Characterization of Novel Antimalarial Compound ACT-451840: Preclinical Assessment of Activity and Dose-Efficacy Modeling.

BACKGROUND: Artemisinin resistance observed in Southeast Asia threatens the continued use of artemisinin-based combination therapy in endemic countries. Additionally, the diversity of chemical mode of action in the global portfolio of marketed antimalarials is extremely limited. Addressing the urgent need for the development of new antimalarials, a chemical class of potent antimalarial compounds with a novel mode of action was recently identified. Herein, the preclinical characterization of one of these compounds, ACT-451840, conducted in partnership with academic and industrial groups is presented. METHOD AND FINDINGS: The properties of ACT-451840 are described, including its spectrum of activities against multiple life cycle stages of the human malaria parasite Plasmodium falciparum (asexual and sexual) and Plasmodium vivax (asexual) as well as oral in vivo efficacies in two murine malaria models that permit infection with the human and the rodent parasites P. falciparum and Plasmodium berghei, respectively. In vitro, ACT-451840 showed a 50% inhibition concentration of 0.4 nM (standard deviation [SD]: ± 0.0 nM) against the drug-sensitive P. falciparum NF54 strain. The 90% effective doses in the in vivo efficacy models were 3.7 mg/kg against P. falciparum (95% confidence interval: 3.3-4.9 mg/kg) and 13 mg/kg against P. berghei (95% confidence interval: 11-16 mg/kg). ACT-451840 potently prevented male gamete formation from the gametocyte stage with a 50% inhibition concentration of 5.89 nM (SD: ± 1.80 nM) and dose-dependently blocked oocyst development in the mosquito with a 50% inhibitory concentration of 30 nM (range: 23-39). The compound's preclinical safety profile is presented and is in line with the published results of the first-in-man study in healthy male participants, in whom ACT-451840 was well tolerated. Pharmacokinetic/pharmacodynamic (PK/PD) modeling was applied using efficacy in the murine models (defined either as antimalarial activity or as survival) in relation to area under the concentration versus time curve (AUC), maximum observed plasma concentration (Cmax), and time above a threshold concentration. The determination of the dose-efficacy relationship of ACT-451840 under curative conditions in rodent malaria models allowed prediction of the human efficacious exposure. CONCLUSION: The dual activity of ACT-451840 against asexual and sexual stages of P. falciparum and the activity on P. vivax have the potential to meet the specific profile of a target compound that could replace the fast-acting artemisinin component and harbor additional gametocytocidal activity and, thereby, transmission-blocking properties. The fast parasite reduction ratio (PRR) and gametocytocidal effect of ACT-451840 were recently also confirmed in a clinical proof-of-concept (POC) study

Time-, stage-, and concentration-dependent effects of ACT-451840 on synchronous cultures of <i>P</i>. <i>falciparum</i> NF54 in vitro.

<p>Parasites were exposed to ACT-451840 for 6 or 24 h at the indicated concentration. Results are expressed as the percentage of growth of the respective development stage relative to an untreated control. Each bar represents the mean + SD of three independent experiments.</p

Therapeutic efficacy against <i>P</i>. <i>falciparum</i> in vivo.

<p>Parasitemia in peripheral blood of mice infected with <i>P</i>. <i>falciparum</i> NF54^0230/N3 and treated with vehicle, chloroquine, or ACT-451840 once daily for 4 d starting on Day 3 after infection. Data shown are mean parasitemia of three mice/group. Abbreviations: 50% effective dose (ED₅₀), 90% effective dose (ED₉₀).</p

In vitro activity against a panel of resistant and sensitive strains of <i>P</i>. <i>falciparum</i>.

<p>In vitro activity against a panel of resistant and sensitive strains of <i>P</i>. <i>falciparum</i>.</p

K13 propeller mutation C580Y confers no cross-resistance to ACT-451840 in ring-stage survival assays (RSA_0-3h).

<p>Graph shows mean ± standard error (SE) ring-stage survival values in the RSA_0-3h. At least two biological replicates were performed per line, each consisting of two technical replicates Cam3.II^C580Y (dotted line) and Cam3.II^rev (solid line). Abbreviation: ring-stage survival assay (RSA_0-3h).</p

Antimalarial activity in <i>P</i>. <i>berghei</i> infected mice plotted against AUC, C_max, and time above threshold over the entire treatment period (1 or 3 d).

<p>Dots are the observed antimalarial activity; the line is the modeled relationship using a maximal effect (E_max) model.</p

Parasite Reduction Ratio.

<p>The number of viable <i>P</i>. <i>falciparum</i> strain 3D7 (MR4) versus treatment time is compared between ACT-451840 and a selection of standard antimalarials (data for the latter was previously reported in reference [<a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1002138#pmed.1002138.ref024" target="_blank">24</a>]). Data are the mean ± SD of four independent replicates.</p

Correlation of the observed survival or antimalarial activity versus modeled maximal effect relationship.

<p>Correlation of the observed survival or antimalarial activity versus modeled maximal effect relationship.</p

Pharmacokinetics parameters of ACT-451840 in healthy mice (<i>n</i> = 4).

<p>Pharmacokinetics parameters of ACT-451840 in healthy mice (<i>n</i> = 4).</p

ACT-451840 blocking transmission.

<p>Standard membrane feeding assays were performed with a 24 h pre-incubation of gametocytes with compound (indirect mode). (A) shows average oocyst intensity per mosquito and (B) shows average oocyst prevalence (percentage of mosquitoes with at least one oocyst). Error bars indicate SE from the measurements of the two groups of the 20 mosquitoes per sample.</p