Formulation of photocleavable liposomes and the mechanism of their content release.

In pursuit of designing photocleavable liposomes as drug delivery vehicles, we synthesized several amphiphilic lipids by connecting stearyl amine (as the non-polar tail) and charged amino acids (as polar heads) via the o-nitrobenzyl derivatives. The lipids containing Glu, Asp, and Lys amino acids were subjected to photocleavage reaction by UV light, and the overall spectral changes of the chromophoric o-nitrobenzyl conjugates were determined as a function of time. The experimental data revealed that the feasibility of the cleavage reaction, nature and magnitude of the spectral changes during the course of the cleavage reaction, and their overall kinetic profiles were dictated by the type of amino acid constituting the polar head groups. The cleavage reactions of the Asp and Glu containing lipids were found to be more facile than that of the lysine-containing lipid. Using these lipids, we formulated photocleavable liposomes, and investigated the photo-triggered release of an encapsulated (within the liposomal lumen) dye as a function of time. The kinetic data revealed that the release of the liposomal content conformed to a two-step mechanism, of which the first (fast) step involved the photocleavage of lipids followed by the slow release of the liposomal content during the second step. The overall mechanistic features intrinsic to the photocleavage of Asp, Glu and Lys containing o-nitrobenzyl conjugated lipids, and their potential applications in formulating liposomes (whose contents can be "unloaded" by the UV light) as drug delivery vehicles are discussed.