Prediction of Human Serotonin and Norepinephrine Transporter Occupancy of Duloxetine by Pharmacokinetic/Pharmacodynamic Modeling in the Rat

Translation of central nervous system occupancy and clinical effect from animal models to humans has remained elusive for many pharmacological targets. The current studies evaluate the ability of a rodent pharmacokinetic/pharmacodynamic (PK/PD) modeling approach to translate ex vivo occupancy determined in rats to that observed after positron emission tomography (PET) imaging in humans for the dual serotonin transporter (SERT) and norepinephrine transporter (NET) inhibitor duloxetine. Ex vivo transporter occupancy in rat spinal cord was evaluated after single oral doses of 0.3 to 60 mg/kg. A novel methodology, based on the initial rates of association of transporter selective radioligands to tissue homogenates, was developed and validated for the assessment of ex vivo transporter occupancy. Duloxetine exhibited selectivity for occupancy of SERT over NET in rat spinal cord with ED(50) values of 1 and 9 mg/kg, respectively. Corresponding EC(50) values for the inhibition of SERT and NET based on unbound duloxetine spinal cord concentrations were 0.5 and 8 nM. An effect compartment PK/PD modeling approach was used to analyze the relationship between the time course of duloxetine plasma concentration and SERT and NET occupancy. Duloxetine inhibited SERT and NET in rat spinal cord with a plasma EC(50) of 2.95 and 59.0 ng/ml, respectively. Similar plasma EC(50) values for the inhibition of SERT (2.29-3.7 ng/ml) have been reported from human PET studies. This study illustrates the value of translational PK/PD modeling approaches and suggests that the preclinical modeling approach used in the current study is capable of predicting plasma concentrations associated with 50% occupancy of SERT in the human central nervous system.