Receptor Occupancy of Nonpeptide Corticotropin-Releasing Factor 1 Antagonist DMP696: Correlation with Drug Exposure and Anxiolytic Efficacy

Abstract

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 ∼60% 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 (∼1.0 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.