Independence of the product of solubility and distribution coefficient of pH

Purpose. The relationship between the pH, solubility, and partition coefficient was investigated to show that the product of intrinsic values of solubility and partition coefficient is equal to the product of total values of solubility and distribution coefficient at different pH. Methods. The pH distribution profiles were obtained from the literature and the pH solubility profiles were obtained from the literature or calculated from their intrinsic solubility and pK_a. Results. The pH solubility and pH distribution coefficient profiles of 25 compounds were investigated to show that the product of intrinsic solubility (S_W) and intrinsic octanol-water partition coefficient (K_OW) is equal to the product of total solubility of a partially ionized solute (S_T) and its octanol-buffer distribution coefficient (K_D) at any pH where ion pair formation and salt precipitation are not present. Conclusions. The fact that S_W·K_OW can be used instead of S_T·K_D to model the absorption of partially ionized drugs in the gastrointestinal tract has important biopharmaceutical implications.