A randomized first-in-human phase I trial of differentially adjuvanted Pfs48/45 malaria vaccines in Burkinabé adults

Abstract

BACKGROUNDMalaria transmission-blocking vaccines aim to interrupt the transmission of malaria from one person to another.METHODSThe candidates R0.6C and ProC6C share the 6C domain of the Plasmodium falciparum sexual-stage antigen Pfs48/45. R0.6C utilizes the glutamate-rich protein (GLURP) as a carrier, and ProC6C includes a second domain (Pfs230-Pro) and a short 36-amino acid circumsporozoite protein (CSP) sequence. Healthy adults (n = 125) from a malaria-endemic area of Burkina Faso were immunized with 3 intramuscular injections, 4 weeks apart, of 30 μg or 100 μg R0.6C or ProC6C each adsorbed to Alhydrogel (AlOH) adjuvant alone or in combination with Matrix-M (15 μg or 50 μg, respectively). The allocation was random and double-blind for this phase I trial.RESULTSThe vaccines were safe and well tolerated with no vaccine-related serious adverse events. A total of 7 adverse events, mild to moderate in intensity and considered possibly related to the study vaccines, were recorded. Vaccine-specific antibodies were highest in volunteers immunized with 100 μg ProC6C-AlOH with Matrix-M, and 13 of 20 (65%) individuals in the group showed greater than 80% transmission-reducing activity (TRA) when evaluated in the standard membrane feeding assay at 15 mg/mL IgG. In contrast, R0.6C induced sporadic TRA.CONCLUSIONAll formulations were safe and well tolerated in a malaria-endemic area of Africa in healthy adults. The ProC6C-AlOH/Matrix-M vaccine elicited the highest levels of functional antibodies, meriting further investigation.TRIAL REGISTRATIONPan-African Clinical Trials Registry (https://pactr.samrc.ac.za) PACTR202201848463189.FUNDINGThe study was funded by the European and Developing Countries Clinical Trials Partnership (grant RIA2018SV-2311).

Keywords: Infectious disease; Malaria; Vaccines.

Figure 1. Study flow.

Overview of trial flow for cohort 1 (low dose) and cohort 2 (high-dose) first-in-human trial of R0.C6-AlOH/MM and ProC6C-AlOH/MM. A total of 207 participants were screened. A total of 9 individuals did not complete the study, of whom 2 individuals withdrew their consent (groups 2A, 2C) and received only 1 and 2 vaccinations, respectively. An additional 2 migrated out the study area (groups 1C, 2B), and 4 individuals were reported as lost to follow-up (groups 2A, 2C, 2E, 2E). One individual (group 2B) tested positive for pregnancy and did not receive the third vaccine dose but has been followed up to the end of the study for safety reasons.

Figure 2. Vaccine-induced immunogenicity (cohort 2).

The immunogenicity to the vaccine immunogens (R0.6C or ProC6C) was evaluated at each study time point (D0, D14, D42, D70, D140, and D180). Groups containing the AlOH adjuvant alone (geometric mean, red line) and AlOH/MM adjuvant (geometric mean, blue line) are plotted independently. Cohort 2 volunteers received 100 μg protein. The control group received Euvax B vaccine (Hep B, G2E) and was plotted to each vaccine immunogen (geometric mean, black line). Individual volunteers are plotted by gray lines for each group. GMT and fold increase/decrease are indicated in Tables 5 and 6, respectively. Vaccine-induced immunogenicity for cohort 1 (low dose) is provided in Supplemental Figure 2.

Figure 3. IgG levels against the vaccine constituent antigens.

IgG antibody levels against Pfs48/45-6C (A), GLURP-R0 (B), Pfs230-Pro (C), and CSP (D) at baseline (D0) and 14 days after last vaccination (D70). Each symbol represents a sample; the horizontal line represents the geometric mean. Data are shown for populations receiving either R0.6C, ProC6C, or hepatitis vaccine as indicated. Antibody levels are given as TB31F equivalence (μg/mL) for Pfs48/45 IgG and in arbitrary units (AU) for GLURP-R0, CSP, and Pfs230-Pro IgG. ^aSignificantly higher than D0 by paired t test. ^bSignificantly lower than G2D (P < 0.0001) by Tukey’s multiple-comparison test. NS, not significant by 1-way ANOVA test. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001 by Tukey’s multiple-comparison test.

Figure 4. Biological activity of antibodies.

The biological activity (functionality) for each group is plotted for each volunteer at D70 from purified IgG at 15 mg/mL in the SMFA (with human complement) as transmission-reducing activity (TRA). Red symbols, groups receiving AlOH adjuvant alone; blue symbols, groups receiving AlOH/MM adjuvant. Control groups receiving Euvax B vaccine (Hep B, G1E and G2E) are combined and reported (gray circles). Cohort 1 (low dose, 30 μg protein) is indicated by open symbols and cohort 2 (high dose, 100 μg protein) by filled symbols. The median for each group is indicated by a line. DP, drug product (either R0.6C-AlOH or ProC6C-AlOH). Statistical significance is indicated among high-dose and control groups by Kruskal-Wallis test with a Dunn-Bonferroni adjustment for multiple comparisons; **P < 0.01, ***P < 0.001, ****P < 0.0001.

Figure 5. Biological and antibody correlation.

Purified IgG used above for analysis in the SMFA was also evaluated in ELISA to the Pfs48/45-6C. (A) Arbitrary units (AU) for 6C titers are reported for D70 IgGs using the Pfs48/45 mAb TB31F as a standard. Red symbols, groups receiving AlOH adjuvant alone; blue symbols, groups receiving AlOH/MM adjuvant. Cohort 1 (low dose, 30 μg protein) is indicated by open symbols and cohort 2 (high dose, 100 μg protein) by filled symbols. Control groups receiving Euvax B vaccine (Hep B, G1E and G2E) are combined and reported (gray circles). The median for each group is indicated by a line. Statistical significance is indicated between groups by Kruskal-Wallis test with a Dunn-Bonferroni adjustment for multiple comparisons; *P < 0.05, ****P < 0.0001. (B) The biological activity is plotted in log of mean oocyst ratio (LMR) between control and test IgGs (y axis) respective to square root (sqrt) of D70 antibody levels of Pfs48/45-6C (x axis). For ease of comprehension, the y axis shows corresponding percent TRA values instead of LMR. The R0.6C groups (all individuals) are plotted in the top panel and ProC6C groups (all individuals) in the bottom panel using the same symbols as in A. The Spearman’s rank P value and correlation coefficient (R) for all individuals in each panel are shown. Antibody correlation for Pfs230-Pro is provided in Supplemental Figure 5.

Figure 6. ProC6C-specific antibodies were depleted from pooled IgG.

From G2D (100 μg ProC6C-AlOH/MM) and Hep B (G2E) groups, 2 pooled IgGs per group were generated based on individual TRA level. G2D-hi pooled IgG contained individual IgGs that showed >95 TRA (n = 6), G2D-lo with <80 TRA (n = 5), Hep B–hi with >80 TRA (n = 2), and Hep B–lo with <50 TRA (n = 5). From the 4 original pooled IgGs, anti-ProC6C-specific antibodies were depleted. (A) Antibody titers (ELISA units) of the original (filled bars) and depleted (open bars) IgGs were determined by ELISA. “ND” indicates that ELISA units were too low to be determined for the depleted IgGs. (B) All IgGs were tested at 15 mg/mL by SMFA. Observed TRA (circles) and the 95% CI (error bars) are shown. The 95% CI and statistical significance (***P < 0.001) were determined by the assay-specific zero-inflated negative binomial model (19).