Fliposomes: pH-controlled Release from Liposomes Containing New trans -2-Morpholinocyclohexanol-based Amphiphiles that Perform a Conformational Flip and Trigger an Instant Cargo Release upon Acidification

A new type of pH-sensitive liposome (fliposomes) was designed based on the amphiphiles that are able to perform a pH-triggered conformational flip (flipids). This flip disrupts the liposome membrane and causes rapid release of the liposome cargo, specifically in the areas of increased acidity. The flipids (1-3) are equipped with a trans-2-morpholinocyclohexanol conformational switch, pH-Sensitive fliposomes containing one of these flipids, POPC and PEG-ceramide (molar ratio 50/45/5) were constructed and characterized. These compositions were stable at 4 degrees C and pH 7.4 for several months. Fliposomes loaded with ANTS/DPX demonstrated an unusually quick content release (in a few seconds) at pH below 5.5, which was more efficient in the case of flipid 1 with the shorter linear C12-tails. The pH-titration curve for the fliposome leakage paralleled the curve for the acid-induced conformational flip of 1-3 studied by 1H NMR. A plausible mechanism of the pH-sensitivity starts with an acid-triggered conformational flip of 1, 2 or 3, which changes the molecular size and shape, shortens the lipid tails, and perturbs the liposome membrane resulting in the content leakage.