Enantiomerically Pure 1,3-Dioxanes as Highly Selective NMDA and σ1 Receptor Ligands

Quinolone bioisosteres of phenolic GluN2B-selective NMDA receptor antagonists

Cyclopenta[g]quinolones of type 4 were designed with the aim to bioisosterically replace the phenol of potent GluN2B ligands such as ifenprodil and Ro 25-6981 by the quinolone system and to restrict the conformational flexibility of the aminopropanol substructure in a cyclopentane system. The designed ligands were synthesized in an eight-step sequence starting with terephthalaldehyde (5). Key steps pf the synthesis were the intramolecular Friedel-Crafts acylation of propionic acids 10 to yield the cyclopenta[g]quinolinediones 11 and the Mannich reaction of diketone 11a followed by conjugate addition at the α,β-unsaturated ketone 12a. Although the quinolones 13a, 15a, and 16a contain an H-bond donor group (secondary lactam) as ifenprodil and Ro 25-6981, they show only moderate GluN2B affinity (Ki > 410 nM). However, the introduction of lipophilic substituents at the quinolone N-atom resulted in more than 10-fold increased GluN2B affinity of the benzyl and benzyloxymethyl derivatives cis-13c (Ko = 36 nM) and 13e (Ko = 27 nM). All compounds are selective over the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor. The benzyl derivative 13c showed six- and threefold selectivity over σ1 and σ2 receptors, respectively.

Phenol-benzoxazolone bioisosteres of GluN2B-NMDA receptor antagonists: Unexpected rearrangement during reductive alkylation with phenylcyclohexanone

Negative allosteric modulators of N-methyl- d-aspartate receptors containing the GluN2B subunit represent promising drug candidates for the treatment of various neurological disorders including stroke, epilepsy, and Parkinson's disease. To increase the bioavailability and GluN2B affinity, the phenol of the potent benzazepine-based inhibitor, WMS-1410 (3), was replaced bioisosterically by a benzoxazolone moiety and the phenylbutyl side chain was conformationally restricted in a phenylcyclohexyl substituent. A four-step, one-pot procedure transformed the oxazolo-benzazepine 7 into the phenylcyclohexyl derivative 11. The same protocol was applied to the methylated analog 12, which unexpectedly led to ring-contracted oxazolo-isoquinolines 18. This rearrangement was explained by the additional methyl moiety in the 8-position inhibiting the formation of the planar intermediate iminium ion with phenylcyclohexanone. The allyl protective group of 11 and 18 was removed with RhCl₃ and HCl to obtain the tricyclic compounds 5 and 19 without substituent at the oxazolone ring. The structures of the rearranged products 18 and 19 were elucidated by X-ray crystal structure analysis. The oxazolo-isoquinoline trans-18 with allyl moiety (K_i  = 89 nM) and the oxazolo-benzazepine 5 without substituent at the oxazolone ring (K_i  = 114 nM) showed GluN2B affinity in the same range as the lead compound 3. In two-electrode voltage clamp measurements, 5 displayed only weak inhibitory activity.

Negative allosteric modulators of NMDA receptors with GluN2B subunit: Alanine-derived benzoxazolone bioisosteres of 2-methyl-3-benzazepine-1,7-diols

Inspired by besonprodil, the phenol of potent negative allosteric modulators of GluN2B-N-methyl-d-aspartate (NMDA) receptors was replaced by a benzoxazolone system. To increase the similarity to the lead compounds, an additional methyl moiety was installed in the 8-position of tricyclic oxazolobenzazepines, resulting in compounds 6. The additional methyl moiety originates from alanine, which was introduced by a Mitsunobu reaction of benzoxazolylethanol 7 with N-triflyl-protected alanine methyl ester. A crucial feature of the synthesis was the protection of the oxazolone ring by an allyl moiety, which was cleaved off at the end of the synthesis by RhCl₃ -catalyzed isomerization. Due to the additional methyl moiety, the intramolecular Friedel-Crafts acylation of acid 10 to afford ketone 11 required careful optimization to minimize the formation of the side product tetrahydroisoquinoline 16. Alkylation or reductive alkylation of secondary amine 13 led to diastereomeric oxazolobenzazepines cis-14 and trans-14, which were separated by flash chromatography. Phenylbutyl derivatives cis-6a and trans-6a revealed twofold higher GluN2B affinity than analog 5a without 8-CH₃ group. The methylated oxazolobenzazepines 6 and 14 did not interact with the phencyclidine binding site of NMDA receptors and σ₂ receptors. However, the σ₁ receptor preferred cis-configured oxazolobenzazepines. The highest σ₁ receptor affinities were obtained for cis-14a (K_i  = 26 nM) and cis-6b (K_i  = 30 nM).

Phenol-Benzoxazolone bioisosteres: Synthesis and biological evaluation of tricyclic GluN2B-selective N-methyl- d-aspartate receptor antagonists

Tricyclic tetrahydrooxazolo[4,5-h]-[3]benzazepin-9-ols 22 were designed as phenol bioisosteres of tetrahydro-3-benzazepine-1,7-diols. Key features of the synthesis are the introduction of the trifluoromethylsulfonyl and allyl protective groups at the heterocyclic N-atoms. Two methods were developed to convert the triflyl-protected ketone 16 into tricyclic alcohols 21 bearing various N-substituents. According to the first method, trifluoromethanesulfinate was removed by K₂ CO₃ . Following the selective reduction of the imino moiety of 17 with NaBH(OAc)₃ afforded the aminoketone 18, which was reductively alkylated and reduced. According to the second method, both the imine and the ketone of the iminoketone 17 were reduced with NaBH₄ to yield the aminoalcohol 20, which was alkylated or reductively alkylated to form tertiary amines 21f-21r. In the last step, the allyl protective group of 21 was removed with RhCl₃ and HCl to obtain oxazolones 22. In receptor binding studies using [³ H]ifenprodil as radioligand ketone, 22m showed the highest GluN2B affinity (K_i  = 88 nM). However, a reduced affinity toward GluN2B subunit-containing N-methyl- d-aspartate (NMDA) receptors was observed for oxazolones 22 compared to bioisosteric 3-benzazepine-1,7-diols. High selectivity of 22m for the ifenprodil binding site of GluN2B-NMDA receptors over the 1-(1-phenylcyclohexyl)piperidine binding site and σ₂ receptors was observed, but only negligible selectivity over σ₁ receptors. In two-electrode voltage clamp experiments, the 4-phenylbutyl derivative 22d (K_i  = 422 nM) demonstrated 80% inhibition of ion flux at a concentration of 1 µM. The differences in GluN2B affinity and inhibitory activity are explained by docking studies. In conclusion, 22d is regarded as a novel scaffold of highly potent GluN1/GluN2B antagonists.

Synthesis of aminoethyl substituted piperidine derivatives as σ1 receptor ligands with antiproliferative properties

A series of novel σ₁ receptor ligands with a 4‐(2‐aminoethyl)piperidine scaffold was prepared and biologically evaluated. The underlying concept of our project was the improvement of the lipophilic ligand efficiency of previously synthesized potent σ₁ ligands. The key steps of the synthesis comprise the conjugate addition of phenylboronic acid at dihydropyridin‐4(1H)‐ones 7, homologation of the ketones 8 and introduction of diverse amino moieties and piperidine N‐substituents. 1‐Methylpiperidines showed particular high σ₁ receptor affinity and selectivity over the σ₂ subtype, whilst piperidines with a proton, a tosyl moiety or an ethyl moiety exhibited considerably lower σ₁ affinity. Molecular dynamics simulations with per‐residue binding free energy deconvolution demonstrated that different interactions of the basic piperidine‐N‐atom and its substituents (or the cyclohexane ring) with the lipophilic binding pocket consisting of Leu105, Thr181, Leu182, Ala185, Leu186, Thr202 and Tyr206 are responsible for the different σ₁ receptor affinities. Recorded logD_7.4 and calculated clogP values of 4a and 18a indicate low lipophilicity and thus high lipophilic ligand efficiency. Piperidine 4a inhibited the growth of human non‐small cell lung cancer cells A427 to a similar extent as the σ₁ antagonist haloperidol. 1‐Methylpiperidines 20a, 21a and 22a showed stronger antiproliferative effects on androgen negative human prostate cancer cells DU145 than the σ₁ ligands NE100 and S1RA.

Chemoenzymatic synthesis of 2,6-disubstituted tetrahydropyrans with high σ1 receptor affinity, antitumor and analgesic activity

1,3-Dioxanes 1 and cyclohexanes 2 bearing a phenyl ring and an aminoethyl moiety in 1,3-relationship to each other represent highly potent σ1 receptor antagonists. In order to increase the chemical stability of the acetalic 1,3-dioxanes 1 and the polarity of the cyclohexanes 2, tetrahydropyran derivatives 3 equipped with the same substituents were designed, synthesized and pharmacologically evaluated. The key step of the synthesis was a lipase-catalyzed enantioselective acetylation of the alcohol (R)-5 leading finally to enantiomerically pure test compounds 3a-g. With respect to σ1 receptor affinity and selectivity over a broad range of related (σ2, PCP binding site) and further targets, the enantiomeric benzylamines 3a and cyclohexylmethylamines 3b represent the most promising drug candidates of this series. However, the eudismic ratio for σ1 binding is only in the range of 2.5-3.3. Classical molecular dynamics (MD) simulations confirmed the same binding pose for both the tetrahydropyran 3 and cyclohexane derivatives 2 at the σ1 receptor, according to which: i) the protonated amino moiety of (2S,6R)-3a engages the same key polar interactions with Glu172 (ionic) and Phe107 (π-cation), ii) the lipophilic parts of (2S,6R)-3a are hosted in three hydrophobic regions of the σ1 receptor, and iii) the O-atom of the tetrahydropyran derivatives 3 does not show a relevant interaction with the σ1 receptor. Further in silico evidences obtained by the application of free energy perturbation and steered MD techniques fully supported the experimentally observed difference in receptor/ligand affinities. Tetrahydropyrans 3 require a lower dissociative force peak than cyclohexane analogs 2. Enantiomeric benzylamines 3a and cyclohexylmethylamines 3b were able to inhibit the growth of the androgen negative human prostate cancer cell line DU145. The cyclohexylmethylamine (2S,6R)-3b showed the highest σ1 affinity (Ki(σ1) = 0.95 nM) and the highest analgesic activity in vivo (67%).

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.

Novel σ1 antagonists designed for tumor therapy: Structure - activity relationships of aminoethyl substituted cyclohexanes

Depending on the substitution pattern and stereochemistry, 1,3-dioxanes 1 with an aminoethyl moiety in 4-position represent potent σ1 receptor antagonists. In order to increase the stability, a cyclohexane ring first replaced the acetalic 1, 3-dioxane ring of 1. A large set of aminoethyl substituted cyclohexane derivatives was prepared in a six-step synthesis. All enantiomers and diastereomers were separated by chiral HPLC at the stage of the primary alcohol 7, and their absolute configuration was determined by CD spectroscopy. Neither the relative nor the absolute configuration had a large impact on the σ1 affinity. The highest σ1 affinity was found for cis-configured benzylamines (1R,3S)-11 (Ki = 0.61 nM) and (1S,3R)-11 (Ki = 1.3 nM). Molecular dynamics simulations showed that binding of (1R,3S)-11 at the σ1 receptor is stabilized by the typical polar interaction of the protonated amino moiety with the carboxy group of E172 which is optimally oriented by an H-bond interaction with Y103. The lipophilic interaction of I124 with the N-substituent also contributes to the high σ1 affinity of the benzylamines. The antagonistic activity was determined in a Ca2+ influx assay in retinal ganglion cells. The enantiomeric cis-configured benzylamines (1R,3S)-11 and (1S,3R)-11 were able to inhibit the growth of DU145 cells, a highly aggressive human prostate tumor cell line. Moreover, cis-11 could also inhibit the growth of further human tumor cells expressing σ1 receptors. The experimentally determined logD7.4 value of 3.13 for (1R,3S)-11 is in a promising range regarding membrane penetration. After incubation with mouse liver microsomes and NADPH for 90 min, 43% of the parent (1R,3S)-11 remained unchanged, indicating intermediate metabolic stability. Altogether, nine metabolites including one glutathione adduct were detected by means of LC-MS analysis.

Fluorinated 2-Arylcyclopropan-1-amines - A new class of sigma receptor ligands

Stereoisomeric 2-aryl-2-fluoro-cyclopropan-1-amines have been discovered as a new class of σ receptor ligands showing different selectivity for the two subtypes of the receptor. Generally, compounds substituted in 4-position are much more active than corresponding 3-substituted isomers. trans-2-Fluoro-2-(4-methoxyphenyl)cyclopropan-1-amine (19a) was the most potent (K_i = 4.8 nM) σ₁ receptor ligand, while cis-2-fluoro-2-(4-trifluoromethylphenyl)cyclopropan-1-amine (20b) was the most potent (K_i = 95 nM) σ₂ receptor ligand.

Synthesis and pharmacological evaluation of fluorinated benzo[7]annulen-7-amines as GluN2B-selective NMDA receptor antagonists

Because of their neuroprotective potential, GluN2B-selective ligands are of great interest for the treatment of various neurological and neurodegenerative disorders. Fluorinated benzo[7]annulen-7-amines, capable for PET, were synthesized by combining fluorinated phenylalkylamines with differently substituted ketones. Relationships between substitution pattern and GluN2B affinity as well as selectivity towards σ₁ and σ₂ receptors were investigated. Two promising ligands (18a and 20c) were selected for further pharmacological evaluation. Besides a slight serotonin transporter (SERT), norepinephrine transporter (NET), and hERG affinity, they did not show interaction with other targets. Furthermore, the pK_a value of a set fluorinated ligands, bearing the fluorine atom in different positions, was determined.

Negative allosteric modulators of the GluN2B NMDA receptor with phenylethylamine structure embedded in ring-expanded and ring-contracted scaffolds

A set of GluN2B NMDA receptor antagonists with conformationally restricted phenylethylamine substructure was prepared and pharmacologically evaluated. The phenylethylamine substructure was embedded in ring expanded 3-benzazocines 4 as well as ring-contracted tetralinamines 6 and indanamines 7. The ligands 4, 6 and 7 were synthesized by reductive alkylation of secondary amine 11, reductive amination of ketones 12 and 16 and nucleophilic substitution of nosylates 14 and 17. The moderate GluN2B affinity of 3-benzazocine 4d (K_i = 32 nM) translated into moderate cytoprotective activity (IC₅₀ = 890 nM) and moderate ion channel inhibition (60% at 10 μM) in two-electrode voltage clamp experiments with GluN1a/GluN2B expressing oocytes. Although some of the tetralinamines 6 and indanamines 7 showed very high GluN2B affinity (e.g. K_i (7f) = 3.2 nM), they could not inhibit glutamate/glycine inducted cytotoxicity. The low cytoprotective activity of 3-benzazocines 4, tetralinamines 6 and indanamines 7 was attributed to the missing OH moiety at the benzene ring and/or in benzylic position. Docking studies showed that the novel GluN2B ligands adopt similar binding poses as Ro 25-6981 with the central H-bond interaction between the protonated amino moiety of the ligands and the carbamoyl moiety of Gln110. However, due to the lack of a second H-bond forming group, the ligands can adopt two binding poses within the ifenprodil binding pocket.

Pharmacological characterization of high-affinity σ1 receptor ligands with spirocyclic thienopyran and thienofuran scaffold

OBJECTIVES

In this study, the pharmacological properties of six spirocyclic piperidines 1-6 showing very high σ1 receptor affinity (Ki  = 0.2-16 nm) were investigated.

METHODS

In vitro receptor binding studies, retinal ganglion assay and in vivo capsaicin assay were used to determine the affinity, selectivity and activity. Influence on human tumour cell growth (cell lines A427, LCLC-103H, 5637 and DAN-G) was determined in different assays. The effect on the ergosterol and cholesterol biosynthesis was determined by GLC/MS analysis.

KEY FINDINGS

Receptor binding studies demonstrated high selectivity for the σ1 receptor. The increased Ca2+ influx mediated by 2 and the analgesic activity of 1, 4, 5 and 6 confirm σ1 receptor antagonistic activity. Inhibition of human tumour cell growth further supports the σ1 antagonistic effects. Treatment of A427 tumour cells with 2 led to cell detachment and cell degradation. Whereas the ergosterol biosynthesis was not affected, the sterol C14-reductase, a key enzyme in the cholesterol biosynthesis, was weakly inhibited.

CONCLUSIONS

Due to the high selectivity, off-target effects are not expected. The antiallodynic activity underlines the clinical potential of the spirocyclic piperidines for the treatment of neuropathic pain. Due to the antiproliferative activity, the spirocyclic σ1 antagonists represent promising antitumour agents.

Impact of hydroxy moieties at the benzo[7]annulene ring system of GluN2B ligands: Design, synthesis and biological evaluation

In this study, the impact of one or two hydroxy moieties at the benzo[7]annulene scaffold on the GluN2B affinity and cytoprotective activity was analyzed. The key intermediate for the synthesis of OH-substituted benzo[7]annulenamines 11-13 and 17 was the epoxyketone 8. Reductive epoxide opening of 8 resulted with high regioselectivity in the 5-hydroxyketone 9 (Pd(OAc)₂, HCO₂H, phosphane ligand) or the 6-hydroxyketone 10 (H₂, Pd/C), whereas hydrolysis in aqueous dioxane led to the dihydroxyketone 14. Reductive amination of these ketones with primary amines and NaBH(OAc)₃ afforded the benzo[7]annulenamines 11-13 and 17. In receptor binding studies 5-OH derivatives 11 and 12 showed higher GluN2B affinity than 6-OH derivatives 13, which in turn were more active than 5,6-di-OH derivative 17a. The same order was found for the cytoprotective activity of the ligands. The tertiary amine 12a with one OH moiety in 5-position represents the most promising GluN2B negative allosteric modulator with a binding affinity of K_i = 49 nM and a cytoprotective activity of IC₅₀ = 580 nM. In the binding pocket 12a shows a crucial H-bond between the benzylic OH moiety and the backbone carbonyl O-atom of Ser132 (GluN1b). It was concluded that a 5-OH moiety is essential for the inhibition of the NMDA receptor associated ion channel, whereas a OH moiety in 6-position is detrimental for binding and inhibition. An OH or CH₂OH moiety at 2-position results in binding at the ifenprodil binding site, but very weak ion channel inhibition.

Tetrahydro-3-benzazepines with fluorinated side chains as NMDA and σ1 receptor antagonists: Synthesis, receptor affinity, selectivity and antiallodynic activity

The class of tetrahydro-1H-3-benzazepines was systematically modified in 1-, 3- and 7-position. In particular, a F-atom was introduced in β- or γ-position of the 4-phenylbutyl side chain in 3-position. Ligands with the F-atom in γ-position possess higher GluN2B affinity than analogs bearing the F-atom in β-position. This effect was attributed to the reduced basicity of β-fluoro amines. 3-Benzazepines with a benzylic OH moiety show moderate GluN2B affinity, but considerable selectivity over the σ2 receptor. However, removal of the benzylic OH moiety led to increased GluN2B affinity, but reduced GluN2B/σ2 selectivity. With respect to GluN2B affinity the phenol 17b with a γ-fluorophenylbutyl moiety in 3-position represents the most interesting fluorinated ligand (Ki(GluN2B) = 16 nM). Most of the synthesized ligands reveal either similar GluN2B and σ1 affinity or higher σ1 affinity than GluN2B affinity. The methyl ether 16b shows high σ1 affinity (Ki(σ1) = 6.6 nM) and high selectivity over a broad panel of receptors and transporters. The high antiallodynic activity in the mouse capsaicin assay proved the σ1 antagonistic activity of 16b.

Modification of the 4-phenylbutyl side chain of potent 3-benzazepine-based GluN2B receptor antagonists

Excitotoxicity driven by overactivation of NMDA receptors represents a major mechanism of acute and chronic neurological and neurodegenerative disorders. Negative allosteric modulators interacting with the ifenprodil binding site of the NMDA receptor are able to interrupt this ongoing neurodamaging process. Starting from the potent 3-benzazepine-1,7-diol 4a novel NMDA receptor antagonists were designed by modification of the N-(4-phenylbutyl) side chain. With respect to developing novel fluorinated PET tracers, regioisomeric fluoroethoxy derivatives 11, 12, 14, and 15 were synthesized. Analogs 19 and 20 with various heteroaryl moieties at the end of the N-side chain were prepared by Sonogashira reaction and nucleophilic substitution. The fluoroethyl triazole 37 was obtained by 1,3-dipolar cycloaddition. In several new ligands, the flexibility of the (hetero)arylbutyl side chain was restricted by incorporation of a triple bond. The affinity towards the ifenprodil binding site was tested in an established competition assay using [³H]ifenprodil as radioligand. Introduction of a fluoroethoxy moiety at the terminal phenyl ring, replacement of the terminal phenyl ring by a heteroaryl ring and incorporation of a triple bond into the butyl spacer led to considerable reduction of GluN2B affinity. The phenol 15 (K_i = 193 nM) bearing a p-fluoroethoxy moiety at the terminal phenyl ring represents the most promising GluN2B ligand of this series of compounds. With exception of 15 showing moderate σ₂ affinity (K_i = 79 nM), the interaction of synthesized 3-benzazepines towards the PCP binding site of the NMDA receptor, σ₁ and σ₂ receptors was rather low (K_i > 100 nM).

Synthesis and Structure-Affinity Relationships of Spirocyclic Benzopyrans with Exocyclic Amino Moiety

σ1 and/or σ2 receptors play a crucial role in pathological conditions such as pain, neurodegenerative disorders, and cancer. A set of spirocyclic cyclohexanes with diverse O-heterocycles and amino moieties (general structure III) was prepared and pharmacologically evaluated. In structure-activity relationships studies, the σ1 receptor affinity and σ1:σ2 selectivity were correlated with the stereochemistry, the kind and substitution pattern of the O-heterocycle, and the substituents at the exocyclic amino moiety. cis-configured 2-benzopyran cis-11b bearing a methoxy group and a tertiary cyclohexylmethylamino moiety showed the highest σ1 affinity ( Ki = 1.9 nM) of this series of compounds. In a Ca2+ influx assay, cis-11b behaved as a σ1 antagonist. cis-11b reveals high selectivity over σ2 and opioid receptors. The interactions of the novel σ1 ligands were analyzed on the molecular level using the recently reported X-ray crystal structure of the σ1 receptor protein. The protonated amino moiety forms a persistent salt bridge with E172. The spiro[benzopyran-1,1'-cyclohexane] scaffold and the cyclohexylmethyl moiety occupy two hydrophobic pockets. Exchange of the N-cyclohexylmethyl moiety by a benzyl group led unexpectedly to potent and selective μ-opioid receptor ligands.

Role of the NMDA Receptor in the Antitumor Activity of Chiral 1,4-Dioxane Ligands in MCF-7 and SKBR3 Breast Cancer Cells

The potent N-methyl-d-aspartate (NMDA) receptor antagonists 1-3 have been demonstrated to show antiproliferative and cytotoxic effects in MCF-7 and SKBR3 breast cancer cell lines. To improve the knowledge about the role played by the NMDA receptor in the antitumor activity of these compounds, the enantiomers of 1 were prepared and evaluated for their affinity for the phencyclidine (PCP) site of the NMDA receptor and for their cytotoxic effect in MCF-7 and SKBR3 cell lines, both expressing the NMDA receptor. The (S)-1 enantiomer, showing negligible affinity for the PCP site, exhibited antiproliferative activity higher than that of (R)-1, which instead bound the PCP site. The downregulation of NMDA GluN1 expression resulted in a decreased (S)-1-induced cytotoxicity and apoptotic cell death, unequivocally demonstrating the involvement of the NMDA receptor in the antitumor effect of this compound. Due to its interesting biological profile, (S)-1 represents a lead compound to develop novel antitumor agents for breast cancer treatment.

2-Methyltetrahydro-3-benzazepin-1-ols - The missing link in SAR of GluN2B selective NMDA receptor antagonists

The NMDA receptor containing GluN2B subunits represents a promising target for the development of drugs for the treatment of various neurological disorders including neurodegenerative diseases. In order to study the role of CH₃ and OH moieties trisubstituted tetrahydro-3-benzazepines 4 were designed as missing link between tetra- and disubstituted 3-benzazepines 2 and 5. The synthesis of 4 comprises eight reaction steps starting from alanine. The intramolecular Friedel-Crafts acylation to obtain the ketone 12 and the base-catalyzed elimination of trifluoromethanesulfinate (CF₃SO₂^-) followed by NaBH₄ reduction represent the key steps. The GluN2B affinity of the cis-configured 3-benzazepin-1-ol cis-4a with a 4-phenylbutyl side chain (K_i = 252 nM) is considerably lower than the GluN2B affinity of (R,R)-2 (K_i = 17 nM) indicating the importance of the phenolic OH moiety for the interaction with the receptor protein. Introduction of an additional CH₃ moiety in 2-position led to a slight decrease of GluN2B affinity as can be seen by comparing the affinity data of cis-4a and 5. The homologous phenylpentyl derivative cis-4b shows the highest GluN2B affinity (K_i = 56 nM) of this series of compounds. According to docking studies cis-4a adopts the same binding mode as the cocrystallized ligand ifenprodil-keto 1A and 5 at the interface of the GluN2B and GluN1a subunits. The same crucial H-bonds are formed between the C(O)NH₂ moiety of Gln110 within the GluN2B subunit and the protonated amino moiety and the OH moiety of (R,R)-cis-4a.

Solid-phase organic synthesis of chiral, non-racemic 1,2,4-trisubstituted 1,4-diazepanes with high σ1 receptor affinity

The aim of this work was to transfer the established chiral-pool synthesis of 1,2,4-trisubstituted 1,4-diazepanes in solution on the solid phase. For this purpose, (S)-configured amino acids, (S)-alanine, and (S)-leucine, with a small methyl and a larger isobutyl moiety were attached to the solid support 9 by reductive amination. After five reaction steps on the solid support, the 1,4-diazepanes (S)-19a,b were cleaved off and reductively alkylated to afford the 1,2,4-trisubstituted 1,4-diazepanes (S)-20a and (S)-21b, respectively. Both compounds show high σ1 affinity and selectivity over the σ2 subtype.

Effect fingerprinting of new psychoactive substances (NPS): What can we learn from in vitro data?

The use of new psychoactive substances (NPS) is increasing and currently >600 NPS have been reported. However, limited information on neuropharmacological and toxicological effects of NPS is available, hampering risk characterization. We reviewed the literature on the in vitro neuronal modes of action to obtain effect fingerprints of different classes of illicit drugs and NPS. The most frequently reported NPS were selected for review: cathinones (MDPV, α-PVP, mephedrone, 4-MEC, pentedrone, methylone), cannabinoids (JWH-018), (hallucinogenic) phenethylamines (4-fluoroamphetamine, benzofurans (5-APB, 6-APB), 2C-B, NBOMes (25B-NBOMe, 25C-NBOMe, 25I-NBOMe)), arylcyclohexylamines (methoxetamine) and piperazine derivatives (mCPP, TFMPP, BZP). Our effect fingerprints highlight the main modes of action for the different NPS studied, including inhibition and/or reversal of monoamine reuptake transporters (cathinones and non-hallucinogenic phenethylamines), activation of 5-HT₂receptors (hallucinogenic phenethylamines and piperazines), activation of cannabinoid receptors (cannabinoids) and inhibition of NDMA receptors (arylcyclohexylamines). Importantly, we identified additional targets by relating reported effect concentrations to the estimated human brain concentrations during recreational use. These additional targets include dopamine receptors, α- and β-adrenergic receptors, GABA_Areceptors and acetylcholine receptors, which may all contribute to the observed clinical symptoms following exposure. Additional data is needed as the number of NPS continues to increase. Also, the effect fingerprints we have obtained are still incomplete and suffer from a large variation in the reported effects and effect sizes. Dedicated in vitro screening batteries will aid in complementing specific effect fingerprints of NPS. These fingerprints can be implemented in the risk assessments of NPS that are necessary for eventual control measures to reduce Public Health risks.

Replacement of benzylic hydroxy group by vinyl or hydroxymethyl moiety at the 3-benzazepine scaffold retaining GluN2B affinity

Since overactivation of NMDA receptors is associated with neurodegenerative disorders, the design and development of subunit-selective NMDA receptor antagonists are of great interest. In order to avoid the formation of quinone-like intermediates as starting point for degradation the benzylic OH group of the lead compounds 2 was replaced by an electron rich vinyl or homologous hydroxymethyl moiety. The Bi(OTf)₃ catalyzed intramolecular Friedel-Crafts alkylation of 9a represents the key step in the synthesis of 1-vinyl substituted tetrahydro-3-benzazepine 10. Ozonolysis of 10 and subsequent reduction led to the hydroxymethyl derivative 14. The GluN2B affinities of the methyl ethers 2a, 3a and 4a and phenols 2b and 3b are very similar, respectively. It can be concluded that the ifenprodil binding site of GluN2B subunit containing NMDA receptors well tolerates a vinyl or hydroxymethyl moiety instead of the benzylic OH group. However, the selectivity has to improved, since the σ₁ affinity of the new ligands is higher than their GluN2B affinity.

Deconstruction - reconstruction approach to analyze the essential structural elements of tetrahydro-3-benzazepine-based antagonists of GluN2B subunit containing NMDA receptors

The role of the phenolic and benzylic OH moieties for the interaction of tetrahydro-3-benzazepine-1,7-diol 3d with GluN2B subunit containing NMDA receptors was analyzed by their stepwise removal. Elimination of trifluormethanesulfinate from 10 and 13 represent the key steps in the synthesis. Removal of phenolic OH moiety led to 5-fold reduced GluN2B affinity of 4d compared with 3d. Additional removal of the benzylic OH moiety (5d) resulted in further reduced GluN2B affinity but increased σ₁ and σ₂ affinities. Introduction of a NO₂ (6d) or NH₂ moiety (7d) decreased the GluN2B affinity. 3-Benzazepin-1-ol 4i with the N-phenylcyclohexyl side chain showed the highest GluN2B affinity of this series of compounds (K_i = 2.2 nM) and, moreover, high selectivity over the PCP binding site, σ₁ and σ₂ receptors. In docking studies 3-benzazepines (S)-4-7 adopt the same binding poses as ifenprodil and display the same crucial interactions. Unexpectedly, the high-affinity ligands (S)-4i, (S)-4j, and (S)-6i were not able to inhibit the glutamate/glycine evoked current in two-electrode voltage clamp measurements and the cytotoxic effects of glutamate/glycine on transfected cell lines.

Do GluN2B subunit containing NMDA receptors tolerate a fluorine atom in the phenylalkyl side chain?

The influence of an F-atom in the side chain of benzo[7]annulen-7-amines on the affinity towards GluN2B subunit containing NMDA receptors and the selectivity over related receptors was investigated. The synthesis of 5a and 5b was performed by reductive amination of the ketone 6 with primary alkanamines 14a and 14b bearing an F-atom in β-position. The GluN2B affinities of non-fluorinated and fluorinated ligands 4 and 5 are almost identical. The low impact of the F-atom on GluN2B affinity was unexpected, as it influences several chemical and physicochemical properties of the ligands. However, introduction of the F-atom led to reduced selectivity over σ receptors. Whereas 5a and 5b display still a 2-3-fold preference for GluN2B over σ1 receptors, they show almost the same affinity to GluN2B and σ2 receptors.

Design and Synthesis of Enantiomerically Pure Decahydroquinoxalines as Potent and Selective κ-Opioid Receptor Agonists with Anti-Inflammatory Activity in Vivo

In order to develop novel κ agonists restricted to the periphery, a diastereo- and enantioselective synthesis of (4aR,5S,8aS)-configured decahydroquinoxalines 5-8 was developed. Physicochemical and pharmacological properties were fine-tuned by structural modifications in the arylacetamide and amine part of the pharmacophore as well as in the amine part outside the pharmacophore. The decahydroquinoxalines 5-8 show single-digit nanomolar to subnanomolar κ-opioid receptor affinity, full κ agonistic activity in the [³⁵S]GTPγS assay, and high selectivity over μ, δ, σ₁, and σ₂ receptors as well as the PCP binding site of the NMDA receptor. Several analogues were selective for the periphery. The anti-inflammatory activity of 5-8 after topical application was investigated in two mouse models of dermatitis. The methanesulfonamide 8a containing the (S)-configured hydroxypyrrolidine ring was identified as a potent (K_i = 0.63 nM) and highly selective κ agonist (EC₅₀ = 1.8 nM) selective for the periphery with dose-dependent anti-inflammatory activity in acute and chronic skin inflammation.

Medicinal Chemistry of σ1 Receptor Ligands: Pharmacophore Models, Synthesis, Structure Affinity Relationships, and Pharmacological Applications

In the first part of this chapter, we summarize the various pharmacophore models for σ1 receptor ligands. Common to all of them is a basic amine flanked by two hydrophobic regions, representing the pharmacophoric elements. The development of computer-based models like the 3D homology model is described as well as the first crystal structure of the σ1 receptor. The second part focuses on the synthesis and biological properties of different σ1 receptor ligands, identified as 1-9. Monocyclic piperazines 1 and bicyclic piperazines 2 and 3 were developed as cytotoxic compounds, thus the IC50 values of cell growth and survival inhibition studies are given for all derivatives. The mechanism of cell survival inhibition, induction of time-dependent apoptosis, of compound ent-2a is discussed. Experimentally determined σ1 affinity shows good correlation with the results from molecular dynamics simulations based on a 3D homology model. Spirocyclic compounds 4 and 5 represent well-established σ1 receptor ligands. The homologous fluoroalkyl derivatives 4 have favorable pharmacological properties for use as fluorinated PET tracers. The (S)-configured fluoroethyl substituted compound (S)-4b is under investigation as PET tracer for imaging of σ1 receptors in the brain of patients affected by major depression. 1,3-Dioxanes 6c and 6d display a very potent σ1 antagonist profile and the racemic 1,3-dioxane 6c has high anti-allodynic activity at low doses. The arylpropenylamines 7 are very potent σ1 receptor ligands with high σ1/σ2 selectivity. The top compound 7g acts as an agonist as defined by its ability to potentiate neurite outgrowth at low concentrations. Among the morpholinoethoxypyrazoles 8, 8c (known as S1RA) reveals the most promising pharmacokinetic and physicochemical properties. Due to its good safety profile, 8c is currently being investigated in a phase II clinical trial for the treatment of neuropathic pain. The most potent ligand 9e of 3,4-dihydro-2(1H)-quinolones 9 shows promising anti-nociceptive activity in the formalin test.

Double Intramolecular Transacetalization of Polyhydroxy Acetals: Synthesis of Conformationally-Restricted 1,3-Dioxanes with Axially-Oriented Phenyl Moiety

The synthesis of conformationally-restricted 1,3-dioxanes with a phenyl moiety fixed in an axial orientation at the acetalic center is described. Starting with diethyl 3-hydroxyglutarate (15), benzaldehyde acetal 12a and acetophenone ketal 12b bearing a protected 1,3,5-trihydroxypentyl side chain in the o-position were prepared. The first acid-catalyzed intramolecular transacetalization gave a mixture of diastereomeric 2-benzofurans 18 (ratio of diastereomers 2:2:1:1). After OH group deprotection, the second intramolecular transacetalization afforded tricyclic alcohol 14a (2-(1,5-epoxy-1,3,4,5-tetrahydro-2-benzoxepin-3-yl]ethan-1-ol). Analogous cyclizations led to the corresponding silyl ether 22a (19%) and azide 23a (13%). Whereas tricyclic alcohol 14a was obtained as a 1:1 mixture of diastereomers, the silyl ether 22a and the azide 23a afforded only one diastereomer. This observation indicates a faster cyclization of the minor diastereomers providing the thermodynamically-favored compounds with equatorially-oriented substituents in the 3-position of the tricyclic 1,5-epoxy-2-benzoxepine system. In general, acetophenone-derived ketalic compounds (b-series) required very mild reaction conditions and gave lower yields than the corresponding acetalic compounds (a-series).

Synthesis and pharmacological evaluation of conformationally restricted κ-opioid receptor agonists

Additional N- and O-atoms in the bicyclic scaffold increase polarity and allow fine tuning of pharmacodynamic and pharmacokinetic properties of novel κ agonists.

Effects of polar κ receptor agonists designed for the periphery on ATP-induced Ca2+ release from keratinocytes

The very polar pyridylmethyl derivative 5a (log D_7.4 = 1.1) represents a potent and selective full κ-opioid receptor agonist (K_i = 0.13 nM, EC₅₀ = 33 nM), which reduced the release of Ca²⁺-ions into the cytoplasm in human keratinocytes.

Novel Potent N-Methyl-d-aspartate (NMDA) Receptor Antagonists or σ1 Receptor Ligands Based on Properly Substituted 1,4-Dioxane Ring

Two series of 1,4-dioxanes (4-11 and 12-19) were rationally designed and prepared to interact either with the phencyclidine (PCP) binding site of the N-methyl-d-aspartate (NMDA) receptor or with σ1 receptors, respectively. The biological profiles of the novel compounds were assessed using radioligand binding assays, and the compounds with the highest affinities were investigated for their functional activity. The results were in line with the available pharmacophore models and highlighted that the 1,4-dioxane scaffold is compatible with potent antagonist activity at NMDA receptor or high affinity for σ1 receptors. The primary amines 6b and 7 bearing a cyclohexyl and a phenyl ring or two phenyl rings in position 6, respectively, were the most potent noncompetitive antagonists at the NMDA receptor with IC50 values similar to those of the dissociative anesthetic (S)-(+)-ketamine. The 5,5-diphenyl substitution associated with a benzylaminomethyl moiety in position 2, as in 18, favored the interaction with σ1 receptors.

Asymmetric synthesis of 3-substituted tetrahydro-2-benzazepines

The enantiomerically and diastereomerically pure tricyclic oxazolidine cis-10 was prepared in a five step synthesis starting with 1-bromo-2-iodobenzene. Me3SiCN and allylSiMe3 reacted with cis-10 in the presence of TiCl4 to form the nitrile (3S)-11 and the allyl derivative (3S)-12 with high diastereoselectivity. The hydrogenolytic removal of the chiral auxiliary failed, since the endocyclic benzyl-N-bond was cleaved simultaneously. Therefore the N-(hydroxyethyl)amide of (3S)-12 was transformed into the enamide 27, which was hydrolyzed to afford the secondary amide 28. The enamide strategy to remove the chiral auxiliary from (3S)-11 led to complete racemization due to fast deprotonation in α-position of the cyano moiety. Two pairs of enantiomers 30a-b/ent-30a-b with prototypical σ substituents at the N-atom were prepared. The low σ1 affinity of the tetrahydro-2-benzazepines (ent-30b, Ki = 407 nM) is attributed to the short distance between the two lipophilic aromatic moieties.

Stereoselective synthesis and pharmacological evaluation of [4.3.3]propellan-8-amines as analogs of adamantanamines

Amantadine (1) exerts its anti-Parkinson effects by inhibition of the NMDA associated cation channel and its antiviral activity by inhibition of the M2 protein channel of influenza A viruses. Herein the synthesis, NMDA receptor affinity and anti-influenza activity of analogous propellanamines 3 are reported. The key steps in the synthesis of the diastereomeric propellanamines syn-3 and anti-3 are diastereoselective reduction of the ketone 7 with L-Selectride to give anti-11, Mitsunobu inversion of the alcohol anti-13 into syn-13, and SN2 substitution of diastereomeric mesylates syn-14 and anti-14 with NaN3. The affinity of the propellanamines syn-3 and anti-3 to the PCP binding site of the NMDA receptor is similar to that of amantadine (Ki=11 μM). However, both propellanamines syn-3 and anti-3 do not exhibit activity against influenza A viruses. Compared to amantadine (1), the structurally related propellanamines syn-3 and anti-3 retain the NMDA antagonistic activity but loose the antiviral activity.

Synthesis and pharmacological evaluation of like- and unlike-configured tetrahydro-2-benzazepines with the α-substituted benzyl moiety in the 5-position

A large set of tetrahydro-2-benzazepines with an α-hydroxy or α-(aryl)alkoxy substituted benzyl moiety in the 5-position was prepared according to the recently reported C6C1 + C3N synthetic strategy. The Heck reaction of 2-iodobenzaldehyde acetal 4 and the subsequent Stetter reaction led to the ketone 7, which was reduced diastereoselectively to form the like-configured alcohol 8. The diastereomeric unlike-configured alcohol 9 was obtained by a Mitsunobu inversion of 8. Alkylation and reductive cyclization of the diastereomeric alcohols 8 and 9 provided like- and unlike-configured 2-benzazepines 13 and 23, which allowed the introduction of various substituents at the N-atom. Analysis of the relationship between the structure and the σ1 affinity revealed that large substituents such as the butyl, benzyl or 4-phenylbutyl moiety at the benzazepine N-atom resulted in high affinity ligands. A p-methoxybenzyl ether is less tolerated by the σ1 receptor than a methyl ether or an alcohol. The unlike-configured alcohols 25d and 27d show slightly higher σ1 affinity than their like-configured diastereomers 15d and 17d. With respect to the σ1 affinity, σ1/σ2 selectivity and lipophilic ligand efficiency, like- and unlike-configured alcohols 15d and 25d represent the most promising σ1 ligands of this series. Interactions of the novel 2-benzazepines with various binding sites of the NMDA receptor were not observed.

Synthesis and pharmacological evaluation of 5-pyrrolidinylquinoxalines as a novel class of peripherally restricted κ-opioid receptor agonists

5-Pyrrolidinyl substituted perhydroquinoxalines were designed as conformationally restricted κ-opioid receptor agonists restricted to the periphery. The additional N atom of the quinoxaline system located outside the ethylenediamine κ pharmacophore allows the fine-tuning of the pharmacodynamic and pharmacokinetic properties. The perhydroquinoxalines were synthesized stereoselectively using the concept of late stage diversification of the central building blocks 14. In addition to high κ-opioid receptor affinity they demonstrate high selectivity over μ, δ, σ1, σ2, and NMDA receptors. In the [35S]GTPγS assay full agonism was observed. Because of their high polarity, the secondary amines 14a (log D7.4=0.26) and 14b (log D7.4=0.21) did not penetrate an artificial blood-brain barrier. 14b was able to inhibit the spontaneous pain reaction after rectal mustard oil application to mice (ED50=2.35 mg/kg). This analgesic effect is attributed to activation of peripherally located κ receptors, since 14b did not affect centrally mediated referred allodynia and hyperalgesia.

Combination of cyclohexane and piperazine based κ-opioid receptor agonists: Synthesis and pharmacological evaluation of trans,trans-configured perhydroquinoxalines

Desymmetrization of the pseudochiral (2r)-configured cyclohexane-1,2,3-triamines 8 with dimethyl oxalate led to racemic aminoquinoxaline-2,3-diones 9. Selective introduction of the κ pharmacophoric structural elements pyrrolidine and 3,4-dichlorophenylacetamide with a two-carbon distance afforded conformationally restricted κ agonists 13-15 based on the quinoxaline ring system. In competitive radioligand receptor binding studies the benzylamine 13b, the secondary amine 14b, and the carbamate 15 displayed high κ receptor affinity. The Ki value of the lead compound derived methoxycarbonyl derivative 15 is 9.7nM. However, the κ affinity of 15 is exceeded by 13b and 14b with a basic functional group instead of the methoxycarbonyl group in 1-position of the quinoxaline system. The chlorine atoms of the dichlorophenylacetyl residue are essential, since the corresponding phenylacetyl analogs show considerably reduced κ affinity. The potent κ ligands 13b, 14b and 15 are selective over the related μ- and δ-opioid receptors, σ1, σ2 and NMDA receptors. In the [(35)S]GTPγS-binding assay 13b behaved as partial agonist with lower activity than U-69,593.