First-in-human, Randomized, Double-blind Clinical Trial of Differentially Adjuvanted PAMVAC, A Vaccine Candidate to Prevent Pregnancy-associated Malaria

Abstract

Background: Malaria in pregnancy has major impacts on mother and child health. To complement existing interventions, such as intermittent preventive treatment and use of impregnated bed nets, we developed a malaria vaccine candidate with the aim of reducing sequestration of asexual "blood-stage" parasites in the placenta, the major virulence mechanism.

Methods: The vaccine candidate PAMVAC is based on a recombinant fragment of VAR2CSA, the Plasmodium falciparum protein responsible for binding to the placenta via chondroitin sulfate A (CSA). Healthy, adult malaria-naive volunteers were immunized with 3 intramuscular injections of 20 μg (n = 9) or 50 μg (n = 27) PAMVAC, adjuvanted with Alhydrogel or glucopyranosyl lipid adjuvant in stable emulsion (GLA-SE) or in a liposomal formulation with QS21 (GLA-LSQ). Allocation was random and double blind. The vaccine was given every 4 weeks. Volunteers were observed for 6 months following last immunization.

Results: All PAMVAC formulations were safe and well tolerated. A total of 262 adverse events (AEs) occurred, 94 (10 grade 2 and 2 grade 3) at least possibly related to the vaccine. No serious AEs occurred. Distribution and severity of AEs were similar in all arms. PAMVAC was immunogenic in all participants. PAMVAC-specific antibody levels were highest with PAMVAC-GLA-SE. The antibodies inhibited binding of VAR2CSA expressing P. falciparum-infected erythrocytes to CSA in a standardized functional assay.

Conclusions: PAMVAC formulated with Alhydrogel or GLA-based adjuvants was safe, well tolerated, and induced functionally active antibodies. Next, PAMVAC will be assessed in women before first pregnancies in an endemic area.

Clinical trials registration: EudraCT 2015-001827-21; ClinicalTrials.gov NCT02647489.

Keywords: VAR2CSA; first-in-human; malaria vaccine; phase 1 clinical trial; pregnancy-associated malaria.

Figure 1.

Study flow chart. Abbreviations: GLA-LSQ, glucopyranosyl lipid adjuvant containing QS21/saponin; GLA-SE, glucopyranosyl lipid adjuvant in stable emulsion; PAMVAC, pregnancy-associated malaria vaccine.

Figure 2.

Pregnancy-associated malaria vaccine (PAMVAC) immunoglobulin G (IgG) over time. Enzyme-linked immunosorbent assay was used to measure IgG antibody reactivity generated against PAMVAC. The y-axis shows the area under the curve (AUC) calculated for each individual at day 7, 28, 35, 56, 63, 84, and 252. The AUC unit was derived from recording the optical density values in plasma after 11 two-fold dilutions (from 1/100 to 1/102 400), yielding a titration curve under which the area was calculated. Shown are the AUC values for individuals who received the 20-µg dose (upper panel) and 50-µg dose (lower panel) according to adjuvant and days after first vaccination. Each individual was vaccinated at time 0, 28, and 56 days (vertical dotted lines). Abbreviations: GLA-LSQ, glucopyranosyl lipid adjuvant containing QS21/saponin; GLA-SE, glucopyranosyl lipid adjuvant in stable emulsion.

Figure 3.

Functional activity of induced antibodies. The binding inhibitory activity of plasma against erythrocytes infected with VAR2CSA expressing FCR3 parasites binding to chondroitin sulfate A (CSA). Samples from individuals enrolled in the study, a pool of plasma from naive Danes, and a pool of plasma from multiparous women living in a highly endemic area in Benin. The plasma was tested at day -1 at a dilution of 1:10 and at day 84 at a 1:20 dilution. Briefly, CSA was coated at the bottom of 96 well plates. Infected erythrocytes were added along with plasma. The plates were washed using a washing robot (Biomek 2000). Values shown are the percentage inhibition according to binding in wells without plasma after subtraction of background binding values in wells without CSA. The upper and lower horizontal lines indicate the positive and negative control plasma samples. All samples were run in triplicate. The 50-µg group is indicated by triangles; 20-µg group is indicated by circles. See also Supplementary Figure S2. Abbreviations: GLA-LSQ, glucopyranosyl lipid adjuvant containing QS21/saponin; GLA-SE, glucopyranosyl lipid adjuvant in stable emulsion.

Figure 4.

Correlation matrix of immunoglobulin G (IgG) reactivity between VAR2CSA variants. Alhydrogel (A), glucopyranosyl lipid adjuvant containing QS21/saponin (B), and glucopyranosyl lipid adjuvant in stable emulsion (GLA-SE) (C). The shading is proportional to the level of cross-reactivity measured by correlating the reactivity of IgG in plasma from individuals in each arm of the study. Numbers indicated according to the shading are r²-values, denoting how close the data are to the fitted regression line. Note the overall higher correlation in the GLA-SE adjuvanted group. See also Supplementary Figure S4.