Demonstration of the Blood-Stage Plasmodium falciparum Controlled Human Malaria Infection Model to Assess Efficacy of the P. falciparum Apical Membrane Antigen 1 Vaccine, FMP2.1/AS01

Abstract

Background: Models of controlled human malaria infection (CHMI) initiated by mosquito bite have been widely used to assess efficacy of preerythrocytic vaccine candidates in small proof-of-concept phase 2a clinical trials. Efficacy testing of blood-stage malaria parasite vaccines, however, has generally relied on larger-scale phase 2b field trials in malaria-endemic populations. We report the use of a blood-stage P. falciparum CHMI model to assess blood-stage vaccine candidates, using their impact on the parasite multiplication rate (PMR) as the primary efficacy end point.

Methods: Fifteen healthy United Kingdom adult volunteers were vaccinated with FMP2.1, a protein vaccine that is based on the 3D7 clone sequence of apical membrane antigen 1 (AMA1) and formulated in Adjuvant System 01 (AS01). Twelve vaccinees and 15 infectivity controls subsequently underwent blood-stage CHMI. Parasitemia was monitored by quantitative real-time polymerase chain reaction (PCR) analysis, and PMR was modeled from these data.

Results: FMP2.1/AS01 elicited anti-AMA1 T-cell and serum antibody responses. Analysis of purified immunoglobulin G showed functional growth inhibitory activity against P. falciparum in vitro. There were no vaccine- or CHMI-related safety concerns. All volunteers developed blood-stage parasitemia, with no impact of the vaccine on PMR.

Conclusions: FMP2.1/AS01 demonstrated no efficacy after blood-stage CHMI. However, the model induced highly reproducible infection in all volunteers and will accelerate proof-of-concept testing of future blood-stage vaccine candidates.

Clinical trials registration: NCT02044198.

Keywords: AMA1; CHMI; blood stage; malaria; vaccine.

Figure 1.

Blood-stage controlled human malaria infection and parasite multiplication rate (PMR) analysis. Individual quantitative polymerase chain reaction data are shown for the VAC054 phase 2a study, including 12 apical membrane antigen 1 (AMA1) vaccinees (group 1; A) and 15 unvaccinated infectivity controls (group 2; B). The lower limit of quantification is indicated by the dotted line at 20 parasites/mL. C, Primary end point analysis of PMRs, showing data for each individual plus the mean ± SD for each group. Both data sets are normally distributed (as determined by the D'Agostino–Pearson test), with similar variance (P = .74, by the F test).

Figure 2.

T-cell and antibody responses in vaccinees and controls. A and B, T-cell responses were assessed in each group by ex vivo interferon γ (IFN-γ) enzyme-linked immunospot analysis, using fresh peripheral blood mononuclear cells (PBMCs). C and D, Serum anti–apical membrane antigen 1 (AMA1; 3D7) immunoglobulin G (IgG) responses were assessed in Oxford for each group by an enzyme-linked immunosorbent assay. Mean and individual responses are shown over time. Blood-stage controlled human malaria infection took place on day 70. Abbreviation: SFU, spot-forming units.

Figure 3.

Assessment of functional growth inhibitory activity (GIA) induced by FMP2.1/Adjuvant System 01 vaccination. A, In vitro GIA of purified immunoglobulin G (IgG) was assessed at 10 mg/mL against 3D7 clone Plasmodium falciparum parasites at the National Institutes of Health (NIH) GIA Reference Center. Individual data and medians are shown for each group on the day before challenge (dC-1), as well as before vaccination (d0) for group 1. Responses >12% are typically regarded as positive for 3D7. B Dilution series of purified IgG from group 1 samples obtained on dC-1. C, Relationship between GIA and anti-3D7 AMA1 serum IgG concentrations, measured by enzyme-linked immunosorbent assay (ELISA) at the NIH. A nonlinear regression curve is also shown (n = 60). The level of anti-3D7 AMA1 response in this ELISA that gives 50% GIA (GIA₅₀), indicated by the dotted line, was 75.5 µg/mL, (95% confidence interval, 68.3–84.2). D, Individual half maximal effective concentration (EC₅₀) for each purified IgG is shown, as well as the GIA₅₀ titers. Individual data and medians are shown for group 1 at dC-1.

Figure 4.

Comparison of blood-stage and mosquito-bite controlled human malaria infection (CHMI). A, Parasite multiplication rates (PMRs) were calculated for infectivity control volunteers in 4 historical mosquito-bite CHMI trials (purple). Data are shown for each trial and are also pooled. Individual data points and mean ± SD are shown. B, Individual PMRs and 95% confidence intervals are shown.