Design and Synthesis of D3R Bitopic Ligands with Flexible Secondary Binding Fragments: Radioligand Binding and Computational Chemistry Studies.

A series of bitopic ligands based on Fallypride with a flexible secondary binding fragment (SBF) were prepared with the goal of preparing a D3R-selective compound. The effect of the flexible linker ((R,S)-trans-2a-d), SBFs ((R,S)-trans-2h-j), and the chirality of orthosteric binding fragments (OBFs) ((S,R)-trans-d, (S,R)-trans-i, (S,S)-trans-d, (S,S)-trans-i, (R,R)-trans-d, and (R,R)-trans-i) were evaluated in in vitro binding assays. Computational chemistry studies revealed that the interaction of the fragment binding to the SBF increased the distance between the pyrrolidine nitrogen and ASP1103.32 of the D3R, thereby reducing the D3R affinity to a suboptimal level.

Dopamine D3 Receptor, Cognition and Cognitive Dysfunctions in Neuropsychiatric Disorders: From the Bench to the Bedside.

The dopamine D3 receptor (D3R) plays a prominent role in the modulation of cognition in healthy individuals, as well as in the pathophysiological mechanism underlying the cognitive deficits affecting patients suffering from neuropsychiatric disorders. At a therapeutic level, a growing body of evidence suggests that the D3R blockade enhances cognitive and thus it may be an optimal therapeutic strategy against cognitive dysfunctions. However, this is not always the case because other ligands targeting the D3R, and behaving as partial agonists or biased agonists, may exert their pro-cognitive effect by maintaining adequate level of dopamine in key brain areas tuning cognitive performances. In this chapter, we review and discuss preclinical and clinical findings with the aim to remark the crucial role of the D3R in cognition and to strengthen the message that drugs targeting D3R may be excellent cognitive enhancers for the treatment of several neuropsychiatric and neurological disorders.

Synthesis of bitopic ligands based on fallypride and evaluation of their affinity and selectivity towards dopamine D2 and D3 receptors.

The difference in the secondary binding site (SBS) between the dopamine 2 receptor (D2R) and dopamine 3 receptor (D3R) has been used in the design of compounds displaying selectivity for the D3R versus D2R. In the current study, a series of bitopic ligands based on Fallypride were prepared with various secondary binding fragments (SBFs) as a means of improving the selectivity of this benzamide analog for D3R versus D2R. We observed that compounds having a small alkyl group with a heteroatom led to an improvement in D3R versus D2R selectivity. Increasing the steric bulk in the SBF increase the distance between the pyrrolidine N and Asp110, thereby reducing D3R affinity. The best-in-series compound was (2S,4R)-trans-27 which had a modest selectivity for D3R versus D2R and a high potency in the ß-arrestin competition assay which provides a measure of the ability of the compound to compete with endogenous dopamine for binding to the D3R. The results of this study identified factors one should consider when designing bitopic ligands based on Fallypride displaying an improved affinity for D3R versus D2R.

Reciprocal cross-sensitization of D1 and D3 receptors following pharmacological stimulation in the hemiparkinsonian rat.

In the majority of Parkinson's disease (PD) patients, long-term dopamine (DA) replacement therapy leads to dyskinesia characterized by abnormal involuntary movements (AIMs). There are various mechanisms of dyskinesia, such as the sensitization of striatal DA D1 receptors (D1R) and upregulation of DA D3 receptors (D3R). These receptors interact physically and functionally in D1R-bearing medium spiny neurons to synergistically drive dyskinesia. However, the cross-receptor-mediated effects due to D1R-D3R cooperativity are still poorly understood. In pursuit of this, we examined whether or not pharmacological D1R or D3R stimulation sensitizes the dyskinetic response to the appositional agonist, a process known as cross-sensitization. First, we established D1R-D3R behavioral synergy in a cohort of 6-OHDA-lesioned female adult Sprague-Dawley rats. Then, in a new cohort, we tested for cross-sensitization in a between-subject design. Five groups received a sub-chronic regimen of either saline, the D1R agonist SKF38393 (1.0 mg/kg), or the D3R agonist PD128907 (0.3 mg/kg). For the final injection, each group received an acute injection of the other agonist. AIMs were monitored following each injection. Sub-chronic administration of both SKF38393 and PD128907 induced the development of dyskinesia. More importantly, cross-agonism tests revealed reciprocal cross-sensitization; chronic treatment with either SKF38393 or PD128907 induced sensitization to a single administration of the other agonist. This reciprocity was not marked by changes to either D1R or D3R striatal mRNA expression. The current study provides key behavioral data demonstrating the role of D3R in dyskinesia and provides behavioral evidence of D1R and D3R functional interactions.

Design, synthesis, and evaluation of indolebutylamines as a novel class of selective dopamine D3 receptor ligands.

A series of indolebutylamine derivatives were designed, synthesized, and evaluated as a novel class of selective ligands for the dopamine 3 receptor. The most potent compound 11q binds to dopamine 3 receptor with a Ki value of 124 nm and displays excellent selectivity over the dopamine 1 receptor and dopamine 2 receptor. Investigation based on structural information indicates that site S182 located in extracellular loop 2 may account for high selectivity of compounds. Interaction models of the dopamine 3 receptor-11q complex and structure-activity relationships were discussed by integrating all available experimental and computational data with the eventual aim to discover potent and selective ligands to dopamine 3 receptor.