Synthesis and monoamine transporter affinity of 3α-arylmethoxy-3β-arylnortropanes

A series of 3-arylnortrop-2-enes and 3α-arylmethoxy-3β-arylnortropanes were synthesized and evaluated for binding affinity at monoamine transporters. The 3-(3,4-dichlorophenyl)nortrop-2-ene (6e) exhibited high affinity for the SERT (K_i = 0.3 nM). The 3α-arylmethoxy-3β-arylnortropanes were generally SERT selective with the 3α-(3.4-dichlorophenylmethoxy)-3βphenylnortrop-2-ene (7c) possessing subnanomolar potency (K_i = 0.061 nM). However, 3α-(3,4-dichlorophenylmethoxy)-3β-phenylnortrop-2-ene (7b) exhibited high affinity at all three transporters [(DAT K_i = 22 nM), (SERT K_i = 6 nM) and (NET K_i = 101 nM)].     

Investigation of various N-heterocyclic substituted piperazine versions of 5/7-{[2-(4-aryl-piperazin-1-yl)-ethyl]-propyl-amino}-5,6,7,8-tetrahydro-naphthalen-2-ol: Effect on affinity and selectivity for dopamine D3 receptor

Here we report on the design and synthesis of several heterocyclic analogues belonging to the 5/7-{[2-(4-aryl-piperazin-1-yl)-ethyl]-propyl-amino}-5,6,7,8-tetrahydro-naphthalen-2-ol series of molecules. Compounds were subjected to [³H]spiperone binding assays, carried out with HEK-293 cells expressing either D2 or D3 dopamine receptors, in order to evaluate their inhibition constant (K_i) at these receptors. Results indicate that N-substitution on the piperazine ring can accommodate various substituted indole rings. The results also show that in order to maintain high affinity and selectivity for the D3 receptor the heterocyclic ring does not need to be connected directly to the piperazine ring as the majority of compounds included here are linked either via an amide or a methylene linker to the heterocyclic moiety. The enantiomers of the most potent racemic compound 10e exhibited differential activity with (?)-10e (K_i; D2 = 47.5 nM, D3 = 0.57 nM) displaying higher affinity at both D2 and D3 receptors compared to its enantiomer (+)-10e (K_i; D2 = 113 nM, D3 = 3.73 nM). Additionally, compound (?)-10e was more potent and selective for the D3 receptor compared to either 7-OH-DPAT or 5-OH-DPAT. Among the bioisosteric derivatives, the indazole derivative 10g and benzo[b]thiophene derivative 10i exhibited the highest affinity for D2 and D3 receptors. In the functional GTPγS binding study, one of the lead molecules, (?)-15, exhibited potent agonist activity at both D2 and D3 receptors with preferential affinity at D3.     

Discovery of novel α1-adrenoceptor ligands based on the antipsychotic sertindole suitable for labeling as PET ligands

The synthesis and in vitro preclinical profile of a series of 5-heteroaryl substituted analogs of the antipsychotic drug sertindole are presented. Compounds 1-(4-fluorophenyl)-3-(1-methylpiperidin-4-yl)-5-(pyrimidin-5-yl)-1H-indole (Lu AA27122, 3i) and 1-(4-fluorophenyl)-5-(1-methyl-1H-1,2,4-triazol-3-yl)-3-(1-methylpiperidin-4-yl)-1H-indole (3l) were identified as high affinity α_1A-adrenoceptor ligands with K_i values of 0.52 and 0.16 nM, respectively, and with a >100-fold selectivity versus dopamine D₂ receptors. Compound 3i showed almost equal affinity for α_1B- (K_i = 1.9 nM) and α_1D-adrenoceptors (K_i = 2.5 nM) as for α_1A, as well as moderate affinity for 5-HT_1B (K_i = 13 nM) and 5-HT₆ (K_i = 16 nM) receptors, whereas 3l showed >40-fold selectivity toward all other targets tested. Based on in vitro assays for assessment of permeability rates and extent, it is predicted that both compounds enter the brain of rats, non-human primates, as well as humans, and as such are good candidates to be carried forward for further evaluation as positron emission tomography (PET) ligands.     

Synthesis and structure–activity relationship of 3β-(4-alkylthio, -methylsulfinyl,and -methylsulfonylphenyl)tropane and 3β-(4-alkylthiophenyl)nortropane derivatives for monoamine transporters

Early studies led to the identification of 3β-(4-methoxyphenyl)tropane-2β-carboxylic acid methyl ester (5) with high affinity at the DAT (IC₅₀ = 6.5 nM) and 5-HTT (K_i = 4.3 nM), while having much less affinity at the NET (K_i = 1110 nM). In the present study, we replaced the 4′-methoxy group of the 3β-phenyl ring with a bioisosteric 4′-methylthio group to give 7a. We also synthesized a number of 3β-(4-alkylthiophenyl)tropanes 7b–e, 3β-(4-methylsulfinylphenyl) and 3β-(4-methylsulfonylphenyl)tropane analogues 7f–h as well as the 3β-(4-alkylthiophenyl)nortropane derivatives 8–11 to further characterize the structure–activity relationship of this type of compound for binding at monoamine transporters. With exception of the 4′-methylsulfonyl analogue 7h, all the tested compounds possessed high binding affinities at the 5-HTT. The K_i values ranged from 0.19 nM to 49 nM. The 3β-(4-methylthiophenyl)tropane 7a and its N-(3-fluoropropyl) analogue 9a and N-allyl analogue 10a are the most selective compounds for the 5-HTT over the NET (NET/5-HTT = 314–364) in the series. However, none of the compounds showed selectivity similar to 5 for both the DAT and 5-HTT relative to the NET. This study provided useful SAR information for rational design of potent and selective monoamine transporter inhibitors.     

A rhodamine-labeled citalopram analogue as a high-affinity fluorescent probe for the serotonin transporter

A novel fluorescent ligand was synthesized as a high-affinity, high specificity probe for visualizing the serotonin transporter (SERT). The rhodamine fluorophore was extended from an aniline substitution on the 5-position of the dihydroisobenzofuran ring of citalopram (2, 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile), using an ethylamino linker. The resulting rhodamine-labeled ligand 8 inhibited [³H]5-HT uptake in COS-7 cells (K_i = 225 nM) with similar potency to the tropane-based JHC 1-064 (1), but with higher specificity towards the SERT relative to the transporters for dopamine and norepinephrine. Visualization of the SERT with compound 8 was demonstrated by confocal microscopy in HEK293 cells stably expressing EGFP–SERT.     

Diether derivatives of homo- or substituted piperidines as non-imidazole histamine H3 receptor ligands

Synthesis and biological activities of a series of homo- or substituted piperidine unsymmetrical diethers are described. The novel compounds were evaluated for histamine H₃ receptor binding affinities at recombinant human H₃ receptor stably expressed in HEK-293 cells. All diethers showed in vitro affinities in nanomolar concentration range. The most potent compounds are 1-[3-(3-(4-chlorophenoxy)propoxy)propyl]-3-methylpiperidine 11 (K_i = 3.2 nM) and 1-[3-(3-(4-chlorophenoxy)propoxy)propyl]azepane 13 (K_i = 3.5 nM).     

Pyrrolidine analogs of GZ-793A: Synthesis and evaluation as inhibitors of the vesicular monoamine transporter-2 (VMAT2)

Central heterocyclic ring size reduction from piperidinyl to pyrrolidinyl in the vesicular monoamine transporter-2 (VMAT2) inhibitor GZ-793A and its analogs resulted in novel N-propane-1,2(R)-diol analogs 11a–i. These compounds were evaluated for their affinity for the dihydrotetrabenazine (DTBZ) binding site on VMAT2 and for their ability to inhibit vesicular dopamine (DA) uptake. The 4-difluoromethoxyphenethyl analog 11f was the most potent inhibitor of [³H]-DTBZ binding (K_i = 560 nM), with 15-fold greater affinity for this site than GZ-793A (K_i = 8.29 μM). Analog 11f also showed similar potency of inhibition of [³H]-DA uptake into vesicles (K_i = 45 nM) compared to that for GZ-793A (K_i = 29 nM). Thus, 11f represents a new water-soluble inhibitor of VMAT function.     

Lobeline esters as novel ligands for neuronal nicotinic acetylcholine receptors and neurotransmitter transporters

Vesicular monoamine transporter-2 (VMAT2) is a viable target for development of pharmacotherapies for psychostimulant abuse. Lobeline (1) is a potent antagonist at α4β21 nicotinic acetylcholine receptors, has moderate affinity (K_i = 5.46 μM) for VMAT2, and is being investigated currently as a clinical candidate for treatment of psychostimulant abuse. A series of carboxylic acid and sulfonic acid ester analogs 2–20 of lobeline were synthesized and evaluated for interaction with α4β21 and α71 neuronal nicotinic acetylcholine receptors (nAChRs), the dopamine transporter (DAT), serotonin transporter (SERT) and VMAT2. Both carboxylic acid and sulfonic acid esters had low affinity at α71 nAChRs. Similar to lobeline (K_i = 4 nM), sulfonic acid esters had high affinity at α4β21 (K_i = 5–17 nM). Aromatic carboxylic acid ester analogs of lobeline (2–4) were 100–1000-fold less potent than lobeline at α4β21 nAChRs, whereas aliphatic carboxylic acid ester analogs were 10–100-fold less potent than lobeline at α4β21. Two representative lobeline esters, the 10-O-benzoate (2) and the 10-O-benzenesulfonate (10) were evaluated in the ³⁶Rb⁺ efflux assay using rat thalamic synaptosomes, and were shown to be antagonists with IC₅₀ values of 0.85 μM and 1.60 μM, respectively. Both carboxylic and sulfonic acid esters exhibited a range of potencies (equipotent to 13–45-fold greater potency compared to lobeline) for inhibiting DAT and SERT, respectively, and like lobeline, had moderate affinity (K_i = 1.98–10.8 μM) for VMAT2. One of the more interesting analogs, p-methoxybenzoic acid ester 4, had low affinity at α4β21 nAChRs (K_i = 19.3 μM) and was equipotent with lobeline, at VMAT2 (K_i = 2.98 μM), exhibiting a 6.5-fold selectivity for VMAT2 over α4β2 nAChRs. Thus, esterification of the lobeline molecule may be a useful structural modification for the development of lobeline analogs with improved selectivity at VMAT2.     

Synthesis and in vitro evaluation of fluorinated diphenyloxide derivatives and sulfur analogs as serotonin transporter ligands

As the serotonin transporter (SERT) is involved in several neurodegenerative and psychiatric disorders; radiopharmaceuticals to image the SERT by PET would be valuable in studying these diseases. To this end we synthesized diphenyloxide derivatives and sulfide analogs, as new tracers, incorporating a fluorine or oxyalkyl fluorinated group on 4′ or 5′-position on phenyl ring B. Three of these exhibited good to high in vitro affinity (7 < K_i < 8 nM) and selectivity for the SERT over the other monoamine transporters.     

Kappa-opioid receptor-selective dicarboxylic ester-derived salvinorin A ligands

Salvinorin A, the active ingredient of the hallucinogenic plant Salvia divinorum is the most potent known naturally occurring hallucinogen and is a selective κ-opioid receptor agonist. To better understand the ligand–receptor interactions, a series of dicarboxylic ester-type of salvinorin A derivatives were synthesized and evaluated for their binding affinity at κ-, δ- and μ-opioid receptors. Most of the analogues show high affinity to the κ-opioid receptor. Methyl malonyl derivative 4 shows the highest binding affinity (K_i = 2 nM), analogues 5, 7, and 14 exhibit significant affinity for the κ-receptor (K_i = 21, 36 and 39 nM).     

Discovery of a potent,selective, and orally bioavailable histamine H3 receptor antagonist SAR110068 for the treatment of sleep–wake disorders

Previous studies have shown that compound 1 displayed high affinity towards histamine H₃ receptor (H₃R), (human (h-H₃R), K_i = 8.6 nM, rhesus monkey (rh-H₃R), K_i = 1.2 nM, and rat (r-H₃R), K_i = 16.5 nM), but exhibited high affinity for hERG channel. Herein, we report the discovery of a novel, potent, and highly selective H₃R antagonist/inverse agonist 5a(SS) (SAR110068) with acceptable hERG channel selectivity and desirable pharmacological and pharmacokinetic properties through lead optimization sequence. The significant awakening effects of 5a(SS) on sleep–wake cycles studied by using EEG recording in rats during their light phase support its potential therapeutic utility in human sleep–wake disorders.     

Synthesis and evaluation of novel azetidine analogs as potent inhibitors of vesicular [3H]dopamine uptake

Lobelane analogs that incorporate a central piperidine or pyrrolidine moiety have previously been reported by our group as potent inhibitors of VMAT2 function. Further central ring size reduction of the piperidine moiety in lobelane to a four-membered heterocyclic ring has been carried out in the current study to afford novel cis-and trans-azetidine analogs. These azetidine analogs (15a–15c and 22a–22c) potently inhibited [³H]dopamine (DA) uptake into isolated synaptic vesicles (K_i ? 66 nM). The cis-4-methoxy analog 22b was the most potent inhibitor (K_i = 24 nM), and was twofold more potent that either lobelane (2a, K_i = 45 nM) or norlobelane (2b, K_i = 43 nM). The trans-methylenedioxy analog, 15c (K_i = 31 nM), was equipotent with the cis-analog, 22b, in this assay. Thus, cis- and trans-azetidine analogs 22b and 15c represent potential leads in the discovery of new clinical candidates for the treatment of methamphetamine abuse.     

Synthesis and CB1 cannabinoid receptor affinity of 4-alkoxycarbonyl-1,5-diaryl-1,2,3-triazoles

A series of 4-alkoxycarbonyl-1,5-diaryl-1,2,3-triazoles were synthesized regioselectively using click chemistry and evaluated at CB1 cannabinoid receptors. The n-propyl ester 11 (K_i = 4.6 nM) and phenyl ester 14 (K_i = 11 nM) exhibited the most potent affinity of the series.     

Synthesis and biological evaluation of phloridzin analogs as human concentrative nucleoside transporter 3 (hCNT3) inhibitors

Nucleoside transporter inhibitors have potential therapeutic applications as anticancer, antiviral, cardioprotective and neuroprotective agents. Although quite a few potent inhibitors of the equilibrative nucleoside transporters are known, largely missing are the concentrative nucleoside transporter inhibitors. Phloridzin (3, K_i = 16.00 μM) is a known moderate inhibitor of the concentrative nucleoside transporters. We have synthesized and evaluated analogs of phloridzin at the hCNT3 nucleoside transporter. Within the series of synthesized analogs compound 16 (K_i = 2.88 μM), possessing a ribofuranose sugar unit instead of a glucopyranose as present in phloridzin, exhibited the highest binding affinity at the hCNT3 transporter. Phloridzin and compound 16 have also been shown to be selective for the hCNT3 transporter as compared with the hENT1 transporter. Compound 16 can serve as a new lead which after further modifications could yield selective and potent hCNT3 inhibitors.     

Nascent structure–activity relationship study of a diastereomeric series of kappa opioid receptor antagonists derived from CJ-15,208

Cyclic tetrapeptide c[Phe-pro-Phe-trp] 2, a diastereomer of CJ-15,208 (1), was identified as a potent dual κ/μ opioid receptor antagonist devoid of δ opioid receptor affinity against cloned human receptors: K_i (2) = 3.8 nM (κ), 30 nM (μ); IC₅₀ ([³⁵S]GTPγS binding) = 140 nM (κ), 21 nM (μ). The d-tryptophan residue rendered 2 ca. eightfold and fourfold more potent at κ and μ, respectively, than the corresponding l-configured tryptophan in the natural product 1. Phe analogs 3–10, designed to probe the effect of substituents on receptor affinity and selectivity, possessed K_i values ranging from 14 to 220 nM against the κ opioid receptor with μ/κ ratios of 0.45–3.0. An alanine scan of 2 yielded c[Ala-pro-Phe-trp] 12, an analog equipotent to 2. Agents 2 and 12 were pure antagonists in vitro devoid of agonist activity. Ac-pro-Phe-trp-Phe-NH₂ 16 and Ac-Phe-trp-Phe-pro-NH₂ 17 two of the eight possible acyclic peptides derived from 1 and 2, were selective, modestly potent μ ligands: K_i (16) = 340 nM (μ); K_i (17) = 360 nM (μ).     

Probes for narcotic receptor mediated phenomena. 47.1 Novel C4a- and N-substituted-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols

A series of N-methyl rac-cis-4a-aralkyl- and alkyl-substituted-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols have been prepared (2a–l) using a simple previously designed synthetic route, in order to find a ligand that would interact with both μ- and δ-opioid receptors. A C4a-phenethyl derivative 2a, was found to have modest receptor affinity both at μ- (K_i = 60 nM) and δ-opioid receptors (K_i = 64 nM). The N-methyl substituent of 2a and that of other ligands in the series was then modified to obtain compounds with different N-substituents that might provide higher affinity at both receptors. A number of compounds differently substituted at C4a and N were synthesized and evaluated. Binding studies and functional assays revealed a moderately selective δ-antagonist (2l), selective μ–δ antagonists (3d, 3g), and a μ–κ antagonist (3f).     

NAD-based inhibitors with anticancer potential

Three classes of novel inhibitors of inosine monophosphate dehydrogenase have been prepared and their anti-proliferative properties were evaluated against several cancer cell lines.(1) Mycophenolic adenine dinucleotide analogues (8–13) containing a substituent at the C2 of adenine ring were found to be potent inhibitors of IMPDH (K_i’s in range of 0.6–82 nM) and sub-μM inhibitors of leukemic K562 cell proliferation. (2) Mycophenolic adenosine (d and l) esters (20 and 21) showed a potent inhibition of IMPDH2 (K_i = 102 and K_i = 231 nM, respectively) and inhibition of K562 cell growth (IC₅₀ = 0.5 and IC₅₀ = 1.6 μM). These compounds serve both as inhibitors of the enzyme and as a depot form of mycophenolic acid. The corresponding amide analogue 22, also a potent inhibitor of IMPDH (K_i = 84 nM), did not inhibit cancer cell proliferation. (3) Mycophenolic-(l)- and (d)-valine adenine di-amide derivatives 25 (K_i = 9 nM) and 28 (K_i = 3 nM) were found to be very potent enzymatically, but did not inhibit proliferation of cancer cells.     

Aerobic oxidation of indolomorphinan without the 4,5-epoxy bridge and subsequent rearrangement of the oxidation product to spiroindolinonyl-C-normorphinan derivative

Aerobic oxidation of indolomorphinan 1 without a 4,5-epoxy bridge proceeded in the presence of platinum catalyst to give indoleninomorphinan 2 or quinolono-C-normorphinan 5. The 4-hydroxy group would play an important role in deciding the course of the reaction. Treatment of 2a with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave spiroindolinonyl-C-normorphinan 3a whose structure resembles that of δ opioid receptor agonist spiroindanyloxymorphone (SIOM). Boron trichloride was effective for the reverse reaction from 3a to 2a without side reaction. This practical interconversion method between hydroxyindolenine and spiroindolinone would be useful for the design and construction of drug-like compound libraries. Although the compound 3b was expected to show the selectivity for δ opioid receptor because of the structural resemblance to SIOM, it was rather selective for μ opioid receptor (μ: K_i = 0.75 nM; δ: K_i = 2.90 nM; κ: K_i = 13.4 nM). The result suggests that the slight difference of the spatial location of the benzene rings in these compounds may definitively affect the binding affinity for δ opioid receptor.     

3-(Arylsulfonyl)-1-(azacyclyl)-1H-indoles are 5-HT6 receptor modulators

Novel 3-(arylsulfonyl)-1-(azacyclyl)-1H-indoles 6 were synthesized as potential 5-HT₆ receptor ligands, based on constraining a basic side chain as either a piperidine or a pyrrolidine. Many of these compounds had good 5-HT₆ binding affinity with K_i values <10 nM. Depending on substitution, both agonists (e.g., 6o: EC₅₀ = 60 nM, E_max = 70%) and antagonists (6y: IC₅₀ = 17 nM, I_max = 86%) were identified in a 5-HT₆ adenylyl cyclase assay.     

Exploring the 2- and 5-positions of the pyrazolo[4,3-d]pyrimidin-7-amino scaffold to target human A1 and A2A adenosine receptors

A new series of 7-aminopyrazolo[4,3-d]pyrimidine derivatives (1–31) were synthesized to evaluate some structural modifications at the 2- and 5-positions aimed at shifting affinity towards the human (h) A_2A adenosine receptor (AR) or both hA_2A and hA₁ ARs. The most active compounds were those featured by a 2-furyl or 5-methylfuran-2-yl moiety at position 5, combined with a benzyl or a substituted-benzyl group at position 2. Several of these derivatives (22–31) displayed nanomolar affinity for the hA_2A AR (K_i = 3.62–57 nM) and slightly lower for the hA₁ ARs, thus showing different degrees (3–22 fold) of hA_2A versus hA₁ selectivity. In particular, the 2-(2-methoxybenzyl)-5-(5-methylfuran-2-yl) derivative 25 possessed the highest hA_2A and hA₁ AR affinities (K_i = 3.62 nM and 18 nM, respectively) and behaved as potent antagonist at both these receptors (cAMP assays). Its 2-(2-hydroxybenzyl) analog 26 also showed a high affinity for the hA_2A AR (K_i = 5.26 nM) and was 22-fold selective versus the hA₁ subtype. Molecular docking investigations performed at the hA_2A AR crystal structure and at a homology model of the hA₁ AR allowed us to represent the hypothetical binding mode of our derivatives and to rationalize the observed SARs.