Evolving approaches on measurements and applications of intracellular free drug concentration and Kp in drug discovery.

: Intracellular-free drug concentration (C) and unbound partition coefficient (Kp) are two important parameters to develop pharmacokinetic and pharmacodynamic relationships, predict drug-drug interaction potentials and estimate therapeutic indices.: Methods on measurements of C, Kp partition coefficient (Kp) and fraction unbound of cells (f) are discussed. Advantages and limitations of several f methods are reviewed. Applications highlighted here are bridging the potency gaps between biochemical and cell-based assays, in vitro hepatocyte assay to predict in vivo liver-to-plasma Kp, the role of Kp in prediction of hepatic clearance for enzyme- and transporter-mediated mechanisms using extended clearance equation, and structural attributes governing tissue Kp.: C and Kp are of growing applications in drug discovery. Methods for measurements of these properties continue to evolve in order to achieve higher precision/accuracy and obtain more detailed information at the subcellular levels. Future directions of the field include the development of in vitro and in silico models to predict tissue Kp, direct measurement of free drug concentration in subcellular organelles, and further investigations into the critical elements governing cell and tissue Kp. Significant innovation is needed to advance this complex, but highly impactful and exciting area of science.