Synthesis and structure-activity relationships of uracil derived human GnRH receptor antagonists: (R)-3-[2-(2-amino)phenethyl]-1-(2,6-difluorobenzyl)-6-methyluracils containing a substituted thiophene or thiazole at C-5

The synthesis of a series of (R)-3-[2-(2-amino)phenethyl]-1-(2,6-difluorobenzyl)-6-methyluracils containing a substituted thiophene or thiazole at C-5 is described. SAR around C-5 of the uracil led to the discovery that a 2-thienyl or (2-phenyl)thiazol-4-yl group is required for optimal receptor binding. The best compound from the series had a binding affinity of 2 nM (K(i)) for the human GnRH receptor. A novel and convenient preparation of N-1-(2,6-difluorobenzyl)-6-methyluracil is also described.     

Synthesis of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole analogues and their binding affinities for dopamine D(2) and D(3) receptors.

A series of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole derivatives was prepared and their affinity for dopamine D₂ and D₃ receptors was measured using in vitro binding assays. Several oxadiazole analogues were also prepared and tested for their affinity for dopamine D₂ and D₃ receptors. The results of receptor binding studies indicated that the incorporation of an imidazole moiety between the phenyl ring and the basic nitrogen did not significantly increase the selectivity for dopamine D₃ receptors, whereas the incorporation of an oxadiazole at the same region resulted in a total loss of affinity for both dopamine receptor subtype binding sites. The most selective compound in this series is 2-(5-bromo-2,3-dimethoxyphenyl)-4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolinomethyl)imidazole (5i), which has a D₃ receptor affinity of 21 nM and a 7-fold selectivity for D₃ versus D₂ receptors. The binding affinity for σ₁ and σ₂ receptors was also measured, and the results showed that several analogues were selective σ₁ receptor ligands.     

1,3-Dipropyl-8-(1-phenylacetamide-1H-pyrazol-3-yl)-xanthine derivatives as highly potent and selective human A(2B) adenosine receptor antagonists

A new series of 1,3-dipropyl-8-(1-phenylacetamide-1H-pyrazol-3-yl)-xanthine derivatives has been identified as potent A(2B) adenosine receptor antagonists. The products have been evaluated for their binding affinities for the human A(2B), A(1), A(2A), and A(3) adenosine receptors. N-(4-chloro-phenyl)-2-[3-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-5-methyl-pyrazol-1-yl] (11c) showed a high affinity for the human A(2B) adenosine receptor K(i)=7nM and good selectivity (A(1), A(2A), A(3)/A(2B)>140). Synthesis and SAR of this novel class of compounds is presented herein.     

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.     

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.     

Synthesis of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole analogues and their binding affinities for dopamine D2 and D3 receptors

A series of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole derivatives was prepared and their affinity for dopamine D₂ and D₃ receptors was measured using in vitro binding assays. Several oxadiazole analogues were also prepared and tested for their affinity for dopamine D₂ and D₃ receptors. The results of receptor binding studies indicated that the incorporation of an imidazole moiety between the phenyl ring and the basic nitrogen did not significantly increase the selectivity for dopamine D₃ receptors, whereas the incorporation of an oxadiazole at the same region resulted in a total loss of affinity for both dopamine receptor subtype binding sites. The most selective compound in this series is 2-(5-bromo-2,3-dimethoxyphenyl)-4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolinomethyl)imidazole (5i), which has a D₃ receptor affinity of 21 nM and a 7-fold selectivity for D₃ versus D₂ receptors. The binding affinity for σ₁ and σ₂ receptors was also measured, and the results showed that several analogues were selective σ₁ receptor ligands.     

Atropisomeric property of 1-(2,6-difluorobenzyl)-3-[(2R)-amino-2-phenethyl]-5-(2-fluoro-3-methoxyphenyl)-6-methyluracil

1-(2,6-Difluorobenzyl)-3-[(2R)-amino-2-phenethyl]-5-(2-fluoro-3-methoxyphenyl)-6-methyluracil (6), a potent and orally active antagonist of the human gonadotropin-releasing hormone receptor, exists as a pair of atropisomers in solution, which was detected by NMR spectroscopy, and separable by HPLC. In addition to a (R)-configured benzylamine, there is a second stereogenic element due to the presence of a chiral axis between the substituted 5-phenyl group and the uracil core. The rate constant of the interconversion (k = 5.07 x 10(-5) s(-1)) of these two atropisomers was determined by proton NMR analysis of a diastereoisomer-enriched sample in aqueous solution at 25 degrees C, and the corresponding Gibbs free energy DeltaG(#) of rotation barrier (97.4 kJ mol(-1)) was calculated using the Eyring equation. The diastereoisomer half-life at physiological temperature (37 degrees C) in aqueous media was estimated to be about 46 min.     

Pyrrolo(iso)quinoline derivatives as 5-HT(2C) receptor agonists

A series of 1-(1-pyrrolo(iso)quinolinyl)-2-propylamines was synthesised and evaluated as 5-HT(2C) receptor agonists for the treatment of obesity. The general methods of synthesis of the precursor indoles are described. The functional efficacy and radioligand binding data for the compounds at 5-HT(2) receptor subtypes are reported. The analogue which showed the highest 5-HT(2C) binding affinity (27, 1.6nM) was found to be successful in reducing food intake in rats.     

NMDA-NR2B subtype selectivity of stereoisomeric 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives

Enantiopure 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives were tested for their affinity to the ifenprodil binding site of the NMDA receptor, their potency to inhibit [3H]MK801 binding and their NMDA-NR2B subtype selectivity. The (1S,1'S)-configurated series displayed the highest affinity to the ifenprodil binding site. A reasonable potency and NMDA-NR2B subtype selectivity was found for (1S,1'S)-4c (R1=Me, R2=OMe). A high affinity to HERG K+ channels, however, suggests that (1S,1'S)-4c may involve an increased risk of cardiovascular side effects.     

Synthesis and anticonvulsant activity of 4-(2-(2,6-dimethylphenylamino)-2-oxoethylamino)-N-(substituted)butanamides: a pharmacophoric hybrid approach

A series of pharmacophoric hybrids of ameltolide-gamma-aminobutyric acid (GABA)-amides was designed, synthesized, and evaluated for their anticonvulsant and neurotoxic properties. Initial anticonvulsant screening was performed using intraperitoneal (ip) maximal electroshock-induced seizure (MES), subcutaneous pentylenetetrazole (scPTZ), and subcutaneous picrotoxin (scPIC)-induced seizure threshold tests. All the compounds had improved lipophilicity and the pharmacological activity profile confirmed their blood-brain barrier penetration. The titled compounds showed promising activity in scPIC screen indicating the involvement of GABA-mediation. Compound 4-(2-(2,6-dimethylaminophenylamino)-2-oxoethylamino)-N-(2,6-dimethylphenyl) butanamide (7) emerged as the most potent derivative effective in all the three animal models of seizure with no neurotoxicity at the anticonvulsant dose.     

2-(3,5-Dimethylphenyl)tryptamine derivatives that bind to the GnRH receptor

A series of 2-(3,5-dimethylphenyl)tryptamine derivatives was prepared and evaluated on a rat gonadotropin releasing hormone receptor assay. Some para-substituents on the 4-phenylbutyl side chain attached to the tryptamine nitrogen led to compounds with potent GnRH receptor binding. The study has helped define structural requirements for GnRH receptor binding for the 2-aryltryptamine GnRH antagonists.     

Synthesis and opioid activity of 2-substituted dynorphin A-(1-13) amide analogues.

A series of 2-substituted dynorphin A-(1-13) amide (Dyn A-(1-13)NH2) analogues was prepared by solid phase peptide synthesis and evaluated for opioid receptor affinities in radioligand binding assays and for opioid activity in the guinea pig ileum (GPI) assay. Amino acid substitution at the 2 position produced marked differences in both opioid receptor affinities and potency in the GPI assay; Ki values for the analogues in the radioligand binding assays and IC50 values in the GPI assay varied over three to four orders of magnitude. The parent peptide, Dyn A-(1-13)NH2, exhibited the greatest affinity and selectivity for kappa receptors and was the most potent peptide examined in the GPI assay. The most important determinant of opioid receptor selectivity and opioid potency for the synthetic analogues was the stereochemistry of the amino acid at the 2 position. Except for [D-Lys2]Dyn A-(1-13)NH2 in the kappa receptor binding assay, the analogues containing a D-amino acid at position 2 were much more potent in all of the assays than their corresponding isomers containing an L-amino acid at this position. The L-amino acid-substituted analogues generally retained some selectivity for kappa opioid receptors. The more potent derivatives with a D-amino acid in position 2, however, preferentially interacted with mu opioid receptors. Introduction of a positively charged amino acid into the 2 position generally decreased opioid receptor affinities and potency in the GPI assay.     

Norbornyllactone-substituted xanthines as adenosine A(1) receptor antagonists

During the search for second-generation adenosine A(1) receptor antagonist alternatives to the clinical candidate 8-(3-oxa-tricyclo[3.2.1.0(2,4)]oct-6-yl)-1,3-dipropyl-3,7-dihydro-purine-2,6-dione (BG9719), we developed a series of novel xanthines substituted with norbornyl-lactones that possessed high binding affinities for adenosine A(1) receptors and in vivo activity.     

Synthesis and in vivo evaluation of (S)-6-(4-fluorophenoxy)-3-((1-[11C]methylpiperidin-3-yl)methyl)-2-o-tolylquinazolin-4(3H)-one, a potential PET tracer for growth hormone secretagogue receptor (GHSR)

The peptide hormone ghrelin mediates through action on its receptor, the growth hormone secretagogue receptor (GHSR), and is known to play an important role in a variety of metabolic functions including appetite stimulation, weight gain, and suppression of insulin secretion. In light of the fact that obesity is one of the major health problems plaguing the modern society, the ghrelin signaling system continues to remain an important and attractive pharmacological target for the treatment of obesity. In vivo imaging of the GHSR could shed light on the mechanism by which ghrelin affects feeding behavior and thus offers a new therapeutic perspective for the development of effective treatments. Recently, a series of piperidine-substituted quinazolinone derivatives was reported to be selective and potent GHSR antagonists with high binding affinities. Described herein is the synthesis, in vitro, and in vivo evaluation of (S)-6-(4-fluorophenoxy)-3-((1-[(11)C]methylpiperidin-3-yl)methyl)-2-o-tolylquinazolin-4(3H)-one ([(11)C]1), a potential PET radioligand for imaging GHSR.     

AMPA receptor agonists: Resolution,configurational assignment,and pharmacology of (+)-(S)- and (-)-(R)-2-amino-3-[3-hydroxy-5-(2-pyridyl)-isoxazol-4-yl]-propionic acid (2-Py-AMPA)

We have previously shown that whereas (RS)-2-amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid (APPA) shows the characteristics of a partial agonist at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors, (S)-APPA is a full AMPA receptor agonist and (R)-APPA a weak competitive AMPA receptor antagonist. This observation led us to introduce the new pharmacological concept, functional partial agonism. Recently we have shown that the 2-pyridyl analogue of APPA, (RS)-2-amino-3-[3-hydroxy-5-(2-pyridyl)isoxazol-4-yl]propionic acid (2-Py-AMPA), is a potent and apparently full AMPA receptor agonist, and this compound has now been resolved into (+)- and (-)-2-Py-AMPA (ee ≥ 99.0%) by chiral HPLC using a Chirobiotic T column. The absolute stereochemistry of the enantiomers of APPA has previously been established by X-ray analysis, and on the basis of comparative studies of the circular dichroism spectra of the enantiomers of APPA and 2-Py-AMPA, (+)- and (-)-2-Py-AMPA were assigned the (S)- and (R)-configuration, respectively. In a series of receptor binding studies, neither enantiomer of 2-Py-AMPA showed detectable affinity for kainic acid receptor sites or different sites at the N-methyl-D-aspartic acid (NMDA) receptor complex. (+)-(S)-2-Py-AMPA was an effective inhibitor of [³H]AMPA binding (IC⁵⁰ = 0.19 ± 0.06 μM) and a potent AMPA receptor agonist in the rat cortical wedge preparation (EC₅₀ = 4.5 ± 0.3 μM) comparable with AMPA (IC₅₀ = 0.040 ± 0.01 μM; EC₅₀ = 3.5 ± 0.2 μM), but much more potent than (+)-(S)-APPA (IC₅₀ = 5.5 ± 2.2 μM; EC₅₀ = 230 ± 12 μM). Like (-)-(R)-APPA (IC₅₀ > 100 μM), (-)-(R)-2-Py-AMPA (IC₅₀ > 100 μM) did not significantly affect [³H]AMPA binding, and both compounds were week AMPA receptor antagonists (K_i = 270 ± 50 and 290 ± 20 μM, respectively). Chirality 9:274–280, 1997. © 1997 Wiley-Liss, Inc.     

Muscarinic M(3) receptor antagonists with (2R)-2-[(1R)-3,3-difluorocyclopentyl]-2-hydroxyphenylacetamide Structures. Part 2

Optimization of the amine part of our original muscarinic M(3) receptor antagonist 1 was performed to identify M(3) receptor antagonists that are superior to 1. Compounds carrying a variety of diamine moieties without hydrophobic substituent on the nitrogen atom were screened against the binding affinity for the M(3) receptor and the selectivity for M(3) over the M(1) and M(2) receptors. This process led to a 4-aminopiperidinamide (2l) with a K(i) value of 5.1 nM and with a selectivity of the M(3) receptor that was 46-fold greater than that of the M(2) receptor. Further derivatization of 2l by inserting a spacer group or by incorporating alkyl group(s) into the amine part resulted in the identification of an 4-(aminoethyl)piperidinamide 2l-b with a K(i) value of 3.7 nM for the M(3) receptor and a selectivity for the M(3) receptor that was 170-fold greater than that of the M(2) receptor.     

Evaluation of 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-(1-methyl-2-(S)-pyrrolidinylmethoxy)pyridine and its analogues as PET radioligands for imaging nicotinic acetylcholine receptors

A novel series of compounds derived from the high-affinity nicotinic acetylcholine receptor (nAChR) ligand, 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-((1-methyl-2-(S)-pyrrolidinyl)methoxy)pyridine (Me-p-PVC), originally developed by Abbott Laboratories, was characterized in vitro in nAChR binding assays at 37 degrees C to show K(i) values in the range of 9-611 pm. Several compounds of this series were radiolabeled with (11)C and evaluated in vivo in mice and monkeys as potential candidates for PET imaging of nAChRs. [(11)C]Me-p-PVC (K(i) =56 pm at 37 degrees C; logD = 1.6) was identified as a radioligand suitable for the in vivo imaging of the alpha 4 beta 2* nAChR subtype. Compared with 2-[(18)F]FA, a PET radioligand that has been successfully used in humans and is characterized by a slow kinetic of brain distribution, [(11)C]Me-p-PVC is more lipophilic. As a result, [(11)C]Me-p-PVC accumulated in the brain more rapidly than 2-[(18)F]FA. Pharmacological evaluation of Me-p-PVC in mice demonstrated that the toxicity of this compound was comparable with or lower than that of 2-FA. Taken together, these results suggest that [(11)C]Me-p-PVC is a promising PET radioligand for studying nAChR occupancy by endogenous and exogenous ligands in the brain in vivo.     

Synthesis of (1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC) derivatives modified at the carbamoyl moiety as a new class of NMDA receptor antagonists

(1S,2R)-1-Phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC, ), which is a conformationally restricted analogue of the antidepressant milnacipran [(+/-)-1], represents a new class of potent NMDA receptor antagonists. A series of PPDC analogues modified at the carbamoyl moiety were synthesized. Among these, (1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-dipropylcyclopropanecarboxamide (4d) was identified as the most potent NMDA receptor antagonist in this series and clearly reduced the MMDA receptor mediated potentiation of rat hippocampal slices, a model of long-term potentiation (LTP). The three-dimensional structure of 4d was also analyzed in detail to clarify the receptor-binding conformation.     

In vitro and in vivo pharmacological profile of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl] ethyl] piperidine (NRA0161)

Atypical antipsychotic properties of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl]ethyl] piperidine (NRA0161) were investigated by in vitro receptor affinities, in vivo receptor occupancies and findings were compared with those of risperidone and haloperidol in rodent behavioral studies. In in vitro receptor binding studies, NRA0161 has a high affinity for human cloned dopamine D(4) and 5-HT(2A) receptor with Ki values of 1.00 and 2.52 nM, respectively. NRA0161 had a relatively high affinity for the alpha(1) adrenoceptor (Ki; 10.44 nM) and a low affinity for the dopamine D(2) receptor (Ki; 95.80 nM). In in vivo receptor binding studies, NRA0161 highly occupied the 5-HT(2A) receptor in rat frontal cortex. In contrast, NRA0161 did not occupy the striatal D(2) receptor. In behavioral studies, NRA0161, risperidone and haloperidol antagonized the locomotor hyperactivity in mice, as induced by methamphetamine (MAP). At a higher dosage, NRA0161, risperidone and haloperidol dose-dependently antagonized the MAP-induced stereotyped behavior in mice and NRA0161 dose-dependently and significantly induced catalepsy in rats. The ED(50) value in inhibiting the MAP-induced locomotor hyperactivity was 30 times lower than that inhibiting the MAP-induced stereotyped behavior and 50 times lower than that which induced catalepsy.These findings suggest that NRA0161 may have atypical antipsychotic activities yet without producing extrapyramidal side effects.     

Synthesis of long-chain amide analogs of the cannabinoid CB1 receptor antagonist N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716) with unique binding selectivities and pharmacological activities

An extended series of alkyl carboxamide analogs of N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl- 1H-pyrazole-3-carboxamide (SR141716; 5) was synthesized. Each compound was tested for its ability to displace the prototypical cannabinoid ligands ([3H]CP-55,940, [3H]2; [3H]SR141716, [3H]5; and [3H]WIN55212-2, [3H]3), and selected compounds were further characterized by determining their ability to affect guanosine 5'-triphosphate (GTP)-gamma-[35S] binding and their effects in the mouse vas deferens assay. This systematic evaluation has resulted in the discovery of novel compounds with unique binding properties at the central cannabinoid receptor (CB1) and distinctive pharmacological activities in CB1 receptor tissue preparations. Specifically, compounds with nanomolar affinity which are able to fully displace [3H]5 and [3H]2, but unable to displace [3H]3 at similar concentrations, have been synthesized. This selectivity in ligand displacement is unprecedented, in that previously, compounds in every structural class of cannabinoid ligands had always been shown to displace each of these radioligands in a competitive fashion. Furthermore, the selectivity of these compounds appears to impart unique pharmacological properties when tested in a mouse vas deferens assay for CB1 receptor antagonism.