Phenothiazines solution complexity - determination of pKa and solubility-pH profiles exhibiting sub-micellar aggregation at 25 and 37°C

Abstract

The ionization constants (pKa) and the pH-dependent solubility (log S-pH) of six phenothiazine derivatives (promazine hydrochloride, chlorpromazine hydrochloride, triflupromazine hydrochloride, fluphenazine dihydrochloride, perphenazine free base, and trifluoperazine dihydrochloride) were determined at 25 and 37 °C. The pKa values of these low-soluble surface active molecules were determined by the cosolvent method (n-propanol/water at 37 °C and methanol/water at 25 °C). The log S-pH profiles were measured at 24 h incubation time in 0.15 M phosphate buffers. The log S-pH "shape-template" method, which critically depends on accurate pKa values (determined independently of solubility data), was used to propose speciation models, which were subsequently refined by rigorous mass-action weighted regression procedure described recently. Differential scanning calorimetry (DSC), UV-visible spectrophotometry, potentiometric, and high performance liquid chromatography (HPLC) measurements were used to characterize the compounds. The intrinsic solubility (S0) values of the three least-soluble drugs (chlorpromazine·HCl, triflupromazine·HCl, and trifluoperazine·2HCl) at 25 °C were 0.5, 1.1, and 2.7 μg/mL (resp.). These values increased to 5.5, 9.2, and 8.7 μg/mL (resp.) at the physiological temperature. The enthalpies of solution for the latter compounds were exceptionally high positive (endothermic) values (99-152 kJ·mol− 1). Cationic sub-micellar aggregates were evident (from the distortions in the log S-pH profiles) for chlorpromazine, fluphenazine, perphenazine, and trifluoperazine at 25 °C. The effects persisted at 37 °C for chlorpromazine and trifluoperazine. The solids in suspension were apparently amorphous in cases where the drugs were introduced as the chloride salts.