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Figure 1. Overview of spray-drying process and customizable operation parameters. GMP: Good manufacturing practice.
(Figure omitted. See article PDF.)
Currently, when a liquid vaccine formulation does not impart an acceptable shelf life, the standard method for improving its stability is to produce a dry vaccine product using lyophilization. Lyophilization involves freezing the liquid solution and then removing water by sublimation of ice at low pressures. Dry vaccine products hold a number of advantages over liquid vaccine presentations. Dry vaccine formulations can have improved stability due to reduced potential for antigen damage by chemical and physical processes that involve hydrolysis, aggregation, denaturation, degradation or fluctuations in pH.
However, lyophilization has its limitations. It usually produces dried cake in the final container, which requires reconstitution prior to administration, and the enhanced stability of lyophilized vaccine is lost once the vaccine is reconstituted. In addition, dried formulations must be accompanied with appropriate diluents, which can lead to complications in transportation and preparation before administration. Alternative drying techniques, such as spray drying, have been pursued in recent decades to stabilize vaccines and in attempts to produce formulations that can be delivered without reconstitution via the gastrointestinal or respiratory mucosa, skin or implantation routes. Mucosal delivery may offer better protection than parental injection by inducing mucosal immunity at the pathogen entry site. Skin delivery may result in dose sparing by targeting antigens to the immune cell-rich skin. Implanted vaccines may slowly release the antigens and stimulate long-lasting immunity [1]. Bulk dried formulations with extended shelf lives have also been identified as beneficial for vaccine stockpiling for bioterrorism and pandemic preparedness [2]. Spray drying technology holds promise for producing stable dry vaccine formulations to meet these aforementioned needs.
Spray-drying technology
Spray drying is a well-established method for producing a dehydrated powder from a liquid feed. Extensively used in industrial food processing and the chemical industry, spray drying operates as a continuous process and produces a bulk product that can be packaged into customizable quantities and filled into a variety of packaging materials without additional processing. The first patent on spray drying was applied for in 1901 by Robert Stauf, with the intention of producing dry powders of milk, blood and other liquid solutions [3].
The steps of spray drying are:...