Hydrophobicity and retention coefficient of selected bile Acid oxo derivatives.

Retention coefficients (k) of cholic acid and its keto derivatives are determined by means of Reversed Phase High Pressure Liquid Chromatography at different temperatures (303K, 309K, and 313K). At each studied temperature, retention factor decreases if the hydroxyl group in the cholic acid molecule replaces oxo group. In addition, the change of retention coefficient in a function of temperature (Δk/ΔT) is dominant for the cholic acid while by increasing the number of oxo groups it decreases. Introduction of an oxo group in a bile acid molecule leads to the lower hydrophobicity of the β side of the steroid nucleus. Because of that, less interaction happens between β side of the steroid nucleus and stationary phase. For dehydrocholic acid (three- oxo derivative), the value for Δk/ΔT shows an exception of this explanation. This suggests that in this molecule the planar polarity is disturbed. Partition coefficient K of nitrazepam (probe molecule) in micelles of bile acid salts at the examined temperatures shows a high linear correlation with retention factors of the selected bile acids. This indicates the importance of hydrophobic interactions in mixed micelles between the examined drug and bile acid salts. Haemolytic potential (erythrocyte haemolysis, log (Lys50)) represents measure of membranotoxicity of bile acids. In addition, it is shown that haemolytic potential correlates highly with the retention coefficient. All experiments that we conducted to obtain the values of K and log (Lys50) as well as their correlations with k, contribute to significance of retention coefficient as a measure of hydrophobicity in biopharmaceutical experiments.