Molecular and Cellular Pharmacology The dopaminergic stabilizers pridopidine (ACR16) and (-)-OSU6162 display dopamine D 2 receptor antagonism and fast receptor dissociation properties Tino Dyhring a, ⁎, Elsebet Ø. Nielsen a , Clas Sonesson b , Fredrik Pettersson b , Jonas Karlsson b , Peder Svensson b , Palle Christophersen a , Nicholas Waters b a Drug Discovery, NeuroSearch A/S, Ballerup, DK-2750, Denmark b NeuroSearch AB, Gothenburg, SE-413 46, Sweden abstract article info Article history: Received 19 August 2009 Received in revised form 23 October 2009 Accepted 10 November 2009 Available online 15 November 2009 Keywords: Dopaminergic stabilizer Dopamine D 2 receptor Pridopidine ACR16 Fast dissociation Antipsychotic A new pharmacological class of CNS ligands with the unique ability to stimulate or suppress motor and behavioral symptoms depending on the prevailing dopaminergic tone has been suggested as “dopaminergic stabilizers”. The molecular mode-of-action of dopaminergic stabilizers is not yet fully understood, but they are assumed to act via normalization of dopaminergic signaling, through interactions with the dopamine D 2 receptor. Here we have evaluated the dopaminergic stabilizers pridopidine (ACR16) and (-)-OSU6162, as well as the new compound N-{[(2S)-5-chloro-7-(methylsulfonyl)-2,3-dihydro-1,4-benzodioxin-2-yl]methyl} ethanamine (NS30678) in a series of cellular in vitro dopamine D 2 receptor functional and binding assays. Neither ACR16, (-)-OSU6162, nor NS30678 displayed detectable dopamine D 2 receptor-mediated intrinsic activity, whereas they concentration-dependently antagonized dopamine-induced responses with IC 50 values of 12.9 μM, 5.8 μM, and 7.0 nM, respectively. In contrast to the high-affinity typical antipsychotics haloperidol and raclopride, the dopaminergic stabilizers ACR16 and (-)-OSU6162 both displayed fast dopamine D 2 receptor dissociation properties, a feature that has previously been suggested as a contributing factor to antipsychotic atypicality and attributed mainly to low receptor affinity. However, the finding that NS30678, which is equipotent to haloperidol and raclopride, also displays fast receptor dissociation, suggests that the agonist-like structural motif of the dopaminergic stabilizers tested is a critical dissociation rate determinant. The results demonstrate that dopaminergic stabilizers exhibit fast competitive dopamine D 2 receptor antagonism, possibly allowing for temporally variable and activity-dependent dopamine D 2 receptor occupancy that may partly account for their unique stabilization of dopamine dependent behaviors in vivo. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Based on a strategy of synthesizing novel dopamine D 2 receptor antagonists, with structural motifs as well as physical/chemical char- acteristics similar to the endogenous agonist dopamine, compounds with “dopaminergic stabilizer” properties have been identified. Dopaminergic stabilizers are believed to constitute a novel class of compounds which, in vivo, act by normalizing hypo- as well as hyperdopaminergic behavior, supposedly via a complex interaction with the population of dopamine D 2 receptors. Pridopidine (ACR16), which is currently in development for symptomatic treatment of Huntington's disease, and its predecessor (-)-OSU6162 have been reported to possess such a profile (Natesan et al., 2006; Rung et al., 2008; Sonesson et al., 1994; Tedroff et al., 1998) and are suggested as drug candidates with a novel mode-of-action with the potential of treating a variety of conditions involving dysregulation of dopami- nergic pathways. Although ACR16 shows intriguing abilities to modify dopamine-related behaviors, the exact mechanism by which these effects are induced is not fully understood. Differences in extrasynaptic versus synaptic dopamine neurotransmission (Carlsson and Carlsson, 2006), partial dopamine receptor agonism (Seeman and Guan, 2007; Seeman et al., 2009) as well as interaction with both an allosteric and orthosteric site of the dopamine D 2 receptor (Lahti et al., 2007; Rung et al., 2008), have been proposed as mechanisms by which ACR16 exerts its stabilizing properties. In the brain, dopamine exerts its action by means of both synaptic and extrasynaptic release followed by re-absorption, degradation or diffusion, creating a complex temporal/spatial dopamine “landscape” in target areas. Dopamine is a high-affinity neurotransmitter on dopamine receptors allowing the dopaminergic system to operate with low concentration tonic signaling while at the same time being capable of responding to short (seconds) dopamine surges elicited by event-related bursting of the dopaminergic neurons, a feature which is fundamental for incentive learning (for a review see Schultz, 2007). This unique feature of dopamine receptors is due to fast association and dissociation properties of dopamine, securing fast response times. European Journal of Pharmacology 628 (2010) 19–26 ⁎ Corresponding author. NeuroSearch A/S, 93 Pederstrupvej, DK-2750, Ballerup, Denmark. Tel.: +45 4460 8277; fax: +45 4460 8080. E-mail address: tdy@neurosearch.dk (T. Dyhring). 0014-2999/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2009.11.025 Contents lists available at ScienceDirect European Journal of Pharmacology journal homepage: www.elsevier.com/locate/ejphar