Since the eradication of smallpox nearly four decades ago, the quest to develop effective vaccines for any number of other diseases has benefitted from significant advances in medical science and technology. In subsequent years, various methods of vaccine delivery have been pioneered. Modern vaccine development goals include improved efficacy, reduced risk, enhanced safety and better stability of vaccines; as well as reduced costs of production and delivery.
However, greater safety often comes at a cost: Many vaccine candidates — so-called subunit vaccines — including highly purified recombinant proteins, synthetic peptides, or T and B cell epitopes — are far safer immunogenic candidates than live attenuated or whole inactivated pathogens. Yet they frequently suffer from limited immunogenicity. Accordingly, enhanced immunogenicity is a primary goal of current vaccine development. Of course, vaccines may be used not only for prophylaxis against pathogens, but also therapeutically against cancer and allergens.
The Search for Suitable Drug Delivery Systems
To that end, various innovative targeted drug delivery systems and vaccine adjuvants have been explored. Adjuvants boost humoral and/or cellular immune responses, but suitably safe — and effective — adjuvants are limited. Adjuvants may take the form of delivery systems or immunostimulators. To date, most human adjuvants elicit humoral immunity only. As such, there is presently a largely unmet need for adjuvants that also stimulate cell-mediated immunity.
Despite decades of research and development, the need for effective vaccines against various dangerous pathogens remains. For example, the recent Ebola and Zika virus pandemic crises underscored the ongoing need for development of additional vaccines to protect us against potential disaster. Among other options, phospholipid-based vesicles (liposomes) are especially promising. These microscopic delivery vessels excel at delivering antigens to antigen-presenting cells, while also serving an adjuvant function. Thus, liposome transfection is an increasingly important tool used in the development of new vaccines.
The Effectiveness of Cationic Lipids
Cationic lipids (and therefore, cationic liposomes) have proven to be especially effective in this role. Liposomes have the distinct advantage of serving especially well as both delivery systems for subunit antigens and as immunopotentiators. They’re also highly versatile vaccine adjuvants that are readily modified to maximize desirable properties. Examples include alterations in the type of lipid used, inclusion of compounds with immunostimulating activity (e.g., lipids or other ligands), formulation method, etc.
The methods used to prepare liposomes also affect their physiochemical properties, including: hydrophobicity, particle size, membrane fluidity, and as previously noted, surface charge. Research suggests that particle size influences both the nature of the immune response elicited, and draining kinetics from the injection site. In general, smaller particles tend to drain from injection sites more rapidly. Smaller particles are also evidently more likely to elicit a response from T-helper cells (Th2), while larger particles tend to induce responses by Th1 cells. However, when cationic liposomes are used, Th1 response tends to be stimulated, regardless of relative particle size.
In vitro research indicates that liposomes with a cationic surface charge are far more readily taken up by antigen presenting cells (APCs) than negatively charged or neutral liposomes. Surface charge is a function of the composition of the lipids comprising the lipid bilayers of the vesicle. A positive overall charge is achieved using charged lipids, such as acetylated ammonium compounds, to synthesize the liposomes.
Moreover, surface charge also influences antibody response. In animal studies, only cationic liposomes elicited a significant antibody response from macrophages. Surface charge also affects the ways in which liposomes interact with proteins, enzymes and cells at the site of injection. This ultimately affects draining kinetics, and thus, the likelihood that a given vaccine administered IM will actually reach the bloodstream, and eventually, its targets.
The Versatility of Liposomes
Subunit vaccines make use of antigens in the form of peptides and proteins. Suitable immunostimulators, on the other hand, may consist of any number of classes of compounds, including lipids, sugars, DNA, siRNA, etc. Because of their exceptional versatility, liposomes can be devised to include different immunostimulators. The amphiphilic and biphasic nature of liposomes allows for extraordinary versatility, in that antigens can be included within the core, on the liposome’s surface, or within the lipid bilayers of the liposome.
Of course, devising suitable liposomes with specific desirable characteristics — and successfully manufacturing them — requires not only refined formulation techniques and processes, but sophisticated machinery. At Microfluidics, we specialize in the design, manufacture and servicing of precision machinery that can help process engineers achieve these demanding goals.
We excel at nanoscale particle production and our high pressure homogenizers produce some of the tightest particle size distributions in the industry.