Nasreen Mullah

Liposomes with detachable polymer coating: destabilization and fusion of dioleoylphosphatidylethanolamine vesicles triggered by cleavage of surface-grafted poly(ethylene glycol)

Plasma‐stable liposomes (100 nm) were prepared from dioleoylphosphatidylethanolamine (DOPE) and 3–6 mol% of a new disulfide‐linked poly(ethylene glycol)‐phospholipid conjugate (mPEG‐DTP‐DSPE). In contrast to similar preparations containing non‐cleavable PEG‐phospholipid conjugate, thiolytic cleavage of the grafted polymer chains facilitated rapid and complete release of the liposome contents. Furthermore, the detachment of PEG from DOPE liposomes resulted in liposomal fusion. Finally, while formulation of pH‐sensitive DOPE/cholesterol hemisuccinate liposomes with mPEG‐DTP‐DSPE abolished the pH sensitivity, cleavage of the PEG chains completely restored this property. These are the first examples of new useful properties of liposomes grafted with cleavable polymer.

Biologically Active Ligand-Bearing Polymer-Grafted Liposomes

Introduction of polymer-grafted lipid vesicles allowed to overcome some of the shortcomings of classical liposomes pertaining to their use in drug delivery, e.g. short circulating lifetimes in vivo with concomitant uptake by liver and spleen (Woodle, 1997). Optimal formulations of polymer-grafted liposomes contain 3–7 mole % of methoxypolyethylene glycol (MW 2000)-distearoylphosphatiyl-ethanolamine (mPEG-DSPE) in addition to various amounts of lecithin and cholesterol (Lasic and Martin, 1995).

New chemoenzymatic approach to glyco-lipopolymers: practical preparation of functionally active galactose–poly(ethylene glycol)–distearoylphosphatidic acid (Gal–PEG–DSPA) conjugate

A practical approach to galactose–PEG–distearoylphosphatidic acid (DSPA) retaining full lectin binding, involves glycosylation of monobenzyl ether–PEG, suitable protection of the sugar hydroxy groups, debenzylation, followed by enzymatic transphosphatidylation with phosphatidylcholine and final deprotection.

Reversible PEGylation: Thiolytic Regeneration of Active Protein from Its Polymer Conjugates

Polyethylene glycol (PEG) is known to convey to its protein conjugates a number of useful properties, including prolonged plasma circulation time, reduced immunogenicity, improved solubility, and resistance to proteolytic degradation [1]. Usually there is a direct relationship between the amount of the linked polymer and the gain in these characteristics. On the other hand, attachment of multiple chains of PEG to a protein, a process known as PEGylation, is often accompanied by a substantial loss of biological activity. This is a particularly severe problem for proteins acting on large substrates, e.g. receptor-binding proteins [1]. To circumvent this problem we introduced new linking chemistry (Figure 1) designed to produce gradual in vivo loss of PEG chains from its conjugates. Just as a promoiety of a prodrug, PEG would be present in these reversible conjugates only temporarily, improving such characteristics as pharmacokinetics and biodistribution. Here we communicate some of our initial results using lysozyme and cysteine as models of a protein and a cleaving agent respectively. Lysozyme is similar in size (14.5 kDa) to various cytokines and chemokines; it has six lysines available for PEGylation; and its activity directed toward a large substrate, peptidomurein of bacterial cell wall.