Abstract
Squalene is a linear triterpene that is extensively utilized as a principal component of parenteral emulsions for drug and vaccine delivery. In this review, the chemical structure and sources of squalene are presented. Moreover, the physicochemical and biological properties of squalene-containing emulsions are evaluated in the context of parenteral formulations. Historical and current parenteral emulsion products containing squalene or squalane are discussed. The safety of squalene-based products is also addressed. Finally, analytical techniques for characterization of squalene emulsions are examined.
Highlights
Introduction to Squalene and EmulsionsSqualene is widely used for numerous vaccine and drug delivery emulsions due to its stabilityenhancing effects and biocompatibility
A required hydrophilic-lypophilic balance (HLB) value for squalene has not been reported in the literature and the various squalene-containing emulsions described below have a wide range of HLBs
Squalene and squalane had been used earlier without antigen to increase nonspecific immunity against tumors, it has been claimed that the first vaccine emulsion to employ squalene or squalane with an antigen was Syntex Adjuvant Formulation (SAF) in the mid-1980s [36]
Summary
Squalene is widely used for numerous vaccine and drug delivery emulsions due to its stabilityenhancing effects and biocompatibility. Squalene and its hydrogenated form, squalane, have unique properties that are ideally suited for making stable and non-toxic nanoemulsions Because of these characteristics, numerous squalene-based emulsions have been effectively developed for drug and vaccine applications. Since emulsions are particulate in nature, they have longer biological residence times and are more effectively phagocytosed by scavenging cells than aqueous formulations [36,37] They can increase drug or vaccine uptake into cells. The most commonly used squalene emulsion for vaccine formulations, MF59®, employs a 50/50 mixture of low HLB and high HLB surfactants to create an overall HLB value of 8.4. A required HLB value for squalene has not been reported in the literature and the various squalene-containing emulsions described below have a wide range of HLBs. Interestingly, vaccine w/o emulsions employing squalane showed that slight variations in surfactant. This review attempts to rectify these uncertainties, where possible, by specifying concentrations both at manufacture and at injection (i.e., at final dilution) as well as explicitly stating % w/v or v/v values
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