Receptor occupancy of nonpeptide corticotropin-releasing factor 1 antagonist DMP696: Correlation with drug exposure and anxiolytic efficacy

4-(1,3-Dimethoxyprop-2-ylamine)-2,7-dimethyl-8-( 2,4- dichlorophenyl)pyrazolo[ 1,5-a]-1,3,5-triazine (DMP696) is a highly selective and potent, nonpeptide corticotropin-releasing factor 1 (CRF1) antagonist. In this study, we measured in vivo CRF1 receptor occupancy of DMP696 by using ex vivo ligand binding and quantitative autoradiography and explored the relationship of receptor occupancy with plasma and brain exposure and behavioral efficacy. In vitro affinity (IC50) of DMP696 to brain CRF1 receptors measured using the brain section binding autoradiography in this study is similar to that assessed using homogenized cell membrane assays previously. The ex vivo binding assay was validated by demonstrating that potential underestimation of receptor occupancy with this procedure could be minimized by identifying an appropriate in vitro incubation time ( 40 min) based upon the dissociation kinetics of DMP696. Orally administrated DMP696 dose dependently occupied CRF1 receptors in the brain, with similar to60% occupancy at 3 mg/kg. In the defensive withdrawal test of anxiety, this dose of DMP696 produced approximately 50% reduction in the exit latency. The time course of plasma and brain drug levels paralleled that of receptor occupancy, with peak exposure at 90 min after dosing. The plasma-free concentration of DMP696 corresponding to 50% CRF1 receptor occupancy (in vivo IC50, 1.22 nM) was similar to the in vitro IC50 (similar to1.0 nM). Brain concentrations of DMP696 were over 150-fold higher than the plasma-free levels. In conclusion, doses of DMP696 occupying over 50% brain CRF1 receptors are consistent with doses producing anxiolytic efficacy in the defense withdrawal test of anxiety, and the IC50 value estimated in vivo based on plasma-free drug concentrations is consistent with the in vitro IC50 value.