Associating in vitro target binding and in vivo CNS occupancy of serotonin reuptake inhibitors in rats: the role of free drug concentrations.

The present study aimed at investigating the theory that free (unbound) active site concentrations are the best predictors of target binding of compounds blocking the serotonin transporter (Sert) in the central nervous system (CNS). Thirteen serotonin reuptake inhibitors were evaluated for their Sert-binding affinities in vitro and in vivo in rats together with their unbound fractions in plasma and brain. Cortical Sert occupancy was used in vivo to acquire EC₅₀-estimates from total plasma, free plasma, whole brain, and free brain concentrations after acute drug administration. The in vitro-in vivo Sert occupancy analyses showed that the best correlation was achieved when unbound brain concentrations were employed. Unbound brain concentrations also provided a better correlation when compared with unbound plasma concentrations, which could be related to lack of equilibrium between plasma and brain at time of measurements or involvement of active brain efflux processes. In addition, brain-free fractions were shown to be directly correlated to the lipophilicity of the compounds. These data emphasize the use and impact of applying free fraction data in assessment of pharmacological in vitro-in vivo correlations and demonstrates its use to validate in vivo Sert occupancy as pharmacodynamic marker for serotonin reuptake inhibitors in rats.