Elusive equilibrium: the challenge of interpreting receptor pharmacology using calcium assays

Tubulin interaction at tubulin-binding sequence 1 (TBS1) is required for proper surface expression and TRPV1 channel activity

TRPV1, a polymodal and nonselective cation channel has unique gating mechanisms which is regulated by supramolecular complexes at the plasma membrane formed with membrane proteins, lipids and kinase pathways. Crosstalk between microtubule cytoskeleton with TRPV1 at various level has been established. Previously we demonstrated that the positively-charged residues present at specific tubulin-binding stretch sequences (i.e. TBS1 and TBS2, AA 710-730 and 770-797 respectively) located at the C-terminus of TRPV1 are crucial for tubulin interaction and such sequences have evolutionary origin. The nature of TRPV1-tubulin complex and its functional importance remain poorly understood. Here, we made several mutations in the TBS1 and TBS2 regions and characterized such mutants. Though these mutations reduce tubulin interaction drastically, a low and basal-level of tubulin interaction remains with these mutants. Substitution of positively-charged residues (Lys and Arg) to Ala in the TBS1, but not in TBS2 region results in reduced ligand-sensitivity. Such ligand-sensitivity is altered in response to Taxol or Nocodazole. We suggest that tubulin interaction at the TBS1 region favours channel opening while interaction in TBS2 favours channel closure. We demonstrate for the first time the functional significance of TRPV1-tubulin complex and endorse microtubule dynamics as a parameter that can alter TRPV1 channel functions. These findings can be relevant for several physiological functions and also in the context of chemotherapy-induced neuropathic pain caused by various microtubule stabilizing chemotherapeutic drugs. Thus, this characterization may indicate TRPV1 as a potential therapeutic target relevant for chemotherapeutic drug-induced peripheral neuropathies, neurodegeneration and other neurological disorders.

In vitro pharmacological characterization of growth hormone secretagogue receptor ligands using the dynamic mass redistribution and calcium mobilization assays

Ghrelin modulates several biological functions via selective activation of the growth hormone secretagogue receptor (GHSR). GHSR agonists may be useful for the treatment of anorexia and cachexia, while antagonists and inverse agonists may represent new drugs for the treatment of metabolic and substance use disorders. Thus, the identification and pharmacodynamic characterization of new GHSR ligands is of high interest. In the present work the label-free dynamic mass redistribution (DMR) assay has been used to evaluate the pharmacological activity of a panel of GHSR ligands. This includes the endogenous peptides ghrelin, desacyl-ghrelin and LEAP2(1-14). Among synthetic compounds, the agonists anamorelin and HM01, the antagonists HM04 and YIL-781, and the inverse agonist PF-05190457 have been tested, together with HM03, R011, and H1498 from patent literature. The DMR results have been compared to those obtained in parallel experiments with the calcium mobilization assay. Ghrelin, anamorelin, HM01, and HM03 behaved as potent full GHSR agonists. YIL-781 behaved as a partial GHSR agonist and R011 as antagonist in both the assays. LEAP2(1-14) resulted a GHSR inverse agonist in DMR but not in calcium mobilization assay. PF-05190457, HM04, and H1498 behaved as GHSR inverse agonists in DMR experiments, while they acted as antagonists in calcium mobilization studies. In conclusion, this study provided a systematic pharmacodynamic characterization of several GHSR ligands in two different pharmacological assays. It demonstrated that the DMR assay can be successfully used particularly to discriminate between antagonists and inverse agonists. This study may be useful for the selection of the most appropriate compounds to be used in future studies.

A miniaturized mode-of-action profiling platform enables high throughput characterization of the molecular and cellular dynamics of EZH2 inhibition

The market approval of Tazemetostat (TAZVERIK) for the treatment of follicular lymphoma and epithelioid sarcoma has established "enhancer of zeste homolog 2" (EZH2) as therapeutic target in oncology. Despite their structural similarities and common mode of inhibition, Tazemetostat and other EZH2 inhibitors display differentiated pharmacological profiles based on their target residence time. Here we established high throughput screening methods based on time-resolved fluorescence energy transfer, scintillation proximity and high content analysis microscopy to quantify the biochemical and cellular binding of a chemically diverse collection of EZH2 inhibitors. These assays allowed to further characterize the interplay between EZH2 allosteric modulation by methylated histone tails (H3K27me3) and inhibitor binding, and to evaluate the impact of EZH2's clinically relevant mutant Y641N on drug target residence times. While all compounds in this study exhibited slower off-rates, those with clinical candidate status display significantly slower target residence times in wild type EZH2 and disease-related mutants. These inhibitors interact in a more entropy-driven fashion and show the most persistent effects in cellular washout and antiproliferative efficacy experiments. Our work provides mechanistic insights for the largest cohort of EZH2 inhibitors reported to date, demonstrating that-among several other binding parameters-target residence time is the best predictor of cellular efficacy.

In vitro pharmacological characterization of standard and new lysophosphatidic acid receptor antagonists using dynamic mass redistribution assay

Lysophosphatidic acid (LPA) is a bioactive phospholipid that acts as an agonist of six G protein-coupled receptors named LPA receptors (LPA1-6). LPA elicits diverse intracellular events and modulates several biological functions, including cell proliferation, migration, and invasion. Overactivation of the LPA-LPA receptor system is reported to be involved in several pathologies, including cancer, neuropathic pain, fibrotic diseases, atherosclerosis, and type 2 diabetes. Thus, LPA receptor modulators may be clinically relevant in numerous diseases, making the identification and pharmacodynamic characterization of new LPA receptor ligands of strong interest. In the present work, label-free dynamic mass redistribution (DMR) assay has been used to evaluate the pharmacological activity of some LPA1 and LPA2 standard antagonists at the recombinant human LPA1 and LPA2 receptors. These results are compared to those obtained in parallel experiments with the calcium mobilization assay. Additionally, the same experimental protocol has been used for the pharmacological characterization of the new compound CHI. KI 16425, RO 6842262, and BMS-986020 behaved as LPA1 inverse agonists in DMR experiments and as LPA1 antagonists in calcium mobilization assays. Amgen compound 35 behaved as an LPA2 antagonist, while Merck compound 20 from WO2012028243 was detected as an LPA2 inverse agonist using the DMR test. Of note, for all the compounds, similar potency values were estimated by DMR and calcium assay. The new compound CHI was found to be an LPA1 inverse agonist, but with potency lower than that of the standard compounds. In conclusion, we have demonstrated that DMR assay can be successfully used to characterize LPA1 and LPA2 ligands. Compared to the classical calcium mobilization assay, DMR offers some advantages, in particular allowing the identification of inverse agonists. Finally, in the frame of this study, a new LPA1 inverse agonist has been identified.

DNA-templated self-assembly of bradykinin into bioactive nanofibrils

Bradykinin (BK) is a peptide hormone that plays a crucial role in blood pressure control, regulates inflammation in the human body, and has recently been implicated in the pathophysiology of COVID-19. In this study, we report a strategy for fabricating highly ordered 1D nanostructures of BK using DNA fragments as a template for self-assembly. We have combined synchrotron small-angle X-ray scattering and high-resolution microscopy to provide insights into the nanoscale structure of BK-DNA complexes, unveiling the formation of ordered nanofibrils. Fluorescence assays hint that BK is more efficient at displacing minor-groove binders in comparison with base-intercalant dyes, thus, suggesting that interaction with DNA strands is mediated by electrostatic attraction between cationic groups at BK and the high negative electron density of minor-grooves. Our data also revealed an intriguing finding that BK-DNA complexes can induce a limited uptake of nucleotides by HEK-293t cells, which is a feature that has not been previously reported for BK. Moreover, we observed that the complexes retained the native bioactivity of BK, including the ability to modulate Ca²⁺ response into endothelial HUVEC cells. Overall, the findings presented here demonstrate a promising strategy for the fabrication of fibrillar structures of BK using DNA as a template, which keep bioactivity features of the native peptide and may have implications in the development of nanotherapeutics for hypertension and related disorders.

Off-target activity of NBOMes and NBOMe analogs at the µ opioid receptor

New psychoactive substances (NPS) are introduced on the illicit drug market at a rapid pace. Their molecular targets are often inadequately elucidated, which contributes to the delayed characterization of their pharmacological effects. Inspired by earlier findings, this study set out to investigate the µ opioid receptor (MOR) activation potential of a large set of psychedelics, substances which typically activate the serotonin (5-HT_2A) receptor as their target receptor. We observed that some substances carrying the N-benzyl phenethylamine (NBOMe) structure activated MOR, as confirmed by both the NanoBiT® βarr2 recruitment assay and the G protein-based AequoScreen® Ca²⁺ release assay. The use of two orthogonal systems proved beneficial as some aspecific, receptor independent effects were found for various analogs when using the Ca²⁺ release assay. The specific 'off-target' effects at MOR could be blocked by the opioid antagonist naloxone, suggesting that these NBOMes occupy the same common opioid binding pocket as conventional opioids. This was corroborated by molecular docking, which revealed the plausibility of multiple interactions of 25I-NBOMe with MOR, similar to those observed for opioids. Additionally, structure-activity relationship findings seen in vitro were rationalized in silico for two 25I-NBOMe isomers. Overall, as MOR activity of these psychedelics was only noticed at high concentrations, we consider it unlikely that for the tested compounds there will be a relevant opioid toxicity in vivo at physiologically relevant concentrations. However, small modifications to the original NBOMe structure may result in a panel of more efficacious and potent MOR agonists, potentially exhibiting a dual MOR/5-HT_2A activation potential.

On the validity of fluorimetric intracellular calcium detection: Impact of lipid components

We investigated the effects of different lipids on the activity of the angiotensin II type 1 receptor (AT1R). As calcium plays a key role in the signaling of the AT1R, we used the calcium-sensitive fluorescence indicators fura-2 to detect intracellular calcium release upon stimulation with the agonist angiotensin II. At first sight, cells preincubated with Very low-density lipoprotein (VLDL) showed a reduced calcium release triggered by angiontensin II compared to untreated control. However, on closer examination, this result seemed to be an artifact. Incubation with VLDL reduced also the amount of intracellular fura-2, as measured by fluorescence in the isosbestic point. Additionally, the maximal obtainable ratio, obtained after complete saturation with calcium ions, was reduced in cells preincubated with VLDL. These findings rendered our initial results questionable. We report the results of our work and our suggestions regarding the experimental setup to contribute to the understanding of the interpretation of fura-2 measurements and to avoid erroneous conclusions.

Optimization of Peptide Linker-Based Fluorescent Ligands for the Histamine H1 Receptor

The histamine H₁ receptor (H₁R) has recently been implicated in mediating cell proliferation and cancer progression; therefore, high-affinity H₁R-selective fluorescent ligands are desirable tools for further investigation of this behavior in vitro and in vivo. We previously reported a H₁R fluorescent ligand, bearing a peptide-linker, based on antagonist VUF13816 and sought to further explore structure-activity relationships (SARs) around the linker, orthostere, and fluorescent moieties. Here, we report a series of high-affinity H₁R fluorescent ligands varying in peptide linker composition, orthosteric targeting moiety, and fluorophore. Incorporation of a boron-dipyrromethene (BODIPY) 630/650-based fluorophore conferred high binding affinity to our H₁R fluorescent ligands, remarkably overriding the linker SAR observed in corresponding unlabeled congeners. Compound 31a, both potent and subtype-selective, enabled H₁R visualization using confocal microscopy at a concentration of 10 nM. Molecular docking of 31a with the human H₁R predicts that the optimized peptide linker makes interactions with key residues in the receptor.

Pharmacology of Kappa Opioid Receptors: Novel Assays and Ligands

The present study investigated the in vitro pharmacology of the human kappa opioid receptor using multiple assays, including calcium mobilization in cells expressing chimeric G proteins, the dynamic mass redistribution (DMR) label-free assay, and a bioluminescence resonance energy transfer (BRET) assay that allows measurement of receptor interaction with G protein and β-arrestin 2. In all assays, dynorphin A, U-69,593, and [D-Pro10]dyn(1-11)-NH2 behaved as full agonists with the following rank order of potency [D-Pro10]dyn(1-11)-NH2 > dynorphin A ≥ U-69,593. [Dmt1,Tic2]dyn(1-11)-NH2 behaved as a moderate potency pure antagonist in the kappa-β-arrestin 2 interaction assay and as low efficacy partial agonist in the other assays. Norbinaltorphimine acted as a highly potent and pure antagonist in all assays except kappa-G protein interaction, where it displayed efficacy as an inverse agonist. The pharmacological actions of novel kappa ligands, namely the dynorphin A tetrameric derivative PWT2-Dyn A and the palmitoylated derivative Dyn A-palmitic, were also investigated. PWT2-Dyn A and Dyn A-palmitic mimicked dynorphin A effects in all assays showing similar maximal effects but 3–10 fold lower potency. In conclusion, in the present study, multiple in vitro assays for the kappa receptor have been set up and pharmacologically validated. In addition, PWT2-Dyn A and Dyn A-palmitic were characterized as potent full agonists; these compounds are worthy of further investigation in vivo for those conditions in which the activation of the kappa opioid receptor elicits beneficial effects e.g. pain and pruritus.

Quantifying the Kinetics of Signaling and Arrestin Recruitment by Nervous System G-Protein Coupled Receptors

Neurons integrate inputs over different time and space scales. Fast excitatory synapses at boutons (ms and μm), and slow modulation over entire dendritic arbors (seconds and mm) are all ultimately combined to produce behavior. Understanding the timing of signaling events mediated by G-protein-coupled receptors is necessary to elucidate the mechanism of action of therapeutics targeting the nervous system. Measuring signaling kinetics in live cells has been transformed by the adoption of fluorescent biosensors and dyes that convert biological signals into optical signals that are conveniently recorded by microscopic imaging or by fluorescence plate readers. Quantifying the timing of signaling has now become routine with the application of equations in familiar curve fitting software to estimate the rates of signaling from the waveform. Here we describe examples of the application of these methods, including (1) Kinetic analysis of opioid signaling dynamics and partial agonism measured using cAMP and arrestin biosensors; (2) Quantifying the signaling activity of illicit synthetic cannabinoid receptor agonists measured using a fluorescent membrane potential dye; (3) Demonstration of multiplicity of arrestin functions from analysis of biosensor waveforms and quantification of the rates of these processes. These examples show how temporal analysis provides additional dimensions to enhance the understanding of GPCR signaling and therapeutic mechanisms in the nervous system.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT_2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca²⁺ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

Cellular and Molecular Mechanisms of Fat Taste Perception

During the last couples of years, a number of studies have increasingly accumulated on the gustatory perception of dietary fatty acids in rodent models and human beings in health and disease. There is still a debate to coin a specific term for the gustatory perception of dietary fatty acids either as the sixth basic taste quality or as an alimentary taste. Indeed, the psycho-physical cues of orosensory detection of dietary lipids are not as distinctly perceived as other taste qualities like sweet or bitter. The cellular and molecular pharmacological mechanisms, triggered by the binding of dietary long-chain fatty acids (LCFAs) to tongue taste bud lipid receptors like CD36 and GPR120, involve Ca²⁺ signaling as other five basic taste qualities. We have not only elucidated the role of Ca²⁺ signaling but also identified different components of the second messenger cascade like STIM1 and MAP kinases, implicated in fat taste perception. We have also demonstrated the implication of Calhm1 voltage-gated channels and store-operated Ca²⁺ (SOC) channels like Orai1, Orai1/3, and TRPC3 in gustatory perception of dietary fatty acids. We have not only employed siRNA technology in vitro and ex vivo on tissues but also used animal models of genetic invalidation of STIM1, ERK1, Orai1, Calhm1 genes to explore their implications in fat taste signal transduction. Moreover, our laboratory has also demonstrated the importance of LCFAs detection dysfunction in obesity in animal models and human beings.

Why Taste Is Pharmacology

The chapter presents an argument supporting the view that taste, defined as the receptor-mediated signaling of taste cells and consequent sensory events, is proper subject matter for the field of pharmacology. The argument develops through a consideration of how the field of pharmacology itself is to be defined. Though its application toward the discovery and development of therapeutics is of obvious value, pharmacology nevertheless is a basic science committed to examining biological phenomena controlled by the selective interactions between chemicals - regardless of their sources or uses - and receptors. The basic science of pharmacology is founded on the theory of receptor occupancy, detailed here in the context of taste. The discussion then will turn to consideration of the measurement of human taste and how well the results agree with the predictions of receptor theory.

Unique Pharmacological Properties of α-Conotoxin OmIA at α7 nAChRs

OmIA, isolated from Conus omaria venom, is a potent antagonist at α7 nAChRs. We determined the co-crystal structure of OmIA with Lymnae stagnalis acetylcholine binding protein (Ls-AChBP) that identified His5, Val10 and Asn11 as key determinants for the high potency of OmIA at α7 nAChRs. Remarkably, despite a competitive binding mode observed in the co-crystal structure, OmIA and analogues displayed functional insurmountable antagonism at α7 and α3β4 nAChRs, except OmIA analogues having long side chain at position 10 ([V10Q]OmIA and [V10L]OmIA), which were partial insurmountable antagonist at α7 nAChRs in the presence of type II positive allosteric modulators (PAMs). A “two-state, two-step” model was used to explain these observations, with [V10Q]OmIA and [V10L]OmIA co-existing in a fast reversible/surmountable as well as a tight binding/insurmountable state. OmIA and analogues also showed biphasic-inhibition at α7 nAChRs in the presence of PNU120596, with a preference for the high-affinity binding site following prolonged exposure. The molecular basis of binding and complex pharmacological profile of OmIA at α7 nAChRs presented in here expands on the potential of α-conotoxins to probe the pharmacological properties of nAChRs and may help guide the development novel α7 modulators.

The Spasmolytic, Bronchodilator, and Vasodilator Activities of Parmotrema perlatum Are Explained by Anti-Muscarinic and Calcium Antagonistic Mechanisms

Parmotremaperlatum is traditionally used in different areas of Pakistan to treat gastrointestinal, respiratory, and vascular diseases. This study evaluates the underlying mechanisms for traditional uses of P. perlatum in diarrhea, asthma, and hypertension. In vitro pharmacological studies were conducted using isolated jejunum, trachea, and aortic preparations, while the cytotoxic study was conducted in mice. Crude extract of P. perlatum(Pp.Cr), comprising appreciable quantities of alkaloids and flavonoids, relaxed spontaneously contracting jejunum preparation, K⁺ (80 mM)-induced, and carbachol (1 µM)-induced jejunum contractions in a concentration-dependent manner similar to dicyclomine and dantrolene. Pp.Cr showed a rightward parallel shift of concentration-response curves (CRCs) of Cch after a non-parallel shift similarto dicyclomine and shifted CRCs of Ca⁺² to rightward much likeverapamil and dantrolene, demonstrating the coexistence of antimuscarinic and Ca⁺² antagonistic mechanism. Furthermore, Pp.Cr, dicyclomine, and dantrolene relaxed K⁺ (80 mM)-induced and Cch (1 µM)-induced tracheal contractions and shifted rightward CRCs of Cch similar to dicyclomine, signifying the dual blockade. Additionally, Pp.Cr also relaxed the K⁺ (80 mM)-induced and phenylephrine (1 µM)-induced aortic contraction, similarly to verapamil and dantrolene, suggesting Ca⁺² channel antagonism. Here, we explored for the first time thespasmolytic and bronchodilator effects of Pp.Crand whether they maybe due to the dual blockade of Ca⁺² channels and muscarinic receptors, while the vasodilator effect might be owing to Ca⁺² antagonism. Our results provide the pharmacological evidence that P. perlatum could be a new potential therapeutic option to treat gastrointestinal, respiratory, and vascular diseases. Hence, there is a need for further research to explore bioactive constituent of P. perlatum as well as further investigation by suitable experimental models are required to further confirm the importance and usefulness of P. perlatum in diarrhea, asthma, and hypertension treatment.

Posttranslational modifications of α-conotoxins: sulfotyrosine and C-terminal amidation stabilise structures and increase acetylcholine receptor binding

Conotoxins are peptides found in the venoms of marine cone snails. They are typically highly structured and stable and have potent activities at nicotinic acetylcholine receptors, which make them valuable research tools and promising lead molecules for drug development. Many conotoxins are also highly modified with posttranslational modifications such as proline hydroxylation, glutamic acid gamma-carboxylation, tyrosine sulfation and C-terminal amidation, amongst others. The role of these posttranslational modifications is poorly understood, and it is unclear whether the modifications interact directly with the binding site, alter conotoxin structure, or both. Here we synthesised a set of twelve conotoxin variants bearing posttranslational modifications in the form of native sulfotyrosine and C-terminal amidation and show that these two modifications in combination increase their activity at nicotinic acetylcholine receptors and binding to soluble acetylcholine binding proteins, respectively. We then rationalise how these functional differences between variants might arise from stabilization of the three-dimensional structures and interactions with the binding sites, using high-resolution nuclear magnetic resonance data. This study demonstrates that posttranslational modifications can modulate interactions between a ligand and receptor by a combination of structural and binding alterations. A deeper mechanistic understanding of the role of posttranslational modifications in structure-activity relationships is essential for understanding receptor biology and could help to guide structure-based drug design.

The Problems of Applying Classical Pharmacology Analysis to Modern In Vitro Drug Discovery Assays: Slow Binding Kinetics and High Target Concentration

The analysis framework used to quantify drug potency in vitro (e.g., K_d or K_i) was initially developed for classical pharmacology bioassays, for example, organ bath experiments testing moderate-affinity natural products. Modern drug discovery can infringe the assumptions of the classical pharmacology analysis equations, owing to the reduction of assay volume in miniaturization, target overexpression, and the increase of compound-target affinity in medicinal chemistry. These assumptions are that (1) the compound concentration greatly exceeds the target concentration (i.e., minimal ligand depletion), and (2) the compound is at equilibrium with the receptor (i.e., rapid ligand binding kinetics). Unappreciated infringement of these assumptions can lead to substantial underestimation of compound affinity, which negatively impacts the drug discovery process, from early-stage lead optimization to prediction of human dosing. This study evaluates the real-world impact of these factors on the target interaction assays used in drug discovery using literature examples, database searches, and simulations. The ranges of compound affinity and the assay types that are prone to depletion and equilibration artifacts are identified. Importantly, the highest-affinity compounds, usually the highest value chemical matter in drug discovery, are the most affected. Methods and simulation tools are provided to enable investigators to evaluate, manage, and minimize depletion or equilibration artifacts. This study enables the correct application of pharmacological data analysis to accurately quantify affinity using modern drug discovery assay technology.

Luciferase Complementation Approaches to Measure GPCR Signaling Kinetics and Bias

An understanding of the kinetic contributions to G protein-coupled receptor pharmacology and signaling is increasingly important in compound profiling. Nonequilibrium conditions are commonly present in vivo, for example, as the drug competes with dynamic changes in hormone or neurotransmitter concentration for the receptor. Under such conditions individual binding kinetic properties of the ligands can influence duration of action, local ligand concentration, and functional properties such as the degree of insurmountable inhibition. Mapping the kinetic patterns of GPCR signaling events elicited by agonists, rather than a peak response at a single timepoint, is often key to predicting their functional impact. This is also a path to a better understanding of the origins of ligand bias, and whether such ligands demonstrate their effects through selection of distinct GPCR conformations, or via their kinetic properties. Recent developments in complementation approaches, based on a small bright shrimp luciferase Nanoluc, provide a new route to kinetic analysis of GPCR signaling in living cells that is amenable to the throughput required for compound profiling. In the NanoBiT luciferase complementation system, GPCRs and effector proteins are tagged with Nanoluc fragments optimized for their low interacting affinity and stability. The interactions brought about by GPCR recruitment of the effector are reproduced by a rapid and reversible increase in NanoBiT luminescence, in the presence of its substrate furimazine. Here we discuss the methods for optimizing and validating the GPCR NanoBiT assays, and protocols for their application to study endpoint and kinetic aspects of agonist and antagonist pharmacology. We also describe how timecourse families of agonist concentration response curves, derived from a single NanoBiT assay experiment, can be used to evaluate the kinetic components in operational model derived parameters of ligand bias.

Establishment of a NanoBiT-Based Cytosolic Ca2+ Sensor by Optimizing Calmodulin-Binding Motif and Protein Expression Levels

Cytosolic Ca2+ levels ([Ca2+]c) change dynamically in response to inducers, repressors, and physiological conditions, and aberrant [Ca2+]c concentration regulation is associated with cancer, heart failure, and diabetes. Therefore, [Ca2+]c is considered as a good indicator of physiological and pathological cellular responses, and is a crucial biomarker for drug discovery. A genetically encoded calcium indicator (GECI) was recently developed to measure [Ca2+]c in single cells and animal models. GECI have some advantages over chemically synthesized indicators, although they also have some drawbacks such as poor signal-to-noise ratio (SNR), low positive signal, delayed response, artifactual responses due to protein overexpression, and expensive detection equipment. Here, we developed an indicator based on interactions between Ca2+-loaded calmodulin and target proteins, and generated an innovative GECI sensor using split nano-luciferase (Nluc) fragments to detect changes in [Ca2+]c. Stimulation-dependent luciferase activities were optimized by combining large and small subunits of Nluc binary technology (NanoBiT, LgBiT:SmBiT) fusion proteins and regulating the receptor expression levels. We constructed the binary [Ca2+]c sensors using a multicistronic expression system in a single vector linked via the internal ribosome entry site (IRES), and examined the detection efficiencies. Promoter optimization studies indicated that promoter-dependent protein expression levels were crucial to optimize SNR and sensitivity. This novel [Ca2+]c assay has high SNR and sensitivity, is easy to use, suitable for high-throughput assays, and may be useful to detect [Ca2+]c in single cells and animal models.

A High-Throughput Assay for the Pancreatic Islet Beta-Cell Potassium Channel: Use in the Pharmacological Characterization of Insulin Secretagogues Identified from Phenotypic Screening

Phenotypic screening is a neoclassical approach for drug discovery. We conducted phenotypic screening for insulin secretion enhancing agents using INS-1E insulinoma cells as a model system for pancreatic beta-cells. A principal regulator of insulin secretion in beta-cells is the metabolically regulated potassium channel Kir6.2/SUR1 complex. To characterize hit compounds, we developed an assay to quantify endogenous potassium channel activity in INS-1E cells. We quantified ligand-regulated potassium channel activity in INS-1E cells using fluorescence imaging and thallium flux. Potassium channel activity was metabolically regulated and coupled to insulin secretion. The pharmacology of channel opening agents (diazoxide) and closing agents (sulfonylureas) was used to validate the applicability of the assay. A precise high-throughput assay was enabled, and phenotypic screening hits were triaged to enable a higher likelihood of discovering chemical matter with novel and useful mechanisms of action.

Analyzing kinetic signaling data for G-protein-coupled receptors

In classical pharmacology, bioassay data are fit to general equations (e.g. the dose response equation) to determine empirical drug parameters (e.g. EC50 and Emax), which are then used to calculate chemical parameters such as affinity and efficacy. Here we used a similar approach for kinetic, time course signaling data, to allow empirical and chemical definition of signaling by G-protein-coupled receptors in kinetic terms. Experimental data are analyzed using general time course equations (model-free approach) and mechanistic model equations (mechanistic approach) in the commonly-used curve-fitting program, GraphPad Prism. A literature survey indicated signaling time course data usually conform to one of four curve shapes: the straight line, association exponential curve, rise-and-fall to zero curve, and rise-and-fall to steady-state curve. In the model-free approach, the initial rate of signaling is quantified and this is done by curve-fitting to the whole time course, avoiding the need to select the linear part of the curve. It is shown that the four shapes are consistent with a mechanistic model of signaling, based on enzyme kinetics, with the shape defined by the regulation of signaling mechanisms (e.g. receptor desensitization, signal degradation). Signaling efficacy is the initial rate of signaling by agonist-occupied receptor (kτ), simply the rate of signal generation before it becomes affected by regulation mechanisms, measurable using the model-free analysis. Regulation of signaling parameters such as the receptor desensitization rate constant can be estimated if the mechanism is known. This study extends the empirical and mechanistic approach used in classical pharmacology to kinetic signaling data, facilitating optimization of new therapeutics in kinetic terms.

Direct and Intervertebral Disc-Mediated Sensitization of Dorsal Root Ganglion Neurons by Hypoxia and Low pH

Objective

Ischemia-related risk factors are consistently correlated with discogenic pain, but it remains unclear how the ischemia-associated hypoxia and acidosis influence the peripheral sensory nervous system, namely the dorsal root ganglion (DRG), either directly or indirectly via intervertebral disc (IVD) mediation.

Methods

Bovine tail IVD organ cultures were preconditioned in different hypoxic and/or acidic conditions for 3 days to collect the conditioned medium (CM). The DRG-derived ND7/23 cells were either treated by the IVD CM or directly stimulated by hypoxic and/or acidic conditions. Neuronal sensitization was evaluated using calcium imaging (Fluo-4) after 3 days.

Results

We found that direct exposure of DRG cell line to hypoxia and acidosis increased both spontaneous and bradykinin-stimulated calcium response compared to normoxia-neutral pH cultures. Hypoxia and low pH in combination showed stronger effect than either parameter on its own. Indirect exposure of DRG to hypoxia-acidosis-stressed IVD CM also increased spontaneous and bradykinin-stimulated response, but to a lower extent than direct exposure. The impact of direct hypoxia and acidosis on DRG was validated in a primary sheep DRG cell culture, showing the same trend.

Conclusion

Our data suggest that targeting hypoxia and acidosis stresses both in IVD and DRG could be a relevant objective in discogenic pain treatment.

Amplification of a calcium channel subunit CACNG4 increases breast cancer metastasis

BACKGROUND

Previously, we found that amplification of chromosome 17q24.1-24.2 is associated with lymph node metastasis, tumour size, and lymphovascular invasion in invasive ductal carcinoma. A gene within this amplicon, CACNG4, an L-type voltage-gated calcium channel gamma subunit, is elevated in breast cancers with poor prognosis. Calcium homeostasis is achieved by maintaining low intracellular calcium levels. Altering calcium influx/efflux mechanisms allows tumour cells to maintain homeostasis despite high serum calcium levels often associated with advanced cancer (hypercalcemia) and aberrant calcium signaling.

METHODS

In vitro 2-D and 3-D assays, and intracellular calcium influx assays were utilized to measure tumourigenic activity in response to altered CANCG4 levels and calcium channel blockers. A chick-CAM model and mouse model for metastasis confirmed these results in vivo.

FINDINGS

CACNG4 alters cell motility in vitro, induces malignant transformation in 3-dimensional culture, and increases lung-specific metastasis in vivo. CACNG4 functions by closing the channel pore, inhibiting calcium influx, and altering calcium signaling events involving key survival and metastatic pathway genes (AKT2, HDAC3, RASA1 and PKCζ).

INTERPRETATION

CACNG4 may promote homeostasis, thus increasing the survival and metastatic ability of tumour cells in breast cancer. Our findings suggest an underlying pathway for tumour growth and dissemination regulated by CACNG4 that is significant with respect to developing treatments that target these channels in tumours with aberrant calcium signaling.

FUNDING

Canadian Breast Cancer Foundation, Ontario; Canadian Institutes of Health Research.

Detailed In Vitro Pharmacological Characterization of the Clinically Viable Nociceptin/Orphanin FQ Peptide Receptor Antagonist BTRX-246040

The peptide nociceptin/orphanin FQ (N/OFQ) is the natural ligand of the N/OFQ receptor (NOP), which is widely expressed in the central and peripheral nervous system. Selective NOP antagonists are worthy of testing as innovative drugs to treat depression, Parkinson disease, and drug abuse. The aim of this study was to perform a detailed in vitro characterization of BTRX-246040 (also known as LY2940094, [2-[4-[(2-chloro-4,4-difluoro-spiro[5H-thieno[2,3-c]pyran-7,4'-piperidine]-1'-yl)methyl]-3-methyl-pyrazol-1-yl]-3-pyridyl]methanol), a novel NOP antagonist that has been already studied in humans. BTRX-246040 has been tested in vitro in the following assays: calcium mobilization in cells expressing NOP and classic opioid receptors and chimeric G proteins, bioluminescence resonance energy transfer assay measuring NOP interaction with G proteins and β-arrestins, the label-free dynamic mass redistribution assay, and the electrically stimulated mouse vas deferens. BTRX-246040 was systematically compared with the standard NOP antagonist SB-612111. In all assays, BTRX-246040 behaves as a pure and selective antagonist at human recombinant and murine native NOP receptors displaying 3-10-fold higher potency than the standard antagonist SB-612111. BTRX-246040 is an essential pharmacological tool to further investigate the therapeutic potential of NOP antagonists in preclinical and clinical studies. SIGNIFICANCE STATEMENT: NOP antagonists may be innovative antidepressant drugs. In this research, the novel clinically viable NOP antagonist BTRX-246040 has been deeply characterized in vitro in a panel of assays. BTRX-246040 resulted a pure, potent, and selective NOP antagonist.

Synthesis and Pharmacological Evaluation of 1-Phenyl-3-Thiophenylurea Derivatives as Cannabinoid Type-1 Receptor Allosteric Modulators

We previously reported diarylurea derivatives as cannabinoid type-1 receptor (CB₁) allosteric modulators, which were effective in attenuating cocaine-seeking behavior. Herein, we extended the structure-activity relationships of PSNCBAM-1 (2) at the central phenyl ring directly connected to the urea moiety. Replacement with a thiophene ring led to 11 with improved or comparable potencies in calcium mobilization, [³⁵S]GTPγS binding, and cAMP assays, whereas substitution with nonaromatic rings led to significant attenuation of the modulatory activity. These compounds had no inverse agonism in [³⁵S]GTPγS binding, a characteristic that is often thought to contribute to adverse psychiatric effects. While 11 had good metabolic stability in rat liver microsomes, it showed modest solubility and blood-brain barrier permeability. Compound 11 showed an insignificant attenuation of cocaine seeking behavior in rats, most likely due to its limited CNS penetration, suggesting that pharmacokinetics and distribution play a role in translating the in vitro efficacy to in vivo behavior.

Prescient Indices of Activity: The Application of Functional System Sensitivity to Measurement of Drug Effect

Through pharmacological procedures, indices of drug activity can be obtained that transcend the systems in which they are measured. If (i) affinity, (ii) efficacies, (iii) orthosteric versus allosteric interaction, and (iv) rate of receptor offset can be determined, activity can be predicted in all systems. This can yield more detailed profiles (fingerprints) of efficacy to better define the required activities of follow-up molecules should the original candidates fail in the clinic. The use of functional assays of varying sensitivity is a major tool in the lead optimization process and the observation of candidate molecule profiles in multiple functional assays can reveal all properties of candidate molecules. In this review, the different indices for agonists, antagonists, and allosteric modulators are defined while highlighting the application of functional assays in deriving these indices.

Pharmacological Assays for Investigating the NOP Receptor

The nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) is a G protein-coupled receptor involved in the regulation of several physiological functions and pathological conditions. Thus, researchers from academia and industry are pursuing NOP to discover and study novel pharmacological entities. In a multidisciplinary effort of pharmacologists, medicinal chemists, and molecular and structural biologists the mechanisms of NOP activation and inhibition have been, at least partially, disentangled. Here, we review the in vitro methodologies employed, which have contributed to our understanding of this target. We hope this chapter guides the reader through the mostly established assay platforms to investigate NOP pharmacology, and gives some hints taking advantage from what has already illuminated the function of other GPCRs. We analyzed the pharmacological results obtained with a large panel of NOP ligands investigated in several assays including receptor binding, stimulation of GTPγS binding, decrease of cAMP levels, calcium flux stimulation via chimeric G proteins, NOP/G protein and NOP/β-arrestin interaction, label-free assays such as dynamic mass redistribution, and bioassays such as the electrically stimulated mouse vas deferens.

Cebranopadol: A Novel First-in-Class Potent Analgesic Acting via NOP and Opioid Receptors

Cebranopadol is a novel first-in-class analgesic with highly potent agonistic activity at nociceptin/orphanin FQ peptide (NOP) and opioid receptors. It is highly potent and efficacious across a broad range of preclinical pain models. Its side effect profile is better compared to typical opioids. Mechanistic studies have shown that cebranopadol's activity at NOP receptors contributes to its anti-hyperalgesic effects while ameliorating some of its opioid-type side effects, including respiratory depression and abuse potential. Phase II of clinical development has been completed, demonstrating efficacy and good tolerability in acute and chronic pain conditions.This article focusses on reviewing data on the preclinical in vitro and in vivo pharmacology, safety, and tolerability, as well as clinical trials with cebranopadol.

Properties of cell signaling pathways and gene expression systems operating far from steady-state

Ligand-receptor systems, covalent modification cycles, and transcriptional networks are basic units of signaling systems and their steady-state properties are well understood. However, the behavior of such systems before steady-state is poorly characterized. Here, we analyzed the properties of input-output curves for each of these systems as they approach steady-state. In ligand-receptor systems, the EC₅₀ (concentration of the ligand that occupies 50% of the receptors) is higher before the system reaches steady-state. Based on this behavior, we have previously defined PRESS (for pre-equilibrium sensing and signaling), a general “systems level” mechanism cells may use to overcome input saturation. Originally, we showed that, given a step stimulation, PRESS operates when the kinetics of ligand-receptor binding are slower than the downstream signaling steps. Now, we show that, provided the input increases slowly, it is not essential for the ligand binding reaction itself to be slow. In addition, we demonstrate that covalent modification cycles and gene expression systems may also operate in PRESS mode. Thus, nearly all biochemical processes may operate in PRESS mode, suggesting that this mechanism may be ubiquitous in cell signaling systems.

Peptide welding technology - A simple strategy for generating innovative ligands for G protein coupled receptors

Based on their high selectivity of action and low toxicity, naturally occurring peptides have great potential in terms of drug development. However, the pharmacokinetic properties of peptides, in particular their half life, are poor. Among different strategies developed for reducing susceptibility to peptidases, and thus increasing the duration of action of peptides, the generation of branched peptides has been described. However, the synthesis and purification of branched peptides are extremely complicated thus limiting their druggability. Here we present a novel and facile synthesis of tetrabranched peptides acting as GPCR ligands and their in vitro and vivo pharmacological characterization. Tetrabranched derivatives of nociceptin/orphanin FQ (N/OFQ), N/OFQ related peptides, opioid peptides, tachykinins, and neuropeptide S were generated with the strategy named peptide welding technology (PWT) and characterized by high yield and purity of the desired final product. In general, PWT derivatives displayed a pharmacological profile similar to that of the natural sequence in terms of affinity, pharmacological activity, potency, and selectivity of action in vitro. More importantly, in vivo studies demonstrated that PWT peptides are characterized by increased potency associated with long lasting duration of action. In conclusion, PWT derivatives of biologically active peptides can be viewed as innovative pharmacological tools for investigating those conditions and states in which selective and prolonged receptor stimulation promotes beneficial effects.

From receptor binding kinetics to signal transduction; a missing link in predicting in vivo drug-action

An important question in drug discovery is how to overcome the significant challenge of high drug attrition rates due to lack of efficacy and safety. A missing link in the understanding of determinants for drug efficacy is the relation between drug-target binding kinetics and signal transduction, particularly in the physiological context of (multiple) endogenous ligands. We hypothesized that the kinetic binding parameters of both drug and endogenous ligand play a crucial role in determining cellular responses, using the NK1 receptor as a model system. We demonstrated that the binding kinetics of both antagonists (DFA and aprepitant) and endogenous agonists (NKA and SP) have significantly different effects on signal transduction profiles, i.e. potency values, in vitro efficacy values and onset rate of signal transduction. The antagonistic effects were most efficacious with slowly dissociating aprepitant and slowly associating NKA while the combination of rapidly dissociating DFA and rapidly associating SP had less significant effects on the signal transduction profiles. These results were consistent throughout different kinetic assays and cellular backgrounds. We conclude that knowledge of the relationship between in vitro drug-target binding kinetics and cellular responses is important to ultimately improve the understanding of drug efficacy in vivo.

Extrapyramidal side effects of antipsychotics are linked to their association kinetics at dopamine D2 receptors

Atypical antipsychotic drugs (APDs) have been hypothesized to show reduced extrapyramidal side effects (EPS) due to their rapid dissociation from the dopamine D₂ receptor. However, support for this hypothesis is limited to a relatively small number of observations made across several decades and under different experimental conditions. Here we show that association rates, but not dissociation rates, correlate with EPS. We measured the kinetic binding properties of a series of typical and atypical APDs in a novel time-resolved fluorescence resonance energy transfer assay, and correlated these properties with their EPS and prolactin-elevating liabilities at therapeutic doses. EPS are robustly predicted by a rebinding model that considers the microenvironment of postsynaptic D₂ receptors and integrates association and dissociation rates to calculate the net rate of reversal of receptor blockade. Thus, optimizing binding kinetics at the D₂ receptor may result in APDs with improved therapeutic profile.

Atypical antipsychotics show reduced extrapyramidal side effects compared to first generation drugs. Here the authors use time-resolved FRET to measure binding kinetics, and show that side effects correlate with drug association rates to the D₂ receptor, while dissociation rates correlate with prolactin elevation.

The Target Residence Time of Antihistamines Determines Their Antagonism of the G Protein-Coupled Histamine H1 Receptor

The pharmacodynamics of drug-candidates is often optimized by metrics that describe target binding (K_d or K_i value) or target modulation (IC₅₀). However, these metrics are determined at equilibrium conditions, and consequently information regarding the onset and offset of target engagement and modulation is lost. Drug-target residence time is a measure for the lifetime of the drug-target complex, which has recently been receiving considerable interest, as target residence time is shown to have prognostic value for the in vivo efficacy of several drugs. In this study, we have investigated the relation between the increased residence time of antihistamines at the histamine H₁ receptor (H₁R) and the duration of effective target-inhibition by these antagonists. Hela cells, endogenously expressing low levels of the H₁R, were incubated with a series of antihistamines and dissociation was initiated by washing away the unbound antihistamines. Using a calcium-sensitive fluorescent dye and a label free, dynamic mass redistribution based assay, functional recovery of the H₁R responsiveness was measured by stimulating the cells with histamine over time, and the recovery was quantified as the receptor recovery time. Using these assays, we determined that the receptor recovery time for a set of antihistamines differed more than 40-fold and was highly correlated to their H₁R residence times, as determined with competitive radioligand binding experiments to the H₁R in a cell homogenate. Thus, the receptor recovery time is proposed as a cell-based and physiologically relevant metric for the lead optimization of G protein-coupled receptor antagonists, like the H₁R antagonists. Both, label-free or real-time, classical signaling assays allow an efficient and physiologically relevant determination of kinetic properties of drug molecules.

4-Aroyl-3-hydroxy-5-phenyl-1H-pyrrol-2(5H)-ones as N-formyl peptide receptor 1 (FPR1) antagonists

Formyl peptide receptors (FPRs) are expressed on a variety of leukocytes and play important roles in inflammation. Thus, FPR antagonists may represent novel therapeutics for modulating innate immunity and treating inflammatory diseases. Previously, 1H-pyrrol-2(5H)-ones were reported to be potent and competitive FPR1 antagonists. In the present studies, 42 additional 1H-pyrrol-2(5H)-one analogs were evaluated for FPR1 antagonist activity. We identified a number of novel competitive FPR1 antagonists that inhibited N-formylmethionyl-leucyl-phenylalanine (fMLF)-induced intracellular Ca²⁺ mobilization in FPR1-transfected HL60 cells and effectively competed with WKYMVm-FITC for binding to FPR1 in FPR1-transfected RBL cells. The most active pyrroles inhibited human neutrophil Ca²⁺ flux, chemotaxis, and adhesion to human epithelial cells, with the most potent being compounds 14 (4-benzoyl-1-hexyl-3-hydroxy-5-(4-hydroxy-3-methoxyphenyl)-2,5-dihydro-1H-pyrrol-2-one) and 17 (4-benzoyl-5-(2,5-dimethoxyphenyl)-3-hydroxy-1-(2-methoxyethyl)-2,5-dihydro-1H-pyrrol-2-one). In addition, these FPR1 antagonists inhibited fMLF-induced phosphorylation of extracellular signal-regulated kinases (ERK1/2) in FPR1-RBL cells, differentiated HL-60 cells, and human neutrophils. Most of the antagonists were specific for FPR1 and did not inhibit WKYMVM/WKYMVm-induced intracellular Ca²⁺ mobilization in FPR2-HL60 cells, FPR3-HL60 cells, or interleukin 8-induced Ca²⁺ flux in human neutrophils. Moreover, molecular modeling showed that the active pyrroles had a significantly higher degree of similarity with the FPR1 antagonist pharmacophore template as compared to inactive analogs. Thus, the 4-aroyl-3-hydroxy-5-phenyl-1H-pyrrol-2(5H)-one scaffold represents an important backbone for the development of novel FPR1 antagonists and could provide important clues for understanding the molecular structural requirements of FPR1 antagonists.

Probe-Dependent Negative Allosteric Modulators of the Long-Chain Free Fatty Acid Receptor FFA4

High-affinity and selective antagonists that are able to block the actions of both endogenous and synthetic agonists of G protein–coupled receptors are integral to analysis of receptor function and to support suggestions of therapeutic potential. Although there is great interest in the potential of free fatty acid receptor 4 (FFA4) as a novel therapeutic target for the treatment of type II diabetes, the broad distribution pattern of this receptor suggests it may play a range of roles beyond glucose homeostasis in different cells and tissues. To date, a single molecule, 4-methyl-N-9H-xanthen-9-yl-benzenesulfonamide (AH-7614), has been described as an FFA4 antagonist; however, its mechanism of antagonism remains unknown. We synthesized AH-7614 and a chemical derivative and demonstrated these to be negative allosteric modulators (NAMs) of FFA4. Although these NAMs did inhibit FFA4 signaling induced by a range of endogenous and synthetic agonists, clear agonist probe dependence in the nature of allosteric modulation was apparent. Although AH-7614 did not antagonize the second long-chain free fatty acid receptor, free fatty acid receptor 1, the simple chemical structure of AH-7614 containing features found in many anticancer drugs suggests that a novel close chemical analog of AH-7614 devoid of FFA4 activity, 4-methyl-N-(9H-xanthen-9-yl)benzamide (TUG-1387), will also provide a useful control compound for future studies assessing FFA4 function. Using TUG-1387 alongside AH-7614, we show that endogenous activation of FFA4 expressed by murine C3H10T1/2 mesenchymal stem cells is required for induced differentiation of these cells toward a more mature, adipocyte-like phenotype.

An overview of Ca2+ mobilization assays in GPCR drug discovery

INTRODUCTION

Calcium ions (Ca²⁺) serve as a second messenger or universal signal transducer implicated in the regulation of a wide range of physiological processes. A change in the concentration of intracellular Ca²⁺ is an important step in intracellular signal transduction. G protein-coupled receptors (GPCRs), the largest and most versatile group of cell surface receptors, transduce extracellular signals into intracellular responses via their coupling to heterotrimeric G proteins. Since Ca²⁺ plays a crucial role in GPCR-induced signaling, measurement of intracellular Ca²⁺ has attracted more and more attention in GPCR-targeted drug discovery. Areas covered: This review focuses on the most popular functional assays measuring GPCRs-induced intracellular Ca²⁺ signaling. These include photoprotein-based, synthetic fluorescent indicator-based and genetically encoded calcium indicator (GECI)-based Ca²⁺ mobilization assays. A brief discussion of the design strategy of fluorescent probes in GPCR studies is also presented. Expert opinion: GPCR-mediated intracellular signaling is multidimensional. There is an urgent need for the development of multiple-readout screening assays capable of simultaneous detection of biased signaling and screening of both agonists and antagonists in the same assay. It is also necessary to develop GECIs offering low cost and consistent assays suitable for investigating GPCR activation in vivo.

P2X7 antagonists for CNS indications: recent patent disclosures

P2X7, a ligand-gated purinergic ion channel, has been at the center of intense efforts in the pharmaceutical industry in the last 15 years due to the growing appreciation of its role in inflammation. Since 2008-2009, increased focus on CNS available compounds has led to the publication of various patents on behalf of several pharmaceutical companies. This patent review aims at analyzing the recent patent literature (2008-2016) with a particular emphasis on those patents that are thought to deal with CNS penetrant compounds on the basis of their physicochemical features, the assays described in the patents and the uses these compounds are claimed for.

Endogenous lysophosphatidic acid (LPA1 ) receptor agonists demonstrate ligand bias between calcium and ERK signalling pathways in human lung fibroblasts

BACKGROUND AND PURPOSE

Human lung fibroblasts (HLF) express high levels of the LPA₁ receptor, a GPCR that responds to the endogenous lipid mediator, lysophosphatidic acid (LPA). Several molecular species or analogues of LPA exist and have been detected in biological fluids such as serum and plasma. The most widely expressed of the LPA receptor family is the LPA₁ receptor, which predominantly couples to G_q/11 , G_i/o and G_12/13 proteins. This promiscuity of coupling raises the possibility that some of the LPA analogues may bias the LPA₁ receptor towards one signalling pathway over another.

EXPERIMENTAL APPROACH

Here, we have explored the signalling profiles of a range of LPA analogues in HLF that endogenously express the LPA₁ receptor. HLF were treated with LPA analogues and receptor activation monitored via calcium mobilization and ERK phosphorylation.

KEY RESULTS

These analyses demonstrated that the 16:0, 17:0, 18:2 and C18:1 LPA analogues appear to exhibit ligand bias between ERK phosphorylation and calcium mobilization when compared with 18:1 LPA, one of the most abundant forms of LPA that has been found in human plasma.

CONCLUSION AND IMPLICATIONS

The importance of LPA as a key signalling molecule is shown by its widespread occurrence in biological fluids and its association with disease conditions such as fibrosis and cancer. These findings have important, as yet unexplored, implications for the (patho-) physiological signalling of the LPA₁ receptor, as it may be influenced not only by the concentration of endogenous ligand but the isoform as well.

Pharmacological studies on the NOP and opioid receptor agonist PWT2-[Dmt1]N/OFQ(1-13)

An innovative chemical strategy named peptide welding technology (PWT) has been developed for the facile synthesis of tetrabranched peptides. [Dmt¹]N/OFQ(1-13)-NH₂ acts as a universal agonist for nociceptin/orphanin FQ (N/OFQ) and classical opioid receptors. The present study investigated the pharmacological profile of the PWT derivative of [Dmt¹]N/OFQ(1-13)NH₂ (PWT2-[Dmt¹]) in several assays in vitro and in vivo after spinal administration in monkeys subjected to the tail withdrawal assay. PWT2-[Dmt¹] mimicked the effects of [Dmt¹]N/OFQ(1-13)-NH₂ displaying full agonist activity, similar affinity/potency and selectivity at human recombinant N/OFQ (NOP) and opioid receptors in receptor binding, stimulation of [³⁵S]GTPγS binding, calcium mobilization in cells expressing chimeric G proteins, and BRET studies for measuring receptor/G-protein and receptor/β-arrestin 2 interaction. In vivo in monkeys PWT2-[Dmt¹] elicited dose-dependent and robust antinociceptive effects being more potent and longer lasting than [Dmt¹]N/OFQ(1-13)-NH₂. The analgesic action of PWT2-[Dmt¹] was sensitive to the NOP receptor antagonist J-113397, but not naltrexone. Thus, the present study demonstrated that the tetrabranched derivative of [Dmt¹]N/OFQ(1-13)-NH₂ obtained with the PWT technology maintains the in vitro pharmacological profile of the parent peptide but displays higher potency and longer lasting action in vivo.

Pharmacological characterization of cebranopadol a novel analgesic acting as mixed nociceptin/orphanin FQ and opioid receptor agonist

The aim of the study was to investigate the in vitro and in vivo pharmacological profile of cebranopadol, a novel agonist for opioid and nociceptin/orphanin FQ (N/OFQ) receptors (NOP). In vitro cebranopadol was assayed in calcium mobilization studies in cells coexpressing NOP or opioid receptors and chimeric G‐proteins and in a bioluminescence resonance energy transfer (BRET) assay for studying receptor interaction with G‐protein and β‐arrestin 2. The mouse tail withdrawal and formalin tests were used for investigating cebranopadol antinociceptive properties. In calcium mobilization studies cebranopadol showed the following rank order of potency NOP = mu > kappa ≥ delta. In BRET studies, cebranopadol promoted NOP and mu receptors interaction with G‐protein with similar high potency and efficacy. However, cebranopadol did not stimulated NOP–β‐arrestin 2 interactions and displayed reduced potency at mu/β‐arrestin 2. In vivo, cebranopadol exhibits highly potent and extremely long‐lasting antinociceptive effects. The effects of cebranopadol in the tail withdrawal assay were sensitive to both SB‐612111 and naloxone. Collectively the present results confirm and extend previous finding demonstrating that cebranopadol, by acting as mixed NOP/opioid receptor agonist, elicits robust analgesic effects in different pain models.

Delineating biased ligand efficacy at 7TM receptors from an experimental perspective

During the last 10 years, the concept of "biased agonism" also called "functional selectivity" swamped the pharmacology of 7 transmembrane receptors and paved the way for developing signaling pathway-selective drugs with increased efficacy and less adverse effects. Initially thought to select the activation of only a subset of the signaling pathways by the reference agonist, bias ligands revealed higher complexity as they have been shown to stabilize variable receptor conformations that associate with distinct signaling events from the reference. Today, one major challenge relies on the in vitro determination of the bias and classification of these ligands, as a prerequisite for future in vivo and clinical translation. In this review, current experimental considerations for the bias evaluation related to choice of the cellular model, of the signaling pathway as well as of the assays are presented and discussed.