Agonist signalling properties of radiotracers used for imaging of dopamine D2/3 receptors

EJNMMI Research, Oct 2014

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 ?-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 ?-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 ?-arrestin-2 pathway may influence the binding of these radiopharmaceuticals.

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Agonist signalling properties of radiotracers used for imaging of dopamine D2/3 receptors

Jan-Peter van Wieringen 0 Martin C Michel 2 Henk M Janssen 1 Anton G Janssen 4 Philip H Elsinga 3 Jan Booij 0 0 Department of Nuclear Medicine, Academic Medical Center, University of Amsterdam , Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands 1 SyMO-Chem BV, Eindhoven, The Netherlands 2 Department of Pharmacology, Johannes Gutenberg University , Mainz, Germany 3 Department of Nuclear Medicine and Molecular Imaging, University Medical Center Groningen, University of Groningen , Groningen, The Netherlands 4 GE Healthcare , Eindhoven, The Netherlands 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 -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 -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 -arrestin-2 pathway may influence the binding of these radiopharmaceuticals. - Background The dopamine (DA) system plays a central role in several neuropsychiatric disorders including Parkinson's disease and schizophrenia. DA D2 receptor (D2R) antagonists are used to reduce psychotic symptoms, whereas D2R agonists are commonly used in the treatment of Parkinson's disease. Consequently, radiopharmaceuticals targeting D2Rs are of value to obtain insight in the pathophysiology of these brain disorders. The D2R is a subfamily within the superfamily of Gprotein-coupled receptors (GPCRs) and contains several subtypes including some in the D2-like subfamily, the D2 (splice variants D2short (D2S) and D2long (D2L) [1]), D3 and D4 receptors [2]. They primarily couple to the Gi/o type of G-proteins to inhibit the enzyme adenylyl cyclase in producing cyclic adenosine mono phosphate (cAMP) [2]. Like other GPCRs, they exhibit interconvertible high- and low-affinity states for agonists in vitro [3]. In the high-affinity state, the receptor is coupled to the Gprotein and this is considered to be the active state of the receptor. In the low-affinity state, the receptor is uncoupled and consequently inactive. Dopamine D2/3 receptor (D2/3R) agonist radiopharmaceuticals for positron emission tomography (PET) have been developed successfully (e.g. 11C-NPA (N-propylnorapomorphine) and 11C-PHNO ((+)-4-propyl-9-hydroxynaphthoxazine)), and such agonists are more sensitive in detecting DA release in humans compared to antagonists like 11C-raclopride [4-7]. It has been proposed that the reason for this increased sensitivity may be that D2/3R agonist radiopharmaceuticals bind preferentially to the high-affinity state of the dopamine D2R, whereas antagonists do not differentiate between the high- and lowaffinity states [8-10]. However, this proposal depends upon the existence of two affinity states in vivo and the results of recent studies challenged this theory [11-13]. Originally, DA receptors (and other GPCRs) were thought to signal intracellularly only through their Gproteins. However, it was shown recently that besides this (canonical) pathway, DA receptors can also exert effects through proteins of which it was initially thought that they regulate receptor desensitization (non-canonical pathway, Figure 1). This cAMP-independent mechanism involves the adaptor protein -arrestin-2. Compounds can have distinct patterns of responses on these pathways; this phenomenon is called biased agonism or ligand-directed signalling [14,15]. Activation of the -arrestin-2 pathway may play a role in the increased sensitivity to detect dopamine release in vivo. Activation of this pathway eventually leads to the regulation of glycogen synthase kinase 3 (GSK3), a protein that is involved in many DA-dependent behaviours [16]. Many drugs (antipsychotics, antidepressants, lithium) affect this cascade, and recently compounds, based on the aripiprazole scaffold, were discovered that are functionally selective for the -arrestin-2 pathway [ (...truncated)


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Jan-Peter van Wieringen, Martin C Michel, Henk M Janssen, Anton G Janssen, Philip H Elsinga, Jan Booij. Agonist signalling properties of radiotracers used for imaging of dopamine D2/3 receptors, EJNMMI Research, 2014, pp. 53, Volume 4, Issue 1, DOI: 10.1186/s13550-014-0053-3