Liposome

Scheme of a liposome formed by phospholipids in an aqueous solution.
Liposomes are composite structures made of phospholipids and may contain small amounts of other molecules. Though liposomes can vary in size from low micrometer range to tens of micrometers, unilamellar liposomes, as pictured here, are typically in the lower size range with various targeting ligands attached to their surface allowing for their surface-attachment and accumulation in pathological areas for treatment of disease.[1]

A liposome is an artificially-prepared spherical vesicle composed of a lamellar phase lipid bilayer. The liposome can be used as a vehicle for administration of nutrients and pharmaceutical drugs.[2] Liposomes can be prepared by disrupting biological membranes (such as by sonication).

Liposomes are often composed of phosphatidylcholine-enriched phospholipids and may also contain mixed lipid chains with surfactant properties such as egg phosphatidylethanolamine. A liposome design may employ surface ligands for attaching to unhealthy tissue.[1]

The major types of liposomes are the multilamellar vesicle (MLV), the small unilamellar liposome vesicle (SUV), the large unilamellar vesicle (LUV), and the cochleate vesicle.[3]

Liposomes should not be confused with micelles and reverse micelles composed of monolayers.[4]

Discovery

The word liposome derives from two Greek words: lipo ("fat") and soma ("body"); it is so named because its composition is primarily of phospholipid.

Liposomes were first described by British haematologist Alec D Bangham[5][6][7] in 1961 (published 1964), at the Babraham Institute, in Cambridge. They were discovered 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 microscope pictures served as the first evidence for the cell membrane being a bilayer lipid structure. Their integrity as a closed, bilayer structure, that could release its contents after detergent treatment (structure-linked latency) was established by Bangham, Standish and Weissmann in the next year.[8] Weissmann - during a Cambridge pub discussion with Bangham - first named the structures "liposomes" after the lysosome, which his laboratory had been studying: a simple organelle the structure-linked latency of which could be disrupted by detergents and streptolysins.[9] Liposomes can be easily distinguished from micelles and hexagonal lipid phases by negative staining transmission electron microscopy.[10]

Alec Douglas Bangham with colleagues Jeff Watkins and Malcolm Standish wrote the 1965 paper that effectively launched the liposome “industry”. Around this time he was joined at Babraham by Gerald Weissmann, an American physician with an interest in lysosomes. Now an emeritus professor at New York University School of Medicine, Weissmann recalls the two of them sitting in a Cambridge pub and reflecting on the role of lipid sheets in separating the interior of the cell from the exterior milieu. This insight, they felt, was to cell function what the discovery of the double helix had been to genetics. Bangham had called his lipid structures “multilamellar smectic mesophases” or sometimes “Banghasomes”. It was Weissmann who proposed the more user-friendly term liposome.[11][12]

Mechanism

A liposome encapsulates a region of aqueous solution inside a hydrophobic membrane; dissolved hydrophilic solutes cannot readily pass through the lipids. Hydrophobic chemicals can be dissolved into the membrane, and in this way liposome can carry both hydrophobic molecules and hydrophilic molecules. To deliver the molecules to sites of action, the lipid bilayer can fuse with other bilayers such as the cell membrane, thus delivering the liposome contents. By making liposomes in a solution of DNA or drugs (which would normally be unable to diffuse through the membrane) they can be (indiscriminately) delivered past the lipid bilayer. A liposome does not necessarily have lipophobic contents, such as water, although it usually does.

Liposomes are used as models for artificial cells. Liposomes can also be designed to deliver drugs in other ways. Liposomes that contain low (or high) pH can be constructed such that dissolved aqueous drugs will be charged in solution (i.e., the pH is outside the drug's pI range). As the pH naturally neutralizes within the liposome (protons can pass through some membranes), the drug will also be neutralized, allowing it to freely pass through a membrane. These liposomes work to deliver drug by diffusion rather than by direct cell fusion.

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.[13] 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 opsonins and ligands to activate endocytosis in other cell types.

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,[14] pesticides to plants, enzymes and nutritional supplements to foods, and cosmetics to the skin.[15]

Liposomes are also used as outer shells of some microbubble contrast agents used in contrast-enhanced ultrasound.

List of drugs

As of 2012, 13 drugs with liposomal delivery systems have been approved and five additional liposomal drugs were in clinical trials.

List of clinically approved liposomal drugs
Name Trade name Company Indication Liposomal Excipients
Liposomal amphotericin B Abelcet Enzon Fungal infections DMPC and DMPG
Liposomal amphotericin B Ambisome Gilead Sciences Fungal and protozoal infections HSPC, Cholesterol, DSPG
Liposomal cytarabine Depocyt Pacira (formerly SkyePharma) Malignant lymphomatous meningitis DOPC, Cholesterol, DPPG
Liposomal daunorubicin DaunoXome Gilead Sciences HIV-related Kaposi’s sarcoma DSPC, Cholesterol
Liposomal doxorubicin Myocet Zeneus Combination therapy with cyclophosphamide in metastatic breast cancer LIPOVA-E120, Cholesterol
Liposomal IRIV vaccine Epaxal Crucell Hepatitis A LECIVA-S70
Liposomal IRIV vaccine Inflexal V Berna Biotech Influenza LECIVA-S90
Liposomal morphine DepoDur SkyePharma, Endo Postsurgical analgesia DOPC, Cholesterol, DPPG
Liposomal verteporfin Visudyne QLT, Novartis Age-related macular degeneration, pathologic myopia, ocular histoplasmosis Egg PG, DMPC
Liposome-proteins SP-B and SP-C Curosurf Chiesi Farmaceutici, S.p.A. pulmonary surfactant for Respiratory Distress Syndrome (RDS) Leciva-S90
Liposome-PEG doxorubicin Doxil/Caelyx Ortho Biotech, Schering-Plough HIV-related Kaposi’s sarcoma, metastatic breast cancer, metastatic ovarian cancer MPEG-DSPE, HSPC, Cholesterol
Micellular estradiol Estrasorb Novavax Menopausal therapy Soybean oil, Polysorbate80
Liposomal vincristine Marqibo Spectrum Pharmaceuticals Acute Lymphoblastic Leukemia (ALL) and Melanoma Cholesterol and egg sphingomyelin
Liposome-PEG doxorubicin Lipo-Dox TTY BIOPHARM HIV-related Kaposi’s sarcoma, metastatic breast cancer, metastatic ovarian cancer, Multiple Myeloma MPEG-DSPE, DSPC, Cholesterol

Dietary and nutritional supplements

Regarding the use of liposomes as a carrier of dietary and nutritional supplements; until very recently the use of liposomes were primarily directed at targeted drug delivery. However, the versatile abilities of liposomes are now being discovered in other settings. Liposomes are presently being cleverly implemented for the specific oral delivery of certain dietary and nutritional supplements.[16]

A very small number of dietary and nutritional supplement companies are currently pioneering the benefits of this unique science towards this new application. This new direction and employment of liposome science is in part due to the low absorption and bioavailability rates of traditional oral dietary and nutritional tablets and capsules. The low oral bioavailability and absorption of many nutrients is clinically well documented.[17] Therefore the natural encapsulation of lypophilic and hydrophilic nutrients within liposomes has made for a very effective method of bypassing the destructive elements of the gastric system and aiding the encapsulated nutrient to be delivered to the cells and tissues.[18]

It is important to note that certain influential factors have far reaching effects on the percentage of liposome that are yielded in manufacturing.[19] These influences also have an effect on the actual amount of realized liposome entrapment and the actual quality of the liposomes themselves. These are very crucial elements which lead to the long term stability of the liposomes. These complex yet significant factors 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 primary and key elements comprise the foundation of an effective liposome carrier for use in increasing the bioavailability of oral dosages of dietary and nutritional supplements.[20]

Manufacturing

The correct choice of liposome preparation method depends on the following parameters:[21][22]

  1. the physicochemical characteristics of the material to be entrapped and those of the liposomal ingredients;
  2. the nature of the medium in which the lipid vesicles are dispersed
  3. the effective concentration of the entrapped substance and its potential toxicity;
  4. additional processes involved during application/delivery of the vesicles;
  5. optimum size, polydispersity and shelf-life of the vesicles for the intended application; and,
  6. batch-to-batch reproducibility and possibility of large-scale production of safe and efficient liposomal products

Formation of liposomes and nanoliposomes is not a spontaneous process. Lipid vesicles are formed when phospholipids such as lecithin are placed in water and consequently form one bilayer or a series of bilayers, each separated by water molecules, once enough energy is supplied.[23]

Liposomes can be created by sonicating phosphatidylcholine rich phospholipids in water.[4] Low shear rates create multilamellar liposomes, which have many layers like an onion. Continued high-shear sonication tends to form smaller unilamellar liposomes. In this technique, the liposome contents are the same as the contents of the aqueous phase. 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 and Mozafari method [24] are employed to produce materials for human use.

Prospect

Further advances in liposome research have been able to allow liposomes to avoid detection by the body's immune system, specifically, the cells of reticuloendothelial system (RES). These liposomes are known as "stealth liposomes", and are constructed with PEG (Polyethylene Glycol) studding the outside of the membrane. The PEG coating, which is inert in the body, allows for longer circulatory life for the drug delivery mechanism. However, research currently seeks to investigate at what amount of PEG coating the PEG actually hinders binding of the liposome to the delivery site. In addition to a PEG coating, most stealth liposomes also have some sort of biological species attached as a ligand to the liposome in order to enable binding via a specific expression on the targeted drug delivery site. These targeting ligands could be monoclonal antibodies (making an immunoliposome), vitamins, or specific antigens. Targeted liposomes can target nearly any cell type in the body and deliver drugs that would naturally be systemically delivered. Naturally toxic drugs can be much less toxic if delivered only to diseased tissues. Polymersomes, morphologically related to liposomes, can also be used this way.

In case of tumor development, certain anticancer drugs such as doxorubicin (Doxil) and daunorubicin are provided through liposomes. Liposomal cisplatin has received orphan drug designation for pancreatic cancer from EMEA.

See also

References

  1. ↑ 1.0 1.1 Torchilin, V (2006). "Multifunctional nanocarriers". Advanced Drug Delivery Reviews 58 (14): 1532–55. doi:10.1016/j.addr.2006.09.009. PMID 17092599.
  2. ↑ Kimball's Biology Pages, "Cell Membranes."
  3. ↑ Explanation on twst.com commercial page, cf. also Int.Patent PCT/US2008/074543 on p.4, section 0014
  4. ↑ 4.0 4.1 Stryer S. (1981) Biochemistry, 213
  5. ↑ Bangham, A. D.; Horne, R. W. (1964). "Negative Staining of Phospholipids and Their Structural Modification by Surface-Active Agents As Observed in the Electron Microscope". Journal of Molecular Biology 8 (5): 660–668. doi:10.1016/S0022-2836(64)80115-7. PMID 14187392.
  6. ↑ Horne, R. W.; Bangham, A. D.; Whittaker, V. P. (1963). "Negatively Stained Lipoprotein Membranes". Nature 200 (4913): 1340. doi:10.1038/2001340a0. PMID 14098499.
  7. ↑ Bangham, A. D.; Horne, R. W.; Glauert, A. M.; Dingle, J. T.; Lucy, J. A. (1962). "Action of saponin on biological cell membranes". Nature 196: 952–955. doi:10.1038/196952a0. PMID 13966357.
  8. ↑ Bangham, A.D., Standish, M.M. and Weissmann, G., The action of steroids and streptolysin S on the permeability of phospholipid structures to cations, J. Molecular Biol., 13:253-259, 1965
  9. ↑ Sessa, G. and Weissmann, G., Incorporation of lysozyme into liposomes: A model for structure-linked latency, J. Biol. Chem., 245:3295-3301, 1970
  10. ↑ YashRoy R.C. (1990) Lamellar dispersion and phase separation of chloroplast membrane lipids by negative staining electron microscopy. Journal of Biosciences, vol 15(2), pp. 93-98.http://www.ias.ac.in/jarch/jbiosci/15/93-98.pdf
  11. ↑ G. Weissmann, G. Sessa, M. Standish and A. D. Bangham, J. Clin. Invest., 1965, 44, 1109–1116
  12. ↑ Geoff Watts (2010-06-12). "Alec Douglas Bangham". The Lancet 375 (9731): 2070. doi:10.1016/S0140-6736(10)60950-6. Retrieved 2014-10-01.
  13. ↑ Bertrand, Nicolas; Bouvet, CéLine; Moreau, Pierre; Leroux, Jean-Christophe (2010). "Transmembrane pH-Gradient Liposomes to Treat Cardiovascular Drug Intoxication". ACS Nano 4 (12): 7552–8. doi:10.1021/nn101924a. PMID 21067150.
  14. ↑ Barani, H; Montazer, M (2008). "A review on applications of liposomes in textile processing". Journal of liposome research 18 (3): 249–62. doi:10.1080/08982100802354665. PMID 18770074.
  15. ↑ Meure, LA; Knott, R; Foster, NR; Dehghani, F (2009). "The depressurization of an expanded solution into aqueous media for the bulk production of liposomes". Langmuir : the ACS journal of surfaces and colloids 25 (1): 326–37. doi:10.1021/la802511a. PMID 19072018.
  16. ↑ Yoko Shojia, Hideki Nakashima (2004). "Nutraceutics and Delivery Systems". Journal of Drug Targeting.
  17. ↑ Williamson, G; Manach, C (2005). "Bioavailability and bioefficacy of polyphenols in humans. II. Review of 93 intervention studies". The American journal of clinical nutrition 81 (1 Suppl): 243S–255S. PMID 15640487.
  18. ↑ Bender, David A. (2003). Nutritional Biochemistry of Vitamins. Cambridge, U.K.
  19. ↑ Szoka Jr, F; Papahadjopoulos, D (1980). "Comparative properties and methods of preparation of lipid vesicles (liposomes)". Annual review of biophysics and bioengineering 9: 467–508. doi:10.1146/annurev.bb.09.060180.002343. PMID 6994593.
  20. ↑ Chaize, B; Colletier, JP; Winterhalter, M; Fournier, D (2004). "Encapsulation of enzymes in liposomes: High encapsulation efficiency and control of substrate permeability". Artificial cells, blood substitutes, and immobilization biotechnology 32 (1): 67–75. doi:10.1081/BIO-120028669. PMID 15027802.
  21. ↑ Gomezhens, A; Fernandezromero, J (2006). "Analytical methods for the control of liposomal delivery systems". TrAC Trends in Analytical Chemistry 25 (2): 167. doi:10.1016/j.trac.2005.07.006.
  22. ↑ Mozafari, MR; Johnson, C; Hatziantoniou, S; Demetzos, C (2008). "Nanoliposomes and their applications in food nanotechnology". Journal of liposome research 18 (4): 309–27. doi:10.1080/08982100802465941. PMID 18951288.
  23. ↑ Mozafari, M.R. and Mortazavi, S.M. (2005). Nanoliposomes: From Fundamentals to Recent Developments. Oxford, UK: Trafford Publishing Ltd.
  24. ↑ Colas, JC; Shi, W; Rao, VS; Omri, A; Mozafari, MR; Singh, H (2007). "Microscopical investigations of nisin-loaded nanoliposomes prepared by Mozafari method and their bacterial targeting". Micron (Oxford, England : 1993) 38 (8): 841–7. doi:10.1016/j.micron.2007.06.013. PMID 17689087.

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