Phase 1/2a study of the malaria vaccine candidate apical membrane antigen-1 (AMA-1) administered in adjuvant system AS01B or AS02A

Abstract

Background: This Phase 1/2a study evaluated the safety, immunogenicity, and efficacy of an experimental malaria vaccine comprised of the recombinant Plasmodium falciparum protein apical membrane antigen-1 (AMA-1) representing the 3D7 allele formulated with either the AS01B or AS02A Adjuvant Systems.

Methodology/principal findings: After a preliminary safety evaluation of low dose AMA-1/AS01B (10 microg/0.5 mL) in 5 adults, 30 malaria-naïve adults were randomly allocated to receive full dose (50 microg/0.5 mL) of AMA-1/AS01B (n = 15) or AMA-1/AS02A (n = 15), followed by a malaria challenge. All vaccinations were administered intramuscularly on a 0-, 1-, 2-month schedule. All volunteers experienced transient injection site erythema, swelling and pain. Two weeks post-third vaccination, anti-AMA-1 Geometric Mean Antibody Concentrations (GMCs) with 95% Confidence Intervals (CIs) were high: low dose AMA-1/AS01B 196 microg/mL (103-371 microg/mL), full dose AMA-1/AS01B 279 microg/mL (210-369 microg/mL) and full dose AMA-1/AS02A 216 microg/mL (169-276 microg/mL) with no significant difference among the 3 groups. The three vaccine formulations elicited equivalent functional antibody responses, as measured by growth inhibition assay (GIA), against homologous but not against heterologous (FVO) parasites as well as demonstrable interferon-gamma (IFN-gamma) responses. To assess efficacy, volunteers were challenged with P. falciparum-infected mosquitoes, and all became parasitemic, with no significant difference in the prepatent period by either light microscopy or quantitative polymerase chain reaction (qPCR). However, a small but significant reduction of parasitemia in the AMA-1/AS02A group was seen with a statistical model employing qPCR measurements.

Significance: All three vaccine formulations were found to be safe and highly immunogenic. These immune responses did not translate into significant vaccine efficacy in malaria-naïve adults employing a primary sporozoite challenge model, but encouragingly, estimation of parasite growth rates from qPCR data may suggest a partial biological effect of the vaccine. Further evaluation of the immunogenicity and efficacy of the AMA-1/AS02A formulation is ongoing in a malaria-experienced pediatric population in Mali.

Trial registration: www.clinicaltrials.gov NCT00385047.

Figure 1. Participant flow diagram.

Figure 2. Geometric mean concentration (GMC) of anti-AMA-1 antibody by ELISA.

Arrows indicate immunization time points and arrowhead indicates day of malaria challenge. Gray solid line with ▪ symbol: GMCs for low dose AMA-1/AS01B vaccinees; black solid line with ▴symbol: GMCs for high dose AMA-1/AS02A vaccinees; small black dashed line with • symbol: GMCs for high AMA-1/AS01B vaccinee;, small dashed gray line with ♦symbol: GMCs for infectivity controls. 95% CIs are shown for each time point.

Figure 3. Mean percent growth inhibition activity of 3D7 parasites by GIA.

(A) WRAIR 20% serum pLDH GIA (B) NIH pLDH GIA at 4 mg/mL purified immunoglobulin. Results expressed as mean percent inhibition with serum from Day 70 using GIA methods described in Outcomes section. Low dose AMA-1/AS01B (n = 5), full dose AMA-1 in AS01B or AS02A (n = 14 in each).

Figure 4. Comparison of IFN-γ ELISPOT results.

Results expressed as mean sfu/million PBMCs with standard error bars. Low dose AMA-1/AS01B (n = 5), full dose AMA-1 in AS01B or AS02A (n = 14 in each). For Day 156, includes only challenged volunteers. Recombinant protein AMA-1 concentration 1.0 µg/mL. Assay also run at 0.1 µg/mL and 10 µg/mL with similar results (data not shown).

Figure 5. Kaplan-Meier (K-M) Survival Curve for prepatent period.

Legend: — Full dose AMA-1/AS01B ^…… Full dose AMA-1/AS02A – – – Infectivity Controls. (A) Prepatent period by thick blood film. Mean prepatent periods: full dose AMA-1/AS01B 10 days (240 hours), full dose AMA-1/AS02A 10 days 21 hours (261 hours), infectivity controls 10 days (240 hours), K-M survival curve log rank 2.94, P = 0.23. (B) Prepatent period by qPCR. Mean prepatent periods: full dose AMA-1/AS01B 7 days 12 hours (180 hours), full dose AMA-1/AS02A 7 days 17 hours (185 hours), infectivity controls 7 days (168 hours), K-M survival curve log rank 3.25, P = 0.19.

Figure 6. In-vivo growth of blood stage parasites after P. falciparum challenge.

Observed parasite densities of individuals (dots) and predicted kinetics (as a group, thick continuous line) in the three immunization groups experimentally infected with Plasmodium falciparum (3D7). Dots represent observed number of parasites per milliliter of blood based on qPCR results. Individuals in the same group are represented in the same color (Infectivity Controls: black (n = 6), Full dose AMA-1/ASO1B: red (n = 6), Full dose AMA-1/ASO2A: blue (n = 10)). All data points represent pretreatment parasitemias.