Use of Intravenous Infusion Study Design to Simultaneously Determine Brain Penetration and Systemic Pharmacokinetic Parameters in Rats

Development of extended pharmacokinetic models for propofol based on measured blood and brain concentrations

Propofol's pharmacokinetics have been extensively studied using human blood samples and applied to target-controlled infusion systems; however, information on its concentration in the brain remains scarce. Therefore, this study aimed to simultaneously measure propofol plasma and brain concentrations in patients who underwent awake craniotomy and establish new pharmacokinetic model. Fifty-seven patients with brain tumors or brain lesions who underwent awake craniotomy were sequentially assigned to model-building and validating groups. Plasma and brain (lobectomy or uncapping margins) samples were collected at five time-points. The concentration of propofol was measured using high-performance liquid chromatography. Population pharmacokinetic analysis was conducted through a nonlinear mixed-effects modeling program using a first-order conditional estimation method with interactions. Propofol's brain concentrations were higher than its plasma concentrations. The measured brain concentrations were higher than the effect site concentrations using the previous models. Extended models were constructed based on measured concentrations by incorporating the brain/plasma partition coefficient (Kp value). Extended models showed good predictive accuracy for brain concentrations in the validating group. The Kp value functioned as a factor explaining retention in the brain. Our new pharmacokinetic models and Kp value can predict propofol's brain and plasma concentrations, contributing to safer and more stable anesthesia.

Optimizing transcardial perfusion of small molecules and biologics for brain penetration and biodistribution studies in rodents

Efficiently removing blood from the brain vasculature is critical to evaluate accurately the brain penetration and biodistribution of drug candidates, especially for biologics as their blood concentrations are substantially higher than the brain concentrations. Transcardial perfusion has been used widely to remove residual blood in the brain; however, the perfusion conditions (such as the perfusion rate and time) reported in the literature are quite varied, and the performance of these methods on blood removal has not been investigated thoroughly. In this study, the effectiveness of the perfusion conditions was assessed by measuring brain hemoglobin levels. Sodium nitrite (NaNO₂ ) as an additive in the perfusate was evaluated at different concentrations. Blood removal was significantly improved with 2% NaNO₂ over a 20 min perfusion in mouse without disrupting the integrity of the blood-brain barrier (BBB). In mice, the optimized perfusion method significantly lowered the measured brain-to-plasma ratio (K_p,brain ) for monoclonal antibodies due to the removal of blood contamination and small molecules with a moderate-to-high BBB permeability and with a high brain-unbound-fraction (f_u,brain ) presumably due to flux out of the brain during perfusion. Perfusion with or without NaNO₂ clearly removed the residual blood in rat brain but with no difference observed in K_p,brain between the perfusion groups with or without 2% NaNO₂ . In conclusion, a perfusion method was successfully developed to evaluate the brain penetration of small molecules and biologics in rodents for the first time. The transcardial perfusion with 2% NaNO₂ effectively removed the residual blood in the brain and significantly improved the assessment of brain penetration of biologics. For small molecules, however, transcardial perfusion may not be performed, as small molecule compounds could be washed away from the brain by the perfusion procedure.