Guanidine-to-piperidine switch affords high affinity small molecule NPFF ligands with preference for NPFF1-R and NPFF2-R subtypes

The Neuropeptide FF (NPFF) receptor system is known to modulate opioid actions and has been shown to mediate opioid-induced hyperalgesia and tolerance. The lack of subtype selective small molecule compounds has hampered further exploration of the pharmacology of this receptor system. The vast majority of available NPFF ligands possess a highly basic guanidine group, including our lead small molecule, MES304. Despite providing strong receptor binding, the guanidine group presents a potential pharmacokinetic liability for in vivo pharmacological tool development. Through structure-activity relationship exploration, we were able to modify our lead molecule MES304 to arrive at guanidine-free NPFF ligands. The novel piperidine analogues 8b and 16a are among the few non-guanidine based NPFF ligands known in literature. Both compounds displayed nanomolar NPFF-R binding affinity approaching that of the parent molecule. Moreover, while MES304 was non-subtype selective, these two analogues presented new starting points for subtype selective scaffolds, whereby 8b displayed a 15-fold preference for NPFF1-R, and 16a demonstrated an 8-fold preference for NPFF2-R. Both analogues showed no agonist activity on either receptor subtype in the in vitro functional activity assay, while 8b displayed antagonistic properties at NPFF1-R. The calculated physicochemical properties of 8b and 16a were also shown to be more favorable for in vivo tool design. These results indicate the possibility of developing potent, subtype selective NPFF ligands devoid of a guanidine functionality.

Design, synthesis and biological evaluation of pyrrolopyrimidine derivatives as novel and selective positive modulator of the small conductance Ca2+-activated K+ channels

The type 2 small conductance Ca²⁺-activated K⁺ channels (SK2) have been considered as one of the most promising therapeutic targets for spinocerebellar ataxias type 2 (SCA2) by playing a critical role in the control of normal purkinje cells (PCs) pacemaking. Herein, a novel series of pyrrolopyrimidine derivatives were designed and synthesized from the lead compound NS13001 as subtype-selective modulators of SK channels. Among them, the halogen-substituted compound 12b (EC₅₀ = 0.34 ± 0.044 μM) was identified with a ∼5.4-fold higher potency on potentiating SK2-a channels at submicromolar concentrations as compared to NS13001 (EC₅₀ = 1.83 ± 0.50 μM). Furthermore, compound 12b exhibited selectivity on SK2-a/SK3 subtype by displaying 93.33 ± 3.26% efficacies on SK2-a channels, and 84.54% ± 7.49% on SK3 channels. In addition, compound 12b demonstrated the potential to cross the blood-brain barrier (BBB) with suitable pharmacokinetic properties and low cytotoxicity. Molecular docking study also unveiled the binding interactions of compound 12b with SK2-CaM protein complex. Overall, the novel pyrrolopyrimidines provide an insightful guidance for future structural optimization of SK channel agonists.

Development of Yin-Yang ligand for cannabinoid receptors

Cannabinoid receptors (CBs), including CB1 and CB2, are the key components of a lipid signaling endocannabinoid system (ECS). Development of synthetic cannabinoids has been attractive to modulate ECS functions. CB1 and CB2 are structurally closely related subtypes but with distinct functions. While most efforts focus on the development of selective ligands for single subtype to circumvent the undesired off-target effect, Yin-Yang ligands with opposite pharmacological activities simultaneously on two subtypes, offer unique therapeutic potential. Herein we report the development of a new Yin-Yang ligand which functions as an antagonist for CB1 and concurrently an agonist for CB2. We found that in the pyrazole-cored scaffold, the arm of N1-phenyl group could be a switch, modification of which yielded various ligands with distinct activities. As such, the ortho-morpholine substitution exerted the desired Yin-Yang bifunctionality which, based on the docking study and molecular dynamic simulation, was proposed to be resulted from the hydrogen bonding with S173 and S285 in CB1 and CB2, respectively. Our results demonstrated the feasibility of structure guided ligand evolution for challenging Yin-Yang ligand.

Optimisation of estrogen receptor subtype-selectivity of a 4-Aryl-4H-chromene scaffold previously identified by virtual screening

4-Aryl-4H-Chromene derivatives have been previously shown to exhibit anti-proliferative, apoptotic and anti-angiogenic activity in a variety of tumor models in vitro and in vivo generally via activation of caspases through inhibition of tubulin polymerisation. We have previously identified by Virtual Screening (VS) a 4-aryl-4H-chromene scaffold, of which two examples were shown to bind Estrogen Receptor α and β with low nanomolar affinity and <20-fold selectivity for α over β and low micromolar anti-proliferative activity in the MCF-7 cell line. Thus, using the 4-aryl-4H-chromene scaffold as a starting point, a series of compounds with a range of basic arylethers at C-4 and modifications at the C3-ester substituent of the benzopyran ring were synthesised, producing some potent ER antagonists in the MCF-7 cell line which were highly selective for ERα (compound 35; 350-fold selectivity) or ERβ (compound 42; 170-fold selectivity).

ER subtype selectivity of m-carborane-containing phenols: C-alkyl groups on the m-carborane cage enhance ERα selectivity

Estrogen receptor (ER) exhibits two subtypes, ERα and ERβ, whose biological functions are quite different despite expression in the same tissues. We developed diiodo-m-carborane derivative 3a, which showed 14-fold selectivity for ERβ with high binding affinity toward ERβ. Interestingly, introduction of an alkyl group into the carbon atom of the m-carborane cage of 3a markedly enhanced the binding affinity toward ERα and decreased affinity toward ERβ. C-n-propyl derivative 3d showed 28-fold selectivity for ERα in an ER binding assay and promoted proliferation of MCF-7 breast cancer cells. Docking simulation studies suggest that the directions of the n-propyl group and the diiodo substituent introduced on the m-carborane cage play important roles for the control of ER subtype selectivity. As 3a and 3d showed ERβ and ERα selectivity with high binding affinity, respectively, these ligands may be useful as biological tools to aid in understanding the different roles of ER subtypes.

Structure-based exploration of an allosteric binding pocket in the NTS1 receptor using bitopic NT(8-13) derivatives and molecular dynamics simulations

Crystal structures of neurotensin receptor subtype 1 (NTS1) allowed us to visualize the binding mode of the endogenous peptide hormone neurotensin and its pharmacologically active C-terminal fragment NT(8-13) within the orthosteric binding pocket of NTS1. Beneath the orthosteric binding pocket, we detected a cavity that exhibits different sequences in the neurotensin receptor subtypes NTS1 and NTS2. In this study, we explored this allosteric binding pocket using bitopic test peptides of type NT(8-13)-Xaa, in which the C-terminal part of NT(8-13) is connected to different amino acids that extend into the newly discovered pocket. Our test compounds showed nanomolar affinities for NTS1, a measurable increase in subtype selectivity compared to the parent peptide NT(8-13), and the capacity to activate the receptor in an IP accumulation assay. Computational investigation of the selected test compounds at NTS1 showed a conserved binding mode within the orthosteric binding pocket, whereas the allosteric cavity was able to adapt to different residues, which suggests a high degree of structural plasticity within that cavity of NTS1.

Development of molecular tools based on the dopamine D3 receptor ligand FAUC 329 showing inhibiting effects on drug and food maintained behavior

Dopamine D₃ receptor-mediated networks have been associated with a wide range of neuropsychiatric diseases, drug addiction and food maintained behavior, which makes D₃ a highly promising biological target. The previously described dopamine D₃ receptor ligand FAUC 329 (1) showed protective effects against dopamine depletion in a MPTP mouse model of Parkinson's disease. We used the radioligand [¹⁸F]2, a [¹⁸F]fluoroethoxy substituted analog of the lead compound 1 as a molecular tool for visualization of D₃-rich brain regions including the islands of Calleja. Furthermore, structural modifications are reported leading to the pyrimidylpiperazine derivatives 3 and 9 displaying superior subtype selectivity and preference over serotonergic receptors. Evaluation of the lead compound 1 on cocaine-seeking behavior in non-human primates showed a substantial reduction in cocaine self-administration behavior and food intake.

Similarities and differences in affinity and binding modes of tricyclic pyrimido- and pyrazinoxanthines at human and rat adenosine receptors

A new series of 32 pyrimido- and 5 tetrahydropyrazino[2,1-f]purinediones was obtained and evaluated for their adenosine receptors (ARs) affinities. The 1,3-dibutyl derivative of 9-(4-(2-(dimethylamino)ethoxy)phenyl)-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)-dione was found to be the most potent A1 AR antagonist of the present series, showing selectivity over the other AR subtypes. The structure-activity for the obtained purinediones was established. Docking experiments of the investigated library to homology models of the human and rat A1 and A2A ARs allowed to compare the expected binding modes for selected compounds. The detailed analysis of binding cavities within individual AR subtypes indicated small but significant structural variations that may underlie the observed differences in binding affinities of purinediones at particular subtypes and species.

Characterization of PCS1055, a novel muscarinic M4 receptor antagonist

Identification of synthetic ligands selective for muscarinic receptor subtypes has been challenging due to the high sequence identity and structural homology among the five muscarinic acetylcholine receptors. Here, we report the pharmacological characterization of PCS1055, a novel muscarinic M4 receptor antagonist. PCS1055 inhibited radioligand [(3)H]-NMS binding to the M4 receptor with a Ki=6.5nM. Though the potency of PCS1055 is lower than that of pan-muscarinic antagonist atropine, it has better subtype selectivity over previously reported M4-selective reagents such as the muscarinic-peptide toxins (Karlsson et al., 1994; Santiago and Potter, 2001a) at the M1 subtype, and benzoxazine ligand PD102807 at the M3-subtype (Bohme et al., 2002). A detailed head-to-head comparison study using [(3)H]-NMS competitive binding assays characterizes the selectivity profiles of PCS1055 to that of other potent muscarinic-antagonist compounds PD102807, tropicamide, AF-DX-384, pirenzapine, and atropine. In addition to binding studies, the subtype specificity of PCS1055 is also demonstrated by functional receptor activation as readout by GTP-γ-[(35)S] binding. These GTP-γ-[(35)S] binding studies showed that PCS1055 exhibited 255-, 69.1-, 342- and >1000-fold greater inhibition of Oxo-M activity at the M4 versus the M1-, M2(-), M3-or M5 receptor subtypes, respectively. Schild analyses indicates that PCS1055 acts as a competitive antagonist to muscarinic M4 receptor, and confirms the affinity of the ligand to be low nanomolar, Kb=5.72nM. Therefore, PCS1055 represents a new M4-preferring antagonist that may be useful in elucidating the roles of M4 receptor signaling.

Development and characterization of the α3β4α5 nicotinic receptor cellular membrane affinity chromatography column and its application for on line screening of plant extracts

The α3β4α5 nAChR has been recently shown to be a useful target for smoking cessation pharmacotherapies. Herein, we report on the development and characterization of the α3β4α5 nicotinic receptor column by frontal displacement chromatography. The binding affinity of the nicotine and minor alkaloids found in tobacco smoke condensates were determined for both the α3β4 and α3β4α5 nicotinic receptors. It was demonstrated that while no subtype selectivity was observed for nicotine and nornicotine, anabasine was selective for the α3β4α5 nicotinic receptor. The non-competitive inhibitor binding site was also studied and it was demonstrated while mecamylamine was not selective between subtypes, buproprion showed subtype selectivity for the α3β4 nicotinic receptor. The application of this methodology to complex mixtures was then carried out by screening aqueous-alcoholic solutions of targeted plant extracts, including Lycopodium clavatum L. (Lycopodiaceae) and Trigonella foenum graecum L. (Fabaceae) against both the α3β4 and α3β4α5 nAChRs.

1,4-Disubstituted aromatic piperazines with high 5-HT2A/D2 selectivity: Quantitative structure-selectivity investigations, docking, synthesis and biological evaluation

Simultaneous targeting of dopamine D2 and 5-HT2A receptors for the treatment of schizophrenia is one key feature of typical and atypical antipsychotics. In most of the top-selling antipsychotic drugs like aripiprazole and risperidone, high affinity to both receptors can be attributed to the presence of 1,4-disubstituted aromatic piperazines or piperidines as primary receptor recognition elements. Taking advantage of our in-house library of phenylpiperazine-derived dopamine receptor ligands and experimental data, we established highly significant CoMFA and CoMSIA models for the prediction of 5-HT2A over D2 selectivity. Subsequently, the models were applied to identify the selective candidates 55-57 from our newly synthesized library of GPCR ligands comprising a pyrazolo[1,5-a]pyridine head group and a 1,2,3-triazole based linker unit. The test compound 57 showed subnanomolar a Ki value (0.64 nM) for 5-HT2A and more than 10- and 30-fold selectivity over the dopamine receptor isoforms D2S and D2L, respectively.

Structural insights into ligand recognition and selectivity for classes A, B, and C GPCRs

The G protein-coupled receptor (GPCR) superfamily constitutes the largest collection of cell surface signaling proteins with approximately 800 members in the human genome. GPCRs regulate virtually all aspects of physiology and they are an important class of drug targets with ~30% of drugs on the market targeting a GPCR. Breakthroughs in GPCR structural biology in recent years have significantly expanded our understanding of GPCR structure and function and ushered in a new era of structure-based drug design for GPCRs. Crystal structures for nearly thirty distinct GPCRs are now available including receptors from each of the major classes, A, B, C, and F. These structures provide a foundation for understanding the molecular basis of GPCR pharmacology. Here, we review structural mechanisms of ligand recognition and selectivity of GPCRs with a focus on selected examples from classes A, B, and C, and we highlight major unresolved questions for future structural studies.

Metal-assisted synthesis of unsymmetrical magnolol and honokiol analogs and their biological assessment as GABAA receptor ligands

We present the synthesis of new derivatives of natural products magnolol (1) and honokiol (2) and their evaluation as allosteric ligands for modulation of GABA_A receptor activity. New derivatives were prepared via metal assisted cross-coupling reactions in two consecutive steps. Compounds were tested by means of two-electrode voltage clamp electrophysiology at the α1β2γ2 receptor subtype at low GABA concentrations. We have identified several compounds enhancing GABA induced current (I_GABA) in the range similar or even higher than the lead structures. At 3 μM, compound 8g enhanced I_GABA by factor of 443, compared to 162 and 338 of honokiol and magnolol, respectively. Furthermore, 8g at EC_10–20 features a much bigger window of separation between the α1β2γ2 and the α1β1γ2 subtypes compared to honokiol, and thus improved subtype selectivity.

Enhanced estrogen receptor beta (ERβ) selectivity of fluorinated carborane-containing ER modulators

We previously showed that fluorination of the carborane-containing selective estrogen receptor modulator (SERM) BE360 altered the agonist/antagonist activity balance and the estrogen receptor (ER) α/β subtype selectivity. Here, we designed and synthesized a series of fluorinated carboranyl phenols as candidate ERβ-selective ligands. Introduction of a fluorine atom onto the carborane cage commonly reduced the binding affinity for ERα, to an extent that depended on the other substituents present. The B-fluorinated m-carboranyl phenol 4a showed fourfold more potent ERβ-binding affinity than the parent non-fluorinated compound 7. 1-Iodo-9-fluoro-m-carboranyl phenol 4f showed high ERβ-binding affinity with an ERβ/ERα selectivity ratio of 8.2. Among the compounds tested, 6 showed the highest ERβ selectivity (10.1-fold) and the highest ER-agonistic activity (EC50: 5.1×10(-10) M) in MCF-7 cell proliferation assay.