rate of freezing and the number of freeze–thaw cycles [7, 8]. Overall, subjecting vaccines adjuvanted with aluminum salts to slow
freezing can significantly comprise their immunogenicity [7–
10]. Unfortunately, it is estimated that 75–100% of vaccines are
exposed to freezing temperatures before being administered
[11]. These vaccines are either discarded under the best practice,
or inadvertently administered to humans, leading to costly waste to
the health care system and poor or no immune protection [12–
17]. For instance, it was reported that the difference in the level of
hepatitis B surface antigen-specific antibody between human subjects living in rural areas (70%) versus urban areas (94%) in Mongolia is due mostly to the exposure of hepatitis B vaccine they
received to freezing temperatures during transportation [18].
Strategies to overcome this problem include adding stabilizing
agent(s) to the liquid vaccines and/or converting the liquid vaccines to dry powder. In the liquid vaccine formulation, water is the
main component that exists between and within the aluminum salt
particles. This water layer helps to maintain the physical state and
shape of the antigen-adsorbed aluminum salt particles when stored
at the recommended storage conditions. However, during a slow
freezing process, the ice crystals that are generated cause distortion
to the hydration shell and eventually lead to aggregation of the
aluminum salt particles [4, 8]. Inhibiting ice crystal formation
using certain excipients (e.g., polyols) could minimize the particle
aggregation and preserve the immunogenicity of the vaccine [7, 8,
19]. For example, Braun and colleagues showed that adding 50%
(v/v) of polyethylene glycol (PEG)300, propylene glycol (PG), or
glycerin to the hepatitis B vaccine that contains insoluble aluminum
salts inhibited particle aggregation after the vaccine was exposed to
multiple freeze–thaw cycles (À20
C), and the vaccine maintained
its potency and induced an antibody level similar to that induced by
the freshly prepared vaccine [19].
Another approach to improve the stability of aluminum saltadjuvanted vaccines is by converting the liquid vaccine into dry
powder using methods such as spray freeze-drying and spraydrying [8, 20–23]. We reported that thin-film freeze-drying
(TFFD) can be used to convert vaccines containing insoluble aluminum salts from liquid to dry powder without causing particle
aggregation or decreasing the immunogenicity after reconstitution
[24, 25]. During TFFD, liquid droplets (~2–4 mm in diameter) of
a vaccine are first applied onto a cryogenically cooled surface
(Fig. 1). Upon impacting the cryogenically cooled surface, the
droplets spread and freeze rapidly (e.g., in ~70–1000 ms), generating frozen thin films that are ~100–400 μm thick [20, 26]. The
solvent in the frozen thin film is then removed by lyophilization.
The applicability of the TFFD technology was confirmed using
model vaccines prepared in-house as well as commercial human
and veterinary vaccines that contain insoluble aluminum salts such
490
Riyad F. Alzhrani et al.
freezing can significantly comprise their immunogenicity [7–
10]. Unfortunately, it is estimated that 75–100% of vaccines are
exposed to freezing temperatures before being administered
[11]. These vaccines are either discarded under the best practice,
or inadvertently administered to humans, leading to costly waste to
the health care system and poor or no immune protection [12–
17]. For instance, it was reported that the difference in the level of
hepatitis B surface antigen-specific antibody between human subjects living in rural areas (70%) versus urban areas (94%) in Mongolia is due mostly to the exposure of hepatitis B vaccine they
received to freezing temperatures during transportation [18].
Strategies to overcome this problem include adding stabilizing
agent(s) to the liquid vaccines and/or converting the liquid vaccines to dry powder. In the liquid vaccine formulation, water is the
main component that exists between and within the aluminum salt
particles. This water layer helps to maintain the physical state and
shape of the antigen-adsorbed aluminum salt particles when stored
at the recommended storage conditions. However, during a slow
freezing process, the ice crystals that are generated cause distortion
to the hydration shell and eventually lead to aggregation of the
aluminum salt particles [4, 8]. Inhibiting ice crystal formation
using certain excipients (e.g., polyols) could minimize the particle
aggregation and preserve the immunogenicity of the vaccine [7, 8,
19]. For example, Braun and colleagues showed that adding 50%
(v/v) of polyethylene glycol (PEG)300, propylene glycol (PG), or
glycerin to the hepatitis B vaccine that contains insoluble aluminum
salts inhibited particle aggregation after the vaccine was exposed to
multiple freeze–thaw cycles (À20
C), and the vaccine maintained
its potency and induced an antibody level similar to that induced by
the freshly prepared vaccine [19].
Another approach to improve the stability of aluminum saltadjuvanted vaccines is by converting the liquid vaccine into dry
powder using methods such as spray freeze-drying and spraydrying [8, 20–23]. We reported that thin-film freeze-drying
(TFFD) can be used to convert vaccines containing insoluble aluminum salts from liquid to dry powder without causing particle
aggregation or decreasing the immunogenicity after reconstitution
[24, 25]. During TFFD, liquid droplets (~2–4 mm in diameter) of
a vaccine are first applied onto a cryogenically cooled surface
(Fig. 1). Upon impacting the cryogenically cooled surface, the
droplets spread and freeze rapidly (e.g., in ~70–1000 ms), generating frozen thin films that are ~100–400 μm thick [20, 26]. The
solvent in the frozen thin film is then removed by lyophilization.
The applicability of the TFFD technology was confirmed using
model vaccines prepared in-house as well as commercial human
and veterinary vaccines that contain insoluble aluminum salts such
490
Riyad F. Alzhrani et al.