Theory of tunable pH-sensitive vesicles of anionic and cationic lipids or anionic and neutral lipids.

The design of vesicles that become unstable at an easily tuned value of pH is of great interest for targeted drug delivery. We present a microscopic theory for two forms of such vesicles. A model of lipids introduced by us previously is applied to a system of ionizable anionic lipid and permanently charged cationic lipid. We calculate the pH at which the lamellar phase becomes unstable with respect to an inverted hexagonal one, a value that depends continuously on the system composition. Identifying this instability with that displayed by unilamellar vesicles undergoing fusion, we obtain very good agreement with the recent experimental data of Hafez, Ansell, and Cullis, (2000, Biophys. J. 79:1438-1446) on the pH at which fusion occurs versus vesicle composition. We explicate the mechanism in terms of the role of the counterions. This understanding suggests that a system of a neutral, nonlamellar-forming lipid stabilized by an anionic lipid would serve equally well for preparing tunable, pH-sensitive vesicles. Our calculations confirm this. Further, we show that both forms of vesicle have the desirable feature of exhibiting a regime in which the pH at instability is a rapidly varying function of the vesicle composition.