Understanding the mode of action and the mechanism of resistance of a therapeutic agent is not mandatory for a compound to be released on the market. However, information regarding these processes may help to predict and control interactions of the compound with the environment and with other drugs deployed in the field. To this purpose, the antiplasmodial action of SC83288 was profiled against artemisinin and artemisinin derivatives in P. falciparum.
The parasite reduction ratio after a single dose, i.e. the speed of action of the molecule, is a quality feature of an anti-malarial drug as recently emphasized by the Medicine for Malaria Venture (MMV) [
61]. Particularly, drugs against severe malaria are expected to rapidly clear sequestered parasites from the patient´s microvasculature, which can make the difference between survival and death. Moreover, fast-acting anti-malarial drugs are favoured for their ability to reduce the risks of resistant parasites arising within the patient. To this day, the artemisinin derivatives remain the gold standards of a rapidly clearing anti-malarial, reducing the parasite burden within hours [
2]. SC83288 has previously been characterized as a fast-acting anti-malarial compound in vitro [
20] using a recrudescence assay [
60]. Under these conditions, the SC83288 speed of action profile was closer to the fast-acting artemisinin than to the slow-acting atovaquone, although the SC83228 showed a slightly longer clearance time and a lower parasite reduction ratio compared to artemisinin [
20]. On a direct clearance-time measurement using trophozoites, SC83288, however, revealed a higher activity than artemisinin [
20], consistent with the potent antiplasmodial activity of SC83288 on trophozoites [
20]. In this study, the killing speed of SC83288 was verified, using two other methods: a relative speed assay [
56], and a re-invasion assay [
57]. The results of these assays were consistent with previous observations, marking SC83288 as a fast-acting anti-malarial compound (Figs.
5 and
6), with an in vitro killing speed profile closer to artesunate than to atovaquone (Fig.
6). Further, combining SC83288 with artesunate did not significantly impair the speed of action (Fig.
6). The speed of action is, however, not the only crucial aspect that needs to be taken into account when developing an anti-malarial compound. As for any xenobiotic molecule, the beneficial versus adverse effects must be carefully weighed, and parameters such as safety, pharmacokinetics and drug–drug interactions need to be considered. In this regard, preliminary results on the safety and toxicology of SC83288 in mice and dogs are promising, although a full assessment would await clinical trials in humans. This study further revealed no obvious in vitro interactions between artemisinin and SC83288 in the
P. falciparum strain Dd2 (Fig.
4). Both the graphical and arithmetical analysis of our isobologram method supported this observation, with the isobologram following a linear trend, and a mean sum of fractional 50% inhibitory concentration falling within the range of an indifferent interaction [
55]. An isobologram may overlook possible similar modes of action, particularly if the two tested compounds are both either competitive inhibitors or mutually exclusive non-competitive inhibitors [
55]. Competition of both compounds for the same binding site, independently of an interference with the binding site of the endogenous substrate, would not be noticed in an isobologram [
55]. However, the chemical and structural differences between SC83288 and the artemisinins (Fig.
1) strongly argue against this possibility.
Resistance is another key aspect of the MMV’s drug candidate profile. Indeed, resistance to most marketed anti-malarial drugs, either previously or still currently used to treat a
P. falciparum infection, has been identified worldwide to various degrees [
62,
63]. Thus, novel drugs need to overcome existing mechanisms of resistance. A previous study has shown that SC83288 is active in vitro against various
P. falciparum laboratory strains that are resistant to quinoline, quinoline-like and antifolate anti-malarials [
20]. This study extends these findings to include the artemisinin resistant parasite strain NF54
ART that carry the
kelch 13 mutation associated with reduced artemisinin responsiveness. SC83288 was found to be active against NF54
ART. Conversely, resistance to SC83288 did not affect the susceptibility of the parasite to other anti-malarial drugs, including artemisinins, chloroquine, mefloquine and the folate antagonists [
20].