Safety and immunogenicity of BK-SE36 in a blinded, randomized, controlled, age de-escalating phase Ib clinical trial in Burkinabe children

Vaccines and monoclonal antibodies: new tools for malaria control

SUMMARYMalaria remains one of the biggest health problems in the world. While significant reductions in malaria morbidity and mortality had been achieved from 2000 to 2015, the favorable trend has stalled, rather significant increases in malaria cases are seen in multiple areas. In 2022, there were 249 million estimated cases, and 608,000 malaria-related deaths, mostly in infants and children aged under 5 years, globally. Therefore, in addition to the expansion of existing anti-malarial control measures, it is critical to develop new tools, such as vaccines and monoclonal antibodies (mAbs), to fight malaria. In the last 2 years, the first and second malaria vaccines, both targeting Plasmodium falciparum circumsporozoite proteins (PfCSP), have been recommended by the World Health Organization to prevent P. falciparum malaria in children living in moderate to high transmission areas. While the approval of the two malaria vaccines is a considerable milestone in vaccine development, they have much room for improvement in efficacy and durability. In addition to the two approved vaccines, recent clinical trials with mAbs against PfCSP, blood-stage vaccines against P. falciparum or P. vivax, and transmission-blocking vaccine or mAb against P. falciparum have shown promising results. This review summarizes the development of the anti-PfCSP vaccines and mAbs, and recent topics in the blood- and transmission-blocking-stage vaccine candidates and mAbs. We further discuss issues of the current vaccines and the directions for the development of next-generation vaccines.

Immune tolerance caused by repeated P. falciparum infection against SE36 malaria vaccine candidate antigen and the resulting limited polymorphism

The call for second generation malaria vaccines needs not only the identification of novel candidate antigens or adjuvants but also a better understanding of immune responses and the underlying protective processes. Plasmodium parasites have evolved a range of strategies to manipulate the host immune system to guarantee survival and establish parasitism. These immune evasion strategies hamper efforts to develop effective malaria vaccines. In the case of a malaria vaccine targeting the N-terminal domain of P. falciparum serine repeat antigen 5 (SE36), now in clinical trials, we observed reduced responsiveness (lowered immunogenicity) which may be attributed to immune tolerance/immune suppression. Here, immunogenicity data and insights into the immune responses to SE36 antigen from epidemiological studies and clinical trials are summarized. Documenting these observations is important to help identify gaps for SE36 continued development and engender hope that highly effective blood-stage/multi-stage vaccines can be achieved.

Malaria prevention in children: an update

PURPOSE OF REVIEW

Malaria cases and deaths decreased from 2000 to 2015 but remain increased since 2019. Several new developments and strategies could help reverse this trend. The purpose of this review is to discuss new World Health Organization (WHO) guidelines and recent research on malaria prevention in children.

RECENT FINDINGS

Fifteen countries have now rolled out seasonal malaria chemoprophylaxis (SMC) in children at highest risk for severe malaria, and new WHO recommendations provide more flexibility for SMC implementation in terms of target age groups, geographic region, and number of cycles. Recent studies confirm that malaria burden in school aged children, and their contribution to transmission, is high. New guidelines permit expanded chemoprevention options for these children. Two vaccines have been approved for use in malaria endemic countries, RTS,S/AS01 E and R21/Matrix-M. Additionally, pyrethroid-chlorfenapyr bed nets are being deployed to combat resistant mosquitoes.

SUMMARY

While challenges remain in malaria control towards elimination, new guidelines and recently approved vaccines offer hope. Monitoring for continued vaccine and chemoprevention effectiveness, and for possible epidemiologic shifts in severe malaria presentation and deaths as additional prevention efforts roll out will be paramount.

Persistence of Anti-SE36 Antibodies Induced by the Malaria Vaccine Candidate BK-SE36/CpG in 5-10-Year-Old Burkinabe Children Naturally Exposed to Malaria

Information on the dynamics and decline/persistence of antibody titres is important in vaccine development. A recent vaccine trial in malaria-exposed, healthy African adults and children living in a malaria hyperendemic and seasonal area (Ouagadougou, Burkina Faso) was the first study in which BK-SE36/CpG was administered to different age groups. In 5- to 10-year-old children, the risk of malaria infection was markedly lower in the BK-SE36/CpG arm compared to the control arm. We report here data on antibody titres measured in this age-group after the high malaria transmission season of 2021 (three years after the first vaccine dose was administered). At Year 3, 83% of children had detectable anti-SE36 total IgG antibodies. Geometric mean antibody titres and the proportion of children with detectable anti-SE36 antibodies were markedly higher in the BK-SE36/CpG arm than the control (rabies) arm. The information obtained in this study will guide investigators on future vaccine/booster schedules for this promising blood-stage malaria vaccine candidate.

The Need for Novel Asexual Blood-Stage Malaria Vaccine Candidates for Plasmodium falciparum

Extensive control efforts have significantly reduced malaria cases and deaths over the past two decades, but in recent years, coupled with the COVID-19 pandemic, success has stalled. The WHO has urged the implementation of a number of interventions, including vaccines. The modestly effective RTS,S/AS01 pre-erythrocytic vaccine has been recommended by the WHO for use in sub-Saharan Africa against Plasmodium falciparum in children residing in moderate to high malaria transmission regions. A second pre-erythrocytic vaccine, R21/Matrix-M, was also recommended by the WHO on 3 October 2023. However, the paucity and limitations of pre-erythrocytic vaccines highlight the need for asexual blood-stage malaria vaccines that prevent disease caused by blood-stage parasites. Few asexual blood-stage vaccine candidates have reached phase 2 clinical development, and the challenges in terms of their efficacy include antigen polymorphisms and low immunogenicity in humans. This review summarizes the history and progress of asexual blood-stage malaria vaccine development, highlighting the need for novel candidate vaccine antigens/molecules.

Safety and immunogenicity of BK-SE36/CpG malaria vaccine in healthy Burkinabe adults and children: a phase 1b randomised, controlled, double-blinded, age de-escalation trial

Background

BK-SE36/CpG is a recombinant blood-stage malaria vaccine candidate based on the N-terminal Plasmodium falciparum serine repeat antigen5 (SE36), adsorbed to aluminium hydroxide gel and reconstituted, prior to administration, with synthetic oligodeoxynucleotides bearing CpG motifs. In healthy Japanese adult males, BK-SE36/CpG was well tolerated. This study assessed its safety and immunogenicity in healthy malaria-exposed African adults and children.

Methods

A double-blind, randomised, controlled, age de-escalating clinical trial was conducted in an urban area of Ouagadougou, Burkina Faso. Healthy participants (n=135) aged 21-45 years (Cohort 1), 5-10 years (Cohort 2) and 12-24 months (Cohort 3) were randomised to receive three vaccine doses (Day 0, 28 and 112) of BK-SE36/CpG or rabies vaccine by intramuscular injection.

Results

One hundred thirty-four of 135 (99.2%) subjects received all three scheduled vaccine doses. Vaccinations were well tolerated with no related Grade 3 (severe) adverse events (AEs). Pain/limitation of limb movement, headache in adults and fever in younger children (all mild to moderate in intensity) were the most frequently observed local and systemic AEs. Eighty-three of BK-SE36/CpG (91%) recipients and 37 of control subjects (84%) had Grade 1/2 events within 28 days post vaccination. Events considered by the investigator to be vaccine related were experienced by 38% and 14% of subjects in BK-SE36/CpG and control arms, respectively. Throughout the trial, six Grade 3 events (in 4 subjects), not related to vaccination, were recorded in the BK-SE36/CpG arm: 5 events (in 3 subjects) within 28 days of vaccination. All serious adverse events (SAEs) (n=5) were due to severe malaria (52-226 days post vaccination) and not related to vaccination. In all cohorts, BK-SE36/CpG arm had higher antibody titres after Dose 3 than after Dose 2. Younger cohorts had stronger immune responses (12-24-month-old > 5-10 years-old > 21-45 years-old). Sera predominantly reacted to peptides that lie in intrinsically unstructured regions of SE36. In the control arm, there were no marked fold changes in antibody titres and participants' sera reacted poorly to all peptides spanning SE36.

Conclusion

BK-SE36/CpG was well-tolerated and immunogenic. These results pave the way for further proof-of-concept studies to demonstrate vaccine efficacy.

Clinical trial registration

https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=1921, PACTR201701001921166.

Malaria Vaccines: From the Past towards the mRNA Vaccine Era

Plasmodium spp. is the etiological agent of malaria, a life-threatening parasitic disease transmitted by infected mosquitoes. Malaria remains a major global health challenge, particularly in endemic regions. Over the years, various vaccine candidates targeting different stages of Plasmodium parasite life-cycle have been explored, including subunit vaccines, vectored vaccines, and whole organism vaccines with Mosquirix, a vaccine based on a recombinant protein, as the only currently approved vaccine for Plasmodium falciparum malaria. Despite the aforementioned notable progress, challenges such as antigenic diversity, limited efficacy, resistant parasites escaping protective immunity and the need for multiple doses have hindered the development of a highly efficacious malaria vaccine. The recent success of mRNA-based vaccines against SARS-CoV-2 has sparked renewed interest in mRNA vaccine platforms. The unique mRNA vaccine features, including their potential for rapid development, scalability, and flexibility in antigen design, make them a promising avenue for malaria vaccine development. This review provides an overview of the malaria vaccines' evolution from the past towards the mRNA vaccine era and highlights their advantages in overcoming the limitations of previous malaria vaccine candidates.

Plasmodium falciparum infection coinciding with the malaria vaccine candidate BK-SE36 administration interferes with the immune responses in Burkinabe children

Background

A vaccine targeting the erythrocyte stages of Plasmodium falciparum could play a role in preventing clinical disease. BK-SE36 is a promising malaria vaccine candidate that has shown a good safety profile and immunological responses during field evaluations. It was observed that repeated natural infections could result in immune tolerance against SE36 molecule.

Methods

The primary trial was conducted to assess the safety and immunogenicity of the BK-SE36 in two cohorts of children aged 25-60 months (Cohort 1) and 12-24 months (Cohort 2). Immunization was at full dose (1.0 mL) administered at 0, 1, and 6 months. Blood samples were collected before each vaccination for immunological assessments and detection of Plasmodium falciparum infection by microscopy. Blood samples were further collected one month post each vaccination to evaluate immunogenicity.

Results

Of seventy-two (72) subjects that have received BK-SE36 vaccination, 71 had available blood smears during vaccination days. One month post Dose 2, the geometric mean of SE36 antibodies was 263.2 (95% CI: 178.9-387.1) in uninfected individuals compared to 77.1 (95% CI: 47.3-125.7) in infected participants. The same trend was observed one-month post booster dose. Participants uninfected at the time of booster vaccination had significantly higher GMTs compared to those who were infected (424.1 (95% CI: 301.9-595.8) vs. 92.8 (95% CI: 34.9-246.6), p = 0.002. There was a 14.3 (95% CI: 9.7-21.1) and 2.4 (95% CI: 1.3-4.4) fold-change, respectively, in uninfected and infected participants between one-month post Dose 2 and booster. The difference was statistically significant (p < 0.001).

Conclusion

Concomitant infection by P. falciparum during BK-SE36 vaccine candidate administration is associated with reduced humoral responses. However, it is to be noted that the BK-SE36 primary trial was not designed to investigate the influence of concomitant infection on vaccine-induced immune response and should be interpreted cautiously.

Trial registration

WHO ICTRP, PACTR201411000934120.

African-specific polymorphisms in Plasmodium falciparum serine repeat antigen 5 in Uganda and Burkina Faso clinical samples do not interfere with antibody response to BK-SE36 vaccination

BK-SE36, based on Plasmodium falciparum serine repeat antigen 5 (SERA5), is a blood-stage malaria vaccine candidate currently being evaluated in clinical trials. Phase 1 trials in Uganda and Burkina Faso have demonstrated promising safety and immunogenicity profiles. However, the genetic diversity of sera5 in Africa and the role of allele/variant-specific immunity remain a major concern. Here, sequence analyses were done on 226 strains collected from the two clinical trial/follow-up studies and 88 strains from two cross-sectional studies in Africa. Compared to other highly polymorphic vaccine candidate antigens, polymorphisms in sera5 were largely confined to the repeat regions of the gene. Results also confirmed a SERA5 consensus sequence with African-specific polymorphisms. Mismatches with the vaccine-type SE36 (BK-SE36) in the octamer repeat, serine repeat, and flanking regions, and single-nucleotide polymorphisms in non-repeat regions could compromise vaccine response and efficacy. However, the haplotype diversity of SERA5 was similar between vaccinated and control participants. There was no marked bias or difference in the patterns of distribution of the SE36 haplotype and no statistically significant genetic differentiation among parasites infecting BK-SE36 vaccinees and controls. Results indicate that BK-SE36 does not elicit an allele-specific immune response.