Coadministration of genetically attenuated Plasmodium falciparum *mei2 (GA2) sporozoites with adjuvants - a proof of principle study

Despite initial successes, the efficacy of whole-sporozoite malaria vaccines in
endemic areas is still limited. Immune hyporesponsiveness in endemic
populations may be responsible for the somewhat disappointing results.
Co-administration of adjuvants may improve the immunogenicity of
whole-sporozoite vaccines. However, given the unconventional method of
immunization, whereby the immunological effector mechanisms require antibody as
well as T cell responses occurring in the skin and in the liver induced by
migrating whole parasites, it remains unclear if co-administered adjuvants will
provide substantial advantage. Because animal models do not sufficiently
replicate immune responses to live sporozoites in humans, this trial will be
investigating the added value of co-administration of adjuvants with Plasmodium
falciparum sporozoites by mosquito bites in a limited number of healthy
volunteers. For this proof-of-concept study the Genetically Attenuated Parasite
2 (GA2 parasite) will be used, a modified Plasmodium falciparum strain that
lacks the mei2 gene, causing it to arrest its development in the late
liver-stage. GA2 thus enables full exposure to the liver stage antigens, whilst
being safe to administer. The GA2 sporozoites will be co-administered with
three products known to activate the immune response and are licensed for human
use. Baccilus Calmette-Guérin (BCG) will target macrophages, a cell type which
has also been shown to phagocytose sporozoites. Yellow fever vaccine strain 17D
(YF-17D) is known for its activation of CD8+ T-cell responses potentially
including the liver, an important effector organism. Imiquimod is a toll-like
receptor ligand known to enhance responses of skin-administered vaccines. We
will investigate whether adding these adjuvants to the GA2 sporozoites may
increase their immunogenicity.

This study has been transitioned to CTIS with ID 2024-516488-99-00 check the CTIS register for the current data.

Primary objectives:
1. To assess the effect of experimental immunization with GA2 sporozoites by
mosquito bite with and without co-administration of different adjuvants on the
pre-patent period after controlled human malaria infection (CHMI).
2. To assess the proportion of participants that acquire sterile protection
against CHMI after single immunization with GA2 sporozoites with and without
co-administration with different adjuvants.
3. To analyze the safety and tolerability of co-administration of GA2
sporozoites with different adjuvants.

Secondary objectives:
1. To compare immune responses after co-administration of GA2 sporozoites with
different adjuvants.

This study will be an adaptive single center, randomized controlled clinical
proof-of-principle trial of GA2 parasites co-administered with adjuvants in 65
healthy, BCG-, YF-17D-, and malaria-naïve male and female participants.
There will be three to four stages in the trial. In stage A, a single
immunization with 50 GA2-infected mosquito bites without co-administration of
adjuvants will be assessed. This stage will consist of two groups:
- Ten participants will receive unadjuvanted immunization with GA2-infected
mosquitoes.
- Five participants (infectivity controls) will be mock immunized with
uninfected mosquitoes.
To assess the added value of adjuvants on the immunizing effects of a single
GA2 immunization, a protective efficacy <=70% must be found in stage A to
progress in stage B. If in stage A the protective efficacy after a single GA2
immunization with 50 mosquito bites is >70%, then we will first progress to
stage A2 with a lower *dose* of GA2 immunization by 10 mosquito bites.
In stage A2, a single immunization with 10 GA2 infected mosquito bites without
co-administration of adjuvants will be assessed. This stage will consist of
three groups:
- Ten participants will receive unadjuvanted immunization with 10 GA2-infected
mosquitoes.
- Five participants will receive unadjuvanted immunization with 50 GA2 infected
mosquitoes. In this group skin biopsies will be done as well to assess innate
skin responses which may orchestrate the findings of high protective efficacy
found in stage A. To have a negative control skin biopsy to compare with, the
participants in this group will also be bitten by 50 uninfected mosquitoes in
the other arm. Skin biopsies of both arms will be taken two days
post-immunization. Participants need to give their consent if they do or do
not want to participate in the skin biopsy group. So this group is not
randomized.
- Five participants (infectivity controls) will be mock immunized with 10
uninfected mosquitoes.

Results from stage A and optional stage A2 will be reviewed by the safety
monitoring committee (SMC). Continuation to stage B will be discussed and
criteria to transition and continue from stage B to C will be designed in
accordance with SMC and submitted to the CCMO. If in stage A2 the protective
efficacy of a single immunization with 10 GA2 infected mosquito bites is again
>70% then there will be no progression to stage B and the study will be stopped.

In stage B of the trial, the co-administration with the three adjuvants will be
assessed. This stage will consist of four groups:
• Five participants will receive immunization with GA2-infected mosquitoes
co-administered with a BCG-vaccination.
• Five participants will receive immunization with GA2-infected mosquitoes
co-administered with an intradermal YF-17D vaccination.
• Five participants will receive immunization with GA2-infected mosquitoes
co-administered imiquimod cream.
• Two infectivity controls will be mock immunized with uninfected mosquitoes.

Based on results of stage B, the most favorable adjuvant can be selected for
further assessment in stage C in consultation with the SMC.
The choice of the adjuvant and whether it is useful to continue with stage C
will be made by the PI after consulting the safety monitoring committee (SMC),
and will be based on results on tolerability, time to patency and immunology.
Criteria for continuation with stage C will be based on findings in stage B and
will be determined before starting stage C. Stage C will consist of three
groups:
• Five additional participants will receive immunization with GA2-infected
mosquitoes co-administered with the adjuvant.
• Five participants will receive unadjuvanted immunization with GA2-infected
mosquitoes.
• Three infectivity controls will be mock immunized with uninfected mosquitoes.

Due to different methods of administration (injection for BCG/YF-17D and
topical application for imiquimod) and due to specific side-effects (e.g.
BCG-ulcer after BCG vaccination), adjuvanted groups and the skin biopsy group
will not be blinded. Unadjuvanted groups and infectivity controls, however,
will be blinded to both participants and clinical study staff.

After immunization, participants will have visits for collection of immunology
samples, safety labs and adverse events (AEs) on day 2, 6, 9 and 14 after
immunization. Six weeks after the immunization, all immunized participants and
infectivity controls will undergo CHMI trough the bites of 5 mosquitos infected
with wild-type Plasmodium falciparum 3D7 sporozoites.
On day 6 until day 21 and on day 28 after infection, participants will be
followed on an out-patient basis to determine AEs and parasite loads detected
by qPCR. As soon as blood stage parasitemia is detected (cut-off >100p/mL) or
at the latest 28 days after CHMI participants will be treated with a curative
regimen of antimalarials (atovaquone/proguanil or alternatively
artemether/lumefantrine) dosed to local hospital guidelines. End of follow-up
will be 6 months after CHMI.

Immunization with the bites of 10 or 50 GA2 infected mosquitos with and without
co-administration of an adjuvant (BCG, YF-17D or imiquimod), followed by a CHMI
after 42 days.

Risks for participants in the studie are related to:
• Potential breakthrough blood-stage infection after exposure to the GA2
parasite (risk deemed very low since no breakthrough blood-stage infections
occurred in previous GA2 trial);
• Systemic adverse events related to exposure to the GA2 parasite and/or the
BCG, YF-17D vaccination and imiquimod;
• Local adverse events related to BCG- YF-17D-vaccination, skin biopsy,
mosquito bites or imiquimod;
• Potential blood-stage infection after CHMI;
• Side effects of antimalarial treatment.
Burden: participants will be exposed to the bites of mosquitoes infected with
GA2 parasites. After the exposure to the GA2, there will be four out-patient
visits.
Subsequently, participant will undergo CHMI and there will be a 28-day-period
of close monitoring with frequent ambulatory visits and blood examinations. In
addition, physical examinations will be performed when clinically indicated and
participants will be asked to complete a diary of adverse events on a daily
basis. When testing positive for malaria, volunteers will be treated with
antimalarials After this period, all participants not yet treated with
antimalarials will have to take antimalarial treatment (when not already
treatment in case of parasitemia after CHMI).
Participants will have to visit the trial center on approximately 30 occasions,
the maximum cumulative amount of blood collected per 4 months will be 500 mL
for each participant.
Group relatedness: in the immunization phase, participant burden is expected to
be equal among all groups apart from known side effects to BCG, YF-17D,
imiquimod and skin biopsies. After CHMI, all infectivity controls are expected
to develop malaria. In other groups, an unknown fraction of the group may not
develop malaria.