The Second Generation Anthrax Vaccine Candidate: rPA102
Monthly Clinical Issues in Biosecurity Series
By Luciana L. Borio, M.D., July 13, 2005
On November 2004, the U.S. Department of Health and Human Services (DHHS) awarded an $877.5 million contract to VaxGen, Inc. to supply 75 million doses of a second generation anthrax vaccine to the U.S. Strategic National Stockpile, which is intended to provide enough vaccine to immunize 25 million people in a three-dose regimen. The contract requires VaxGen to obtain licensure from the U.S. Food and Drug Administration (FDA) to use the new vaccine in both pre- and post-anthrax exposure settings. The final determination of the vaccine’s exact content of antigen and adjuvant is still pending data derived from ongoing clinical trials.
Bacillus anthracis is a spore-forming gram-positive bacterium whose principal virulence determinants include secreted protein exotoxins. The three secreted factors are protective antigen (PA), lethal factor, and edema factor. PA combines with lethal factor to form lethal toxin, and PA combines with edema factor to form edema toxin. PA allows the binding of lethal and edema factors to the cell membrane and facilitates their transport across the cell membrane, where they exert severe damage. Thus, a vaccine against PA would prevent the action of both, lethal and edema factors.
Little is known about immunity to infection with B. anthracis in humans. PA is the most important antigen required for specific immunity to anthrax in animals. PA cloned into B. subtilis induced protection from lethal challenge with virulent B. anthracis spores in animal models . Protection can be transferred with serum from animals vaccinated with protective antigen alone , suggesting that antibodies are the main mechanism of vaccine-induced immunity. Antibodies to PA prevent it from binding to its cellular receptor and prevent lethal factor and edema factor from binding to PA, which would normally enable their action.
Vaccination prior to exposure to B. anthracis provides a high level of protection against the disease. Available data suggests that the best method for preventing inhalational anthrax after inhalation of spores is prolonged antibiotic therapy in conjunction with vaccination because anthrax spores have been shown to persist in a host for long periods of time and, in the absence of immunity, may germinate once antibiotics are discontinued. It has been shown as well that post-exposure treatment with antibiotics does not confer protective immunity, leaving victims susceptible to repeat exposures. In September 2004, the CDC revised its recommendations to include 60 days of selected oral antibiotics in conjunction with anthrax vaccination to persons potentially exposed to aerosolized B. anthracis spores .
The only U.S. licensed vaccine against anthrax is AVA (BioThrax®), which has been FDA-approved since 1970. AVA consists of aluminum hydroxide-adsorbed supernatant material (which includes PA), from synthetic medium fermentor cultures of a nonencapsulated, toxinogenic strain of B. anthracis (V770-NP1-R, derived from the Sterne strain). AVA is stabilized with formaldehyde and preserved with benzethonium chloride. The approved regimen requires six doses over an 18-month period, administered subcutaneously. Specifically, AVA is administered in 0.5 ml doses at 0, 2, and 4 weeks, then at 6, 12, and 18 months, followed by yearly boosters. However, a recent study found that a schedule omitting the 2-week dose (i.e. administered at 0 and 4 weeks) induced an equivalent anti-PA antibody response at 6 and 24 weeks when compared with the standard dosing regimen .
AVA’s dosing schedule is an impediment to an agile response in the aftermath of a bioterrorist attack. In addition, the vaccine is perceived as having a high degree of reactogenicity among its users, and its safety has been called into question. However, a review by the Institute of Medicine concluded that the vaccine is not substantially more reactogenic than many other licensed vaccines .
There are no human efficacy data for this vaccine. Its development was based on the results of clinical trials conducted in the 1950s with a similar vaccine, which consisted of alum-precipitated supernatant material from cultures of the virulent B. anthracis Vollum R1-NP strain, which conferred a significant degree of protection (92.5% protection for both cutaneous and inhalational anthrax cases) in U.S. goat hair mill workers in 1955-58.
In 1998, members of the U.S. Armed Forces began to receive the AVA given heightened concerns about the possible use of B. anthracis spores as a biological weapon. Shortly after the anthrax attacks of 2001, adequate supplies of AVA were problematical because of production problems. Production of AVA is laborious and undertaken with older vaccine technology, yielding uncertain quantities of PA.
The rPA102 vaccine candidate has been under development for more than a decade by the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID). The U.S. government renewed its interest in developing an anthrax vaccine with a more practical dosing schedule (and, preferably, an improved safety profile) in the aftermath of the 2001 terrorist attacks. rPA anthrax vaccine candidate is alum-adjuvanted and produced through recombinant technology, yielding consistent quantities of rPA derived from an asporogenic mutant of the B. anthracis delta-Stern-1 (pPA102) strain. Its efficacy and safety have been documented in numerous animal studies. The rPA vaccine candidate works by inducing antibodies to PA, which neutralize this central anthrax toxin.
A phase I clinical trial of rPA102 was undertaken in 2003 to compare the safety and immunogenicity of AVA with varying doses of rPA102. In the study, subjects received one of four different rPA102 formulations (containing 5, 25, 50 or 75 ug rPA with 82.5 ug aluminum adjuvant), or AVA (containing 830 ug of aluminum adjuvant). There were 20 subjects in each group; each was immunized intramuscularly at 4 week intervals. Results indicated that rPA102 at the higher doses (50 ug and 75 ug) elicited an immune response comparable to that of AVA, even though AVA contained more than 10 times the amount of adjuvant . The study also demonstrated a dose-response relationship in all rPA 102 treatment groups. Local reactions (mostly arm pain) occurred in 34.4% of those who received rPA and in 89.5% of those who received AVA. However, systemic reactions (mostly headache and fatigue) occurred in 39% of those who received rPA and in 18% of those who received AVA. There were no serious adverse effects in any of the groups.
Phase II clinical trials among 480 healthy volunteers to determine the optimal combination of rPA and the adjuvant required to induce the most robust immune response are underway. An additional phase II clinical study to determine the optimal dosing schedule for the anthrax vaccine candidate is also planned.
The new rPA 102 vaccine candidate appears to be as immunologically effective as AVA, the current licensed anthrax vaccine in the U.S. and has several advantages, including ease of manufacture, lot consistency with a predictable rPA content, simpler dosing schedule, and a perceived improved safety profile. Future efforts should focus on developing a vaccine with a very prolonged shelf-life, an even simpler dosing regimen, and an alternative delivery method (such as oral or transdermal routes) that would facilitate immunization of large numbers of people in the event of a bioterrorist attack.
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