Poly(4-vinylpyridine)-coated liposomes: stability studies and release of acetylsalicylic acid

Liposomes of dimyristoylphosphatidylcholine (DMPC) and dicetylphosphate (DCP) reacted with 4-vinylpyridine (4-VP) to form a salt and, subsequently, autopolymerized for form poly(4-vinylpyridine) (poly(4-VP))-coated liposomes. The conditions for optimization of polymer coating have been determined; also, the effects of polymer coating on liposome stability, the encapsulation of ASA and its release kinetics have been measured. The coating efficiency was maximum at a DMPC:DCP 1:1 mole ratio, at pH 4.0 in acetate buffer, and a polymerization time of 40 min. The polymer-coated liposomes were stable in 2 mM sodium cholate and 4 per cent isopropanol solutions, as determined from turbidity measurements, versus a 20-25% decrease in stability of uncoated liposomes. The encapsulation efficiency of ASA reached a maximum of 9 per cent at DMPC:DCP 1:1 mole ratio. The release of ASA at 37 degrees C, pH 7.0 was characterized by an initial fast release (85 and 63 per cent in 20 min from uncoated and polymer-coated liposomes, respectively) followed by a slow, constant release rate up to 140 min. Thus, autopolymerization of a polymerizable monomer at liposome surfaces represents a potentially feasible stabilization approach for liposomes exposed to sodium cholate solutions with greater retention of solute than uncoated liposomes.