Solubility and ionization characteristics of doxepin and desmethyldoxepin.

The theromdynamic pKa values for doxepin and its metabolite desmethyldoxepin were determined by the solubility method to be 8.96 and 9.75, respectively at 25 degrees. The intrinsic solubilities for doxepin and desmethyldoxepin were linearly dependent upon ionic strength. The intrinsic solubilities at zero ionic strength and 25 degrees were determined to be 1,13 x 10(-4) M for doxepin and 3.95 x 10(-4) M for desmethyldoxepin. The solubility experiment was repeated at different temperatures and a constant ionic strength of 0.167 M. The change in enthalpy (6.71 kcal/mole) and entropy (-4.16 cal/mole degrees K) of solution for doxepin was determined from a van't Hoff plot for this nonideal system. The apparent partition coefficient between hexane and water for the doxepin free base was determined to be 13,615 at an ionic strength of 0.067 M.