Insight into tissue unbound concentration: utility in drug discovery and development

In a preclinical setting, plasma or whole tissue drug concentrations are often correlated with pharmacodynamics, although according to the free drug hypothesis, unbound drug concentration should be more pharmacologically relevant. Alternatively, blood concentrations may be a good surrogate for tissue concentration for passively permeable compounds. However, for a large number of compounds that are substrates for uptake and/or efflux transporters expressed at the tissue level, significant discrepancies are expected between unbound concentrations in blood and those in tissues. Consequently, attempts have been made to measure tissue unbound drug concentrations using tissue homogenates, slices and microdialysis. Mathematical expressions for calculating the rate and extent of drug distribution into tissues have also been established. For example, a ratio of unbound concentration in the tissue to that in plasma (K(p,uu)) is the best indicator of the extent of tissue distribution. Despite these technical advances, however, very few examples demonstrate a focus on tissue unbound drug concentrations in a preclinical setting. This review will illustrate various techniques to estimate tissue unbound drug concentrations, relevant parameters to calculate the rate and extent of tissue distribution and different factors affecting tissue unbound concentration. The review will also highlight various examples from the literature where tissue unbound drug concentrations have demonstrated a superior correlation with efficacy. The impact of tissue unbound drug concentrations on the projection of human efficacious dose is also discussed.