Phase IIa clinical trial immunogenicity and efficacy results of potential Plasmodium falciparum malaria vaccine R21 mixed with adjuvant Matrix M (MM), were presented at the 2018 European Congress of Clinical Microbiology and Infectious Diseases (ECCMID).
Encouraging immunogenicity results comparable with standard 50μg Mosquirix vaccination in the UK group and testing in Burkina Faso also showed a similar immunological response to R21. The R21 with MM vaccine given in a dosing regimen of 10μg at Weeks 0, 4 and 8 demonstrated the highest protective efficacy, 82%, when challenged with a controlled human malaria infection (CHMI). Given the existing efficacy drawbacks to Mosquirix and these encouraging early-stage data, GlobalData views R21 as a promising pipeline vaccine.
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Currently, there are no WHO-licensed vaccines for the prevention of malaria. However, the most promising malaria vaccine candidate is GlaxoSmithKline’s (GSK’s) Mosquirix, which is a subunit vaccine consisting of P. falciparum circumsporozoite proteins (CSP) fused with hepatitis B surface antigen (HBsAg) which forms  virus-like particles (VLPs), and is co-formulated with a proprietary chemical adjuvant, AS01. R21 is based on the same CSP-HBsAg fusion protein; however, it lacks the excess HBsAg found in Mosquirix because the protein is produced by a yeast species Pichia Pastoris. In this trial, R21 was combined with Novovax’s saponin-based adjuvant MM.
The efficacy of R21 was investigated in the UK in malaria-naïve patients, four study groups containing a total of 37 participants. The groups were split into dosing regimens of 10/10/10μg, 10/10/2μg, 10/10μg of R21/MM and an unvaccinated control group. Each group were then challenged with CHMI delivered through a mosquito bite, they were then followed-up on through an outpatient protocol for three weeks. The 10/10/10μg regimen, given at Weeks 0, 4 and 8, demonstrated the highest efficacy, with 82% of participants in this group not diagnosed with malaria within three weeks post-challenge.
A follow-up study was also conducted with five of the participants from the 10/10/10μg regimen group agreeing to be re-challenged with CHMI, in an investigation into the longer term efficacy of R21/MM. These participants each received a second CHMI challenge 8.5 months after receiving the last vaccination dose; this was conducted alongside two volunteers from an earlier Phase I trial receiving the CHMI challenge 18 months post-vaccination. Three of the five volunteers from the 10/10/10 group remained malaria-free following the second challenge, raising hopes that longer-term efficacy is achievable. Neither volunteer from the 18 month post vaccination group were protected but did still show an immunological response.
The short-term efficacy results of this trial are encouraging; however, because of the small study size and signs of ineffective CHMI challenge from the control group they should be viewed cautiously. GlobalData believes that a significantly larger Phase IIb study conducted in a malaria endemic region is required before R21/MM can be considered a potential successor to Mosquirix.
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