Liposomes represent promising carriers for drug delivery applications. To maximize this potential, there has been significant interest in developing liposomal systems encapsulating molecular cargo that are highly stable until their contents are released remotely in a controlled manner. Herein, we describe the design, synthesis, and analysis of a photocleavable analogue of the ubiquitous lipid phosphoatidylcholine (PC) for the development of highly stable and controllable photodisruptable membranes. Our strategy was to develop a lipid that closely mimics the structure of PC to optimize favorable properties including biocompatibility and stability of subsequent liposomes when mixed with lipids possessing a broad range of physicochemical properties. Thus, NB-PC was designed, which contains a photocleavable 2-nitrobenzyl group embedded within the acyl chain at the sn-2 position. Following the synthesis of NB-PC, liposome disruption efficacy was evaluated through photolysis studies involving the detection of nile red release. Studies performed using a range of liposomes with different percentages of NB-PC, PC, phosphatidylethanolamine (PE), cholesterol, and polyethylene glycol-PE (PEG-PE) demonstrated minimal background release in controls, release efficacies that correlate directly with the amount of NB-PC incorporation, and that release is only minimally impacted by the inclusion of the lipids PE and cholesterol that possess disparate properties. These results demonstrate that the NB-PC system is a highly stable, flexible, and tunable system for photoinitiated release from liposomal systems.

Liposomal drug delivery would benefit from enhanced control over content release. Here, we report a novel avenue for triggering release driven by chemical composition using liposomes sensitized to calcium-a target chosen due to its key roles in biology and disease. To demonstrate this principle, we synthesized calcium-responsive lipid switch 1, designed to undergo conformational changes upon calcium binding. The conformational change perturbs membrane integrity, thereby promoting cargo release. This was shown through fluorescence-based release assays via dose-dependent response depending on the percentage of 1 in liposomes, with minimal background leakage in controls. DLS experiments indicated dramatic changes in particle size upon treatment of liposomes containing 1 with calcium. In a comparison of ten naturally occurring metal cations, calcium provided the greatest release. Finally, STEM images showed significant changes in liposome morphology upon treatment of liposomes containing 1 with calcium. These results showcase lipid switches driven by molecular recognition principles as an exciting avenue for controlling membrane properties.

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Artificial systems for controlled membrane fusion applicable for drug delivery would ideally use triggers that are orthogonal to biology. To apply the strain-promoted alkyneazide cycloaddition (SPAAC) to drive membrane fusion, ODIBO lipid 1 was designed, synthesized and studied alongside ADIBO-lipids 2–4 to assess fusion with liposomes containing azido-lipid 5. Lipids 1–2 were first shown to be effective for liposome derivatization. Next, fusion was evaluated using liposomes containing 1 and varying ratios of PC and PE via a FRET dilution fusion assay, and a 1/1 PC/PE ratio yielded the greatest signal change attributed to fusion. Finally, lipids 1–4 were compared, and 1 yielded the greatest triggering of fusion, while 2–4 yielded varying efficacies depending on the structural features of each lipid. Fusion was further validated through STEM studies showing larger multilamellar assemblies after liposome mixing. This work provides a platform for triggered fusion towards drug delivery applications and an understanding of the effects of lipid structure and membrane composition on fusion.

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