Cleavable lipids

In addition to enzymatic hydrolysis of natural lipids in polymeric membranes as discussed in chapter 4.2.2., other methods have been applied to trigger the release of vesicle-entrapped compounds as depicted in Fig. 37. Based on the investigations of phase-separated and only partially polymerized mixed liposomes 101, methods to uncork polymeric vesicles have been developed. One specific approach makes use of cleavable lipids such as the cystine derivative (63). From this fluorocarbon lipid mixed liposomes with the polymerizable dienoic acid-containing sulfolipid (58) were prepared in a molar ratio of 1 9 101115>. After polymerization of the matrix forming sulfolipids, stable spherically shaped vesicles are obtained as demonstrated in Fig. 54 by scanning electron microscopy 114>. 

In such phase-separated partially polymerized liposomes the corks consisting of cleavable lipids can be visualized by freeze-fracture electron microscopy101) and photobleaching 100). 

In order to circumvent such a drawback, there has been a recent focus on the design of cleavable lipids whose hydrolysis is catalyzed by the drop in pH.10 This approach exploits acid-labile chemical groups such as acetals, ketals, orthoesters, and vinyl ethers. Such functional groups... 

Hatakeyama H, Akita H, Kogure K, Oishi M, Nagasaki Y, Kihira Y, Ueno M, Kobayashi H, Kikuchi H, Harashima H (2007) Development of a novel systemic gene delivery system for cancer therapy with a tumor-specific cleavable PEG-lipid. Gene Ther 14 68-77... 

Grafting of PEG on the liposome surface interferes with the ability of the liposome to undergo membrane fusion and destabilization in the endosome. Meyer et al. observed this point (33). The stabilization of the lipopiexes into a lamellar phase would be a possible reason for this transfection inhibition, by lack of destabilization into the endosome (34). Thus, cleavable PEG-lipid has been designed to limit the nonspecific interaction with proteins, although restoring the ability of the particles to interact with the endosomal cellular membranes and to release their therapeutic payload. 

Fig. 8. Two pairs of a polymerizable zwitterionic dienoyl lipid and a cleavable disulfide amphiphile derived from cysteine. In each pair, one amphiphile has a hydrocarbon tail and the other a fluorocarbon tail.

Our retrosynthetic analysis for lipid I is presented below [Scheme 2], Our protected version of lipid I employed acetate protective groups for the carbohydrate hydroxyls, methyl esters for each of the carboxyl groups in the pentapeptide side chain, and trifluoroacetate for the terminal amino group of the lysine residue. These base-cleavable protective groups could be removed in a single operation in the final step of our synthesis and would not subject the sensitive diphosphate linkage to acidic reagents or reaction conditions. 

The presence of the unsaturated undecaprenylside chain makes lipids I and II challenging synthetic targets. As a key strategy for the synthesis of lipid I (21), the undecaprenyl-linked diphosphate moiety was infroduced at a late stage in the synthesis to avoid potential solubility complications caused by the enhanced lipophilic character of the undecaprenyl-linked substrate. The anticipated acid sensitivity of the anomeric diphosphate dictated the use of base-cleavable protective groups for all peripheral functionality, including the side chains of peptapeptide. 

Terada T, Iwai M, Kawakami S et al. (2006) Novel PEG-matrix metalloproteinase-2 cleavable peptide-lipid containing galactosylated liposomes for hepatocellular carcinoma-selective targeting. J Control Release 111 333-342... 

Researchers have tested whether the membrane lipid environment affects the dynamics and cleavability of a model peptide corresponding to the amino acid sequence 684-726 of the amyloid precursor protein APP reconstituted in liposomes. SS NMR with H-Ala713, which is located within the putative transmembrane domain, suggested that the peptide observes less rotational motion in egg phosphatidylcholine (egg PC) membranes than in DMPC bilayers above the main phase transition temperature. They conclude stating that the dynamics of APP(684-726) on the transmembrane level as well as the motion of the ot-cleavage site and its hydrolysis by a model enzyme are dependent on the bilayer characteristics. ... 

Xu H, Deng Y, Chen D, Hong W, Lu Y, Dong X (2008) Esterase-catalyzed dePEGylation of pH-sensitive vesicles modified with cleavable PEG-lipid derivatives. J Contr Release 130 238-245... 

Wong JB, Grosse S, TabOT AB, Hart SL, Hailes HC (2008) Acid cleavable PEG-lipids for applications in a ternary gene delivery vector. Mol BioSyst 4 532-541... 

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