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A liposome is a small artificial vesicle, spherical in shape, having at least one lipid bilayer. Due to their hydrophobicity and/or hydrophilicity, biocompatibility, particle size and many other properties, liposomes can be used as drug delivery vehicles for administration of pharmaceutical drugs and , such as lipid nanoparticles in , and DNA vaccination. Liposomes can be prepared by disrupting biological membranes (such as by sonication).
Liposomes are most often composed of ,Mashaghi S., et al. Lipid Nanotechnology. Int J Mol Sci. 2013 Feb; 14(2): 4242–4282. especially phosphatidylcholine, and cholesterol, but may also include other lipids, such as those found in egg and phosphatidylethanolamine, as long as they are compatible with lipid bilayer structure. A liposome design may employ surface for attaching to desired cells or tissues.
Based on vesicle structure, there are seven main categories for liposomes: multilamellar large (MLV), oligolamellar (OLV), small unilamellar (SUV), medium-sized unilamellar (MUV), large unilamellar (LUV), giant unilamellar (GUV) and multivesicular vesicles (MVV). The major types of liposomes are the multilamellar vesicle (MLV, with several lamellar phase ), the small unilamellar liposome vesicle (SUV, with one lipid bilayer), the large unilamellar vesicle (LUV), and the cochleate vesicle. A less desirable form is multivesicular liposomes in which one vesicle contains one or more smaller vesicles.
Liposomes should not be confused with , or with and reverse micelles.Stryer S. (1981) Biochemistry, 213 In contrast to liposomes, micelles typically contain a monolayer of fatty acids or surfactants.Mashaghi S., et al. Lipid Nanotechnology. Int J Mol Sci. 2013 Feb; 14(2): 4242–4282.
Liposomes were first described by British hematologist Alec Douglas Bangham in 1961 at the Babraham Institute, in Cambridge—findings that were published 1964. The discovery came about when Bangham and R. W. Horne were testing the institute's new electron microscope by adding negative stain to dry phospholipids. The resemblance to the plasmalemma was obvious, and the microscopic pictures provided the first evidence that the cell membrane is a bilayer lipid structure. The following year, Bangham, his colleague Malcolm Standish, and Gerald Weissmann, an American physician, established the integrity of this closed, bilayer structure and its ability to release its contents following detergent treatment (structure-linked latency). During a Cambridge pub discussion with Bangham, Weissmann first named the structures "liposomes" after something which laboratory had been studying, the lysosome: a simple organelle whose structure-linked latency could be disrupted by detergents and streptolysins. Liposomes are readily distinguishable from micelles and hexagonal lipid phases through negative staining transmission electron microscopy.
Bangham, with colleagues Jeff Watkins and Standish, wrote the 1965 paper that effectively launched what would become the liposome "industry." Around that same time, Weissmann joined Bangham at the Babraham. Later, Weissmann, then an emeritus professor at New York University School of Medicine, recalled the two of them sitting in a Cambridge pub, reflecting on the role of lipid sheets in separating the cell interior from its exterior milieu. This insight, they felt, would be to cell function what the discovery of the double helix had been to genetics. As Bangham had been calling his lipid structures "multilamellar smectic mesophases," or sometimes "Banghasomes," Weissmann proposed the more user-friendly term liposome.
A similar approach can be exploited in the biodetoxification of drugs by injecting empty liposomes with a transmembrane pH gradient. In this case the vesicles act as sinks to scavenge the drug in the blood circulation and prevent its toxic effect. Another strategy for liposome drug delivery is to target endocytosis events. Liposomes can be made in a particular size range that makes them viable targets for natural macrophage phagocytosis. These liposomes may be digested while in the macrophage's phagosome, thus releasing its drug. Liposomes can also be decorated with and to activate endocytosis in other cell types.
Regarding pH-sensitive liposomes, there are three mechanisms of drug delivery intracellularly, which occurs via endocytosis. This is possible because of the acidic environment within endosomes. The first mechanism is through the destabilization of the liposome within the endosome, triggering pore formation on the endosomal membrane and allowing diffusion of the liposome and its contents into the cytoplasm. Another is the release of the encapsulated content within the endosome, eventually diffusing out into the cytoplasm through the endosomal membrane. Lastly, the membrane of the liposome and the endosome fuse together, releasing the encapsulated contents onto the cytoplasm and avoiding degradation at the lysosomal level due to minimal contact time.
Certain anticancer drugs such as doxorubicin (Doxil) and daunorubicin may be administered encapsulated in liposomes. Liposomal cisplatin has received orphan drug designation for pancreatic cancer from EMEA. A study provides a promising preclinical demonstration of the effectiveness and ease of preparation of valrubicin-loaded immunoliposomes (Val-ILs) as a novel nanoparticle technology. In the context of hematological cancers, Val-ILs have the potential to be used as a precise and effective therapy based on targeted vesicle-mediated cell death.
The use of liposomes for transformation or transfection of DNA into a host cell is known as lipofection.
In addition to gene and drug delivery applications, liposomes can be used as carriers for the delivery of dyes to textiles, pesticides to plants, enzymes and nutritional supplements to foods, and cosmetics to the skin.
Liposomes are also used as outer shells of some microbubble used in contrast-enhanced ultrasound.
The term nutraceutical combines the words nutrient and Medication, originally coined by Stephen DeFelice, who defined nutraceuticals as "food or part of a food that provides medical or health benefits, including the prevention and/or treatment of a disease". However, currently, there is no conclusive definition of nutraceuticals yet, to distinguish them from other food‐derived categories, such as food (dietary) supplements, herbal products, pre‐ and probiotics, , and fortified foods. Generally, this term is used to describe any product derived from food sources which is expected to provide health benefits additionally to the nutritional value of daily food. A wide range of nutrients or other substances with nutritional or physiological effects (EU Directive 2002/46/EC) might be present in these products, including , , , essential fatty acids, Dietary fiber and various plants and herbal extracts. Liposomal nutraceuticals contain bioactive compounds with health-promoting effects. The encapsulation of bioactive compounds in liposomes is attractive as liposomes have been shown to be able to overcome serious hurdles bioactives would otherwise encounter in the gastrointestinal (GI) tract upon oral intake.
Certain factors have far-reaching effects on the percentage of liposome that are yielded in manufacturing, as well as the actual amount of realized liposome entrapment and the actual quality and long-term stability of the liposomes themselves. They are the following: (1) The actual manufacturing method and preparation of the liposomes themselves; (2) The constitution, quality, and type of raw phospholipid used in the formulation and manufacturing of the liposomes; (3) The ability to create homogeneous liposome particle sizes that are stable and hold their encapsulated payload. These are the primary elements in developing effective liposome carriers for use in dietary and nutritional supplements.
Liposomes can hence be created by sonication a dispersion of amphipatic lipids, such as phospholipids, in water. Low create multilamellar liposomes. The original aggregates, which have many layers like an onion, thereby form progressively smaller and finally unilamellar liposomes (which are often unstable, owing to their small size and the sonication-created defects). Sonication is generally considered a "gross" method of preparation as it can damage the structure of the drug to be encapsulated. Newer methods such as extrusion, micromixing and Mozafari method are employed to produce materials for human use. Using lipids other than phosphatidylcholine can greatly facilitate liposome preparation.
Liposomes are used as models for artificial cells.
Liposomes can be used on their own or in combination with traditional antibiotics as neutralizing agents of bacterial toxins. Many bacterial toxins evolved to target specific lipids of the host cells membrane and can be baited and neutralized by liposomes containing those specific lipid targets..
A study published in May 2018 also explored the potential use of liposomes as "nano-carriers" of fertilizing nutrients to treat malnourished or sickly plants. Results showed that these synthetic particles "soak into plant leaves more easily than naked nutrients", further validating the utilization of nanotechnology to increase crop yields.
Machine learning has started to contribute to liposome research. For example, deep learning was used to monitor a multistep bioassay containing sucrose-loaded and nucleotides-loaded liposomes interacting with a lipid membrane-perforating peptide. Artificial neural networks were also used to optimize formulation parameters of leuprolide acetate loaded liposomes and to predict the particle size and the polydispersity index of liposomes.
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