Evaluation of the compatibility of a second generation
recombinant anthrax vaccine with aluminum-containing adjuvants
Scott Jendrek, , a,
Stephen F. Littleb, Stanley Hemc,
Gautam Mitraa and Steven Giardinaa
a Building 320, SAIC-Frederick Inc., National Cancer Institute,
Frederick Cancer Research and Development Center, Frederick, MD 21702, USA b Bacteriology Division, US Army Medical Research Institute of
Infectious Diseases, Fort Detrick, Frederick, MD 21702, USA c Department of Industrial and Physical Pharmacy, Purdue University,
West Lafayette, IN 47907, USA
Received 10 September 2002; accepted 29 January 2003. ; Available online 11
March 2003.
Abstract
Recombinant protective antigen (rPA) is the active pharmaceutical
ingredient in a second generation anthrax vaccine undergoing pre-clinical
evaluation. This rPA vaccine differs from the currently licensed vaccine,
anthrax vaccine adsorbed (AVA), in that the sole component is a
recombinant form of protective antigen (PA). Unlike AVA the rPA vaccine
contains no lethal factor (LF) or edema factor (EF), components of the two
bipartite toxins, nor many other Bacillus anthracis-related
contaminating proteins that are present in AVA. The proposed clinical
protocol involves adsorption of the rPA to an aluminum-based adjuvant. The
adsorptive characteristics of rPA and two aluminum-containing adjuvants
were examined in a physiological buffer with and without EDTA. Based on
the pI of rPA (pI=5.6) and the zero charge point of aluminum
hydroxide adjuvant (11.5) and aluminum phosphate adjuvant (4.5), it was
predicted and demonstrated that rPA bound in a more efficient manner to
aluminum hydroxide adjuvant than to aluminum phosphate adjuvant in the
physiological buffer. Binding of the rPA to the aluminum hydroxide
adjuvant was decreased by increasing amounts of phosphate in the buffer.
The adsorptive capacity for rPA onto aluminum hydroxide adjuvant in the
physiological buffer and in water were calculated to be 0.46 mg rPA/mg
aluminum in DPBS and 0.73 mg rPA/mg aluminum in water. This study also
demonstrated that upon desorption from the aluminum hydroxide adjuvant the
rPA was physically intact and free of detectable aggregates. Further, the
eluted material was biologically active in an in vitro cytotoxicity assay.
Desorption was only possible after an overnight incubation of 28 °C and
not after a room temperature incubation reflecting increased contact with
the aluminum hydroxide adjuvant over time. These data suggest that the
interaction between rPA and aluminum hydroxide adjuvant is predominantly
electrostatic in character.
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