Vaccines in silica need no refrigeration

ENCAPSULATING vaccines and protein-based therapeutics such as antivenom could mean they can be stored without the need for refrigeration, according to new research.

Biological substances such as these require continual cold storage to prevent them breaking down and losing their structure, rendering them unusable. This causes a significant global public health issue, for example in the developing world where refrigeration may not be possible. Millions of doses of vaccines, which are generally protein-based, are thought to be lost each year through breaks in the so-called ‘cold chain’. Researchers at the Universities of Bath and Newcastle, led by Bath’s Asel Sartbaeva, have now developed a way to encapsulate these proteins in silica cages to prevent them from denaturing in heat and acids.

The ‘ensilication’ process works on the basis of confining the protein in its correct shape and structure, therefore physically preventing the protein from unfolding and denaturing. Sartbaeva and the team begin by taking a solution of the desired protein, and mixing it with a solution of the silica precursor material, pre-hydrolysed tetraethylorthosilicate. The mixture is then stirred for 20 minutes. The silica forms a shell around the protein molecules spontaneously, and these precipitate out of solution. The researchers then dry the ensilicated powders for 24 hours in an extractor.

Sartbaeva and the team found that approximately 93% of the protein is ensilicated in the process. To release the protein from the cages, the researchers subject the powder to an acidified dilute solution of sodium fluoride. Silica is vulnerable to the solution and the cages break down.

The researchers tested their process on hen egg white lysozyme, horse haemoglobin, and tetanus toxin C-fragment (TTCF), part of the tetanus virus commonly used in vaccines. They subjected these ensilicated proteins to temperatures of up to 100^oC for between 2–5 hours, as well as incubating them with a 10 M solution of hydrochloric acid for three hours. Some samples were stored at 22^oC for six months to test for ageing. The samples were found to be stable. The lysozyme, for example, retains 75–100% of its enzymatic activity, while TTCF retains its antibody binding capacity.

“The ability to keep proteins intact in solid form until they are needed would be valuable for the storage on industrial enzymes, vaccines and biological therapeutics such as antibodies and antivenom treatments. The ensilication process we describe here has the potential to transform vaccine availability worldwide by elimination of the cold chain,” the researchers said.

Sartbaeva says that the next step in the research will be to test the technique more widely on other vaccines, antibodies, antiviral and antivenom drugs and other biopharmaceuticals. They are also investigating other ways to break down the silica cages.

Scientific Reports doi.org/f94n2h