Phase 1 clinical trial of apical membrane antigen 1: an asexual blood-stage vaccine for Plasmodium falciparum malaria

Apical membrane antigen 1 (AMA1), a polymorphic merozoite surface protein, is a leading blood-stage malaria vaccine candidate. A phase 1 trial was conducted with 30 malaria-naive volunteers to assess the safety and immunogenicity of the AMA1-C1 malaria vaccine. AMA1-C1 contains an equal mixture of recombinant proteins based on sequences from the FVO and 3D7 clones of Plasmodium falciparum. The proteins were expressed in Pichia pastoris and adsorbed on Alhydrogel. Ten volunteers in each of three dose groups (5 mug, 20 mug, and 80 mug) were vaccinated in an open-label study at 0, 28, and 180 days. The vaccine was well tolerated, with pain at the injection site being the most commonly observed reaction. Anti-AMA1 immunoglobulin G (IgG) was detected by enzyme-linked immunosorbent assay (ELISA) in 15/28 (54%) volunteers after the second immunization and in 23/25 (92%) after the third immunization, with equal reactivity to both AMA1-FVO and AMA1-3D7 vaccine components. A significant dose-response relationship between antigen dose and antibody response by ELISA was observed, and the antibodies were predominantly of the IgG1 isotype. Confocal microscopic evaluation of sera from vaccinated volunteers demonstrated reactivity with P. falciparum schizonts in a pattern similar to native parasite AMA1. Antigen-specific in vitro inhibition of both FVO and 3D7 parasites was achieved with IgG purified from sera of vaccinees, demonstrating biological activity of the antibodies. To our knowledge, this is the first AMA1 vaccine candidate to elicit functional immune responses in malaria-naive humans, and our results support the further development of this vaccine.

rDEN4delta30, a live attenuated dengue virus type 4 vaccine candidate, is safe, immunogenic, and highly infectious in healthy adult volunteers

BACKGROUND

The live attenuated dengue virus type 4 (DEN-4) vaccine candidate virus rDEN4 Delta 30 was previously found to be safe and immunogenic at a dose of 10(5) plaque-forming units (pfu).

METHODS

In a follow-up placebo-controlled phase 2 clinical trial, rDEN4 Delta 30 was administered as a single inoculation to 3 separate dose cohorts (10(3) pfu, 10(2) pfu, or 10(1) pfu), for further evaluation. Each dose cohort consisted of 20 vaccinees and 4 placebo recipients. Volunteers were monitored closely for adverse events, and serum was collected on study days 28 and 42 for determination of neutralizing antibody titer.

RESULTS

The vaccine was well tolerated at all doses studied. The most common adverse events observed were a transient asymptomatic rash in >50% of vaccinees and a mild neutropenia in approximately 20% of vaccinees. No vaccinee developed a dengue-like illness. The vaccine was highly infectious and immunogenic, with 95%-100% of vaccinees in each dose cohort developing a >/=4-fold increase in titers of serum neutralizing antibodies against DEN-4.

CONCLUSIONS

The rDEN4 Delta 30 vaccine is safe and induced an antibody response that was broadly neutralizing against genotypically diverse DEN-4 viruses. It is a promising vaccine candidate for inclusion in a tetravalent dengue vaccine formulation.

Attenuation and immunogenicity in humans of a live dengue virus type-4 vaccine candidate with a 30 nucleotide deletion in its 3'-untranslated region

The recombinant dengue virus type-4 vaccine candidate 2AA30 was attenuated in rhesus monkeys due to an engineered 30-nucleotide deletion in the 3'-untranslated region of the viral genome. A clinical trial to evaluate the safety and immunogenicity of a single dose of 2Adelta30 was conducted with 20 adult human volunteers. The vaccine candidate was well tolerated and did not cause systemic illness in any of the 20 volunteers. Viremia was detectable in 14 volunteers at a mean level of 1.6 log10 plaque-forming units/ml of serum, although all 20 volunteers seroconverted with a seven-fold or greater increase in serum neutralizing antibody titer on day 28 post-vaccination (mean titer = 1:580). A mild, asymptomatic, macular rash developed in 10 volunteers, and a transient elevation in the serum level of alanine aminotransferase was noted in five volunteers. The low level of reactogenicity and high degree of immunogenicity of this vaccine candidate warrant its further evaluation and its use to create chimeric vaccine viruses expressing the structural genes of dengue virus types 1, 2, and 3.