Phospholipid Stabilised Emulsions for Parenteral Nutrition and Drug Delivery

Abstract

Emulsions have been used in medical practice from the very earliest times. They comprise the dispersion of one immiscible liquid in another, and consequently can take different physical forms.⁷ For example, the dispersion of an oil in water provides the oil-in-water (o/w) emulsion system whereas the converse is the water-in-oil system (w/o). The physical nature of the system often dictates its route of administration into the body. Water-in-oil emulsions are normally given topically to the skin or the eye, or used as controlled release injectables (intramuscular or subcutaneous). Oil-in-water emulsions are those that can be given orally or by injection into the bloodstream. More complicated types of emulsion also exist, where a prototype system is reemulsified to give a so-called multiple emulsion system. Some of these, in the form of water-oil-water (w/o/w) systems, have been used as vaccine carriers.¹⁰ While these multiple emulsions may have advantage in terms of controlled release, they are difficult to stabilise and therefore have found limited use in medical and pharmaceutical practice. One further class of emulsions is known as microemulsions. These are thermodynamically stable solubilized systems and are rarely employed in pharmaceutical practice.¹⁴ As far as the author is aware, they have not been used as injectable systems. Unfortunately, within the pharmaceutical world, there is presently confusion over the term “microemulsion”. Emulsions can be produced that have a small particle size, for example less than 500 nm average diameter. If needed, such systems can be referred to as “fine” emulsions but they should not be termed “microemulsions”. This confusion seems to have arisen from the promotional activities of certain manufacturers of homogenisation equipment.