Liposome peptide modification

Peptide modification of liposomes offers the potential for enhancing the packaging process and for enhancing each stage of the lipofection process, to ultimately improve transfection efficiency. 

Peptide modification of liposomes has been found to enhance gene transfer up to a 1000-fold (150). They have potential as an equally active and yet safer alternatives to the viral vectors. 

Bourel-Bonnet L, GrasMasse H, Melnyk O. A novel family of amphilic alpha-oxo aldehydes for the site-specific modification of peptides by two palmi-toyl groups in solution or in liposome suspensions. Tetrahedron Lett 2001 42 6851. 

Methylation has also been found to enhance the membrane affinity of a number of different Icmt substrates. Of particular interest has been the effect of methylation on the biology of the proto-oncoprotein Ras. In an early study, K-Ras produced by in vitro translation was found to be farne-sylated, but further modification including proteolysis and methylation required incubation with intracellular membranes. Unmethylated K-Ras produced by in vitro translation in the presence of pancreatic microsomes and an inhibitor of methylation, methylthioadenosine (MTA), was found to associate less efficiently with PlOO membrane fractions from COS cells than the fully modified protein [55]. In other in vitro studies, farnesylated peptides corresponding to the C-terminus of Ras had 20-fold higher affinity for liposomes when methylated [56]. 

Structural modifications and formulation are two additional approaches proposed for overcoming the enzymatic barrier (and references therein). Using recombinant or synthetic techniques, selective modifications to the protein or peptide sequence can be introduced effectively reducing proteolytic susceptibility, but these changes must not have significant impact on the pharmacological properties of the molecule (e.g., reduced potency or altered selectivity). Moreover, modifications to address a specific enzymatic action will not eliminate vulnerability to others. The formulation approach essentially involves encapsulation systems to protect the protein or peptide from reactions with enzymes, and selected examples include emulsions, liposomes, or enteric-coated capsules (and references therein). 

In spite of these formidable challenges, the attractiveness of oral route has fueled the exploration of an incredibly diverse set of strategies to deliver proteins and peptides and the subject has been exhaustively reviewed. The various approaches include permeation enhancers, enzyme inhibitors, mucoadhesives, multifunctional matrices that simultaneously incorporate the above strategies, enteric coatings that offer protection from the acidic environment of the stomach, encapsulation (liposomes, microspheres, and nanoparticles), pH-sensitive polymers, microemulsions, carriers (delivery agents), and protein modification either to simply enhance permeability or to exploit specific transporters. While proof-of-concept has been demonstrated with most of these delivery systems in animal... 

Targeting Plasmids themselves are barely suited for targeting, because the monotone chemistry of naked plasmid DNA polymer does not provide unique modification sites. For molecular targeting, the tools of viruses should be optimized, reduced to the size of peptides, and added to the liposomal cover. 

Cationic liposomes Inefficient binding and entry Poor nuclear translocation New formulations, host modification Fusogenic peptides, altemative formulations, weak bases... 

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