Experimental infection and protection against ... - TI Pharma

120 Chapter 6
It is thus unclear whether blood-stage parasite growth inhibition can be
determined with sufficient precision after sporozoite challenge.
The advent of molecular techniques allows for quantification of submicroscopic
parasitemia [2]. In addition to the number of protected volunteers, Q-PCR allows
for estimations of liver stage parasite load (for pre-erythrocytic vaccines) or
blood-stage multiplication rate (for erythrocytic vaccines), which may be
particularly useful in vaccines that do not induce full protection, e.g. “leaky
vaccines” [9]. Here we use Q-PCR data from 11 years of experience of sporozoite
challenge trials at the Radboud University Nijmegen Medical Centre (RUNMC) to
determine inter-individual variation in parasite kinetics and calculate the power
of sporozoite challenge trials for both pre-erythrocytic and asexual erythrocytic
vaccines.
Methods
Data and study population
Data were available from 48 volunteers participating in seven different
sporozoite challenge trials at the RUNMC from 1999 to 2010. The data set
included subjects from immunological studies (N=20), infectivity controls from
immunisation trials (N=16) and subjects from a malaria vaccine trial not showing
any protection (N= 12). All volunteers were challenged by the bites of four to
seven infected mosquitoes for ten minutes. Mosquitoes were laboratory-reared
and infected with the NF54 strain of Pf [10]. Presence of sporozoites in
mosquitoes was confirmed by salivary gland dissection. Trial subjects were
followed two to three times daily from day five after challenge until three days
after start of antimalarial treatment. At every visit, blood samples were collected
and assessed for presence of parasites by microscopy (threshold 4 Pf/µl) and
quantified by Q-PCR (threshold 20-100 Pf/ml) [2]. Antimalarial treatment was
provided as soon parasites were detected by microscopy. Ethical approval was
obtained for each trial separately (Review board numbers: 0011-0262,
2001/203, 2002/170, 2004/129, 2006/207, NL14715.000.06, NL24193.091.09).
Statistical analysis
Analyses were performed using log-transformed data. We calculated geometric
mean parasitemia for every cycle in individual volunteers as well as group
geometric mean and standard deviation (SD). We determined individual parasite
multiplication rate by division of parasite densities of two subsequent cycles and