Effect of Lipid Composition on Insulin-Mediated Fusion of Small Unilamellar Liposomes: A Kinetic Study

Abstract

Liposome-entrapped insulin could be used to prolong the hypoglycemic action of insulin. Also, the conjugation of insulin to the surface of liposomes allows the potential application of using insulin as a transporting molecule to deliver liposome-entrapped drugs to insulin-receptor rich tissues. The success of these two approaches of drug delivery depends on how insulin may interact with liposomes. The present study describes the application of the principle of kinetics to investigate the effect of insulin on the stability of various preparations of liposomal drug carriers. The technique of fluorescence resonance energy transfer was employed to study the destabilization of liposomal formulations through the process of insulin-mediated fusion of liposomes. The kinetics of insulin-mediated fusion appeared to be compatible with a model whereby the initial rate of fusion is governed by the mechanism of fusion of two small unilamellar, unfused liposomal particles. The rate constants of insulin-mediated fusion of various liposomal formulations were estimated from the initial rate of fusion, using the model of two-particle fusion. Arrhenius analysis of the rate constants of fusion at different temperatures suggests that the mechanism of insulin-mediated fusion of small unilamellar vesicles is not governed merely by the energy and frequency of collision between liposomal particles. Other factors, such as the binding of insulin with the surface of liposomes and the temperature effect on the dynamics of the liposomal membrane, as well as the conformation of insulin, could potentially be important. The effect of the destabilization of liposomes by insulin can be reduced by using bovine brain sphingomyelin instead of phosphatidylcholine and by including cholesterol in the liposomal formulation.