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However, sterically stabilized liposomes have longer circulation times and successfully reach the tumor sites. This results in attack of conventional liposomes by the macrophage cells and subsequent clearance by RES. Figure 1 represents the effects of steric stabilization on RES clearance opsonins (shown in green) adhere only to conventional liposomes and not to the sterically stabilized liposomes (polymer chains shown in pink). One of the commonly used techniques is introduction of steric stabilization by augmenting the liposomal surface with hydrophilic polymers as shown in Figure 1 35, 40, 41. Therefore, various strategies have been exploited to induce stealth-ness in liposomes to prevent their opsonization and impart longer circulation in the bloodstream 32, 38, 39. A comprehensive review on opsonization mechanisms of conventional liposomes is previously presented 34. This phenomenon is also known as liposomal opsonization 35, 36, 37. The presence of opsonins on the liposomal surface makes them susceptible to macrophage attack and eventually, their removal through the reticuloendothelial system (RES). Opsonins attach themselves electrostatically to the surface of the lipids. The opsonins serve as identification markers for macrophages 33, 34. However, conventional liposomes were found to attract plasma proteins, also called as opsonin proteins, from the blood stream.
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These liposomes comprised of a combination of cationic, anionic and zwitterionic phospholipids in conjunction with cholesterol. The classification of liposomal drug delivery is broadly divided into conventional and long circulation liposomes.Ĭonventional or the first-generation liposomes were developed commercially in the beginning of 1980s for the delivery of hydrophobic doxorubicin and amphotericin 31, 32. Besides, liposomes can be customized according to size, charge and number of lamellae depending on the applications 29, 30.
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The properties of liposomes can be tuned by inclusion of polymers 20, 21, 22, cholesterol 23, 24, 25 or membrane proteins 26, 27, choice of lipids and sizing methods to improve their biodegradability, biocompatibility and versatility 7, 8, 28 for potential applications in encapsulation of drugs used in cancer therapy. 1, 2, 3, 4, 5, 6 The presence of hydrophilic aqueous core and hydrophobic lipid bilayer region enables them to encapsulate both hydrophobic and hydrophilic molecules, making them ideal candidates for vaccines 7, 8, 9 and drug carriers 10, 11 compared to metal 12, 13, polymer 14, 15, 16, and dendrimer 17, 18, 19-based carries. The lipid molecules comprise of a hydrophilic head group and hydrophobic tail region, in presence of an aqueous medium the lipids self-assemble to form liposomes.