Pharmacological characterization of CGP 12177 at the human β 2-adrenoceptor

Abstract

1 It has recently been reported that CGP 12177 can act as an agonist at a novel secondary site within the human β 1-adrenoceptor. The aim of this study was to undertake a detailed pharmacological study of the effects of CGP 12177 on the human β 2-adrenoceptor. 2 CGP 12177 acted as a potent partial agonist of 3H-cyclic AMP accumulation (log EC 50-8.90±0.06) and CRE-mediated reporter gene transcription (log EC 50-9.66±0.04) in CHO-K1 cells expressing the human β 2-adrenoceptor. These CGP-induced responses were antagonized by the β 2-selective antagonist ICI 118551 (apparent log K D values of -8.84±0.15 and -9.51±0.02 for the cyclic AMP and reporter gene responses respectively). 3 CGP 12177 was also able to antagonize both cyclic AMP and reporter gene responses to more efficacious β 2-agonists with similar log K D values (e.g. -9.57±0.15 and -10.04±0.096 respectively with salbutamol as agonist). 4 3H-CGP 12177 binding to β 2-adrenoceptors in intact CHO-β 2 cells yielded a log K D value of -9.84±0.06, but indicated that the ligand dissociates very slowly from the receptor (t1/2 for dissociation=65 min). However, studies with a Green Fluorescent Protein (GFP)-tagged β 2-adrenoceptor indicated that CGP 12177 does not stimulate β 2-adrenoceptor internalization. 5 This study demonstrates that CGP 12177 is a high affinity partial agonist of both camp accumulation and CRE-mediated gene transcription at the human β 2-adrenoceptor. It provides no evidence that CGP 12177 can discriminate a secondary site on the β 2-adrenoceptor analogous to that observed for the human β 1-adrenoceptor. However, despite its very weak actions on camp accumulation, the potent agonist effects of CGP 12177 on CRE-mediated gene transcription at the human β 2-adrenoceptor, coupled with its long duration of action, offers a potential lead for drug development for the treatment of chronic inflammatory airway diseases.