Direct venous inoculation of Plasmodium falciparum sporozoites for controlled human malaria infection: a dose-finding trial in two centres

Abstract

Background: Controlled human malaria infection (CHMI) accelerates development of anti-malarial interventions. So far, CHMI is done by exposure of volunteers to bites of five mosquitoes carrying Plasmodium falciparum sporozoites (PfSPZ), a technique available in only a few centres worldwide. Mosquito-mediated CHMI is logistically complex, exact PfSPZ dosage is impossible and live mosquito-based interventions are not suitable for further clinical development.

Methods: An open-labelled, randomized, dose-finding study in 18-45 year old, healthy, malaria-naïve volunteers was performed to assess if intravenous (IV) injection of 50 to 3,200 aseptic, purified, cryopreserved PfSPZ is safe and achieves infection kinetics comparable to published data of mosquito-mediated CHMI. An independent study site verified the fully infectious dose using direct venous inoculation of PfSPZ. Parasite kinetics were assessed by thick blood smear microscopy and quantitative real time PCR.

Results: IV inoculation with 50, 200, 800, or 3,200 PfSPZ led to parasitaemia in 1/3, 1/3, 7/9, and 9/9 volunteers, respectively. The geometric mean pre-patent period (GMPPP) was 11.2 days (range 10.5-12.5) in the 3,200 PfSPZ IV group. Subsequently, six volunteers received 3,200 PfSPZ by direct venous inoculation at an independent investigational site. All six developed parasitaemia (GMPPP: 11.4 days, range: 10.4-12.3). Inoculation of PfSPZ was safe. Infection rate and pre-patent period depended on dose, and injection of 3,200 PfSPZ led to a GMPPP similar to CHMI with five PfSPZ-infected mosquitoes. The infectious dose of PfSPZ predicted dosage of radiation-attenuated PfSPZ required for successful vaccination.

Conclusions: IV inoculation of PfSPZ is safe, well tolerated and highly reproducible. It shall further accelerate development of anti-malarial interventions through standardization and facilitation of CHMI. Beyond this, rational dose selection for whole PfSPZ-based immunization and complex study designs are now possible.

Trial registration: ClinicalTrials.gov NCT01624961 and NCT01771848 .

Figures

Figure 1

Trial profile. In Tübingen the IV dose of PfSPZ Challenge IV was increased sequentially in 4 steps from 50 (1), to 200 (2), to 800 (3) and to 3,200 PfSPZ (4). In steps 1 and 2, volunteers were randomly assigned to receive PfSPZ Challenge ID or IV. An independent verification group (3,200 PfSPZ DVI) in Barcelona was added after completion of the 3,200 PfSPZ IV group in Tübingen.

Figure 2

Effect of IV PfSPZ Challenge dose on probability of infection. Observed values are given as grey dots, with size representing weight. Model estimates are represented as the black line and the 95% confidence interval as grey ribbon.

Figure 3

Kaplan-Meier plot of time to infection.

Figure 4

Effect of number of mosquito bites on pre-patent period. Published data are from 44 volunteers bitten by 1–7 infected mosquitoes at one centre (Department of Medical Microbiology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands) [26]. Observed values are given as grey dots with size representing weight. A Loess smooth is displayed with the estimate (black line) and 95% confidence interval (grey ribbon).

Figure 5

Parasite quantification by qPCR. Blood was assessed every other day beginning on Day 5 and on the day of microscopically detected parasitaemia if no sampling was scheduled for that day in Tübingen. In Barcelona blood was sampled every day from Days 6 to 9 and twice daily from Day 9 onward. The dotted line indicates the limit of quantification of qPCR (30 parasites per mL).

Figure 6

Adverse event pattern. Adverse event (AE) episodes are given over time from Day 0 through Day 21. Shades of blue represent AE grading from Grade 1 (dark), to Grade 2 (blue), to Grade 3 (light blue). Volunteer IDs with a star indicate those volunteers who did not develop parasitaemia. The time of parasite detection by microscopy is given as the letter ‘M’ in red.