Human studies have shown that immunization by the bite Plasmodium falciparum (Pf) sporozoite(SPZ)-infected mosquitoes under drug coverage with chloroquine, an approach called chemoprophylaxis with sporozoites (CPS) or infection treatment vaccination (ITV), can provide high level, long term protection against homologous controlled human malaria infection (CHMI). The Sanaria PfSPZ chemoprophylaxis vaccination (PfSPZ CVac) approach duplicates this with an injectable SPZ regimen. In both approaches, whether mosquitoes or syringes are used for SPZ administration, when chloroquine is used as the chemoprophylactic agent, transient, limited, asexual erythrocytic stage is seen in the majority of participants. Thus the question remains whether the significant protective efficacy seen can be achieved with pre-erythrocytic (sporozoite/liver stage) exposure only.

Previously, we performed a phase 1 study to investigate the safety, tolerability, immunogenicity, and protective efficacy of Sanaria PfSPZ CVac with chloroquine (sporozoites, liver, and blood stage) or pyrimethamine with chloroquine (sporozoites and liver stage only) to further describe stage specific sterile protection (NIAID protocol #15-I-0169). In this study, we demonstrated that pyrimethamine is safe to administer, well tolerated, and can prevent subpatent and patent parasitemia (qPCR and blood smear negative) 100% of the time during Sanaria PfSPZ CVac. The study also duplicated the results previously reported with Sanaria PfSPZ CVac with chloroquine in terms of safety profile and protective efficacy against homologous CHMI. Although a combination of Sanaria PfSPZ- CVac with pyrimethamine and PfSPZ Challenge at 51,200 PfSPZ did not provide a significant protection level against homologous CHMI, we demonstrated that some subjects did develop protective immunity without any evidence of blood stage exposure during PfSPZ-CVac.

Building on these results and taking the lessons learned from other pre-erythrocytic vaccine studies and models that have shown the importance of reaching a minimal antigen threshold required for the development of sterile immunity, this proposed study will assess the safety, tolerability, immunogenicity, and protective efficacy of increasing the dose of Sanaria PfSPZ Challenge sporozoites while receiving the same, or, if needed to successfully prevent parasitemia, a higher dose of pyrimethamine. Unlike the first study, however, pyrimethamine will be administered by itself as the partner drug, and will not be co-administered with chloroquine. The efficacy of PfSPZ-CVac with pyrimethamine will be assessed against CHMI with homologous parasites (Arm 2a) and CHMI with heterologous parasites (Arm 2b). Additionally, we will explore the impact of increasing the dose of Sanaria PfSPZ Challenge sporozoites while receiving chloroquine alone prophylaxis. The efficacy of PfSPZ-CVac with chloroquine will be assessed only against CHMI with heterologous parasites (Arm 3), as protection against homologous parasites has now been shown in two separate trials. It will thus be possible to compare the efficacy of the two partner drugs against heterologous CHMI. The results of this study will contribute to understanding the targets and mechanisms of immunity against Pf malaria infection and how the degree of exposure to the parasite (pre-erythrocytic or erythrocytic stage only or both) impacts these responses and subsequent protective efficacy.