pH-metric solubility. 2: Correlation between the acid-base titration and the saturation shake-flask solubility-pH methods

Purpose. The objective of this study was to compare the results of a normal saturation shake-flask method to a new potentiometric acid-base titration method for determining the intrinsic solubility and the solubility-pH profiles of ionizable molecules, and to report the solubility constants determined by the latter technique. Methods. The solubility-pH profiles of twelve generic drugs (atenolol, diclofenac. Na, famotidine, flurbiprofen, furosemide, hydrochlorothiazide, ibuprofen, ketoprofen, labetolol. HCl, naproxen, phenytoin, and propranolol. HCl), with solubilities spanning over six orders of magnitude, were determined both by the new pH-metric method and by a traditional approach (24 hr shaking of saturated solutions, followed by filtration, then HPLC assaying with UV detection). Results. The 212 separate saturation shake-flask solubility measurements and those derived from 65 potentiometric titrations agreed well. The analysis produced the correlation equation: log(1/S)titration = -0.063(+/- 0.032) + 1.025(+/-0.011) log(1/S)(shake-flask), s = 0.20, r(2) = 0.978. The potentiometrically-derived intrinsic solubilities of the drugs were: atenolol 13.5 mg/mL, diclofenac. Na 0.82 mu g/mL, famotidine 1.1 mg/mL, flurbiprofen 10.6 mu g/mL, furosemide 5.9 mu g/mL, hydrochlorothiazide 0.70 mg/mL, ibuprofen 49 mu g/mL, ketoprofen 118 mu g/mL, labetolol. HCl 128 mu g/mL, naproxen 14 mu g/mL, phenytoin 19 mu g/mL, and propranolol. HCl 70 mu g/mL. Conclusions. The new potentiometric method was shown to be reliable for determining the solubility-pH profiles of uncharged ionizable drug substances. Its speed compared to conventional equilibrium measurements, its sound theoretical basis, its ability to generate the full solubility-pH profile from a single titration, and its dynamic range (currently estimated to be seven orders of magnitude) make the new pi-I-metric method an attractive addition to traditional approaches used by preformulation and development scientists. It may be useful even to discovery scientists in critical decision situations (such as calibrating computational prediction methods).