Agonist signalling properties of radiotracers used for imaging of dopamine D-2/3 receptors

Background: Dopamine D-2/3 receptor (D2/3R) agonist radiopharmaceuticals are considered superior to antagonists to detect dopamine release, e.g. induced by amphetamines. Agonists bind preferentially to the high-affinity state of the dopamine D2R, which has been proposed as the reason why agonists are more sensitive to detect dopamine release than antagonist radiopharmaceuticals, but this theory has been challenged. Interestingly, not all agonists similarly activate the classic cyclic adenosine mono phosphate (cAMP) and the beta-arrestin-2 pathway, some stimulate preferentially one of these pathways; a phenomenon called biased agonism. Because these pathways can be affected separately by pathologies or drugs (including dopamine releasers), it is important to know how agonist radiotracers act on these pathways. Therefore, we characterized the intracellular signalling of the well-known D2/3R agonist radiopharmaceuticals NPA and PHNO and of several novel D2/3R agonists. Methods: cAMP accumulation and beta-arrestin-2 recruitment were measured on cells expressing human D2R. Results: All tested agonists showed (almost) full agonism in both pathways. Conclusions: The tested D2/3R agonist radiopharmaceuticals did not exhibit biased agonism in vitro. Consequently, it is likely that drugs (including psychostimulants like amphetamines) and/or pathologies that influence the cAMP and/or the beta-arrestin-2 pathway may influence the binding of these radiopharmaceuticals

Synthesis and evaluation in rats of homologous series of [F-18]-labeled dopamine D-2/3 receptor agonists based on the 2-aminomethylchroman scaffold as potential PET tracers

Background: Agonist positron emission tomography (PET) tracers for dopamine D-2/3 receptors (D(2/3)Rs) offer greater sensitivity to changes in endogenous dopamine levels than D2/3R antagonist tracers. D2/3R agonist tracers currently available for clinical research are labeled with the short-lived isotope carbon-11, which limits their use. We aimed to develop high-affinity D2R agonists amenable for labeling with the longer-living fluorine-18. Here, we report the evaluation as potential PET tracers of two homologous series of [F-18]fluorinated tracers based on the 2-aminomethylchroman-7-ol (AMC) scaffold: (R)-2-((4-(2-fluoroalkoxy)benzylamino)methyl)chroman-7-ols (AMC13 homologues) and (R)-2-((2-(4-(4-(fluoroalkoxy)phenyl)piperazin-1-yl)ethylamino)methyl)chroman-7-ols (AMC15 homologues). We varied the length of the F-18-fluoroalkyl chain in these structures to balance brain penetration and non-specific binding of the radioligands by adjusting their lipophilicity. Methods: The tracers were evaluated in brain slices of Sprague-Dawley rats by in vitro autoradiography and in living rats by microPET imaging and ex vivo autoradiography. PET data were analyzed with one- and two-tissue compartmental models (1TCM/2TCM), simplified reference tissue model (SRTM), and Logan graphical analysis. Specificity of binding was tested by blocking D2/3R with raclopride. Results: Homologues with a shorter fluoroalkyl chain consistently showed greater D2/3R-specific-to-total binding ratios in the striatum than those with longer chains. The fluoroethoxy homologue of AMC13 ([F-18]FEt-AMC13) demonstrated the highest degree of D2/3R-specific binding among the evaluated tracers: mean striatum-to-cerebellum uptake ratio reached 4.4 in vitro and 2.1/2.8 in vivo/ex vivo (PET/autoradiography). Striatal binding potential (BPND) relative to cerebellum was 0.51-0.63 depending on the estimation method. Radiometabolites of [F-18]FEt-AMC13 did not enter the brain. In vitro, application of 10 mu mol/L raclopride reduced D2/3R-specific binding of [F-18]FEt-AMC13 in the striatum by 81 %. In vivo, pre-treatment with 1 mg/kg (2.9 mu mol/kg) raclopride led to 17-39 % decrease in D2/3R-specific binding in the striatum. Conclusions: Varying the length of the [F-18]fluoroalkyl chain helped improve the characteristics of the original candidate tracers. Further modifications of the current lead [F-18]FEt-AMC13 can provide an agonist radiopharmaceutical suitable for D2/3R imaging by PET

A cancer drug atlas enables synergistic targeting of independent drug vulnerabilities.

Personalized cancer treatments using combinations of drugs with a synergistic effect is attractive but proves to be highly challenging. Here we present an approach to uncover the efficacy of drug combinations based on the analysis of mono-drug effects. For this we used dose-response data from pharmacogenomic encyclopedias and represent these as a drug atlas. The drug atlas represents the relations between drug effects and allows to identify independent processes for which the tumor might be particularly vulnerable when attacked by two drugs. Our approach enables the prediction of combination-therapy which can be linked to tumor-driving mutations. By using this strategy, we can uncover potential effective drug combinations on a pan-cancer scale. Predicted synergies are provided and have been validated in glioblastoma, breast cancer, melanoma and leukemia mouse-models, resulting in therapeutic synergy in 75% of the tested models. This indicates that we can accurately predict effective drug combinations with translational value

Agonist high- and low-affinity states of dopamine D-2 receptors:methods of detection and clinical implications

Dopamine D-2 receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D-2 receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [H-3]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D-2 receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D-2 receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed

PET and SPECT imaging of the central dopamine system in humans

The neurotransmitter dopamine plays a role in many different functions of the human brain, ranging from psychomotor planning to cognition. This short review addresses which parts of the dopamine system can be imaged quantitatively in the living human brain using positron-emission tomography (PET) or single-photon emission computed tomography (SPECT). Nowadays, imaging of the nigrostriatal dopaminergic pathway in humans can be performed quantitatively using radiotracers like the aromatic amino acid decarboxylase (AADC) substrate [18F]FDOPA, vesicular monoamine transporter 2 (VMAT-2) radioligands derived from tetrabenazine or PET/SPECT radioligands that bind to the dopamine transporter (DAT). Using PET, also several other dopaminergic projection pathways (e.g. mesocortical projections) can be assessed in humans. Several antagonist PET radioligands for the dopamine D1 receptor have been developed successfully. In addition, well-validated antagonist PET and SPECT radioligands are available for imaging of dopamine D2/3 receptors in the living human brain. Recently, also agonist PET radioligands for the dopamine D2/3 receptors have become available, which afford the opportunity to evaluate the existence of the high-affinity state of these receptors in vivo. These agonist radiopharmaceuticals may also prove more sensitive to changes in dopamine concentrations (e.g. induced by the dopamine releaser amphetamine). Finally, selective antagonist PET radioligands for the dopamine D4 receptor have recently been synthesized and evaluated successfully in small laboratory animals, although these radioligands have not yet been reported as applied in human subjects. In conclusion, after almost three decades of research, several relevant parts of the central dopamine system can be assessed quantitatively in the living human brain using PET or SPECT. Future studies may include application of agonist radioligands and more dopamine receptor subtype selective radioligands

The antagonist SPECT tracer 123I-iododexetimide binds preferentially to the muscarinic M1 receptor in-vivo, but is it also a potential tool to assess the occupancy of muscarinic M1 receptors by agonists?

Cognitive deterioration in neuropsychiatric disorders is associated with high attrition rates giving an urgent need to develop better pharmaceutical therapies. The underlying mechanisms of cognitive impairments are unclear but research has shown that the muscarinic receptor subtype 1 (M1 receptor) plays a critical role. Blocking the M1 receptor gives rise to profound cognitive deficits, while the administration of M1 agonist drugs improves cognitive functioning. In this research highlight we will outline supporting data that the radiotracer 123I-iododexetimide preferentially binds to the M1 receptor in-vivo and can be used to assess changes in M1 receptor expression in-vivo associated with cognitive decline. These findings come from a previously published paper extensively examining binding characteristics of 123/127I-iododexetimide to muscarinic receptors. Results of biodistribution studies also has shown that acute administration of the M1/4 receptor agonist xanomeline could inhibit 127I-iododexetimide binding in M1-rich brain areas in rats, suggesting that 123I-iododexetimide may also be used to evaluate the occupancy of M1 receptors by M1 agonists in-vivo. This may be of clinical relevance considering the efficacy of M1 agonist drugs in the treatment of cognitive deficits. Here we show the results from new biodistribution experiments in rats conducted to test the hypothesis that 123I-iododexetimide may be a useful radiotracer to evaluate the M1 receptor occupancy by M1 agonists in-vivo. Contrary to our expectations, no significant change in 123I-iododexetimide ex-vivo binding was observed after acute administration of xanomeline in M1 receptor-rich brain areas, whereas significantly decreased 123I-iododexetimide binding was found after chronic treatment with xanomeline. 123I-iododexetimide single photon emission computed tomography (SPECT) may therefore be a useful imaging tool to further evaluate M1 receptor changes in neuropsychiatric disorders, as a potential stratifying biomarker, to assess the occupancy of M1 receptors after M1 antagonist treatment, or after chronic treatment with M1 agonists, although it may be less suited to evaluate the M1 receptor occupancy after acute treatment with M1 agonists. Future studies should concentrate efforts towards finding also an M1 agonist radiotracer for positron emission tomography (PET) or SPECT to assess the working mechanism of M1 agonists

Agonist signalling properties of radiotracers used for imaging of dopamine D_2/3 receptors

Background: Dopamine D2/3 receptor (D2/3R) agonist radiopharmaceuticals are considered superior to antagonists to detect dopamine release, e.g. induced by amphetamines. Agonists bind preferentially to the high-affinity state of the dopamine D2R, which has been proposed as the reason why agonists are more sensitive to detect dopamine release than antagonist radiopharmaceuticals, but this theory has been challenged. Interestingly, not all agonists similarly activate the classic cyclic adenosine mono phosphate (cAMP) and the β-arrestin-2 pathway, some stimulate preferentially one of these pathways; a phenomenon called biased agonism. Because these pathways can be affected separately by pathologies or drugs (including dopamine releasers), it is important to know how agonist radiotracers act on these pathways. Therefore, we characterized the intracellular signalling of the well-known D2/3R agonist radiopharmaceuticals NPA and PHNO and of several novel D2/3R agonists. Methods: cAMP accumulation and β-arrestin-2 recruitment were measured on cells expressing human D2R. Results: All tested agonists showed (almost) full agonism in both pathways. Conclusions: The tested D2/3R agonist radiopharmaceuticals did not exhibit biased agonism in vitro. Consequently, it is likely that drugs (including psychostimulants like amphetamines) and/or pathologies that influence the cAMP and/or the β-arrestin-2 pathway may influence the binding of these radiopharmaceuticals.</p

Novice drivers’ performance after different alcohol dosages and placebo in the divided-attention steering simulator

Abstract Rationale The divided-attention steering simulator (DASS) is designed to measure lane-keeping (i.e., a tracking task using a steering wheel) while performing a secondary visual task (responding to digits that appear in the corners of the computer screen). Some studies have already used the DASS, but the magnitude of impairment is difficult to interpret because reference values are lacking. Objective To examine the magnitude of impairment after administration of four different dosages of alcohol and placebo. Materials and methods Thirty-two healthy young adults participated in this randomized, single-blind crossover trial. Subjects received alcohol to gain a blood alcohol concentration (BAC) of 0.02%, 0.05%, 0.08%, and 0.10% or alcohol-placebo. Sixteen subjects performed a 30-min test in DASS (dual-task condition). Outcome measures were steering error, reaction time, and percentage of errors. Sixteen other subjects performed the test without performing the secondary peripheral task (single-task condition). Results Twenty-eight subjects (novice drivers; drivers&apos; license up to 5 years) were included in the analyses. Dose-dependent impairment was found in both the singletask condition (F (4,11) =10.86, p&lt;0.001) and the dual-task condition (F (4,9) =5.58, p&lt;0.015). Performance at all BAC levels differed significantly (p&lt;0.05) from alcohol-placebo, except BAC 0.02%. With increasing BAC levels, subjects made more errors and reacted slower on the peripheral visual search task, but these effects did not reach significance. Conclusion With increasing BAC, dose-dependent impairment was found. The DASS seems to be a suitable dividedattention task that is useful in psychopharmacological research and training of novice drivers

Ex Vivo Characterization of a Novel Iodine-123-Labelled Aminomethylchroman as a Potential Agonist Ligand for SPECT Imaging of Dopamine D2/3 Receptors

For imaging of dopamine D2/3 receptors, agonist tracers are favoured over antagonists because they are more sensitive to detection of dopamine release and because they may selectively label the high-affinity receptor state. We have developed novel D2/3 receptor selective agonists that can be radiolabelled with [123I], which label is advantageous over most other labels, such as carbon-11, as it has a longer half-life. Particularly, we considered (R) N-[7-hydroxychroman-2-yl]-methyl 4-iodobenzyl amine (compound 1) as an attractive candidate for development as it shows high binding affinity to D2/3 receptors in vitro, and here we report on the characterization of this first [123I]-labelled D2/3 receptor agonist radiopharmaceutical intended for SPECT imaging. The appropriate tin precursor for [123I]-1 was developed and was successfully radiolabelled with iodine-123 giving a moderate yield (30–35%) and a good purity (>95%) for [123I]-1. In biodistribution experiments in Wistar rats intravenous injection of [123I]-1 resulted in a fast brain uptake, where the observed binding in the D2/3 receptor-rich striatum was slightly higher than that in the cerebellum 30 min to 4 h p.i. Storage phosphor imaging experiments, however, did not show specific D2/3 receptor binding. In conclusion, despite promising in vitro data for 1, neither specific ex vivo binding nor high signal-to-noise ratios were found in rodents for [123I]-1