G protein coupled receptors (GPCRs) exhibit a spectrum of functional behaviours in response to natural and synthetic ligands. Recent crystal structures provide insights into inactive states of several GPCRs. Efforts to obtain an agonist-bound active-state GPCR structure have proven difficult due to the inherent instability of this state in the absence of a G protein. We generated a camelid antibody fragment (nanobody) to the human β2 adrenergic receptor (β2AR) that exhibits G protein-like behaviour, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex. Comparison with the inactive β2AR structure reveals subtle changes in the binding pocket; however, these small changes are associated with an 11 Å outward movement of the cytoplasmic end of transmembrane segment 6, and rearrangements of transmembrane segments 5 and 7 that are remarkably similar to those observed in opsin, an active form of rhodopsin. This structure provides insights into the process of agonist binding and activation. Two papers by Brian Kobilka and colleagues describe the X-ray crystal structure of the human β2 adrenergic receptor (β2AR) bound to various agonists. β2AR is a member of the G protein coupled receptor (GPCR) family of membrane-spanning receptors that sense molecules outside the cell and activate internal signalling pathways. With a ubiquitous role in human physiology, GPCRs are prime targets for drug discovery. A third paper by Christopher Tate and his team describes crystal structures of a similar GPCR, the turkey β1-adrenergic receptor (β1AR), bound to full and partial agonists. Together, these new structures reveal the subtle structural changes that accompany agonist binding, showing how binding events inside and outside the cell membrane stabilize the receptor's active state. Agonist binding to β1AR is shown to induce a contraction of the catecholamine-binding pocket relative to the antagonist-bound receptor, and molecular-dynamics simulations of the β2AR agonist complex suggest that the agonist-bound active state spontaneously relaxes to an inactive-like state in the absence of a G protein. The X-ray crystal structure of the human β2 adrenergic receptor, a G-protein-coupled receptor, in an agonist-bound 'active' state is solved. Comparison of this structure with a previously published structure of the same GPCR in an inactive state indicates that minor changes in the binding pocket of the protein lead to major changes elsewhere — there is a large outward movement of the cytoplasmic end of one of the transmembrane segments and rearrangements of two other transmembrane segments. This structure provides insights into the process of agonist binding and activation.

Compound 4p was identified from a series of 6-hydroxy-4H-benzo[1,4]oxazin-3-ones as potent agonist of the human beta(2)-adrenoceptor with a high beta(1)/beta(2)-selectivity. A complete reversal of acetylcholine-induced bronchoconstriction which lasted over the whole study period of 5 h was demonstrated for 4p in a guinea pig in vivo model without any signs of cardiovascular effects up to 10-fold above the first dose reaching 100% bronchoprotection. The enantiomerically pure (R)-form of 4p exerted a bronchodilatory efficacy over 24 h in dogs and guinea pigs in the absence of systemic pharmacodynamic effects. Formoterol which was tested as comparator in the same in vivo models of acetylcholine-induced bronchoconstriction did not retain efficacy after 24 h. In summary, the preclinical pro. le of compound (R)-4p (olodaterol, also known as BI 1744 CL) suggests a potential for once-daily dosing in man accompanied with an improved safety pro. le. (C) 2010 Elsevier Ltd. All rights reserved.

Novel beta(2)-agonists with a 5-hydroxy-4H-benzo[1,4]oxazin-3-one moiety as head group are described. Systematic chemical variations at the phenethylamine residue of these compounds lead to the discovery of compound 6m as potent, full agonist of the beta(2)-adrenoceptor with a high beta(1)/beta(2)-selectivity. Molecular modeling revealed an interaction between the carboxylic acid group of 6m and a lysine residue (K305) of the beta(2)-receptor as putative explanation for the high observed selectivity. Further, compound 6m displayed in a guinea pig in vivo model a complete reversal of acetylcholine induced bronchoconstriction which lasted over the complete study time of 5h.

Beta2-adrenoceptor agonists with basic and acidic groups attached via an alkyl linker to the phenyl ethanolamine core were prepared and investigated in vitro and in vivo. The compounds exhibited a high potency in a functional cellular assay and a bronchoprotective effect in a guinea pig model which lasted over the complete study period of 5h.