PHOSPHOLIPIDS

Lipid substances, containing a phosphate group and one or more fatty acid residues, which are essential components of cell membranes. Hydrolysis yields fatty acids, phosphoric acid, and a base. They are amphoteric with a polar and a nonpolar region. Lecithins, cephalins, and related com_x0002_pounds are based on a glycerol backbone, with a phosphate group. In lecithin (phosphatidylcholine), R0 and Rv are fatty acid residues, usually one saturated and the other unsaturated. In the cephalins, ethanolamine (H2NCH2CH2OH) or serine replaces choline. They are used in the food industry as surfactants, emulsifiers, and antioxidants. In sphingomyelins, R is a fatty acid residue, usually tetracosanoic acid. They occur abundantly in brain tissues in association with cerebrosides, which are similar. On hydrolysis they split into choline, sphingosine, phosphoric acid, and a fatty acid.

Phospholipids occur as white powders. They are sometimes supplied as clear, nearly colorless chloroform or methylene chloride solutions. Phosphatidylglycerols, phosphatidic acids, and phosphatidylserines are available as sodium or ammonium salts. Phospholipids can be purified from natural sources, such as eggs or soybeans, or can be chemically synthesized. Lecithins are partially purified mixtures of naturally occurring phospholipids.

phospholipids (phosphatides) is used topically as a moisturizer and emollient because of their inherent compatibility with skin lipids. In general, natural phospholipids have a short-lived effect when topically applied, and are a primary material in the manufacture of liposomes. Phospholipids are complex fat substances that, together with protein, form the membrane of all living cells.

Phospholipids can be manufactured from naturally occurring materials, especially soybean and egg. The manufacturing process typically involves extraction, fractionation, and purification. They can also be synthesized chemically by reacting glycerol phosphocholine (PC), glycerol phosphoglycerol (PG), glycerol phosphoserine (PS), glycerol phosphoethanolamine (PE), or glycerol phosphoinositol (PI) with purified fatty acids.

Phospholipids are a class of lipids, which helps to maintain the structural definition of cells. These lipids are key components of cell and organelle membranes, blood lipoproteins and lung surfactants. Phospholipids plays an important role in signal transduction. It is also involved in the regulation of various cellular process including cell growth.

Phospholipids are amphiphilic molecules and are the major component of most cell membranes.They are able to selfassociate and form a variety of structures, including micelles and liposomes.
Numerous pharmaceutical formulations use phospholipids to form various types of liposomes, including unilamellar (one bilayer membrane surrounding an aqueous chamber), multilamellar (two chamber), and multivesicular (numerous aqueous chambers joined in a honeycomb-like arrangement) liposomes.Modified phospholipids have been used to enhance the properties of the resulting liposomes. The covalent attachment of polyethylene glycol (PEG) to the phospholipid, or PEGylation, provides steric hindrance to the surface of the liposomes, resulting in decreased uptake by the reticuloendothelial system (RES), also known as the mononuclear phagocyte system, and a prolonged circulation half-life following intravenous administration; the so-called ‘stealth liposomes.’ Conjugation with antibodies produces immunoliposomes, which are able to target specific cell types and deliver a payload of encapsulated drug.
Phospholipids can be anionic, cationic, or neutral in charge. Because of their amphiphilic nature, phospholipids will associate at hydrophobic/hydrophilic interfaces. The charged lipids can be used to provide electrostatic repulsion and physical stability to suspended particles. Thus, they have been used to physically stabilize emulsions and suspensions.Phospholipids have also been used in formulations administered as lung surfactants, in intravenous fat emulsions, and in oral solutions (e.g. Rapamune).

Phospholipids are esters of glycerol and are similar in structure to fats. However, unlike fats, they contain only two fatty acids. The third ester linkage involves a phosphate group which gives phospholipids two distinct parts: a long non-polar tail and a polar substituted phosphate head. Because of this dual nature, phospholipids tend to form bi-layers in an aqueous solution, with the tails in the interior and the polar heads interfacing with the polar water molecules.
Phospholipids form a significant portion of cell membranes and perform two important functions. They protect the cell from extra cellular fluids, and allow nutrients and other chemicals to enter the cell, while letting the waste products leave the cell. Lecithin is an example of phospholipids.

Generally, phospholipids have little or no acute toxicity (i.e. they are well tolerated even when administered at doses in the g/kg range).The clearance of most phospholipids occurs by wellknown metabolic pathways.
Liposomes containing stearylamines (cationic liposomes) have been found to induce cytotoxicity through apoptosis in the macrophage-like cell line RA W2647 and inhibit the growth of cells in vitro.In nine cancer-derived cell lines and one normal cultured human cell line, stearylamine- and cardiolipin-containing liposomes were toxic () at 200 μM liposomal lipid concentration or less, whereas PG- and PS-containing liposomes were toxic in the range 130–3000μM. Positively charged lipids such as stearylamine can increase the toxicity of liposomes.These studies reported an (IV) of 1.1 g/kg and 7.5 g/kg with and without stearylamine, respectively.
The safety of phospholipids delivered by the intravenous route is complicated by their tendency to form particles that are recognized by macrophages of the RES. Uptake by the RES is dependent on particle size and composition.

Phospholipids are stable in the solid state if protected from oxygen, heat, and light. Chloroform or dichloromethane solutions are also stable. Both the solid-state and solution forms should be stored at -20°C. Liposomal phospholipids are known to degrade via oxidation and hydrolysis. To minimize oxidation, liposomes can be prepared under oxygen-free environments and antioxidants, such as butylated hydroxytoluene (BHT), can be added. To minimize hydrolysis, water can be removed from liposomes by lyophilization. In cases where liposomes are unstable to lyophilization, long-term storage at 2–8°C is recommended. The ester hydrolysis of phospholipids in liposomes typically follows a Vshaped curve, with the minimum at around pH 6.5.

Included in the FDA Inactive Ingredient Database (oral, otic, buccal, vaginal, topical, epidural, intravenous, intramuscular, and inhalation aerosol). A number of phospholipids such as DPPG and DOPC are present in approved products in Europe and the USA.