The concept of ligand bias at G protein-coupled receptors broadens the possibilities for agonist activities and provides the opportunity to develop safer, more selective therapeutics. Morphine pharmacology in b-arrestin-2 knockout mice suggested that a ligand that promotes coupling of the m-opioid receptor (MOR) to G proteins, but not b-arrestins, would result in higher analgesic efficacy, less gastrointestinal dysfunction, and less respiratory suppression than morphine. Here we report the discovery of TRV130 ([(3-methoxythiophen-2-yl)methyl]({2-[(9R)-9-(pyridin-2-yl)-6-oxaspiro[4.5]decan-9-yl]ethyl})amine), a novel MOR G protein-biased ligand. In cell-based assays, TRV130 elicits robust G protein signaling, with potency and efficacy similar to morphine, but with far less b-arrestin recruitment and receptor internalization. In mice and rats, TRV130 is potently analgesic while causing less gastrointestinal dysfunction and respiratory suppression than morphine at equianalgesic doses. TRV130 successfully translates evidence that analgesic and adverse MOR signaling pathways are distinct into a biased ligand with differentiated pharmacology. These preclinical data suggest that TRV130 may be a safer and more tolerable therapeutic for treating severe pain.

Seven transmembrane receptors (7TMRs), commonly referred to as G protein-coupled receptors, form a large part of the "druggable" genome. 7TMRs can signal through parallel pathways simultaneously, such as through heterotrimeric G proteins from different families, or, as more recently appreciated, through the multifunctional adapters, ␤-arrestins. Biased agonists, which signal with different efficacies to a receptor's multiple downstream pathways, are useful tools for deconvoluting this signaling complexity. These compounds may also be of therapeutic use because they have distinct functional and therapeutic profiles from "balanced agonists." Although some methods have been proposed to identify biased ligands, no comparison of these methods applied to the same set of data has been performed. Therefore, at this time, there are no generally accepted methods to quantify the relative bias of different ligands, making studies of biased signaling difficult. Here, we use complementary computational approaches for the quantification of ligand bias and demonstrate their application to two well known drug targets, the ␤2 adrenergic and angiotensin II type 1A receptors. The strategy outlined here allows a quantification of ligand bias and the identification of weakly biased compounds. This general method should aid in deciphering complex signaling pathways and may be useful for the development of novel biased therapeutic ligands as drugs.

Biased G protein-coupled receptor ligands engage subsets of the receptor signals normally stimulated by unbiased agonists. However, it is unclear whether ligand bias can elicit differentiated pharmacology in vivo. Here, we describe the discovery of a potent, selective ␤-arrestin biased ligand of the angiotensin II type 1 receptor. TRV120027 (Sar-Arg-Val-Tyr-Ile-His-Pro-DAla-OH) competitively antagonizes angiotensin II-stimulated G protein signaling, but stimulates ␤-arrestin recruitment and activates several kinase pathways, including p42/44 mitogenactivated protein kinase, Src, and endothelial nitric-oxide synthase phosphorylation via ␤-arrestin coupling. Consistent with ␤-arrestin efficacy, and unlike unbiased antagonists, TRV120027 increased cardiomyocyte contractility in vitro. In rats, TRV120027 reduced mean arterial pressure, as did the unbiased antagonists losartan and telmisartan. However, unlike the unbiased antagonists, which decreased cardiac performance, TRV120027 increased cardiac performance and preserved cardiac stroke volume. These striking differences in vivo between unbiased and ␤-arrestin biased ligands validate the use of biased ligands to selectively target specific receptor functions in drug discovery.

Presenilins are integral membrane protein involved in the production of amyloid ␤-protein. Mutations of the presenilin-1 and -2 gene are associated with familial Alzheimer's disease and are thought to alter ␥-secretase cleavage of the ␤-amyloid precursor protein, leading to increased production of longer and more amyloidogenic forms of A␤, the 4-kDa ␤-peptide. Here, we show that radiolabeled ␥-secretase inhibitors bind to mammalian cell membranes, and a benzophenone analog specifically photocross-links three major membrane polypeptides. A positive correlation is observed among these compounds for inhibition of cellular A␤ formation, inhibition of membrane binding and cross-linking. Immunological techniques establish N-and C-terminal fragments of presenilin-1 as specifically cross-linked polypeptides. Furthermore, binding of ␥-secretase inhibitors to embryonic membranes derived from presenilin-1 knockout embryos is reduced in a gene dose-dependent manner. In addition, C-terminal fragments of presenilin-2 are specifically cross-linked. Taken together, these results indicate that potent and selective ␥-secretase inhibitors block A␤ formation by binding to presenilin-1 and -2.␤-Amyloid precursor protein (␤APP) 1 is a transmembrane protein that undergoes processing to A␤ by proteolytic activities known as ␤-and ␥-secretases (for review, see Refs. 1-3). The ␤-secretase cleavage occurs in the extracellular domain by a recently identified aspartyl protease variously termed BACE, memapsin, and Asp2 (4 -9), whereas the heterogeneous ␥-secretase cleavage occurs in the transmembrane domain (2, 10). Dominant mutations in either of the two human presenilin (PS-1 and PS-2) genes lead to familial Alzheimer's disease (AD). PS-1 and -2 are polytopic membrane proteins (for review, see Refs. 11-13). Presenilins are proteolytic processed. In vivo, only small amounts of the holoprotein can be detected, primarily in the nuclear envelope, whereas 30-kDa N-terminal and 20-kDa C-terminal fragments of presenilin are observed in all mammalian tissues and cell lines analyzed so far. Coimmunoprecipitation experiments revealed that presenilin fragments are assembled into a high molecular weight complex together with other proteins (for review see 11-13). The proposed mechanism through which the presenilin mutations cause AD is an alteration in the predominant ␥-secretase cleavage site which increases the amount of the longer, more amyloidogenic A␤ 1-42(43) fragments produced (11-13). A null mutation of the mouse PS-1 selectively reduces ␥-secretase activity (14), and site-directed mutagenesis of PS-1 and PS-2 at two conserved aspartyl residues, which resemble the catalytic center of aspartyl proteases, also reduces ␥-secretase activity (15, 16). These observations indicate that PS-1 and PS-2 either stimulate the activity of ␥-secretase by trafficking to appropriate cellular compartments, serve as cofactors of the ␥-secretase, or are ␥-secretase themselves.Here, we report that a series of potent and selective ␥-secretase inhibitors bind to mam...

Background: Biased agonism is an incompletely understood phenomenon describing the unequal activation of different signal transduction pathways by a G protein-coupled receptor (GPCR). Results: A cell-free approach using GPCR-transducer fusion proteins (G-protein or ␤-arrestin) quantifies signaling in vitro to elucidate the molecular basis of biased agonism. Conclusion: Differences in ligand-receptor-transducer coupling account for biased agonism in cells. Significance: Biased agonism is a bona fide molecular property of GPCR ligands.

The concept of "ligand bias" at G protein coupled receptors has been introduced to describe ligands which preferentially stimulate one intracellular signaling pathway over another. There is growing interest in developing biased G protein coupled receptor ligands to yield safer, better tolerated, and more efficacious drugs. The classical μ opioid morphine elicited increased efficacy and duration of analgesic response with reduced side effects in β-arrestin-2 knockout mice compared to wild-type mice, suggesting that G protein biased μ opioid receptor agonists would be more efficacious with reduced adverse events. Here we describe our efforts to identify a potent, selective, and G protein biased μ opioid receptor agonist, TRV130 ((R)-30). This novel molecule demonstrated an improved therapeutic index (analgesia vs adverse effects) in rodent models and characteristics appropriate for clinical development. It is currently being evaluated in human clinical trials for the treatment of acute severe pain.

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