Human studies have shown that immunization by the bite of Plasmodium falciparum (Pf) sporozoite (SPZ)-infected mosquitoes (chemoprophylaxis with sporozoites \[CPS\]) or by injecting aseptic, purified, cryopreserved sporozoites (SPZ) with needle and syringe (PfSPZ chemoprophylaxis vaccination, Sanaria PfSPZ-CVac) under drug coverage with chloroquine can provide \>80% protection against homologous controlled human malaria infection (CHMI) (Roestenberg, McCall et al. 2009, Roestenberg, Teirlinck et al. 2011, Mordmuller, Surat et al. 2017). The protective immunity induced by CPS/PfSPZ-CVac is thought to target sporozoite and liver stage antigens, but the transient parasitemia observed under chloroquine prophylaxis may additionally induce immune responses targeting blood stage parasites that contribute to protection. It is important to understand the contribution of each stage in this model as transient parasitemia in the immunization process adds a safety concern and may impair the development of immunity.

We have conducted two phase 1 studies to explore whether significant protective efficacy can be achieved with exclusively pre-erythrocytic (sporozoite/liver stage) exposure in this model using PYR chemoprophylaxis. This drug, unlike chloroquine, kills the parasites in the liver, so there is no period of transient parasitemia following immunization. These studies in malaria na(SqrRoot) ve adults demonstrated that PfSPZ-CVac (with PYR administered 2 \& 3 days after injection) is safe, well-tolerated, and can prevent subpatent and patent parasitemia after PfSPZ injection. The NCT02511054 study used the same dose (5.12x104 PfSPZ) that has resulted in \>80% protection in previous trials of PfSPZ-CVac (chloroquine), showed only 2/9 vaccinees were protected against a homologous (same strain of Pf as the vaccine) CHMI. In trials using irradiated SPZ, increasing the dose of PfSPZ used for immunization has resulted in higher protective efficacy. Therefore, in an ongoing trial, NCT03083847, we have increased the PfSPZ dose about 4-fold up to 2.0x105 PfSPZ, with preliminary results show markedly improved protection against both homologous and heterologous CHMI in healthy US adults (7/8 and 4/4 study subjects uninfected, respectively). This is the first demonstration that the PfSPZ-CVac approach can confer sterile protection without blood stage exposure.

Building on these promising results from PfSPZ-CVac (pyrimethamine) in the US, this proposed study in a malaria-endemic, adult population in Bancoumana, Mali will initially assess, in a pilot phase, the safety and tolerability of still higher doses of PfSPZ Challenge (4.0x105 PfSPZ) in combination with PYR (day 0;) or chloroquine (days -2, +5). The higher PfSPZ dose is hypothesized to be necessary to achieve significant efficacy against naturally transmitted malaria in malaria-experienced populations. Once this higher dose of PfSPZ is determined to be safe and tolerable and that earlier PYR dosing prevents patent parasitemia, the main phase will explore the effect of both earlier PYR dosing and a higher dosage of PfSPZ used for immunization in the development of protective immunity against natural malaria infection. The results of this study will contribute to understanding the targets and mechanisms of immunity against Pf malaria infection, and how the degree of prior exposure to the parasite (pre-erythrocytic and/or erythrocytic stages) impacts these responses and subsequent protective efficacy in field settings.

After completion of follow up in the main phase, all participants that are still enrolled in the study will be offered a booster dose of the vaccine (4th dose) with a single dose of 4x10\^5 PfSPZ Challenge or normal saline (aligned with what they received in the main phase of the study) at approximately 11 months post 3rd vaccination. The booster dose is timed to allow administration well before the beginning of the ensuing malaria transmission season. Participants will then be followed for approximately 6 months to assess safety and vaccine efficacy through this second transmission season.