise a newly synthesised library of compounds for imaging of the translocator protein (TSPO) in brain using SPECT. Results showed that compound LS 1 was the most likely to succeed within the library investigated, but the high %PPB observed for LS 1 suggested novel compounds with improved %PPB were needed. Thus, a novel library of compounds for imaging of TSPO in brain using SPECT is currently been developed for future testing using the HPLC tool developed here and competition binding assays. Pre-clinical research: radiotracers for imaging the noradrenaline transporter (NAT) in brain using SPECT. In this thesis, NKJ64, a novel iodinated analogue of reboxetine, was successfully radiolabelled via electrophilic iododestannylation and evaluated as a potential SPECT radiotracer for imaging the NAT in brain using rodents and non-human primates. Biological evaluation of the novel radiotracer, 123/125I-NKJ64, in rodents included: in vitro ligand binding assays; in vitro and ex vivo autoradiography; in vivo biodistribution studies and ex vivo pharmacological blocking studies. In rats, 123/125I-NKJ64 displayed saturable binding with nanomolar affinity for the NAT in cortical homogenates, regional distribution consistent with the known density of NAT in the rodent brain and high maximum brain uptake of around 2.93 % of the injected dose. The specific: non-specific ratio (locus coeruleus:caudate putamen) of 123I-NKJ64 uptake was 2.8 at 30 minutes post intravenous injection and prior administration of reboxetine significantly reduced the accumulation of 123I-NKJ64 in the locus coeruleus (> 50% reduction). Data obtained using rodents indicated that further evaluation of 123I-NKJ64 in non-human primates was needed to determine its utility as a SPECT radiotracer for imaging of NAT in brain. Consequently, in vivo kinetic modelling studies using SPECT imaging with 123I-NKJ64 and two baboons were carried out to determine 123I-NKJ64 brain binding kinetics, brain distribution and plasma metabolism in non-human primates. Even though a high brain uptake of around 3.0% of the injected dose was determined, the high non-specific binding observed throughout the brain, a low binding potential (BPND<2) in NAT rich regions and a brain distribution that was inconsistent with the known NAT distribution in non-human primate brain precludes the translation of 123I-NKJ64 into humans. Another NAT radiotracer, 123I-INER, developed by Tamagnan and colleagues at Yale University and Institute for Degenerative Disorders, New Haven, USA, was also investigated as part of this thesis. Kinetic modelling analysis of 123I-INER in baboon brain was investigated for different models, namely invasive and reference tissue models. Bolus plus constant infusion experiments with displacement at equilibrium using six different doses of atomoxetine and four different doses of reboxetine were carried out in several baboons to obtain occupancy measurements as a function of injected dose (mg/kg) for the two NAT selective drugs. Results showed that reference tissue models were able to determine BPND values of 123I-INER in different brain regions. In addition the volume of distribution could be determined by dividing concentration in tissue by the concentration in venous blood at 3 hours post-injection. After administration of atomoxetine or reboxetine, dose-dependent occupancy was observed in brain regions known to contain high densities of NATs. Results supported the translation of 123I-INER into humans studies, despite the slow kinetics determined over the imaging period. Pharmacokinetic properties of 123I-INER described in this thesis may be used to simplify future data acquisition and image processing. Conclusion In conclusion, this thesis reported: (1) the development of novel radiotracers for brain imaging, namely NAT and TSPO; and (2) the development of a new methodology for aiding lead molecule identification at early stages of radiotracer discovery (i.e. prior to radiolabelling). In addition, an overview of radiotracer discovery and development process is provided in a single document, with a focus on brain radiotracers

Agriculture handbook

1996 handbook for the faculty of Agricultur

Challenges in imaging and predictive modeling of rhizosphere processes

Background Plant-soil interaction is central to human food production and ecosystem function. Thus, it is essential to not only understand, but also to develop predictive mathematical models which can be used to assess how climate and soil management practices will affect these interactions. Scope In this paper we review the current developments in structural and chemical imaging of rhizosphere processes within the context of multiscale mathematical image based modeling. We outline areas that need more research and areas which would benefit from more detailed understanding. Conclusions We conclude that the combination of structural and chemical imaging with modeling is an incredibly powerful tool which is fundamental for understanding how plant roots interact with soil. We emphasize the need for more researchers to be attracted to this area that is so fertile for future discoveries. Finally, model building must go hand in hand with experiments. In particular, there is a real need to integrate rhizosphere structural and chemical imaging with modeling for better understanding of the rhizosphere processes leading to models which explicitly account for pore scale processes

Infrastructure dynamics: A selected bibliography

The term infrastructure is used to denote the set of life support and public service systems which is necessary for the development of growth of human settlements. Included are some basic references in the field of dynamic simulation, as well as a number of relevant applications in the area of infrastructure planning. The intent is to enable the student or researcher to quickly identify such applications to the extent necessary for initiating further work in the field

Investigation of the Effects of Image Signal-to-Noise Ratio on TSPO PET Quantification of Neuroinflammation

Neuroinflammation may be imaged using positron emission tomography (PET) and the tracer [11C]-PK11195. Accurate and precise quantification of 18 kilodalton Translocator Protein (TSPO) binding parameters in the brain has proven difficult with this tracer, due to an unfavourable combination of low target concentration in tissue, low brain uptake of the tracer and relatively high non-specific binding, all of which leads to higher levels of relative image noise. To address these limitations, research into new radioligands for the TSPO, with higher brain uptake and lower non-specific binding relative to [11C]-PK11195, is being conducted world-wide. However, factors other than radioligand properties are known to influence signal-to-noise ratio in quantitative PET studies, including the scanner sensitivity, image reconstruction algorithms and data analysis methodology. The aim of this thesis was to investigate and validate computational tools for predicting image noise in dynamic TSPO PET studies, and to employ those tools to investigate the factors that affect image SNR and reliability of TSPO quantification in the human brain. The feasibility of performing multiple (n≥40) independent Monte Carlo simulations for each dynamic [11C]-PK11195 frame- with realistic modelling of the radioactivity source, attenuation and PET tomograph geometries- was investigated. A Beowulf-type high performance computer cluster, constructed from commodity components, was found to be well suited to this task. Timing tests on a single desktop computer system indicated that a computer cluster capable of simulating an hour-long dynamic [11C]-PK11195 PET scan, with 40 independent repeats, and with a total simulation time of less than 6 weeks, could be constructed for less than 10,000 Australian dollars. A computer cluster containing 44 computing cores was therefore assembled, and a peak simulation rate of 2.84x105 photon pairs per second was achieved using the GEANT4 Application for Tomographic Emission (GATE) Monte Carlo simulation software. A simulated PET tomograph was developed in GATE that closely modelled the performance characteristics of several real-world clinical PET systems in terms of spatial resolution, sensitivity, scatter fraction and counting rate performance. The simulated PET system was validated using adaptations of the National Electrical Manufacturers Association (NEMA) quality assurance procedures within GATE. Image noise in dynamic TSPO PET scans was estimated by performing n=40 independent Monte Carlo simulations of an hour-long [11C]-PK11195 scan, and of an hour- long dynamic scan for a hypothetical TSPO ligand with double the brain activity concentration of [11C]-PK11195. From these data an analytical noise model was developed that allowed image noise to be predicted for any combination of brain tissue activity concentration and scan duration. The noise model was validated for the purpose of determining the precision of kinetic parameter estimates for TSPO PET. An investigation was made into the effects of activity concentration in tissue, radionuclide half-life, injected dose and compartmental model complexity on the reproducibility of kinetic parameters. Injecting 555 MBq of carbon-11 labelled TSPO tracer produced similar binding parameter precision to 185 MBq of fluorine-18, and a moderate (20%) reduction in precision was observed for the reduced carbon-11 dose of 370 MBq. Results indicated that a factor of 2 increase in frame count level (relative to [11C]-PK11195, and due for example to higher ligand uptake, injected dose or absolute scanner sensitivity) is required to obtain reliable binding parameter estimates for small regions of interest when fitting a two-tissue compartment, four-parameter compartmental model. However, compartmental model complexity had a similarly large effect, with the reduction of model complexity from the two-tissue compartment, four-parameter to a one-tissue compartment, two-parameter model producing a 78% reduction in coefficient of variation of the binding parameter estimates at each tissue activity level and region size studied. In summary, this thesis describes the development and validation of Monte Carlo methods for estimating image noise in dynamic TSPO PET scans, and analytical methods for predicting relative image noise for a wide range of tissue activity concentration and acquisition durations. The findings of this research suggest that a broader consideration of the kinetic properties of novel TSPO radioligands, with a view to selection of ligands that are potentially amenable to analysis with a simple one-tissue compartment model, is at least as important as efforts directed towards reducing image noise, such as higher brain uptake, in the search for the next generation of TSPO PET tracers

Mathematical Models of Microbial Evolution: Cooperative Systems

Microbes usually live in large communities, where they interact with other organisms and species. These interactions include cooperation, when individuals facilitate each others growth and reproduction. Such cooperation has been for instance observed within pathogens in the process of infection. Therefore, given the number and the frequency of infectious diseases, understanding the nature and the dynamics of microbial cooperation may be a crucial step in modern medicine. Microbes often secrete costly enzymes which extracellularly metabolise resources available in the environment. This external metabolism is a form of ’public good cooperation’, in which individuals invest their energy in producing ’public goods’, available to other organisms. To study this phenomenon we deploy mathematical models which are based on biologically relevant assumptions. Our models not only aim to capture the dynamics of studied microbial communities, but also to remove the natural complexity arising in the empirical studies and thus to provide a mechanistic understanding of their results. We first recover and explain the recent empirical finding, about mixed strain infections, showing that an addition of a low virulent strain which does not produce public goods (termed ’cheat’) may counter-intuitively enhance the total population virulence. What drives this result turns out to be an interaction of two different cooperative traits and the presence of spatial structure. Next we study the competition between the strains that do and do not produce public goods. Our results depend on environmental conditions, such as resource concentration and population density, but they are also determined by the degree of spatial structure - the ecological trait which so far has been treated only as a binary variable. Finally, we identify some environmental threats for the external metabolism feeding strategy, and we examine its competitiveness in comparison to ’internal metabolism’, in which the costly enzymes are private.EPSR

Imaging Neuroinflammation in Progressive Multiple Sclerosis

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system CNS), where inflammation and neurodegeneration lead to irreversible neuronal damage. In MS, a dysfunctional immune system causes auto‐reactive lymphocytes to migrate into CNS where they initiate an inflammatory cascade leading to focal demyelination, axonal degeneration and neuronal loss. One of the hallmarks of neuronal injury and neuroinflammation is the activation of microglia. Activated microglia are found not only in the focal inflammatory lesions, but also diffusely in the normal‐appearing white matter (NAWM), especially in progressive MS. The purine base, adenosine is a ubiquitous neuromodulator in the CNS and also participates in the regulation of inflammation. The effect of adenosine mediated via adenosine A2A receptors has been linked to microglial activation, whereas modulating A2A receptors may exert neuroprotective effects. In the majority of patients, MS presents with a relapsing disease course, later advancing to a progressive phase characterised by a worsening, irreversible disability. Disease modifying treatments can reduce the severity and progression in relapsing MS, but no efficient treatment exists for progressive MS. The aim of this research was to investigate the prevalence of adenosine A2A receptors and activated microglia in progressive MS by using in vivo positron emission tomography (PET) imaging and [11C]TMSX and [11C](R)‐PK11195 radioligands. Magnetic resonance imaging (MRI) with diffusion tensor imaging (DTI) was performed to evaluate structural brain damage. Non‐invasive input function methods were also developed for the analyses of [11C]TMSX PET data. Finally, histopathological correlates of [11C](R)‐PK11195 radioligand binding related to chronic MS lesions were investigated in post‐mortem samples of progressive MS brain using autoradiography and immunohistochemistry. [11C]TMSX binding to A2A receptors was increased in NAWM of secondary progressive MS (SPMS) patients when compared to healthy controls, and this correlated to more severe atrophy in MRI and white matter disintegration (reduced fractional anisotropy, FA) in DTI. The non‐invasive input function methods appeared as feasible options for brain [11C]TMSX images obviating arterial blood sampling. [11C](R)‐PK11195 uptake was increased in the NAWM of SPMS patients when compared to patients with relapsing MS and healthy controls. Higher [11C](R)‐PK11195 binding in NAWM and total perilesional area of T1 hypointense lesions was associated with more severe clinical disability, increased brain atrophy, higher lesion load and reduced FA in NAWM in the MS patients. In autoradiography, increased perilesional [11C](R)‐PK11195 uptake was associated with increased microglial activation identified using immunohistochemistry. In conclusion, brain [11C]TMSX PET imaging holds promise in the evaluation of diffuse neuroinflammation in progressive MS. Being a marker of microglial activation, [11C](R)‐ PK11195 PET imaging could possibly be used as a surrogate biomarker in the evaluation of the neuroinflammatory burden and clinical disease severity in progressive MS.Siirretty Doriast