Safety and immunogenicity of a malaria vaccine, Plasmodium falciparum AMA-1/MSP-1 chimeric protein formulated in montanide ISA 720 in healthy adults

Abstract

Background: The P. falciparum chimeric protein 2.9 (PfCP-2.9) consisting of the sequences of MSP1-19 and AMA-1 (III) is a malaria vaccine candidate that was found to induce inhibitory antibodies in rabbits and monkeys. This was a phase I randomized, single-blind, placebo-controlled, dose-escalation study to evaluate the safety and immunogenicity of the PfCP-2.9 formulated with a novel adjuvant Montanide ISA720. Fifty-two subjects were randomly assigned to 4 dose groups of 10 participants, each receiving the test vaccine of 20, 50, 100, or 200 microg respectively, and 1 placebo group of 12 participants receiving the adjuvant only.

Methods and findings: The vaccine formulation was shown to be safe and well-tolerated, and none of the participants withdrew. The total incidence of local adverse events (AEs) was 75%, distributed among 58% of the placebo group and 80% of those vaccinated. Among the vaccinated, 65% had events that were mild and 15% experienced moderate AEs. Almost all systemic adverse reactions observed in this study were graded as mild and required no therapy. The participants receiving the test vaccine developed detectable antibody responses which were boosted by the repeated vaccinations. Sixty percent of the vaccinated participants had high ELISA titers (>1:10,000) of antigen-specific antibodies which could also recognize native parasite proteins in an immunofluorescence assay (IFA).

Conclusion: This study is the first clinical trial for this candidate and builds on previous investigations supporting PfCP-2.9/ISA720 as a promising blood-stage malaria vaccine. Results demonstrate safety, tolerability (particularly at the lower doses tested) and immunogenicity of the formulation. Further clinical development is ongoing to explore optimizing the dose and schedule of the formulation to decrease reactogenicity without compromising immunogenicity.

Trial registration: Chinese State Food and Drug Administration (SFDA) 2002SL0046; Controlled-Trials.com ISRCTN66850051 [66850051].

Figure 1. Study enrollment flow diagram.

Figure 2. Antibody response to the cultured parasite measured by IFA in sera from participants in groups receiving 20 µg, 50 µg, 100 µg and 200 µg of the test vaccine formulation and the placebo, respectively.

The IFA titers in sera collected from the days 30, 60, 80, 180, 194 and 240 were indicated by different symbols. The x-axis is the volunteer's consecutive number: No.1∼10 receiving 20 µg, No.11∼20 receiving 50 µg, No.21∼30 receiving 1000 µg, and No.31∼40 receiving 200 µg of the vaccine while No.41∼52 receiving the placebo.

Figure 3. Kinetics of geometric mean values of the IFA titers to the parasite for the groups receiving 20 µg(A, ▪), 50 µg(B,Δ), 100 µg(C, ×) and 200 µg(D, *) of the test vaccine formulation and the placbo(♦).

The value was shown at the time-point days 0, 30, 60, 90,180,194 and 240.

Figure 4. T-cell response to the PfCP-2.9 protein by Lymphocyte Proliferation Assay.

Peripheral blood mononuclear cells were separated from blood of participants receiving either the vaccine formulation (group A to D) or the placeb(group P) on the days 0, 30, 90 and 194 and tested for their proliferation. The results were expressed as the stimulation index (SI), SI = mean cpm of culture with the antigen/mean cpm of culture without the antigen.