Most pharmaceutical and gene therapy applications of targeted liposomes presently suffer from inefficient contents delivery to the cytoplasm of target cells. We report a plasma-stable liposome, composed of synthetic, naturally occurring diplasmenylcholine (1,2-di-O-(Z-1‘-hexadecenyl)-sn-glycero-3-phosphocholine; DPPlsC), that rapidly and efficiently releases its contents at endosomal pHs. Acid-catalyzed hydrolysis of these liposomes produces glycerophosphocholine and fatty aldehydes, leading to greatly enhanced liposome permeability (t50%  release ≅ 1−4 h between pH 4.5−5.5) when >20% of the vinyl ether lipid has been hydrolyzed. Plasma stability of nonhydrolyzed 9:1 DPPlsC/dihydrocholesterol liposomes exceeds 48 h at 37 °C, pH 7.4 in 50% serum; pure DPPlsC liposomes remain stable in 10% serum under the same conditions. Fluorescence assays of KB cells treated with 99.5:0.5 DPPlsC/DSPE−PEG3350−folate liposomes containing encapsulated propidium iodide (PI) indicate that 83% of the PI escapes the endosomal compartment within 8 h to produce intensely stained nucleii. The IC50 value of 1-β-arabinofuranosylcytosine (Ara-C) encapsulated in DPPlsC/DSPE−PEG3350−folate liposomes is 0.49 μM in KB cell cultures, a ∼6000-fold enhancement in cytotoxicity compared with free drug (2.8 mM). Empty DPPlsC/DSPE−PEG3350−folate liposomes had no effect on DNA synthesis, indicating that DPPlsC and its degradation products are benign to cell function at these lipid concentrations. Our results suggest that concurrent application of selective targeting and membrane translocation mechanisms in drug carriers can significantly increase their efficacy.

A photoactivated liposome release system that is generally applicable for triggered release of encapsulated hydrophilic materials is described. This approach to phototriggered release, derived from the known effects of plasmalogen photooxidation on membrane permeability in whole cells and model membrane systems, relies on producing a lamellar phase change or increase in permeability upon cleaving its constitutive lipids to single-chain surfactants using 630-820 nm light to sensitize the photooxidation of the plasmalogen vinyl ether linkage. Semi-synthetic plasmenylcholine liposomes containing encapsulated calcein and a membrane-bound sensitizer, such as zinc phthalocyanine, tin octabutoxyphthalocyanine, or bacteriochlorophyll a, were prepared by extrusion. Irradiation of air-saturated liposome solutions enhanced membrane permeability toward calcein and Mn2+, and promoted membrane fusion processes compared to non-irradiated or anaerobic controls. Bacteriochlorophyll a sensitization produced the fastest observed photoinitiated release rate from these liposomes (100% calcein release in less than 20 min; 800 nm irradiation at 300 mW); the observed release rate was two orders of magnitude slower for egg lecithin liposomes prepared and irradiated under identical experimental conditions. Liposome aggregation, interlipidic particle formation, and membrane fusion between adjoining liposomes was observed by 31P-NMR, freeze-fracture/freeze-etch TEM, and cryo-TEM as a function of irradiation time. The use of near-infrared sensitizers and the capacity of photolyzed plasmenylcholine liposomes to undergo membrane fusion processes make photodynamic therapy with these liposome-borne sensitizers an attractive adjunct to biochemical targeting methods.

The first practical total chemical synthesis of a plasmenylcholine (1-0-l'-(Z)-hexadecenyl-2-hexadecanoyl-snglycero-3-phosphocholine) with pure (Z) olefin stereochemistry is reported. Monopalmitin was doubly protected as the 3-TBDPS-2-TBDMS ethers (tertbutyldiphenylsilyl-, tert-butyldimethylsilyl-) and converted to the corresponding The efficiency and flexibility of this route makes it well-suited to the preparation of a wide variety of 1-, 2-, and 3-substituted as well as isotopically labeled plasmenylcholines for biophysical and biochemical studies.

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