General approach for the calculation of tissue to plasma partition coefficients

A new mechanistic, universal model for the calculation of steady state tissue:plasma partition coefficients (Kt:p) of organic chemicals in mammalian species was developed. The approach allows the estimation of Kt:p-values based on the composition of the tissues in terms of water, neutral lipids, neutral and acidic phospholipids and proteins using the lipophilicity, the binding to phospholipid membranes, the pKa and the unbound fraction in blood plasma as compound specific parameters. Taking explicitly into account the sign and fraction of the charge of the compounds at the physiological pH the method is universally applicable to neutral, acidic, basic or multiply charged substances and has thus a significantly extended applicability compared to previously published approaches. The model was applied to 59 chemically diverse drug compounds for which tissue:plasma partition coefficients are reported in the literature. In total 474 experimentally observed Kt:p values for 12 tissues and the red blood cells were available and could be compared to model results. For 73% of the calculated values a deviation less than 3-fold from the respective observed value was found, proving the validity of the approach.