Influence of protein binding and lipophilicity on the distribution of chemical compounds in in vitro systems

Abstract Concentration-effect relationships determined in vitro depend not only on the activity of the chemical compounds or the sensitivity of the targets but also on the distribution of the compounds in the in vitro systems. In cell cultures, protein binding and lipid/water partition can be major determinants of distribution. Both are dependent on substance-specific factors, such as affinity to protein binding sites and lipophilicity, and on system-specific factors, such as protein concentration and lipid/water relationships. With this in mind, an algorithm has been developed that relates the total concentration of a compound to the free concentration in the aqueous phase. This algorithm can be used to describe theoretically the quantitative influence of the various factors relevant for distribution on concentration-response relationships. Conditions can be defined where protein binding and lipid/water partition are quantitatively important and have to be considered when comparing EC 50 values obtained in different in vitro systems. Provided that substance- and system-related parameters are known, the algorithm can be used in comparative cell toxicology to estimate equivalent effective concentrations for differently composed systems.