Morano N, et al. formalin-treated/alum-adsorbed rPA or by the two dosages. The antibody levels declined in all groups during the 1-year intervals after the 3rd and 4th injections but less so during the 2nd year, after the 4th injection (fold decreases were 10 to 25 versus 3.4 to 7.0, 0.001). There were too few AVA recipients for statistical comparisons, but their antibody levels followed those of rPA. Anti-rPA measured by ELISA correlated with TNA titers (= 0.97). These data support studying alum-adsorbed rPA in children. INTRODUCTION plasmids control the synthesis of these factors: pXO1 for the toxin and pXO2 for the capsule. Anthrax toxin KT182 conforms to the AB model of toxin. The B (binding) subunit is designated protective antigen (PA). The A (active) subunit is composed of two polypeptides designated lethal factor (LF), a metalloprotease, and edema factor (EF), an adenylate cyclase. Serum antibodies to PA confer immunity to anthrax in humans and in KT182 laboratory animals (4, 27). The active principle of the licensed anthrax Rabbit Polyclonal to USP13 vaccine adsorbed (AVA) is its PA (26). Animal studies and two clinical trials provide the basis for considering a vaccine containing only PA to be effective against anthrax. The recombinant PA (rPA) used in this study was mutated to remove two sites that are highly susceptible to proteolysis. This was intended to facilitate production of the protein in a homogeneous, intact state, by limiting the protein’s susceptibility to proteases secreted into the supernatant of the producer strain. Decreased protease susceptibility may also help stabilize the final vaccine product against trace amounts of protease in the final product. This protein may therefore not experience the reported stability issues that impacted a previous candidate vaccine, also produced from (1). The two sites altered are the furin site, residues RKKR at positions 164 to 167, which were changed to SNKE, and residues FF at positions 314 to 315, which were deleted. Removal of the RKKR sequence also prevents the PA from forming an oligomer that is responsible for pore formation and toxin action. Pore formation is highly unlikely to occur even with native PA when it is bound to alum and (in some formulations) treated with formaldehyde, but elimination of the pore-forming ability eliminates this hypothetical possibility of forming a toxic entity. AVA is prepared from the cell-free filtrate of a mutant strain of National Institute of Child Health and Human Development (protocol 04-CH-0283), the U.S. Food and Drug Administration (BB IND 11154), and the Institutional Review Board of Georgetown University (protocol 2003-080). The study was randomized and partially blinded; formalin-only-treated rPA was a clear fluid, distinguishable from alum-adsorbed or formalin-treated/alum-adsorbed rPA. The latter two formulations were indistinguishable. The dosages were blinded. AVA was in vials distinguishable from rPA vials. The study was conducted at Georgetown University Hospital, Washington, DC, and at the KT182 Clinical Center (CC), NIH, Bethesda, MD. Participants were healthy volunteers of either sex, 18 to 45 years of age. Excluded were subjects with abnormal liver or renal function, hepatitis B and/or hepatitis C, HIV infection, history of anthrax, or previous anthrax vaccination. After signing the informed consent, eligible volunteers were randomized to 1 1 of 7 groups, receiving one of 3 rPA formulations.