Interactions of the AT1 antagonist valsartan with dipalmitoyl-phosphatidylcholine bilayers

Valsartan is a marketed drug with high affinity to the type 1 angiotensin (AT1) receptor. It has been reported that AT1 antagonists may reach the receptor site by diffusion through the plasma membrane. For this reason we have applied a combination of differential scanning calorimetry (DSC), Raman spectroscopy and small and wide angle X-ray scattering (SAXS and WAXS) to investigate the interactions of valsartan with the model membrane of dipalmitoyl-phosphatidylcholine (DPPC). Hence, the thermal, dynamic and structural effects in bulk as well as local dynamic properties in the bilayers were studied with different valsartan concentrations ranging from 0 to 20 mol%. The DSC experimental results showed that valsartan causes a lowering and broadening of the phase transition. A splitting of the main transition is observed at high drug concentrations. In addition, valsartan causes an increase in enthalpy change of the main transition, which can be related to the induction of interdigitation of the lipid bilayers in the gel phase. Raman spectroscopy revealed distinct interactions between valsartan with the lipid interface localizing it in the polar head group region and in the upper part of the hydrophobic core. This localization of the drug molecule in the lipid bilayers supports the interdigitation view. SAXS measurements confirm a monotonous bilayer thinning in the fluid phase, associated with a steady increase of the root mean square fluctuation of the bilayers as the valsartan concentration is increased. At high drug concentrations these fluctuations are mainly governed by the electrostatic repulsion of neighboring membranes. Finally, valsartans' complex thermal and structural effects on DPPC bilayers are illustrated and discussed on a molecular level.