Benzodiazepine localisation at the lipid-water interface: effect of membrane composition and drug chemical structure

The effect of membrane chemical composition and drug chemical structure on the localisation of several benzodiazepines (BZDs) (DZ, diazepam; CZ, clonazepam; CX, chlordiazepoxide) within model membranes was investigated. We used a spectrophotometric method presented in a previous paper (B.A. García, M.A. Perillo, Biochim. Biophys. Acta 1324 (1997) 76-84) based on the study of BZD acid-base equilibrium. 'Intrinsic pK' values (pKi) were calculated according to the theory of M.S. Fernández and P. Fromherz (J. Phys. Chem. 81 (1977) 1755-1761). Homogeneous media of known dielectric constant (dioxane 0-80% v/v in water) were used to construct a curve of DeltapKi (pKi-pKw) vs. dielectric constant (D) where DeltapKi values obtained in lipidic dispersions were interpolated. In heterogeneous media consisting of aqueous dispersions of Triton X-100 micelles we determined the relative localisation depth of BZDs according to their DTriton values (36, 37 and 62 for DZ, CX and CZ respectively) taking into account that lower D values correspond to deeper localisation. pKi determined in dispersions of dipalmitoylphosphatidylcholine (dpPC) and egg phosphatidylcholine (egg-PC) mixed multilamellar vesicles showed that, when cholesterol content increased from 0 to 20 mole%, D values decreased (from 59 to 40) in dpPC vesicles and increased (from 51 to 72) in egg-PC vesicles, indicating a tendency of BZDs to penetrate deeper into less ordered interfaces. These results should be considered to understand the non-specific pharmacological effects of BZDs as well as to evaluate the actual relevance of their pharmacological concentrations.