A Randomized Trial Evaluating the Prophylactic Activity of DSM265 Against Preerythrocytic Plasmodium falciparum Infection During Controlled Human Malarial Infection by Mosquito Bites and Direct Venous Inoculation.

Background: DSM265 is a selective inhibitor of Plasmodium dihydroorotate dehydrogenase that fully protected against controlled human malarial infection (CHMI) by direct venous inoculation of Plasmodium falciparum sporozoites when administered 1 day before challenge and provided partial protection when administered 7 days before challenge. Methods: A double-blinded, randomized, placebo-controlled trial was performed to assess safety, tolerability, pharmacokinetics, and efficacy of 1 oral dose of 400 mg of DSM265 before CHMI. Three cohorts were studied, with DSM265 administered 3 or 7 days before direct venous inoculation of sporozoites or 7 days before 5 bites from infected mosquitoes. Results: DSM265-related adverse events consisted of mild-to-moderate headache and gastrointestinal symptoms. DSM265 concentrations were consistent with pharmacokinetic models (mean area under the curve extrapolated to infinity, 1707 µg*h/mL). Placebo-treated participants became positive by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and were treated 7-10 days after CHMI. Among DSM265-treated subjects, 2 of 6 in each cohort were sterilely protected. DSM265-treated recipients had longer times to development of parasitemia than placebo-treated participants (P < .004). Conclusions: This was the first CHMI study of a novel antimalarial compound to compare direct venous inoculation of sporozoites and mosquito bites. Times to qRT-PCR positivity and treatment were comparable for both routes. DSM265 given 3 or 7 days before CHMI was safe and well tolerated but sterilely protected only one third of participants.

A genetically engineered Plasmodium falciparum parasite vaccine provides protection from controlled human malaria infection.

Genetically engineered live Plasmodium falciparum sporozoites constitute a potential platform for creating consistently attenuated, genetically defined, whole-parasite vaccines against malaria through targeted gene deletions. Such genetically attenuated parasites (GAPs) do not require attenuation by irradiation or concomitant drug treatment. We previously developed a P. falciparum (Pf) GAP with deletions in P52, P36, and SAP1 genes (PfGAP3KO) and demonstrated its safety and immunogenicity in humans. Here, we further assessed safety, tolerability, and immunogenicity of the PfGAP3KO vaccine and tested its efficacy against controlled human malaria infection (CHMI) in malaria-naïve subjects. The vaccine was delivered by three (n = 6) or five (n = 8) immunizations with ~200 PfGAP3KO-infected mosquito bites per immunization. PfGAP3KO was safe and well tolerated with no breakthrough P. falciparum blood stage infections. Vaccine-related adverse events were predominately localized urticaria related to the numerous mosquito bites administered per vaccination. CHMI via bites with mosquitoes carrying fully infectious Pf NF54 parasites was carried out 1 month after the last immunization. Half of the study participants who received either three or five PfGAP3KO immunizations remained P. falciparum blood stage negative, as shown by a lack of detection of Plasmodium 18S rRNA in the blood for 28 days after CHMI. Six protected study participants received a second CHMI 6 months later, and one remained completely protected. Thus, the PfGAP3KO vaccine was safe and immunogenic and was capable of inducing protection against sporozoite infection. These results warrant further evaluation of PfGAP3KO vaccine efficacy in dose-range finding trials with an injectable formulation.

Complete attenuation of genetically engineered Plasmodium falciparum sporozoites in human subjects.

Immunization of humans with whole sporozoites confers complete, sterilizing immunity against malaria infection. However, achieving consistent safety while maintaining immunogenicity of whole parasite vaccines remains a formidable challenge. We generated a genetically attenuated Plasmodium falciparum (Pf) malaria parasite by deleting three genes expressed in the pre-erythrocytic stage (Pf p52-/p36-/sap1-). We then tested the safety and immunogenicity of the genetically engineered (Pf GAP3KO) sporozoites in human volunteers. Pf GAP3KO sporozoites were delivered to 10 volunteers using infected mosquito bites with a single exposure consisting of 150 to 200 bites per subject. All subjects remained blood stage-negative and developed inhibitory antibodies to sporozoites. GAP3KO rodent malaria parasites engendered complete, protracted immunity against infectious sporozoite challenge in mice. The results warrant further clinical testing of Pf GAP3KO and its potential development into a vaccine strain.

Plasmodium yoelii vitamin B5 pantothenate transporter candidate is essential for parasite transmission to the mosquito.

In nearly all non-photosynthetic cells, pantothenate (vitamin B5) transport and utilization are prerequisites for the synthesis of the universal essential cofactor Coenzyme A (CoA). Early studies showed that human malaria parasites rely on the uptake of pantothenate across the parasite plasma membrane for survival within erythrocytes. Recently, a P. falciparum candidate pantothenate transporter (PAT) was characterized by functional complementation in yeast. These studies revealed that PfPAT mediated survival of yeast cells in low pantothenate concentrations and restored sensitivity of yeast cells lacking pantothenate uptake to fenpropimorph. In addition, PfPAT was refractory to deletion in P. falciparum in vitro, but nothing is known about the in vivo functions of PAT in Plasmodium life cycle stages. Herein, we used gene-targeting techniques to delete PAT in Plasmodium yoelii. Parasites lacking PAT displayed normal asexual and sexual blood stage development compared to wild-type (WT) and WT-like p230p(-) parasites. However, progression from the ookinete to the oocyst stage and sporozoite formation were completely abolished in pat(-) parasites. These studies provide the first evidence for an essential role of a candidate pantothenate transport in malaria transmission to Anopheles mosquitoes. This will set the stage for the development of PAT inhibitors against multiple parasite life cycle stages.