Design, synthesis and dopamine D4 receptor binding activities of new N-heteroaromatic 5/6-ring Mannich bases

A series of phenylpiperazine-methyl-substituted 1H-pyrrolo[2,3-c]pyridine, imidazo[1,2-c]-, pyrrolo[2,3-d]- and pyrrolo[3,2-d]pyrimidines were prepared as selective dopamine D4-ligands. The pyrrolo[2,3-d]pyrimidine derivatives 12d (K(i)=1,9 nM) and 34 d (K(i)=2,4 nM) as well as the pyrrolo[3,2-d]pyrimidine Mannich base 49f (K(i)=2,8 nM) showed high dopamine D4 receptor activity superior to the atypical antipsychotic agent clozapine.

Novel D3 selective dopaminergics incorporating enyne units as nonaromatic catechol bioisosteres: synthesis, bioactivity, and mutagenesis studies

Enynes of type 4 and 5 as long chain derivatives of the nonaromatic dopamine D 3 receptor agonist 3 (FAUC 73) were prepared by exploiting chemoselective functionalization of the azido-substituted vinyl triflate 9. Radioligand binding studies indicated excellent D 3 affinity and selectivity over related GPCRs for the terminal alkynes 4c (FAUC 460) and 5c. Biphasic displacement curves gave picomolar K i values for the high affinity binding site of D 3. According to mitogenesis experiments and bioluminescence based cAMP assays, the biphenylcarboxamide 4c and its click chemistry derived triazole analogue 5c behaved as strong partial agonists but relative ligand efficacy significantly depended on the type of functional assay. Site directed mutagenesis involving the mutants D 3 D3.32E, and D 3 F6.51W implied that ligand interactions with D3.32 and F6.51 are highly crucial, giving rise to analogous binding modes for dopamine, classical and enyne type agonists.

18F-Labeled FAUC 346 and BP 897 derivatives as subtype-selective potential PET radioligands for the dopamine D3 receptor

Disturbances of neutrotransmission at the dopamine D3 receptor are related to several neuropsychiatric diseases and in particular to drug addiction. Herein, we report the computer-assisted prediction of D3 selectivities of new fluoroalkoxy-substituted receptor ligands by means of 3D-QSAR analysis. As close analogues of the D3-selective lead compound FAUC 346 and BP 879, the (19)F-substituted test compounds 4 a-d were synthesized and evaluated. In vitro investigation of their binding characteristics in transfected Chinese Hamster Ovary (CHO) cells led to excellent K(i) values between 0.12 and 0.69 nM at the dopamine D3 subtype. The benzothiophene-substituted carboxamide 4 a (K(i)=0.12 nM) displayed 133 and 283-fold selectivity over the structurally related D2(Long) and D4 subtypes, respectively. Mitogenesis assays showed the behavior of partial agonists. Based on these data, we synthesized the [(18)F]fluoroethoxy-substituted radioligands [(18)F]4 a-d. The N-[4-[4-(2-hydroxyphenyl)piperazin-1-yl]butyl]-2-carboxamides 3 a-d were prepared and labeled with 2-[(18)F]fluoroethyltosylate in a two-step procedure. Optimization of the (18)F-labeling conditions led to radiochemical yields between 24 and 65 %.

Click chemistry based solid phase supported synthesis of dopaminergic phenylacetylenes

'Click resins' enable solid phase supported reactions to work under nearly perfect conditions fulfilling the requirements of click chemistry. Utilizing the formylpyrrolylmethyltriazole (FPMT) linker 6, which is readily available via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), a BAL strategy could be successfully applied for a parallel synthesis of dopaminergic phenylacetylens. A focused library of 20 test compounds revealing three points of diversity was generated by a four-step SPOS approach including microwave assisted Sonogashira coupling. GPCR-ligand binding assays indicated excellent dopamine D3 and D4 receptor binding affinities which were identified to cause a partial agonist activity for the most potent test compounds 2c,e,i,k.

In vitro affinities of various halogenated benzamide derivatives as potential radioligands for non-invasive quantification of D2-like dopamine receptors

Benzamide derivatives as radiotracers have played an important role in diagnosing malfunction in dopaminergic neurotransmission. A variety of halogenated and two unsubstituted benzamide derivatives were synthesised and their in vitro affinities to dopaminergic, serotonergic and adrenergic receptors and their lipophilicities were determined. As references IBZM (3), raclopride (4) and FLB457 (5) were tested as well. The two iodinated compounds NAE (27) and NADE (28) displayed K(i) values of 0.68 and 14 nM for the D(2) receptor. The well-established radiotracers FP (1) and DMFP (2) showed affinities in the same range as did the brominated compounds NABrE (29) and NABrDE (30). The log D(7.4) values of 2.91 for NAE (27) and of 2.81 for NADE (28) are in the range of those found for IBZM (3), FP (1) and DMFP (2). These facts allow to expect good properties for the two iodinated compounds NAE (27) and NADE (28) regarding in vivo imaging with SPECT.

Structure−Selectivity Investigations of D2-Like Receptor Ligands by CoMFA and CoMSIA Guiding the Discovery of D3 Selective PET Radioligands

Elucidation of the physiological role of the D3 receptor and its distribution in the brain using positron emission tomography (PET) is hampered by the lack of bioavailable subtype selective tracer ligands. To develop appropriate D3 radioligands, we designed an integrative procedure involving the elucidation of structural features determining D3 selectivity over both congeners D2 and D4 by comparative molecular analysis. Thus, we have successfully generated CoMFA and CoMSIA models based on the affinitiy differences of a series of 79 ligands representing a broad range of selectivities. These models yielded highly significant cross-validations (q2cv(D3/D2) = 0.86; q2cv(D3/D4) = 0.92) and excellent predictions of a 16-ligand test set (r2pred = 0.79-0.93). Exploiting this information, synthesis and receptor binding studies directed us to the fluorinated lead compounds 78 and 79, featuring subnanomolar D3 affinities and considerable selectivities over D2 and D4 and, subsequently, to the subtype selective PET tracers [18F]78 and [18F]79.

CoMFA and CoMSIA investigations of dopamine D3 receptor ligands leading to the prediction, synthesis, and evaluation of rigidized FAUC 365 analogues

Taking advantage of our in-house experimental data on dopamine D3 receptor modulators, we have successfully established highly significant CoMFA and CoMSIA models (q(cv)2 = 0.82/0.76). These models were carefully investigated to assure their stability and predictivity (r(pred)2 = 0.65/0.61) and subsequently applied to guide experimental investigations on the synthesis and receptor binding of three conformationally restricted D3 ligands. Besides the high D3 affinity, the test compound 45, incorporating a trans-1,4-cyclohexylene partial structure, exhibited improved (approximately 3200-fold) selectivity over the D4 subtype.

2-[(4-phenylpiperazin-1-yl)methyl]imidazo(di)azines as selective D4-ligands. Induction of penile erection by 2-[4-(2-methoxyphenyl)piperazin-1-ylmethyl]imidazo[1,2-a]pyridine (PIP3EA), a potent and selective D4 partial agonist

A series of novel 2-[(4-phenylpiperazin-1-yl)methyl]imidazoazines and aza-analogues were prepared and screened at selected dopamine, serotonin, and adrenergic receptor subtypes. 2-Substituted imidazopyridines and pyridazines presented high affinities and selectivities for D4 dopamine receptors. Whereas functional experiments indicated neutral antagonists or weak partial agonist effects for most of the target compounds, the 2-methoxyphenyl substituted 2-piperazinylmethylimidazopyridine 3c (PIP3EA) displayed substantial agonist efficacy in mitogenesis experiments and GTPgammaS binding tests, resulting in EC50 values of 3.0 (46%) and 4.5 nM (57%), respectively. Our D4 agonist 3c induced penile erection in vivo when administered to rats. This effect was inhibited by L-745,870 a D4 selective antagonist, confirming the mechanistic pathway.

Fancy bioisosteres: novel paracyclophane derivatives as super-affinity dopamine D3 receptor antagonists

The exploration of the chemical diversity space depends on the discovery of novel bioisosteric elements. As a continuation of our project on bilayered arene surrogates, we herein report on [2.2]paracyclophane-derived dopamine D3 receptor antagonists of type 4 and 6. For the most promising test compound 6a, bearing a 2-methoxyphenyl substituent, a stereocontrolled preparation was performed when the planar chirality of enantiomers (R)-6a (FAUC 418) and (S)-6a caused a considerable differentiation of D3 binding, which is indicated by K(i) values of 0.19 and 3.0 nM, respectively. Functional experiments showed D3 antagonist properties for the paracyclophane derivatives of type 6. To elucidate putative bioactive low-energy conformations, DFT-based studies including the calculation of diagnostic magnetic shielding properties were performed. An 89% increase in volume for the [2.2]paracyclophane moiety compared to that of the monolayered benzofurane of lead compound 3b indicates higher plasticity of GPCR binding regions than usually expected.

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).

Tissue distribution of radioiodinated FAUC113: assessment of a pyrazolo(1,5-a) pyridine based dopamine D4 receptor radioligand candidate

AIM

Disturbances of the D4 receptor subtype have been implicated in the genesis of a broad range of psychiatric disorders. In order to assess the suitability of a radioiodinated analogue of the D4-selective ligand FAUC 113 for tracer studies in vivo, we investigated the in-vivo stability, biodistribution and brain-uptake of 7-(131)I-FAUC 113 in Sprague-Dawley rats.

METHODS

Radiolabelling was carried out with high radiochemical yield and specific activity. After intravenous injection, blood and tissue samples, taken at designated time intervals, were collected for analysis. Analyses of metabolites were performed by radio-hplc and radio-tlc. For in-vivo evaluation, sagittal cryo-sections of the rat brain were investigated by in-vitro and ex-vivo autoradiography on a mu-Imager system.

RESULTS

7-(131)I-FAUC 113 was rapidly cleared from blood. Highest uptake was observed in kidney (0.603 +/- 0.047% ID/g, n = 4) and liver (0.357 +/- 0.070% ID/g, n = 4) at 10 min p.i.; 7-(131)I-FAUC 113 displayed rapid uptake (0.21-0.26% ID/g) and fast clearance in various brain regions consistent with the determined logP-value of 2.36 +/- 0.15 (n = 4). In-vivo stability of 7-(131)I-FAUC 113 was confirmed in the frontal cortex (>95%). Ex-vivo autoradiography revealed a frontal cortex-to-cerebellum ratio of 1.57 +/- 0.13 at 10 min p.i. (n = 6). Coinjection with L-750667 could not suppress any putative specific binding of 7-(131)I-FAUC 113. In-vitro autoradiography using authentic 7-iodo-FAUC 113 or L-750667 failed to cause significant displacement of the radioligand.

CONCLUSIONS

Radioiodinated FAUC 113 does not allow imaging of D4 receptors in the rat brain in vivo nor in vitro. Further work should aim at the development of selective dopamine D4 radioligands with improved tracer characteristics, such as receptor affinity and subtype selectivity, specific activity or blood-brain barrier permeability.