New liposomal technology has developed vehicles for transporting substances into the body effectively via absorption in the mouth, bypassing the digestive system.
Liposomes are nano-sized vesicles with spherical, closed structures, composed of curved phospholipid bilayers (for water-soluble nutrients) or a phospholipid mono-layer (for fat-soluble nutrients) that enclose part of the surrounding solvent/aqueous solution into their interior. They are identical to the phospholipids that make up cell membranes. The structure of liposomes allows them to engulf biologically active substances that are readily absorbed and utilised by the body.
A significant advantage of liposomes is they can incorporate and release two materials with different solubilities simultaneously. One example of this is the incorporation of two antioxidant agents namely cholecalciferol/vitamin D3 (a lipid-soluble molecule) and glutathione (a water-soluble molecule) in the same lipid vesicle.
Liposomes are administered orally for transmucosal absorption in the mouth. Delivery efficiency is size-dependent with smallest sizes (<150nm) showing fastest absorption.
Liposomes, and other lipid particles, are cleared from the blood by a system of immune cells, monocytes and macrophages, in the liver and spleen that clear particles out of the blood. Clearance is inversely related to size, with the smallest particle sized lipsosomes circulating the blood the longest. Capture by this system starts to decrease as sizes drop below 200nm and best evasion rates are below 125nm.
Further advances in liposome research have been able to allow liposomes to avoid detection by the body's immune system, thus protecting it from renal clearance and consequently prolonging its circulatory time. As a result of these studies, a kind of liposome so-called “stealth particles” has emerged. Stealth liposomes are made by covering the carrier surface with hydrophilic (water-loving) chains, such as polyethylene glycol (PEG).
PEGs increase the bioavailability of substances, keeps them stable, increases long circulation in the body and increases uptake on the blood brain barrier. PEGs, also known as Tocofersolan are made from a water soluble form of vitamin E. Small liposomes are the results of refined chemistry and high shear equipment. Small liposomes are more efficient at intracellular delivery of encapsulated compounds.
- Klibanov AL, Maruyama K, Beckerleg AM, et al. Activity of amphipathic poly(ethylene glycol) 5000 to prolong the circulation time of liposomes depends on the liposome size and is unfavorable for immunoliposome binding to target. Biochim Biophys Acta 1991;1062(2):142-148.
- Allen TM, Hansen C, Martin F, et al. Liposomes containing synthetic lipid derivatives of poly(ethylene glycol) show prolonged circulation half-lives in vivo. Biochim Biophys Acta 1991;1066(1):29-36.