Purpose: More than 15 years have passed since the first description of the unbound brain-to-plasma partition coefficient (K_p,uu,brain) by Prof. Margareta Hammarlund-Udenaes, which was enabled by advancements in experimental methodologies including cerebral microdialysis. Since then, growing knowledge and data continue to support the notion that the unbound (free) concentration of a drug at the site of action, such as the brain, is the driving force for pharmacological responses. Towards this end, K_p,uu,brain is the key parameter to obtain unbound brain concentrations from unbound plasma concentrations. Methods: To understand the importance and impact of the K_p,uu,brain concept in contemporary drug discovery and development, a survey has been conducted amongst major pharmaceutical companies based in Europe and the USA. Here, we present the results from this survey which consisted of 47 questions addressing: 1) Background information of the companies, 2) Implementation, 3) Application areas, 4) Methodology, 5) Impact and 6) Future perspectives. Results and conclusions: From the responses, it is clear that the majority of the companies (93%) has established a common understanding across disciplines of the concept and utility of K_p,uu,brain as compared to other parameters related to brain exposure. Adoption of the K_p,uu,brain concept has been mainly driven by individual scientists advocating its application in the various companies rather than by a top-down approach. Remarkably, 79% of all responders describe the portfolio impact of K_p,uu,brain implementation in their companies as ‘game-changing’. Although most companies (74%) consider the current toolbox for K_p,uu,brain assessment and its validation satisfactory for drug discovery and early development, areas of improvement and future research to better understand human brain pharmacokinetics/pharmacodynamics translation have been identified.

The high-throughput brain slice method is a precise and robust technique for estimating the overall uptake of drugs into brain tissue through determination of the unbound volume of distribution in the brain (V_u,brain; ml·g brain^-1). V_u,braindescribes the relationship between the total drug concentration in the brain and the concentration of unbound drug in the brain interstitial fluid, regardless of blood–brain barrier function. The brain slice method is more physiologically based than the brain homogenate method with respect to the assessment of drug distribution in the brain because the cell-cell interactions, pH gradients and active transport systems are all conserved. The method provides information that is directly relevant to issues such as nonspecific binding to brain tissue, lysosomal trapping, and active uptake into the cells. For these reasons, the brain slice method is recommended for estimation of target-site pharmacokinetics in the early drug discovery process and fundamental pharmacological studies. This article provides a detailed protocol for the rat and mouse brain slice methods, with the aim of enabling simple, cost-effective profiling of compounds with diverse physicochemical properties. The procedure for assessing the viability of the brain slices after the 5 h incubation period is also described. The results are interpreted for a set of compounds covering a wide range of physicochemical properties and various pharmacological targets. Application of the method for evaluating the unbound intracellular-to-extracellular concentration ratio (K_p,uu,cell) and the unbound brain-to-plasma concentration ratio (K_p,uu,brain) is discussed.