Basic NeuroscienceBrain penetration assessment in vivo: A reliable and simple method in anesthetized rats at steady state

- Brain penetration is measured in a robust and efficient manner at steady state.
- CNS drug discovery improved by simplified brain/plasma-ratio method in rats.
- Steady state method robust with respect to variable plasma-brain equilibration rate.
- One day experiment yielding brain/plasma-ratio in triplicates with simple equipment.
- The 3Rs benefits: animal reduction and steady state model refinement.

BackgroundFor CNS drugs, brain disposition is of critical importance during drug discovery. In vitro methods are used early followed by more predictive in vivo methods later on in the drug discovery process. Current in vivo methods are costly, have long turnover times or do not measure brain disposition at steady state.New methodA new method to evaluate drug brain disposition in vivo was developed in anaesthetized rats. Seven reference compounds were administered as an initial IV bolus (loading dose) followed by IV infusion for 4.5 h in order to obtain a steady state plasma concentration before brain sampling. The loading dose was estimated from a preliminary single dose IV pharmacokinetic study and was found to successfully bring plasma concentrations to steady state for compounds exhibiting either mono- or bi-compartmental pharmacokinetics.ResultsUsing this method, a steady state lasting at least 2 h was obtained, thus making the in vivo method robust with respect to differences in the pharmacokinetics and/or blood-to-brain equilibration rate of the compounds tested. The method produced highly reproducible results, with substantial advantages in terms of cost, turnaround time and animal welfare.Comparison with existing methodsThe results agreed with those reported in other, more elaborate preclinical models and in humans, enabling brain disposition to be assessed in a simple, efficient and robust in vivo model for new chemical entities.ConclusionsIntroducing the presented method in drug discovery allows brain disposition to be assessed earlier in the drug discovery pipeline and thus facilitate the selection of potent and penetrant CNS drugs.