On the Accuracy of Determination of Unbound Drug Fraction in Tissue using Diluted Tissue Homogenate

The unbound drug fraction in tissue, fut, is commonly measured in vitro using the diluted tissue homogenate. An appraisal of the calculation procedure that is routinely applied to obtain fut is presented. An accurate detailed calculation that takes into account the drug protein binding in tissue extracellular water and the pH difference between extra- and intracellular water is considered. It turns out that for neutral compounds, the routine calculation provides fut quite accurately. Though for acidic compounds, the routine calculation can considerably underestimate fut up to 1.8-fold for monoprotic and up to 2.4-fold for diprotic ones, whereas for basic compounds, fut can be substantially overestimated up to 1.9-fold for monoprotic and up to 4.3-fold for diprotic ones. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:1909-1916, 2012