Liposomes are spherical vesicles composed of one or more phospholipid bilayers whose structure closely resembles that of cell membranes.
Fig. 1 Classification of liposomes
The advantages of anionic liposomes include:
- Anionic liposomes exhibit higher stability in solution
- Lower aggregation compared to neutrally charged liposomes
- Enhanced endocytosis compared to cationic liposomes
Anionic liposomes are usually applied to study drug uptake and release in vivo and to establish cellular models for drug-like molecules. However, due to the interaction between anionic liposomes and non-specific receptors, especially B-type scavenger receptors, it is easy to be phagocytosed by mononuclear macrophages system (MPS), along with the activation and binding of complement system, which lead to the blocking or inhibition of anionic function of liposomes as a drug carrier. Therefore, Lume and Klibanov first modified liposomes through polyethylene glycol (PEG), which is a superior hydrophilic polymer that stabilizes liposomes (Fig. 1).
The use of anionic liposomes covalently linked to PEG chains, i.e. PEGylated-anionic-liposomes, improves surface properties related to steric hindrance effects and charge shielding. It will protect anionic liposomes from circulating proteins and prolong their circulation time in the blood, improving their plasma clearance and their therapeutic efficacy (Fig. 1).
Fig. 2 Schematic diagram of PEGylated-anionic-liposomes delivery system
Fig. 3 Main components of PEGylated-anionic-liposomes
Anionic liposomes can be prepared by ultrasonic treatment, extrusion and thin film methods, Before anionic liposome formation, the lipid mixture is mixed with PEG-containing lipids (e.g. DPPE-mPEG) at different molar ratios on the thin film step.
The approximate thickness of the 2000 kD PEG chain surface coating is 5 nm. With the addition of 5% PEG-2000, approximately 45% of the surface will be covered. As far as the conformation of PEG chains is concerned, the higher the molar PEG-lipid/lipid composition ratio, the higher the coverage density. At very low coverage, below 5% PEG, the polymer exhibits mushroom-like shape and lacks extension into the aqueous bulk phase (Fig. 1). For medium to high coverage (more than 5% PEG), the conformation transforms into the brush-like shape (Fig. 1). Liposomes with brush structures of polymers are less susceptible to phagocytosis by macrophage type cells than liposomes with less extended mushroom structures, in brief, PEGylation of liposomes prolong cycling time by suppressing MPS uptake, that is, preventing liposome-macrophage interplays.
On account of the nano-scale, biofilm-like structure, excellent biocompatibility and safety, liposomes have been widely used as delivery systems in drug development, especially in antimicrobial, antiviral and anti-tumor applications. Interestingly, compared with cationic liposomes, PEGylated-anionic-liposomes can improve the targeting characteristics, self-assembly and receptor-mediated transfection efficiency of receptor-bound peptide ligands, which can be used as a novel vector for gene delivery in vivo. Mignet et al. have developed a PH-sensitive pegylated anionic lipid complex that is sensitive to pH changes in the 5.5 to 6.5 range (reduced pH due to hypoxia in the tumor microenvironment) to efficiently deliver DNA to the tumor, to improve treatment effectiveness.
Nie et al. successfully encapsulated doxorubicin with fluorescence and therapy in charged liposomal preparations of anionic cholesterol derivatives, which can be used as a model platform to further study in vitro and in vivo behavior with the assistance of fluorescence images, FACS technology, and some other classical methods (Fig. 1). The first FDA-approved nanomedicine doxorubicin was delivered using pegylated liposomes. The charged liposomes combined with PEG showed prolonged and improved release profile of doxorubicin, accompanied by reduced cytotoxicity and hemolysis. Charged liposomes, especially anionic liposomes, significantly enhanced the uptake and cell inhibition of various cancer cells compared with neutral liposomes. In vivo tumor suppression further confirmed the greater tumor suppressive effect of charged liposomes, suggesting that PEGylated cationic and anionic liposomes preparations with modified cholesterol derivatives may be potential drug carriers to improve therapeutic efficacy.
Pegylated-anionic-liposomes are used to treat the overuse of antidepressants drugs (e.g. amitriptyline), adjust their absorption properties and subsequent decrease in serum drug concentrations, thereby alleviating a range of cardiovascular diseases and even death, as well as the severe economic burden.
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