Dissociation Constant of Chlorpromazine Incorporated in Lecithin Vesicles Determined by ¹H Nuclear Magnetic Resonance Titration

Abstract

The pK_a value of chlorpromazine incorporated in lecithin vesicles, denoted by pK__(av), was determined by proton nuclear magnetic resonance (¹H-NMR) spectrometric titration in D₂O containing 50 mM NaCl and 2.5 mM Na₂B₄O₇·10H₂O at 25°C for the drug and lipid concentrations of 1.76-3.5 mM and 20-67 mM, respectively. The pK_av value was calculated from the relation between the chemical shift of the N-dimethyl signal of the incorporated chlorpromazine and the bulk pH value using a nonlinear least-squares method. Six experiments gave the mean value of pK_av 8.48±0.07 (S.D.). The value was significantly lower than pK_a 9.8 in D₂O, which was determined by extrapolating the pK_a's obtained in mixtures of methanol-d₄ and D₂O by the same ¹H-NMR titration method. The results indicated that about 10% of chlorpromazine incorporated in lecithin vesicles was in a deprotonated free base state at physiological pH 7.5, although it was considered to be mostly in a protonated cationic state in aqueous solution of the same pH.