Chemoprotective Antimalarial Activity of P218 against Plasmodium falciparum: A Randomized, Placebo-Controlled Volunteer Infection Study

Abstract

P218 is a highly selective dihydrofolate reductase inhibitor with potent in vitro activity against pyrimethamine-resistant Plasmodium falciparum. This single-center, randomized, double-blind, placebo-controlled phase Ib study evaluated P218 safety, pharmacokinetics, and chemoprotective efficacy in a P. falciparum sporozoite (PfSPZ) volunteer infection study (VIS). Consecutive dose safety and tolerability were evaluated (cohort 1), with participants receiving two oral doses of P218 1,000 mg 48 hours apart (n = 6), or placebo (n = 2). P218 chemoprotective efficacy was assessed (cohorts 2 and 3) with direct venous inoculation of 3,200 aseptic, cryopreserved PfSPZ (NF54 strain) followed 2 hours later with two P218 doses of 1,000 mg (cohort 2, n = 9) or 100 mg (cohort 3, n = 9) administered 48 hours apart, or placebo (n = 6). Parasitemia was assessed from day 7 using quantitative PCR targeting the var gene acidic terminal sequence (varATS qPCR). By day 28, all participants in cohort 2 (P218 1,000 mg) and 8/9 in cohort 3 (P218 100 mg) were sterilely protected post-PfSPZ VIS, confirming P218 P. falciparum chemoprotective activity. With placebo, all six participants became parasitemic (geometric mean time to positive parasitemia 10.6 days [90% CI: 9.9-11.4]). P218 pharmacokinetics were similar in participants with or without induced infection. Adverse events of any cause occurred in 45.8% (11/24) of participants who received P218 and 50.0% (4/8) following placebo; all were mild/moderate in severity, transient, and self-limiting. There were no clinically relevant changes in laboratory parameters, vital signs, or electrocardiograms. P218 displayed excellent chemoprotective efficacy against P. falciparum with favorable safety and tolerability.

Figure 1.

Study design for cohort 1, and cohorts 2 and 3.

Figure 2.

In vivo efficacy in a humanized mouse model and population pharmacokinetics for dose selection. (A) P218 oral efficacy in a standard 4-day test in female NOD-scid IL-2Rγ^null mice engrafted with human erythrocytes and infected with 20 × 10⁶ Plasmodium falciparum–infected erythrocytes (P. falciparum Pf3D7^0087/N9). (B) Population PK simulations of P218 plasma concentration over time following oral administration of two 1,000 mg doses administered 48 hours apart or two 100 mg doses administered 48 hours apart (see also Supplemental Materials Methods S1). Data are median with fifth and 95th population percentiles generated from 1,000 trials of six participants each. MIC = minimum inhibitory concentration; LLOQ = lower limit of quantitation.

Figure 3.

Participant disposition.

Figure 4.

P218 chemoprotective efficacy. (A) Kaplan–Meier estimates of absence of detectable parasites with P218 chemoprevention following Plamodium falciparum sporozoite inoculation. One participant in cohort 3 had their final quantitative PCR (qPCR) assessment on day 27 and was censored at this point. (B) Parasitemia values from qPCR for individual participants following P. falciparum sporozoite inoculation following placebo, two doses of 1,000 mg P218 48 hours apart (cohort 2), or two doses of 100 mg P218 48 hours apart (cohort 3). Protocol-defined positive parasitemia was ≥ 250 parasites/mL blood. Zero values are not shown. Closed symbols indicate the start of rescue therapy in the placebo group. Only one participant had a positive blood smear (placebo group day 9) with a corresponding qPCR parasite count of 257 parasites/mL blood.

Figure 5.

Mean plasma concentration–time profiles. (A) P218 after two doses of 1,000 mg P218 48 hours apart (cohort 1), or following Plasmodium falciparum sporozoite inoculation for two doses of 1,000 mg P218 48 hours apart (cohort 2), or two doses of 100 mg P218 48 hours apart (cohort 3), and (B) P218 and metabolites following two doses of 1,000 mg P218 48 hours apart (cohort 1).