N-[2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide: a potent and selective dopamine D4 ligand

Design, synthesis, radiolabeling, and in vivo evaluation of carbon-11 labeled N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide, a potential positron emission tomography tracer for the dopamine D(4) receptors

Here we describe the design, synthesis, and evaluation of physicochemical and pharmacological properties of D(4) dopamine receptor ligands related to N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (2). Structural features were incorporated to increase affinity for the target receptor, to improve selectivity over D(2) and σ(1) receptors, to enable labeling with carbon-11 or fluorine-18, and to adjust lipophilicity within the range considered optimal for brain penetration and low nonspecific binding. Compounds 7 and 13 showed the overall best characteristics: nanomolar affinity for the D(4) receptor, >100-fold selectivity over D(2) and D(3) dopamine receptors, 5-HT(1A), 5-HT(2A), and 5-HT(2C) serotonin receptors and σ(1) receptors, and log P = 2.37-2.55. Following intraperitoneal administration in mice, both compounds rapidly entered the central nervous system. The methoxy of N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide (7) was radiolabeled with carbon-11 and subjected to PET analysis in non-human primate. [(11)C]7 time-dependently accumulated to saturation in the posterior eye in the region of the retina, a tissue containing a high density of D(4) receptors.

Chemical Modifications on 4-Arylpiperazine-Ethyl Carboxamide Derivatives Differentially Modulate Affinity for 5-HT1A, D4.2, and α2A Receptors: Synthesis and In Vitro Radioligand Binding Studies

A series of substituted 4-aryl-piperazine-ethyl heteroarylcarboxamides were prepared and tested in in vitro radioligand binding studies. The presence of a quinoxaline has a favourable impact in terms of serotonin 5-HT1A versus dopamine D4.2 receptor selectivity. Compounds with a 3-CF3 group at the distal phenyl ring are the most effective in terms of affinity and selectivity for 5-HT1A versus D4.2 receptors. A 4-phenyl-1,2,3,6-tetrahydropyridine in place of the corresponding 4-phenyl-piperazine side chain is also favourable not only for the affinity for 5-HT1A and D4.2 receptors but also in some cases for α 2A-adrenoceptors.

Antibacterial alkoxybenzamide inhibitors of the essential bacterial cell division protein FtsZ

3-Methoxybenzamide is a weak inhibitor of the essential bacterial cell division protein FtsZ. Exploration of the structure-activity relationships of 3-methoxybenzamide analogues led to the identification of potent anti-staphylococcal compounds.

Structure−Affinity Relationship Study onN-(1,2,3,4-Tetrahydronaphthalen-1-yl)-4-Aryl-1-Piperazinealkylamides, a New Class of 5-Hydroxytryptamine7Receptor Agents

A series of N-(1,2,3,4-tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinealkylamides was prepared and their affinity for serotonin (5-hydroxytryptamine, 5-HT) 5-HT7, 5-HT(1A), and 5-HT(2A) receptors was measured by in vitro binding assays. In relation to 5-HT7 receptor affinity, receptor binding studies indicated that (i) the optimal alkyl chain length was five methylenes, (ii) an unsubstituted 1,2,3,4-tetrahydronaphthalenyl nucleus was preferred, and (iii) the substitution pattern of the aryl ring linked to the piperazine ring played a crucial role. Several compound with high affinity for 5-HT7 receptors were identified. Among them, 4-(2-methoxyphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (28), 4-(2-acetylphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (34), 4-(2-methylthiophenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (44), 4-(2-hydroxyphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (46), and 4-(2-methylphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (49) were assayed for the 5-HT7 receptor-mediated relaxation of substance P-induced guinea pig ileum contraction. Compounds 28, 44, and 49 behaved as full agonists and compound 34 as a partial agonist, whereas derivative 46 acted as an antagonist. Among the compounds presented here, it emerged that 44 was identified as a potent 5-HT7 receptor agonist (Ki = 0.22 nM, EC50 = 2.56 microM), endowed with selectivity over 5-HT(1A) and 5-HT(2A) receptors (200-fold and >1000-fold, respectively).

Development of Bombesin Analogs with Conformationally Restricted Amino Acid Substitutions with Enhanced Selectivity for the Orphan Receptor Human Bombesin Receptor Subtype 3

The human bombesin receptor subtype 3 (hBRS-3) orphan receptor, which has a high homology to bombesin (Bn) receptors [gastrin-releasing peptide (GRP) and neuromedin B (NMB) receptors], is widely distributed in the rat central nervous system. Its natural ligand or role in physiology is unknown due to lack of selective ligands. Its target disruption leads to obesity, diabetes, and hypertension. A synthetic high-affinity agonist, [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14), has been described, but it is nonselective for hBRS-3 over other Bn receptors; however, substitution of (R)- or (S)-amino-3-phenylpropionic acid (Apa) for beta-Ala11 resulted in a modestly selective ligand. In the present study, we have attempted to develop a more selective hBRS-3 ligand by using two strategies: substitutions on phenyl ring of Apa11 and the substitution of other conformationally restricted amino acids into position 11 of [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14). Fifteen analogs were synthesized and affinities were determined for hBRS-3 and Bn receptors (hGRP-R and hNMB-R). Selective analogs were tested for their ability to activate each receptor by stimulating phospholipase C. One analog, [D-Tyr6,Apa-4Cl,Phe13,Nle14]Bn(6-14), retained high affinity for the hBRS-3 (Ki=8 nM) and had enhanced selectivity (>230-fold) for hBRS-3 over hGRP-R or hNMB-R. This analog specifically interacted with hBRS-3, fully activated hBRS-3 receptors, and was a potent agonist at the hBRS-3 receptor. This enhanced selectivity should allow this analog to be useful for investigating the possible role of hBRS-3 in physiological or pathological processes.

Structure-affinity relationship study on N-[4-(4-arylpiperazin-1-yl)butyl]arylcarboxamides as potent and selective dopamine D(3) receptor ligands

The benzamide PB12 (N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide) (1), already reported as potent and selective dopamine D(4) receptor ligand, has been modified searching for structural features that could lead to D(3) receptor affinity. Changes in the aromatic ring linked to N-1 piperazine ring led to the identification of 2-methoxyphenyl and 2,3-dichlorophenyl derivatives (compounds 6 and 13) displaying moderate D(3) affinity (K(i) = 145 and 31 nM, respectively). Intermediate alkyl chain elongation in compounds 1, 6, and 13 improved binding affinity for the D(3) receptor and decreased the D(4) affinity (compounds 18-26). Among these latter compounds, the N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-3-methoxybenzamide (19) was further modified with the replacement or of the 2,3-dichlorophenyl moiety (compounds 27-30) or of the 3-methoxyphenyl ring (compounds 31-41). In this way, we identified several high-affinity D(3) ligands (0.13 nM < K(i)'s < 4.97 nM) endowed with high selectivity over D(2), D(4), 5-HT(1A), and alpha(1) receptors. In addition, N-[4-[4-(2,3-dimethylphenyl)piperazin-1-yl]butyl]-3-methoxybenzamide (27) and N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-methoxy-2-benzofurancarboxamide (41) appear to be valuable candidates for positron emission tomography (PET) because of their affinity values, lipophilicity properties, and liability of (11)C labeling in the O-methyl position.

Syntheses and pharmacological evaluation of two potent antagonists for dopamine D4 receptors: [11C]YM-50001 and N-[2-[4-(4-Chlorophenyl)-piperizin-1-yl]ethyl]-3-[11C]methoxybenzamide

Two benzamide derivatives as dopamine D4 receptor antagonists, YM-50001(4) and N- [2-[4-(4-chlorophenyl]piperizin-1-yl]ethyl]-3-methoxybenzamide (9), were labeled by positron-emitter (11C), and their pharmacological specificities to dopamine D4 receptors were examined by quantitative autoradiography and positron emission tomography (PET). Radiosyntheses were accomplished by O-methylation of corresponding phenol precursors (5 and 10) with [11C]CH3I followed by HPLC purifications. In vitro binding on rat brain slices showed different distribution patterns and pharmacological properties between the two radioligands. The [11C]4 showed the highest binding in the striatum, which was inhibited not only by 10 microM 4 but also by 10 microM raclopride, a selective dopamine D2 receptor antagonist. In contrast, [11C]9 showed the highest binding in the cerebral cortex, which was inhibited by several D4 receptor antagonists (9, RBI-254, L-745,870), but not by any other receptor ligands (D1/D5, D2/D3, 5-HT1A, 5-HT2A, sigma1 and alpha1) tested. In vivo brain distribution of [11C]9 in rat showed the highest uptake in the frontal cortex, a region that has a high density of D4 receptors. These results indicate that the pharmacological property of [11C]9 matches the rat brain D4 receptors, but that of [11C]4 rather appears to match the rat brain D2 receptors. The results for the benzamide [11C]9 prompted us to further evaluate its potential as a PET radioligand for D4 receptors by employing PET on monkey brain. Unfortunately, in contrast to rats, neither specific binding nor differences in regional uptake of radioactivity were observed in monkey brain after intravenous 11C]9 injection. Based on that specific activities of radioligands might be critical in mapping the neurotransmitter receptors if they are only faintly expressed in the brain, 11C]9 with an extremely high specific activity (1810 GBq/micromol) was used for PET study. However, the effort to determine the specific binding for D4 failed. These results indicate that both of the benzamide derivatives would not be suitable radioligands for D4 receptors with PET.

Determination of dopamine D(4) receptor density in rat striatum using PB12 as a probe

N-[2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (PB12), a potent and selective dopamine D(4) receptor ligand, was used as a probe for the direct determination of the dopamine D(4) receptor density in rat striatum as an alternative to the subtraction method. The experiment was performed using [(3)H]spiroperidol to label D(2), D(3) and D(4) receptors and PB12 to determine directly dopamine D(4) receptor specific binding. The determined B(max) value was 82 fmol/mg protein. The contribution of the dopamine D(4) receptor to the overall population of D(2)-like receptors was 63%; however, this value cannot be considered reliable because of the observed difference in the kinetic profiles of D(2), D(3) and D(4) receptors.

Carbon-11 pb-12: an attempt to visualize the dopamine d(4) receptor in the primate brain with positron emission tomography

The dopamine D(4) receptor (D(4)R) is expressed in low density in various extrastriatal brain regions. This receptor subtype is discussed in relation to the pathophysiology and treatment of schizophrenia but no selective positron emission tomography (PET) ligand is available to date to study the distribution in vivo. The arylpiperazine derivative N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (PB-12) is a novel, high-affinity ( K(i)=0.040 nM) and selective D(4)R ligand. We radiolabeled PB-12 with carbon-11 (t(1/2) 20.4 min) by O-methylation of the corresponding desmethyl analogue N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-hydroxybenzamide (LM-190) with [(11)C]methyl triflate. Derivative LM-190 was prepared by condensing 3-hydroxybenzoic acid with the appropriate amine. For the radiolabeling, the incorporation yield was >90% and the total synthesis time including high performance liquid chromatography (HPLC) purification was about 35 min. The specific radioactivity of [(11)C]PB-12 at time of injection was 67-118 GBq x micromol(-1). PET studies in a cynomolgus monkey showed a high uptake and widespread distribution of radioactivity in the brain, including the neocortex and thalamus. About 40% of total radioactivity in plasma represented unchanged radioligand at 60 min after injection as determined by HPLC. Pretreatment with the D(4)R ligand 3-[[4-(4-chlorophenyl)piperazin-1-yl]methyl]-1H-pyrollo[2,3-b]pyridine (L-745,870) prior to radioligand injection failed to demonstrate receptor-specific binding in the monkey brain. Furthermore, the brain radioactivity distribution was left unaffected by pretreating with unlabeled PB-12. This failure to detect a D(4)R-specific signal may be related to a very low density of the D(4)R in primate brain, insufficient binding affinity of the radioligand, and a high background of nonspecific binding. It can be concluded from these findings that [(11)C]PB-12 is not suitable to visualize the D(4)R in the primate brain with PET.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

A structure-affinity relationship study on derivatives of N-[2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide, a high-affinity and selective D(4) receptor ligand

N-[2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (1), a high-affinity and selective dopamine D(4) receptor ligand, was chosen as a lead, and structural modifications were done on its amide bond and on its alkyl chain linking the benzamide moiety to the piperazine ring and by preparing some semirigid analogues. The binding profile at dopamine D(4) and dopamine D(2), serotonin 5-HT(1A), and adrenergic alpha(1) receptors of 16 new compounds was determined. From the results emerged that the modification of the amide bond and the elongation of the intermediate alkyl chain caused a decrease in dopamine D(4) receptor affinity. All prepared semirigid analogues displayed D(4) receptor affinity values in the same range of the opened counterparts.

Dopamine D4 receptors

Initial investigations on dopamine D4 receptors generated much interest in the role of this receptor in schizophrenia and other aspects of human behavior, as well as new opportunities for novel therapeutics. However, attempts to treat patients suffering from schizophrenia with dopamine D4 agents have failed to yield satisfactory results so far. An examination of the dopamine D4 literature shows that contrasting and conflicting data seemed to be the norm in this field of research. This paper reviews the literature on the dopamine D4 receptor and discusses many of the associated methodological problems that might have contributed to the paradoxical findings.