Heart failure therapeutics on the basis of a biased ligand of the angiotensin-2 type 1 receptor. Rationale and design of the BLAST-AHF study (Biased Ligand of the Angiotensin Receptor Study in Acute Heart Failure)

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.

From biased signalling to polypharmacology: unlocking unique intracellular signalling using pepducins

For over a decade, pepducins have been utilized to develop unique pharmacological profiles that have been particularly challenging for traditional drug discovery methods. It is becoming increasingly clear that these cell-penetrating lipopeptides can access receptor conformations that are currently not accessible through orthosteric targeting. This review addresses the emerging concepts in the development of pepducins including the elicitation of biased signalling, pepducin polypharmacology and recent insight into their mechanism of action.

Composite End Points in Acute Heart Failure Research: Data Simulations Illustrate the Limitations

Composite end points are frequently used in clinical trials of investigational treatments for acute heart failure, eg, to boost statistical power and reduce the overall sample size. By incorporating multiple and varying types of clinical outcomes they provide a test for the overall efficacy of the treatment. Our objective is to compare the performance of popular composite end points in terms of statistical power and describe the uncertainty in these power estimates and issues concerning implementation. We consider several composites that incorporate outcomes of varying types (eg, time to event, categorical, and continuous). Data are simulated for 5 outcomes, and the composites are derived and compared. Power is evaluated graphically while varying the size of the treatment effects, thus describing the sensitivity of power to varying circumstances and eventualities such as opposing effects. The average z score offered the most power, although caution should be exercised when opposing effects are anticipated. Results emphasize the importance of an a priori assessment of power and scientific basis for construction, including the weighting of individual outcomes deduced from data simulations. The interpretation of a composite should be made alongside results from the individual components. The average z score offers the most power, but this should be considered in the research context and is not without its limitations.

Biased Agonism of the Angiotensin II Type I Receptor

Angiotensin II (AngII) type I receptor (AT1R) recognizes AngII, a cardiovascular peptide hormone that acts as a terminal effector of the renin-angiotensin system (RAS). AT1R belongs to the rhodopsin-like peptidergic family of G protein-coupled receptors (GPCRs) and serves as a therapeutic target for the treatment of cardiovascular diseases, such as hypertension, cardiac hypertrophy and heart failure. Classically, AT1R was considered to signal only through G proteins. However, recent studies have revealed that AT1R is capable of activating G protein-independent signaling that is mediated by β-arrestins. β-arrestin is a cytosolic scaffold that is recruited to the activated GPCRs. In vitro and ex vivo studies have demonstrated that the activation of the AT1R-β-arrestin pathway stimulates contractility and exerts prosurvival effects in cardiomyocytes. TRV027, a potent synthetic β-arrestin-biased ligand for AT1R, specifically activates AT1R-β-arrestin signaling without stimulating G proteins. In preclinical studies, TRV027 not only produced vasodilation by antagonizing the AT1R-Gαq pathway but also enhanced cardiac performance by activating AT1R-β-arrestin signaling. Because of this unique pharmacological profile, TRV027 is now being evaluated in a phase II clinical trial as a novel therapeutic for acute heart failure (AHF).

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Receptor, Ligand and Transducer Contributions to Dopamine D2 Receptor Functional Selectivity

Functional selectivity (or biased agonism) is a property exhibited by some G protein-coupled receptor (GPCR) ligands, which results in the modulation of a subset of a receptor's signaling capabilities and more precise control over complex biological processes. The dopamine D2 receptor (D2R) exhibits pleiotropic responses to the biogenic amine dopamine (DA) to mediate complex central nervous system functions through activation of G proteins and β-arrestins. D2R is a prominent therapeutic target for psychological and neurological disorders in which DA biology is dysregulated and targeting D2R with functionally selective drugs could provide a means by which pharmacotherapies could be developed. However, factors that determine GPCR functional selectivity in vivo may be multiple with receptors, ligands and transducers contributing to the process. We have recently described a mutagenesis approach to engineer biased D2R mutants in which G protein-dependent ([Gprot]D2R) and β-arrestin-dependent signaling ([βarr]D2R) were successfully separated (Peterson, et al. PNAS, 2015). Here, permutations of these mutants were used to identify critical determinants of the D2R signaling complex that impart signaling bias in response to the natural or synthetic ligands. Critical residues identified in generating [Gprot]D2R and [βarr]D2R conferred control of partial agonism at G protein and/or β-arrestin activity. Another set of mutations that result in G protein bias was identified that demonstrated that full agonists can impart unique activation patterns, and provided further credence to the concept of ligand texture. Finally, the contributions and interplay between different transducers indicated that G proteins are not aberrantly activated, and that receptor kinase and β-arrestin activities are inextricably linked. These data provide a thorough elucidation of the feasibility and malleability of D2R functional selectivity and point to means by which novel in vivo therapies could be modeled.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

Biasing the odds: Approaches to capturing, understanding and exploiting functional selectivity in GPCRs

There is significant expectation in the pharmacological community that an understanding of biased signalling will lead to the development of new drugs and a better understanding of molecular targets in the in vivo context. I think it is safe to say that Pharma is withholding judgment on the promise and potential of what they view as an interesting pharmacological curiosity. That said, beyond successes of biased ligands in clinical trials and their appearance on the market, what it is need is a clear plan and the right tools and analytical methods to characterize functional selectivity from in cellulo to in vivo. In this issue of Methods, we have put together a series of articles that help lay out a methodological and analytical framework to help get us there.

The Impact of Worsening Heart Failure in the United States

In-hospital worsening heart failure represents a clinical scenario wherein a patient hospitalized for acute heart failure experiences a worsening of their condition, requiring escalation of therapy. Worsening heart failure is associated with worse in-hospital and postdischarge outcomes. Worsening heart failure is increasingly being used as an endpoint or combined endpoint in clinical trials, as it is unique to episodes of acute heart failure and captures an important event during the inpatient course. While prediction models have been developed to identify worsening heart failure, there are no known FDA-approved medications associated with decreased worsening heart failure. Continued study is warranted.

Pharmacological Profile of Nociceptin/Orphanin FQ Receptors Interacting with G-Proteins and β-Arrestins 2

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions by selectively activating an opioid like receptor named N/OFQ peptide receptor (NOP). Biased agonism is emerging as an important and therapeutically relevant pharmacological concept in the field of G protein coupled receptors including opioids. To evaluate the relevance of this phenomenon in the NOP receptor, we used a bioluminescence resonance energy transfer technology to measure the interactions of the NOP receptor with either G proteins or β-arrestin 2 in the absence and in presence of increasing concentration of ligands. A large panel of receptor ligands was investigated by comparing their ability to promote or block NOP/G protein and NOP/arrestin interactions. In this study we report a systematic analysis of the functional selectivity of NOP receptor ligands. NOP/G protein interactions (investigated in cell membranes) allowed a precise estimation of both ligand potency and efficacy yielding data highly consistent with the known pharmacological profile of this receptor. The same panel of ligands displayed marked differences in the ability to promote NOP/β-arrestin 2 interactions (evaluated in whole cells). In particular, full agonists displayed a general lower potency and for some ligands an inverted rank order of potency was noted. Most partial agonists behaved as pure competitive antagonists of receptor/arrestin interaction. Antagonists displayed similar values of potency for NOP/Gβ₁ or NOP/β-arrestin 2 interaction. Using N/OFQ as reference ligand we computed the bias factors of NOP ligands and a number of agonists with greater efficacy at G protein coupling were identified.