[Colloidal, partition and binding properties of phenothiazine derivatives, V: Relationships between albumin binding and physical properties (author's transl)]

Ionization constants and distribution coefficients of phenothiazines and calcium channel antagonists determined by a pH-metric method and correlation with calculated partition coefficients

The pH-metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n-octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH-dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure-activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.

Relationship between the solubility parameter and the binding of drugs by plasma proteins

An equation, based on regular solution theory, was used to relate the solubility parameter to the binding of drugs by plasma proteins. The equation was tested on a homologous series and a good correlation was found. Sulfonamides showed maximum binding when their solubility parameters were similar to the solubility parameters of the amino acids situated in a sequence with one tryptophan residue. This observation supports the assumption that this sequence is the primary binding site for the sulfonamides. Binding peaks were also found at solubility parameters in other drug series corresponding to the solubility parameters of human serum albumin (HSA) or bovine serum albumin (BSA) amino acids. It is suggested that the solubility parameter could be used to predict the binding of drugs to plasma proteins.