Evidence of blood stage efficacy with a virosomal malaria vaccine in a phase IIa clinical trial

Abstract

Background: Previous research indicates that a combination vaccine targeting different stages of the malaria life cycle is likely to provide the most effective malaria vaccine. This trial was the first to combine two existing vaccination strategies to produce a vaccine that induces immune responses to both the pre-erythrocytic and blood stages of the P. falciparum life cycle.

Methods: This was a Phase I/IIa study of a new combination malaria vaccine FFM ME-TRAP+PEV3A. PEV3A includes peptides from both the pre-erythrocytic circumsporozoite protein and the blood-stage antigen AMA-1. This study was conducted at the Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, UK. The participants were healthy, malaria naïve volunteers, from Oxford. The interventions were vaccination with PEV3A alone, or PEV3A+FFM ME-TRAP. The main outcome measure was protection from malaria in a sporozoite challenge model. Other outcomes included measures of parasite specific immune responses induced by either vaccine; and safety, assessed by collection of adverse event data.

Results: We observed evidence of blood stage immunity in PEV3A vaccinated volunteers, but no volunteers were completely protected from malaria. PEV3A induced high antibody titres, and antibodies bound parasites in immunofluorescence assays. Moreover, we observed boosting of the vaccine-induced immune response by sporozoite challenge. Immune responses induced by FFM ME-TRAP were unexpectedly low. The vaccines were safe, with comparable side effect profiles to previous trials. Although there was no sterile protection two major observations support an effect of the vaccine-induced response on blood stage parasites: (i) Lower rates of parasite growth were observed in volunteers vaccinated with PEV3A compared to unvaccinated controls (p = 0.012), and this was reflected in the PCR results from PEV3A vaccinated volunteers. These showed early control of parasitaemia by some volunteers in this group. One volunteer, who received PEV3A alone, was diagnosed very late, on day 20 compared to an average of 11.8 days in unvaccinated controls. (ii). Morphologically abnormal parasites were present in the blood of all (n = 24) PEV3A vaccinated volunteers, and in only 2/6 controls (p = 0.001). We describe evidence of vaccine-induced blood stage efficacy for the first time in a sporozoite challenge study.

Trial registration: ClinicalTrials.gov NCT00408668.

Conflict of interest statement

Competing Interests: Rinaldo Zurbriggen is Chief Scientific Officer and a member of the Executive Board of Pevion Biotech Ltd., which provided the vaccine.

Figures

Figure 1. Study Design.

P = Vaccination with PEV3A, F = Vaccination with FP9 ME-TRAP, M = Vaccination with MVA ME-TRAP, C = Sporozoite challenge

Figure 2. CONSORT Flow Chart

Figure 3. Anti-peptide responses.

A. (Upper panel) Geometric mean anti-AMA49-C1 (PEV301) and anti UK-39 (PEV302) endpoint-titres (log10) with 95% confidence intervals for group 1 (PEV3A), group 2 (PEV3A+ME-TRAP) and unvaccinated controls. B. (Lower panel) Mean avidity increase (relative to avidity index after first immunisation) with 95% confidence intervals. Arrows along the x axis represent the timing of each vaccination, and the asterisk denotes the challenge.

Figure 4. Antibody reactivity with P. falciparum.

A. (Upper panel) IgG endpoint titres measured in IFA with P. falciparum (K1) blood stage parasites. B. (Lower panel) IgG endpoint titres measured in IFA with P. falciparum (NF54) sporozoites. Sera were tested before vaccination, after every vaccination and after sporozoite challenge. Individual titres and the geometric mean for every time point are shown on a log scale.

Figure 5. Number of infected hepatocytes.

Estimated numbers of infected hepatocytes for individual volunteers are shown. Lines represent the geometric mean for each group with IQ ranges.

Figure 6. Kaplan Meier Survival curve.

A Kaplan Meier plot showing time to diagnosis of malaria for each group of volunteers after the challenge.

Figure 7. PCR data.

Number of parasites per mL, estimated using a calibration curve, plotted by day post sporozoite challenge. A. Three volunteers selected to demonstrate expected pattern of PCR results: an exponential increase in the numbers of parasites over time, with some cycling seen as parasites are sequestered and released. B. This figure shows data from all 4 volunteers who had unusual PCR results. Up to 150 parasites/mL are detected, and are subsequently undetectable for one or more time points, before they are measured again. This pattern occurs several times, up to a maximum of 5 times in volunteer N525.

Figure 8. Growth rates, mean and IQ ranges, by group.

Growth rate (parasites per mL per cycle) for each individual is shown; the lines represent the group means and inter quartile ranges.

Figure 9. Normal parasites and crisis forms.

Crisis forms differ from normal parasites in their slightly more ‘ragged’ morphology and altered staining of the nuclear material-live parasites have nuclei that stain red/blue, these abnormal parasites lose all red chromatin staining. Magnified high power images are shown in A&B. Normal parasites are seen in Figure 9A, whilst 9B shows a film containing a crisis form. The remaining films are unmagnified and white blood cell nuclei are clearly visible in blue, with smaller headphone shaped parasites. Figure 9C & D show single crisis forms, whilst 9E & F show multiple normal parasites, with the characteristic red/blue nuclear staining.