Lipoic acid derived polymerizable liposomes

Two polymerizable lipoic acid-derived lipids, 1,2-bis (12-(lipoyloxy)-dodecanoyl) -sn-glycero-3-phosphocholine (1), and 1-palmitoyl-2- (12-(lipoyloxy)dodecanoyl) -sn-glycero-3-phosphocholine (2) were synthesized and characterized. The liposomes formed from these lipids were proven to be unilamellar with mean diameters of 0.1 um by $\sp{31}$P NMR, TEM, and dynamic light scattering. Polymerization of these lipids was achieved under very mild conditions using dithiothreitol as an initiator. Spectrophotometry and quantitative thin-layer chromatography were used to estimate that the extent of polymerization was $\u3e$93% for 1 and 89% for 2. The permeability characteristics of liposomes of 1, a cross-linkable lipid, 2, a non-cross-linkable lipid, and mixtures of 1 and 2 were determined by efflux methods using ($\sp{14}$C) sucrose. Membranes derived from monomeric 1 were more permeable toward sucrose than those derived from monomeric 2. Increasing the mole percentage of 2 in mixed nonpolymerized liposomes decreased the permeability. Upon polymerization there was a significant decrease in the permeability of 1, while 2 exhibited a significant increase in release rates of sucrose. Increasing the mole percentage of 1 in mixed polymerized liposomes decreased the permeability. These observations are interpreted in terms of the relative packing efficiency of 1 and 2. Defects in the bilayer caused by polymer boundaries is the rationalize used to explain the permeability behavior of the polymerized lipids. Polymerized cross-linked liposomes exhibit resistance towards disruption by SDS and Triton X-100. Non-cross-linked liposomes were readily lysed. Polymerized liposomes of 1 exhibited a shelf-life over 1 year. Phospholipase A$\sb2$ exhibited no activity towards polymerized liposomes of 1. Phospholipase C had minimal (ca. 5%) activity while phospholipase D exhibited a high degree of activity, hydrolyzing over 50% of the polymerized lipids. The polymerization was readily reversed via thiol reduction with dithiothreitol and octanethiol. Glutathione did not reduce the polymer. In Appendix I, liposomes of 1 and 2 are further characterized by FTIR, DSC, and digital-enhanced video light microscopy. In Appendix II, three other polymerizable lipoic acid-derived lipids, 1,2-bis(16-(lipoyloxy)-hexadecanoyl) -sn-glycero-3-phosphocholine, and 1-(12-(lipoyloxy)dodecanoyl) -2-palmitoyl-sn-glycero-3-phosphocholine and 1,2-bis(lipoyl) -sn-glycero-3-phosphocholine were synthesized and characterized. In Appendix III, the biodegradation of polymerized liposomes of 1,2-bis(12-(methacryloyloxy)-dodecanoyl) -sn-glycero-3-phosphocholine by the phospholipases A$\sb2$, C, D was examined