Immobilized Artificial Membrane (lAM)-HPLC for Partition Studies of Neutral and Ionized Acids and Bases in Comparison with the Liposomal Partition System
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Purpose. To study the partitioning of model acids ((RS)-warfarin and salicylic acid), and bases (lidocaine, (RS)-propranolol and diazepam), with immobilized artificial membrane (lAM)-HPLC, as compared to partitioning in the standardized phosphatidylcholine liposome/buffer system.
Methods. The pH-dependent apparent partition coefficients D were calculated from capacity factors (k′IAM) obtained by IAM-HPLC, using a 11-carboxylundecylphosphocholine column. For lipophilic compounds k′IAM, values were determined with organic modifiers and extrapolation to 100% water phase (k′IAMw) was optimized. Temperature dependence was explored (23 to 45° C), and Gibbs free energy (ΔG), partial molar enthalpy (ΔH) and change in entropy (ΔS) were calculated. Equilibrium dialysis was used for the partitioning studies with the liposome/buffer system.
Results. For extrapolation of k′IAMw, linear plots were obtained both with the respective dielectric constants and the mole fractions of the organic modifier. All tested compounds showed a similar pH-D diagram in both systems; however, significant differences were reproducibly found in the pH range of 5 to 8. In all cases, ΔG and ΔH were negative, whereas ΔS values were negative for acids and positive for bases.
Conclusions. In both partitioning systems, D values decreased significantly with the change from the neutral to the charged ionization state of the solute. The differences found under physiological conditions, i.e. around pH 7.4, were attributed to nonspecific interactions of the drug with the silica surface of the IAM column.
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