It is proposed that AT1 antagonists (ARBs) exert their biological action by inserting into the lipid membrane
and then diffuse to the active site of AT1 receptor. Thus, lipid bilayers are expected to be actively involved and
play a critical role in drug action. For this reason, the thermal, dynamic and structural effects of olmesartan
alone and together with cholesterol were studied using differential scanning calorimetry (DSC),
13
C magicangle spinning (MAS) nuclear magnetic resonance (NMR), cross-polarization (CP) MAS NMR, and Raman
spectroscopy as well as small- and wide angle X-ray scattering (SAXS and WAXS) on dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles.
13
C CP/MAS spectra provided direct evidence for the
incorporation of olmesartan and cholesterol in lipid bilayers. Raman and X-ray data revealed how both
molecules modify the bilayer's properties. Olmesartan locates itself at the head-group region and upper
segment of the lipid bilayers as
13
C CP/MAS spectra show that its presence causes significant chemical
shift changes mainly in the A ring of the steroidal part of cholesterol. The influence of olmesartan on
DPPC/cholesterol bilayers is less pronounced. Although, olmesartan and cholesterol are residing at the
same region of the lipid bilayers, due to their different sizes, display distinct impacts on the bilayer's
properties. Cholesterol broadens significantly the main transition, abolishes the pre-transition, and decreases the
membrane fluidity above the main transition. Olmesartan is the only so far studied ARB that increases the
gauche:trans ratio in the liquid crystalline phase. These significant differences of olmesartan may in part
explain its distinct pharmacological profile.