Synthesis of 2-(5-bromo-2,3-dimethoxyphenyl)-5-(aminomethyl)-1H-pyrrole analogues and their binding affinities for dopamine D2, D3, and D4 receptors

A series of 2-(5-bromo-2,3-dimethoxyphenyl)-5-(aminomethyl)-1H-pyrrole analogues was prepared and their affinity for dopamine D(2), D(3), and D(4) receptors was measured using in vitro binding assays. The results of receptor binding studies indicated that the incorporation of a pyrrole moiety between the phenyl ring and the basic nitrogen resulted in a significant increase in the selectivity for dopamine D(3) receptors. The most selective compound in this series is 2-(5-bromo-2,3-dimethoxyphenyl)-5-(2-(3-pyridal)piperidinyl)methyl-1H-pyrrole (6p), which has a D(3) receptor affinity of 4.3 nM, a 20-fold selectivity for D(3) versus D(2) receptors, and a 300-fold selectivity for D(3) versus D(4) receptors. This compound is predicted to be a useful ligand for studying the functional role of dopamine D(3) receptors in vivo.     

Novel 2,3,4,5-tetrahydro-1H-3-benzazepines with high affinity and selectivity for the dopamine D3 receptor

Starting from the dopamine D3 receptor antagonist SB-277011 1, a series of 2,3,4,5-tetrahydro-1H-3-benzazepines has been identified with high affinity for the dopamine D3 receptor and selectivity over the D2 receptor. The 3-acetamido-2-fluorocinnamide derivative 20 gave high D3 receptor affinity (pKi 8.4) with 130-fold selectivity over the 2, receptor.     

Synthesis and radioiodination of selective ligands for the dopamine D3 receptor subtype

Starting from FAUC 365, a series of iodine substituted heteroaryl carboxamides has been synthesized revealing high affinity and selectivity for the dopamine D3 receptor. Binding data showed a 15-560-fold selectivity for the dopamine D3 over D2. A 2,3-dichloro substitution pattern on the phenylpiperazine moiety led to the highest subtype selectivity, whereas the 2-methoxy substituted compounds showed superior D3 affinity. Suitable precursors were radioiodinated with high radiochemical yields (53-85%) leading to potential imaging agents for the D3 receptor by SPET.     

Synthesis and characterization of selective dopamine D2 receptor antagonists

A series of indole compounds have been prepared and evaluated for affinity at D2-like dopamine receptors using stably transfected HEK cells expressing human D2, D3, or D4 dopamine receptors. These compounds share structural elements with the classical D2-like dopamine receptor antagonists, haloperidol, N-methylspiperone, and benperidol. The compounds that share structural elements with N-methylspiperone and benperidol bind non-selectively to the D2 and D3 dopamine receptor subtypes. However, several of the compounds structurally similar to haloperidol were found to (a) bind to the human D2 receptor subtype with nanomolar affinity, (b) be 10- to 100-fold selective for the human D2 receptor compared to the human D3 receptor, and (c) bind with low affinity to the human D4 dopamine receptor subtype. Binding at sigma (sigma) receptor subtypes, sigma1 and sigma2, were also examined and it was found that the position of the methoxy group on the indole was pivotal in both (a) D2 versus D3 receptor selectivity and (b) affinity at sigma1 receptors. Adenylyl cyclase studies indicate that our indole compounds with the greatest D2 receptor selectivity are neutral antagonists at human D2 dopamine receptor subtypes. With stably transfected HEK cells expressing human D2 (hD2-HEK), these compounds (a) have no intrinsic activity and (b) attenuated quinpirole inhibition of adenylyl cyclase. The D2 receptor selective compounds that have been identified represent unique pharmacological tools that have potential for use in studies on the relative contribution of the D2 dopamine receptor subtypes in physiological and behavioral situations where D2-like dopaminergic receptor involvement is indicated.     

Selectivity and activation of dopamine D3R from molecular dynamics

D₃ receptor, a member of dopamine (DA) D₂-like receptor family, which belongs to class A of G-protein coupled receptors (GPCRs), has been reported to play a critical role in neuropsychiatric disorders. Recently, the crystal structure of human dopamine D3 receptor was reported, which facilitates structure-based drug discovery of D3R significantly. We dock D3R-selective compounds into the crystal structure of D3R and homology structure of D2R. Then we perform 20?ns molecular dynamics (MD) of the receptor with selective compounds bound in explicit lipid and water. Our docking and MD results indicate the important residues related to the selectivity of D3R. Specifically, residue Thr^7.39 in D3R may contribute to the high selectivity of R-22 with D3R. Meanwhile, the 4-carbon linker and phenylpiperazine of R-22 improve the binding affinity and the selectivity with D3R. We also dock the agonists, including dopamine, into D3R and perform MD. Our molecular dynamics results of D3R with agonist bound show strong conformational changes from TM5, TM6, and TM7, outward movement of intracellular part of TM6, fluctuation of “ionic lock” motif and conformational change of Tyr^7.53, which is consistent with recent crystal structures of active GPCRs and illustrates the dynamical process during activation. Our results reveal the mechanism of selectivity and activation for D3R, which is important for developing high selective antagonists and agonists for D3R.     

Heterocyclic analogues of 2-aminotetralins with high affinity and selectivity for the dopamine D3 receptor.

A novel series of 5,6,7,8-tetrahydroquinazolines, 4,5,6,7-tetrahydroindazoles and 4,5,6,7-tetrahydrobenzothiazoles has been prepared, having high affinity and selectivity for the dopamine D3 receptor. The 4-methoxy-5,6,7,8-tetrahydroquinazoline 6i and 2-amino-4,5,6,7-tetrahydrobenzothiazole 8 proved to be agonists with among the highest D3 receptor affinities and selectivities reported to date.     

Design and synthesis of a piperazinylalkylisoxazole library for subtype selective dopamine receptor ligands

A piperazinylbutylisoxazole libary was designed, synthesized and screened for the binding affinities to dopamine D2, D3, and D4 receptors. Several ligands were identified to possess high binding affinity and selectivity for the D3 and D4 receptors over the D2 receptor. Compounds 6s and 6t showed K(i) values of 2.6 nM and 3.9 nM for the D3 receptor with 46- and 50-fold selectivity over the D2 receptor, respectively.     

Synthesis and biological characterization of novel hybrid 7-[[2-(4-phenyl-piperazin-1-yl)-ethyl]-propyl-amino]-5,6,7,8-tetrahydro-naphthalen-2-ol and their heterocyclic bioisosteric analogues for dopamine D2 and D3 receptors

In a recent preliminary communication we described the development of a series of hybrid molecules for the dopamine D2 and D3 receptor subtypes. The design of these compounds was based on combining pharmacophoric elements of aminotetralin and piperazine molecular fragments derived from known dopamine receptor agonist and antagonist molecules. Molecules developed from this approach exhibited high affinity and selectivity for the D3 receptor as judged from preliminary [(3)H]spiperone binding data. In this report, we have expanded our previous finding by developing additional novel molecules and additionally evaluated functional activities of these novel molecules in the [(3)H]thymidine incorporation mitogenesis assay. The binding results indicated highest selectivity in the bioisosteric benzothiazole derivative N6-[2-(4-phenyl-piperazin-1-yl)-ethyl]-N6-propyl-4,5,6,7-tetrahydro-benzothiazole-2,6-diamine (14) for the D3 receptor whereas the racemic compound 7-([2-[4-(2,3-dichloro-phenyl)-piperazin-1-yl]-ethyl]-propyl-amino)-5,6,7,8-tetrahydro-naphthalen-2-ol (10c) showed the strongest potency. Mitogenesis studies to evaluate functional activity demonstrated potent agonist properties in these novel derivatives for both D2 and D3 receptors. In this regard, compound 7-[[4-(4-phenyl-piperazin-1-yl)-butyl]-prop-2-ynyl-amino]-5,6,7,8-tetrahydro-naphthalen-2-ol (7b) exhibited the most potent agonist activity at the D3 receptor, 10 times more potent than quinpirole and was also the most selective compound for the D3 receptor in this series. Racemic compound 10a was resolved; however, little separation of activity was found between the two enantiomers of 10a. The marginally more active enantiomer (-)-10a was examined in vivo using the 6-OH-DA induced unilaterally lesioned rat model to evaluate its activity in producing contralateral rotations. The results demonstrated that in comparison to the reference compound apomorphine, (-)-10a was quite potent in inducing contralateral rotations and exhibited longer duration of action.     

Dopamine D3 receptor ligands: recent advances in the control of subtype selectivity and intrinsic activity

Various pharmacological studies have implicated the dopamine D(3) receptor as an interesting therapeutic target in the treatment of different neurological disorders. Because of these putative therapeutic applications, D(3) receptor ligands with diverse intrinsic activities have been an active field of research in recent years. Separation of purely D(3)-mediated drug effects from effects produced by interactions with similar biogenic amine receptors allows to verify the therapeutic impact of D(3) receptors and to reduce possible side-effects caused by "promiscuous" receptor interactions. The requirement to gain control of receptor selectivity and in particular subtype selectivity has been a challenging task in rational drug discovery for quite a few years. In this review, recently developed structural classes of D(3) ligands are discussed, which cover a broad spectrum of intrinsic activities and show interesting selectivities.     

Synthesis and SAR of highly potent and selective dopamine D3-receptor antagonists: variations on the 1H-pyrimidin-2-one theme

In our efforts to further pursue one of the most selective dopamine D(3)-receptor antagonists reported to date, we now describe the synthesis and SAR of novel and highly selective dopamine D(3) antagonists based on a 1H-pyridin-2-one or on a urea scaffold. The most potent compounds exhibited K(i) values toward the D(3) receptor in the nano- to subnanomolar range and high selectivity versus the related D(2) dopamine receptor. Thus, 1H-pyridin-2-one 7b displays oral bioavailability (F=37%) as well as brain penetration (brain plasma ratio 3.7) in rat. Within the urea series, an excellent D(3) versus D(2) selectivity (>100-fold) could be achieved by removal of one NH group (compound 6), although bioavailability (rat) was suboptimal (F<10%). These data significantly enhance our understanding of the D(3) pharmacophore and are expected to lead to novel approaches for the treatment of schizophrenia.     

Conformationally-flexible benzamide analogues as dopamine D3 and sigma 2 receptor ligands

A series of conformationally-flexible analogues was prepared and their affinities for D2-like dopamine (D2, D3 and D4) were determined using in vitro radioligand binding assays. The results of this structure-activity relationship study identified one compound, 15, that bound with high affinity (K(i) value=2nM) and moderate selectivity (30-fold) for D3 compared to D2 receptors. In addition, this series of compounds were also tested for affinity at sigma1 and sigma2 receptors. We evaluated the affinity of these dopaminergic compounds at sigma receptors because (a) several antipsychotic drugs, which are high affinity antagonists at dopamine D2-like receptors, also bind to sigma receptors and (b) sigma receptors are expressed ubiquitously and at high levels (picomoles per mg proteins). It was observed that a number of analogues displayed high affinity and excellent selectivity for sigma2 versus sigma1 receptors. Consequently, these novel compounds may be useful for characterizing the functional role of sigma2 receptors and for imaging the sigma2 receptor status of tumors in vivo with PET.     

Functional Differentiation of Multiple Dopamine D1-Like Receptors by NNC 01-0012

Abstract: Although members of the multiple vertebrate/mammalian dopamine D1 receptor gene family can be selectively classified on the basis of their molecular/phylogenetic, structural, and tissue distribution profiles, no subtype-specific discriminating agents have yet been identified that can functionally differentiate these receptors. To define distinct pharmacological/functional attributes of multiple D1-like receptors, we analyzed the ligand binding profiles, affinity, and functional activity of 12 novel NNC compounds at mammalian/vertebrate D1/D1A and D5/D1B, as well as vertebrate D1C/D1D, dopamine receptors transiently expressed in COS-7 cells. Of all the compounds tested, only NNC 01-0012 displayed preferential selectivity for vertebrate D1C receptors, inhibiting [³H]SCH-23390 binding with an estimated affinity (∼0.6 n M ) 20-fold higher than either mammalian/vertebrate D1/D1A or D5/D1B receptors or the D1D receptor. Functionally, NNC 01-0012 is a potent antagonist at D1C receptors, inhibiting to basal levels dopamine (10 µ M )-stimulated adenylyl cyclase activity. In contrast, NNC 01-0012 (10 µ M ) exhibits weak antagonist activity at D1A receptors, inhibiting only 60% of maximal cyclic AMP production by dopamine, while acting as a partial agonist at vertebrate D1B and D1D receptors, stimulating adenylyl cyclase activity by ∼33% relative to the full agonist dopamine (10 µ M ), an effect that was blocked by the selective D1 receptor antagonist NNC 22-0010. These data clearly suggest that the benzazepine NNC 01-0012, despite lacking the N -methyl residue in the R3 position, is a selective and potent D1C receptor antagonist. Moreover, the differential signal transduction properties exhibited by NNC 01-0012 at these receptor subtypes provide further evidence, at least in vertebrates, for the classification of the D1C receptor as a distinct D1 receptor subtype.     

8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (dinoxyline), a high affinity and potent agonist at all dopamine receptor isoforms

The synthesis and preliminary pharmacological evaluation of 8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (5, now named dinoxyline) is described. This molecule was designed as a potential bioisostere that would conserve the essential elements of our beta-phenyldopamine D(1) pharmacophore (i.e., position and orientation of the nitrogen, hydroxyls, and phenyl rings). Previously, we have rigidified these elements using alkyl bridges, as exemplified in the dopamine D(1) full agonist molecules dihydrexidine (1) and dinapsoline (2). This approach has been modified and we now show that it is possible to tether these elements using an ether linkage. Preliminary pharmacology has revealed that 5 is a potent full D(1) agonist (K(0.5) <10 nM; EC(50)=30 nM), but also has high affinity for brain D(2)-like and cloned D(2) and D(3) receptors. Interestingly, whereas 1 and 2 and their analogues have only moderate affinity for the human D(4) receptor, 5 also has high affinity for this isoform. Moreover, although N-alkylation of 1 and 2 increases D(2) affinity, the N-allyl (15) and N-n-propyl (17) derivatives of 5 had decreased D(2) affinity. Therefore, 5 may be engaging different amino acid residues than do 1 and 2 when they bind to the D(2) receptor. This is the first example of a ligand with high affinity at all dopamine receptors, yet with functional characteristics similar to dopamine. These rigid ligands also will be useful tools to determine specific residues of the receptor transmembrane domains that are critical for agonist ligand selectivity for the D(4) receptor.     

Synthesis and biological investigations of dopaminergic partial agonists preferentially recognizing the D4 receptor subtype

Aminomethyl-substituted biaryls bearing a pyrazole or triazole moiety were synthesized and investigated for dopamine and serotonin receptor binding. The N-arylpyrazoles 3b,f,g and 4 revealed Ki values in the subnanomolar range (0.28-0.70 nM) for the dopamine D4 receptor subtype. Employing both mitogenesis and GTPgammaS assays, ligand efficacy was evaluated indicating partial agonist properties. Interestingly, the tetrahydropyrimidine 4 (FAUC 2020) displayed significant intrinsic selectivity for D2(long) over D2(short).     

Design,synthesis and evaluation of bitopic arylpiperazinephenyl-1,2,4-oxadiazoles as preferential dopamine D3 receptor ligands

The dopamine D3 receptor (D3R) was proposed as a therapeutic target for drug development to treat drug abuse and addiction and neuropsychiatric disorders. Several D3R-selective modulators over the dopamine D2 receptor (D2R) can avoid extrapyramidal symptoms (EPS) and hyperprolactinemia. However, few biased D3R ligands were identified or showed a narrow range of selectivity at the D3R over D2R because of their high sequence homology. Herein, we designed, synthesized and evaluated the binding affinity of a series of bitopic ligands: arypiperazine-phenyl-1,2,4-oxadiazoles. Compound 9e·HCl was the most potent and selective D3R modulator among these bitopic ligands. Molecular modeling revealed that D3R selectivity depends on the divergence of secondary binding pocket (SBP) in D3R and D2R. Specifically, non-conserved Tyr36, EL1 especially non-conserved Thr92 and Gly94, and EL2 Val180, Cys181 and Ser182 of D3R may contribute to D3R specificity over D2R.     

Synthesis and pharmacological evaluation of substituted naphth[1,2,3-de]isoquinolines (dinapsoline analogues) as D1 and D2 dopamine receptor ligands

Dinapsoline ((2); (+/-)-dihydroxy-2,3,7,11b-tetrahydro-1H-naphth[1,2,3-de]isoquinoline) is a full D(1) dopamine agonist that also has significant D(2) receptor affinity. Based on a similar pharmacophore, dinapsoline has pharmacological similarities to dihydrexidine ((1); (+/-)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine), the first high affinity full D(1) agonist. Small alkyl substitutions on the dihydrexidine backbone are known to alter markedly the D(1):D(2) selectivity of dihydrexidine, and it was of interest to determine whether similar SAR exists within the dinapsoline series. This report describes the synthesis and pharmacological evaluation of six analogues of dinapsoline: N-allyl-(3);N-n-propyl- (4); 6-methyl- (5); 4-methyl- (6); 4-methyl-N-allyl- (7); and 4-methyl-N-n-propyl-dinapsoline (8). As expected from earlier studies with the dihydrexidine backbone, N-allyl (3) or N-n-propyl (4) analogues had markedly decreased D(1) affinity. Unexpectedly, and unlike the dihydrexidine series, these same substituents did not markedly increase D(2) affinity. The addition of a methyl group to position 6 (5) increased D(1):D(2) selectivity, but less markedly than did the analogous 2-methyl substituent added to 1. Unlike the analogous 4-methyl substituent of 1, the addition of a 4-methyl-group (6) actually decreased D(1) affinity without affecting D(2) affinity. These data demonstrate that the dinapsoline (2) backbone can be modified to produce dopamine agonists with novel properties. Moreover, as rigid ligands in which small substituents can cause significant changes in selectivity, they are important tools for deriving 'differential' SARs of the dopamine receptor isoforms.     

CODA-RET reveals functional selectivity as a result of GPCR heteromerization

Here we present a new method that combines protein complementation with resonance energy transfer to study conformational changes in response to activation of a defined G protein-coupled receptor heteromer, and we apply the approach to the putative dopamine D1-D2 receptor heteromer. Remarkably, the potency of the D2 dopamine receptor (D2R) agonist R-(-)-10,11-dihydroxy-N-n-propylnoraporphine (NPA) to change the Gα(i) conformation via the D2R protomer in the D1-D2 heteromer was enhanced ten-fold relative to its potency in the D2R homomer. In contrast, the potencies of the D2R agonists dopamine and quinpirole were the same in the homomer and heteromer. Thus, we have uncovered a molecular mechanism for functional selectivity in which a drug acts differently at a G protein-coupled receptor (GPCR) protomer depending on the identity of the second protomer participating in the formation of the signaling unit--opening the door to enhancing pharmacological specificity by targeting differences between homomeric and heteromeric signaling.     

Synthesis and sar of 2- and 3-substituted 7-azaindoles as potential dopamine D4 ligands

7-azaindole compounds bearing a cyclic amine moiety linked by a one or two carbon chain attached at the 2- or 3-position were synthesised and evaluated as potential dopamine D4 ligands. Highest affinity and selectivity for the D4 receptor resided in the 3-aminomethyl-7-azaindole series.     

Design of novel hexahydropyrazinoquinolines as potent and selective dopamine D3 receptor ligands with improved solubility

We have recently reported hexahydropyrazinoquinolines as a new class of dopamine 3 (D(3)) receptor ligands with high-affinity to the D(3) receptor and excellent selectivity over the closely related D(1)-like and D(2)-like receptors. However, our previously reported most potent and selective D(3) ligands have poor aqueous solubility, which greatly hinders our in vivo studies aimed at evaluation of their therapeutic potential in animal models. In this study, we wish to report the design, synthesis, and evaluation of a series of new hexahydropyrazinoquinolines as D(3) ligands with improved solubility. Among them, compound 4g has a K(i) value of 9.7 nM for the D(3) receptor and displays a selectivity of >5000 and 466 times over the D(1)-like and D(2)-like receptors, respectively. Importantly, the hydrochloride salt form of compound 4g has a good aqueous solubility (>50 mg/mL) and represents a promising D(3) ligand for further in vivo evaluations of its therapeutic potential for the treatment of drug abuse, restless legs syndrome, schizophrenia, Parkinson's disease, and depression.     

Discovery of 3-aryl-3-methyl-1H-quinoline-2,4-diones as a new class of selective 5-HT6 receptor antagonists

A 5,7-dichloro-3-phenyl-3-methyl-quinoline-2,4-dione (11a) has been identified in a random screen as a lead for 5-HT(6) antagonist. During the lead optimization process, several analogs were synthesized and their biological activities were investigated. Within this series, several compounds display high binding affinity and selectivity for the 5-HT(6) receptor. In particular, 3-(4-hydroxyphenyl)-3-methyl-quinoline-2,4-dione (12f) exhibits high affinity (K(i)=12.3 nM) for 5-HT(6) receptor with good selectivity over other serotonin and dopamine (D(1)-D(4)) receptor subtypes. In a functional adenylyl cyclase stimulation assay, this compound exhibited considerable antagonistic activity (IC(50)=0.61 microM).