7-Azabicyclo[2.2.1]heptane as a scaffold for the development of selective sigma-2 (σ2) receptor ligands

A series of N-substituted 7-azabicyclo[2.2.1]heptanes (12-17 and 22-25) and similarly substituted pyrrolidines (32-36 and 41-44) were synthesized as sterically-reduced, achiral analogs of adamantane- and trishomocubane-derived σ ligands. In vitro competition binding assays against σ receptors revealed that arylalkyl N-substituents conferred selectivity for the σ(2) subtype, while alicyclic or polycarbocyclic substituents imparted high affinity for both subtypes. The σ(2) binding and subtype selectivities of N-arylalkyl-7-azanorbornanes was generally greater than the analogously-substituted pyrrolidines, indicating that steric bulk and conformational restriction around the nitrogen atom are likely important for subtype discrimination.     

N-Arylalkyl-2-azaadamantanes as cage-expanded polycarbocyclic sigma (σ) receptor ligands

A series of racemic N-arylalkyl-2-azaadamantan-1-ols (9-15) and the corresponding deoxygenated, achiral N-arylalkyl-2-azaadamantanes (23-29) were synthesized and screened in competition binding assays against a panel of CNS targets. Adamantyl hemiaminals 9-15 displayed generally low affinity for both σ(1) (K(i) values= 294-1950 nM) and σ(2) receptors (K(i) values=201-1020 nM), and negligible affinity for 42 other CNS proteins. Deoxygenation of 9-15 to give the corresponding achiral azaadamantanes 23-29 greatly improved affinity for σ(1) (K(i) values=8.3-239 nM) and σ(2) receptors (K(i) values=34-312 nM).     

Reevaluation of fenpropimorph as a σ receptor ligand: Structure-affinity relationship studies at human σ1 receptors

Fenpropimorph (1) is considered a “super high-affinity” σ₁ receptor ligand (K_i = 0.005 nM for guinea pig σ₁ receptors). Here, we examine the binding of 1 and several of its deconstructed analogs at human σ₁ (hσ₁) receptors. We monitored their subtype selectivity by determining the binding affinity at σ₂ receptors. In addition, we validated an existing pharmacophore model at the molecular level by conducting 3D molecular modeling studies, using the crystal structure of hσ₁ receptors, and Hydrophatic INTeractions (HINT) analysis. Our structure affinity relationship studies showed that 1 binds with lower affinity at hσ₁ receptors (K_i = 17.3 nM) compared to guinea pig; moreover, we found that none of the fenpropimorph methyl groups is important for its binding at hσ₁ receptors, nor is stereochemistry. For example, removal of all methyl groups as seen in 4 resulted in an almost 5-fold higher affinity at hσ₁ receptors compared to 1 and 350-fold selectivity versus σ₂ receptors. In addition, although the O atom of the morpholine ring does not contribute to affinity at hσ₁ receptors (and might even detract from it), it plays role in subtype (σ₁ versus σ₂ receptor) selectivity.     

3D-QSAR and docking studies of pentacycloundecylamines at the sigma-1 (σ1) receptor

Pentacycloundecylamine (PCU) derived compounds have been shown to be promising lead structures for the development of novel drug candidates aimed at a variety of neurodegenerative and psychiatric diseases. Here we show for the first time a 3D quantitative structure–activity relationship (3D-QSAR) for a series of aza-PCU-derived compounds with activity at the sigma-1 (σ₁) receptor. A comparative molecular field analysis (CoMFA) model was developed with a partial least squares cross validated (q²) regression value of 0.6, and a non-cross validated r² of 0.9. The CoMFA model was effective at predicting the sigma-1 activities of a test set with an r² >0.7. We also describe here the docking of the PCU-derived compounds into a homology model of the sigma-1 (σ₁) receptor, which was developed to gain insight into binding of these cage compounds to the receptor. Based on docking studies we evaluated in a [³H]pentazocine binding assay an oxa-PCU, NGP1-01 (IC₅₀ = 1.78 μM) and its phenethyl derivative (IC₅₀ = 1.54 μM). Results from these studies can be used to develop new compounds with specific affinity for the sigma-1(σ₁) receptor.     

Alkylsulfanyl analogs as potent α2δ ligands

We identified novel (3R, 5S)-3-aminomethyl-5-methanesulfanyl hexanoic acid (5a: DS75091588) and (3R, 5S)-3-aminomethyl-5-ethanesulfanyl hexanoic acid (6a: DS18430756) as sulfur-containing γ-amino acid derivatives that were useful for the treatment of neuropathic pain. These two compounds exhibited a potent analgesic effect in animal models of both type I diabetes and type II diabetes, and good pharmacokinetics.     

Activity–lipophilicity relationship studies on P-gp ligands designed as simplified tariquidar bulky fragments

A series of alkyloxyquinoline derivatives has been developed to evaluate the relationship between P-gp potency and lipophilicity. The results show a satisfactory lipophilicity-activity correlation although a series of derivatives showing higher P-gp potency is needed in order to confirm this hypothesis.     

Synthesis and structure–activity relationship of pyripyropene A derivatives as potent and selective acyl-CoA:cholesterol acyltransferase 2 (ACAT2) inhibitors: Part 2

Synthesis and structure–activity relationships of 7-O-p-cyanobenzoyl pyripyropene A derivatives with modification at C1 and 11 are described. Regioselective mono-deprotection of di-tert-butylsilylene acetal was critical in their synthesis.     

Diazaspirocyclic compounds as selective ligands for the α4β2 nicotinic acetylcholine receptor

Diazaspirocyclic ligands have been synthesized in four steps as selective α4β2 nicotinic acetylcholine receptor antagonists. Structural assignment of 1-(pyridin-3-yl)-2-spiropyrrolidino-3,2'-1-azabiclo[2.2.1]heptane 2, was confirmed using a combination of NMR experiments on a key intermediate, spirolactam 9. All three target compounds synthesized in this diazaspirocyclic series exhibited high affinity (K(i)<35 nM) at the human α4β2 nAChR subtype, and very low affinity for the human α7, α3β4 (ganglion) and α1β1γδ (muscle) subtypes (K(i)>500 nM).     

Novel anti-viability ceramide analogs: Design,synthesis, and structure–activity relationship studies of substituted (S)-2-(benzylideneamino)-3-hydroxy-N-tetradecylpropanamides

A group of novel l-serinamides, substituted (S)-2-(benzylideneamino)-3-hydroxy-N-tetradecylpropanamides (3a–o) and substituted (S)-2-(benzylamino)-3-hydroxy-N-tetradecyl propanamides (4c, 4i, 4l, and 4o), were synthesized as potential anti-tumor lead compounds. In vitro cell viability assay results indicate treatment with 3a–o compounds resulted in significant inhibition of cell viability in the chemoresistant breast cancer cell line, MCF-7TN-R. Compounds 3i and 3l, both ortho-substituted analogs, show the greatest efficacy with IC₅₀ values of 10.3 μM and 12.5 μM, respectively. The SAR analysis indicate that the imine functional group of 3a–o is critical for the cellular anti-viability effect, and the partial atomic charge (PAC) value of imine C atom is a valuable structural parameter for predicting the activity of these ceramide analogs.     

Synthesis and structure–activity relationship of pyripyropene A derivatives as potent and selective acyl-CoA:cholesterol acyltransferase 2 (ACAT2) inhibitors: Part 1

In an effort to develop potent and selective inhibitors toward ACAT2, structure–activity relationship studies were carried out using derivatives based on pyripyropene A (PPPA, 1). We have successfully developed novel PPPA derivatives with a 7-O-substituted benzoyl substituent that significantly exhibit more potent ACAT2 inhibitory activity and higher ACAT2 isozyme selectivity than 1.     

Synthesis and structure–activity relationship of pyripyropene A derivatives as potent and selective acyl-CoA:cholesterol acyltransferase 2 (ACAT2) inhibitors: Part 3

In an effort to develop potent and selective inhibitors toward ACAT2, structure–activity relationship studies were carried out using derivatives based on pyripyropene A (PPPA, 1). In particular, we investigated the possibility of introducing appropriate 1,11-O-benzylidene and 7-O-substituted benzoyl moieties into PPPA (1). The new o-substituted benzylidene derivatives showed higher selectivity for ACAT2 than PPPA (1). Among them, 1,11-O-o-methylbenzylidene-7-O-p-cyanobenzoyl PPPA derivative 7q and 1,11-O-o,o-dimethylbenzylidene-7-O-p-cyanobenzoyl PPPA derivative 7z proved to be potent ACAT2 inhibitors with unprecedented high isozyme selectivity.     

Syntheses and structure–activity relationship (SAR) studies of 2,5-diazabicyclo[2.2.1]heptanes as novel α7 neuronal nicotinic receptor (NNR) ligands

Biaryl substituted 2,5-diazabicyclo[2.2.1]heptanes have been synthesized and tested for their affinity toward α7 neuronal nicotinic receptors (NNRs). SAR studies established that 5-N-methyl substituent, heteroaryl linker and the nature of terminal aryl group are critical for the ligand to achieve potent α7 NNR agonist activity.     

Discovery of benzamide analogs as negative allosteric modulators of human neuronal nicotinic receptors: Pharmacophore modeling and structure–activity relationship studies

The present study describes our ongoing efforts toward the discovery of drugs that selectively target nAChR subtypes. We exploited knowledge on nAChR ligands and their binding site that were previously identified by our laboratory through virtual screenings and identified benzamide analogs as a novel chemical class of neuronal nicotinic receptor (nAChR) ligands. The lead molecule, compound 1 (4-(allyloxy)-N-(6-methylpyridin-2-yl)benzamide) inhibits nAChR activity with an IC₅₀ value of 6.0 (3.4–10.6) μM on human α4β2 nAChRs with a ～5-fold preference against human α3β4 nAChRs. Twenty-six analogs of compound 1 were also either synthesized or purchased for structure–activity relationship (SAR) studies and provided information relating the chemical/structural properties of the molecules to their ability to inhibit nAChR activity. The discovery of subtype-selective ligands of nAChRs described here should contribute significantly to our understanding of the involvement of specific nAChR subtypes in normal and pathophysiological states.     

Structure–activity relationship study of acridine analogs as haspin and DYRK2 kinase inhibitors

Haspin is a serine/threonine kinase required for completion of normal mitosis that is highly expressed during cell proliferation, including in a number of neoplasms. Consequently, it has emerged as a potential therapeutic target in oncology. A high throughput screen of approximately 140,000 compounds identified an acridine analog as a potent haspin kinase inhibitor. Profiling against a panel of 270 kinases revealed that the compound also exhibited potent inhibitory activity for DYRK2, another serine/threonine kinase. An optimization study of the acridine series revealed that the structure–activity relationship (SAR) of the acridine series for haspin and DYRK2 inhibition had many similarities. However, several structural differences were noted that allowed generation of a potent haspin kinase inhibitor (33, IC₅₀ <60 nM) with 180-fold selectivity over DYRK2. In addition, a moderately potent DYRK2 inhibitor (41, IC₅₀ <400 nM) with a 5.4-fold selectivity over haspin was also identified.     

Design synthesis and structure–activity relationship of 5-substituted (tetrahydronaphthalen-2yl)methyl with N-phenyl-N-(piperidin-2-yl)propionamide derivatives as opioid ligands

Here, we report the design, synthesis and structure activity relationship of novel small molecule opioid ligands based on 5-amino substituted (tetrahydronaphthalen-2-yl)methyl moiety with N-phenyl-N-(piperidin-2-yl)propionamide derivatives. We synthesized various molecules including amino, amide and hydroxy substitution on the 5th position of the (tetrahydronaphthalen-2-yl)methyl moiety. In our further designs we replaced the (tetrahydronaphthalen-2-yl)methyl moiety with benzyl and phenethyl moiety. These N-phenyl-N-(piperidin-2-yl)propionamide analogues showed moderate to good binding affinities (850–4 nM) and were selective towards the μ opioid receptor over the δ opioid receptors. From the structure activity relationship studies, we found that a hydroxyl substitution at the 5th position of (tetrahydronapthalen-2yl)methyl group, ligands 19 and 20, showed excellent binding affinities 4 and 5 nM, respectively, and 1000 fold selectivity towards the μ opioid relative to the delta opioid receptor. The ligand 19 showed potent agonist activities 75 ± 21 nM, and 190 ± 42 nM in the GPI and MVD assays. Surprisingly the fluoro analogue 20 showed good agonist activities in MVD assays 170 ± 42 nM, in contrast to its binding affinity results.     

Structure–toxicity relationship of phenolic analogs as anti-melanoma agents: An enzyme directed prodrug approach

The aim of this study was to identify a phenolic prodrug compound that is minimally metabolized by rat liver microsomes, but yet could form quinone reactive intermediates in melanoma cells as a result of its bioactivation by tyrosinase. In current work, we investigated 24 phenolic compounds for their metabolism by tyrosinase, rat liver microsomes and their toxicity towards murine B16-F0 and human SK-MEL-28 melanoma cells. A linear correlation was found between toxicities of phenolic analogs towards SK-MEL-28 and B16-F0 melanoma cells, suggesting similar mechanisms of toxicity in both cell lines. 4-HEB was identified as the lead compound. 4-HEB (IC₅₀ 48 h, 75 μM) showed selective toxicity towards five melanocytic melanoma cell lines SK-MEL-28, SK-MEL-5, MeWo, B16-F0 and B16-F10, which express functional tyrosinase, compared to four non-melanoma cells lines SW-620, Saos-2, PC3 and BJ cells and two amelanotic SK-MEL-24, C32 cells, which do not express functional tyrosinase. 4-HEB caused significant intracellular GSH depletion, ROS formation, and showed significantly less toxicity to tyrosinase specific shRNA transfected SK-MEL-28 cells. Our findings suggest that presence of a phenolic group in 4-HEB is critical for its selective toxicity towards melanoma cells.     

Part 1: N-alkylated glycines as potent α2δ ligands

A new series of glycine-derived ligands of the α₂δ subunit of voltage gated calcium channels is described. Several novel compounds (7) based on (6) were prepared that possessed a potency <100 nM in the α₂δ binding assay.     

Synthesis and structure–activity relationship studies of MI-2 analogues as MALT1 inhibitors

Recent studies revealed that MALT1 is a promising therapeutic target for the treatment of ABC-DLBCL. Among several reported MALT1 inhibitors, MI-2 as an irreversible inhibitor represents a new class of ABC-DLBCL therapeutics. Due to its inherent potential cross-reactivity, further structure–activity relationship (SAR) study is imperative. In this work, five focused compound libraries based on the chemical structure of MI-2 are designed and synthesized. The systematic SARs revealed that the side chain of 2-methoxyethoxy has little impact on the activity and can be replaced by other functionalized groups, providing new MI-2 analogues with retained or enhanced potency. Compounds 81–83 with terminal hydroxyl group as side chain displayed enhanced activities against MALT1. Replacement of triazole core with pyrazole is also tolerant, while structural modifications on other sites are detrimental. These findings will facilitate further development of small-molecule MALT1 inhibitors.     

Triptolide derivatives as potential multifunctional anti-Alzheimer agents: Synthesis and structure–activity relationship studies

Owning to the promising neuroprotective profile and the ability to cross the blood–brain barrier, triptolide has attracted extensive attention. Although its limited solubility and toxicity have greatly hindered clinical translation, triptolide has nonetheless emerged as a promising candidate for structure–activity relationship studies for Alzheimer’s disease. In the present study, a series of triptolide analogs were designed and synthesized, and their neuroprotective and anti-neuroinflammatory effects were then tested using a cell culture model. Among the triptolide derivatives tested, a memantine conjugate, compound 8, showed a remarkable neuroprotective effect against Aβ_1–42 toxicity in primary cortical neuron cultures as well as an inhibitory effect against LPS-induced TNF-α production in BV2 cells at a subnanomolar concentration. Our findings provide insight into the different pharmacophores that are responsible for the multifunctional effects of triptolide in the central nervous system. Our study should help in the development of triptolide-based multifunctional anti-Alzheimer drugs.