DSC study of the action of phenylbutazone on DMPC and DPPC bilayers

Abstract Phenylbutazone (PhB) is a powerful anti-inflammatory drug that is very sparingly soluble in water (solubility ⋍ 1 × 10 −3 M). This drug is capable of altering the phase transition temperature of phospholipid bilayers consisting of pure dimyristoyl- or dipalmitoylphosphatidylcholine without changing their calorimetric enthalpy ( ΔH cal ), but decreasing their cooperativity unit size, as shown by differential scanning calorimetric (DSC) measurements. On interaction with PhB, multilamellar liposomes (MLV) of dimyristoylphosphatidylcholine (DMPC) undergo lateral phase separation owing to the immiscibility in the bilayer plane. In the authors' view, phenylbutazone lies preferentially in the polar region of the bilayer. In this way, the intercalated drug molecules can disrupt hydrogen bonds spanning between adjacent head-groups, thereby destroying the specific structural arrangement of a specific polar head-group region. The pH of the medium is the determining factor — through the degree of ionization of the drug and the interface — on which qualitative and quantitative changes in the main thermotropic transition of phospholipids induced by PhB depend.