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

Established solid state P-31-NMR and novel F-19-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 a nti-inflammatory agent FFA was dissolved at 5% (w/w) in dimyristoylphosphat idylcholine (DMPC), and the system was investigated at low hydration (3 H2O per lipid) where morphological transitions of the lipid are strongly affec ted by additives. It is demonstrated that FFA induces a fluid H-II phase in DMPC at ambient temperatures, i.e. much below its regular chain-melting tr ansition which occurs around 50 degrees C at low hydration. The guest molec ules are preferentially accommodated in the hexagonal phase of the lipid, w hich coexists with the usual crystalline state of pure DMPC. The peculiar t ransition sequence L-C --> H-II --> L-alpha 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 th e existence of the hexagonal and bilayer phases, and to determine their res pective dimensions. When the drug FFA dissolves in the bilayer, its structu ral effect on the surrounding lipid molecules may be related to its pharmac ological activity in membranes. For example, FFA is known to modulate ion c hannel function, and it has been suggested that it inhibits phospholipase a ctivity by accelerating the transbilayer flip-flop of lipids.