The amphiphilic drug flufenamic acid can induce a hexagonal phase in DMPC: a solid state 31P- and 19F-NMR study

Established solid state 31P-NMR and novel 19F-NMR experiments are used in a complementary approach to describe the behaviour of a fluorinated drug, flufenamic acid (FFA), in phospholipid model membranes. The non-steroidal anti-inflammatory agent FFA was dissolved at 5% (w/w) in dimyristoylphosphatidylcholine (DMPC), and the system was investigated at low hydration (3 H2O per lipid) where morphological transitions of the lipid are strongly affected by additives. It is demonstrated that FFA induces a fluid HII phase in DMPC at ambient temperatures, i.e. much below its regular chain-melting transition which occurs around 50°C at low hydration. The guest molecules are preferentially accommodated in the hexagonal phase of the lipid, which coexists with the usual crystalline state of pure DMPC. The peculiar transition sequence LC→HII→Lα with increasing temperature is explained by a re-distribution of FFA in the lipid matrix and a concomitant phase separation under conditions of limiting hydration. Small-angle X-ray diffraction and freeze–fracture electron microscopy are used to confirm the existence of the hexagonal and bilayer phases, and to determine their respective dimensions. When the drug FFA dissolves in the bilayer, its structural effect on the surrounding lipid molecules may be related to its pharmacological activity in membranes. For example, FFA is known to modulate ion channel function, and it has been suggested that it inhibits phospholipase activity by accelerating the transbilayer flip-flop of lipids.