The Spiroindolone KAE609 Does Not Induce Dormant Ring Stages in Plasmodium falciparum Parasites

Abstract

In vitro drug treatment with artemisinin derivatives, such as dihydroartemisinin (DHA), results in a temporary growth arrest (i.e., dormancy) at an early ring stage in Plasmodium falciparum. This response has been proposed to play a role in the recrudescence of P. falciparum infections following monotherapy with artesunate and may contribute to the development of artemisinin resistance in P. falciparum malaria. We demonstrate here that artemether does induce dormant rings, a finding which further supports the class effect of artemisinin derivatives in inducing the temporary growth arrest of P. falciparum parasites. In contrast and similarly to lumefantrine, the novel and fast-acting spiroindolone compound KAE609 does not induce growth arrest at the early ring stage of P. falciparum and prevents the recrudescence of DHA-arrested rings at a low concentration (50 nM). Our findings, together with previous clinical data showing that KAE609 is active against artemisinin-resistant K13 mutant parasites, suggest that KAE609 could be an effective partner drug with a broad range of antimalarials, including artemisinin derivatives, in the treatment of multidrug-resistant P. falciparum malaria. Over the last 2 decades, artemisinin-based combination therapies (ACTs) have been the most efficacious treatments against malaria and have contributed greatly to the decline in malaria mortality and morbidity (1, 2). Recent reports of falling efficacy rates of ACTs in Southeast Asia, as evidenced by increased treatment failures (3-5) and prolonged parasite clearance times following ACT treatment (6-11), are of great concern. Molecular markers of artemisinin resistance, K13 propeller mutations, have been identified (12, 13), and several recent reports suggest that resistance to the currently used ACTs is developing and spreading sooner than expected (14, 15). It remains unclear whether this resistance is a result of changes in the parasite molecular machinery required for the mode of action of artemisinin derivatives (16) or due to a recently described phenomenon of dormancy (growth retardation), where Plasmodium falciparum rings are able to survive dihydroartemisinin (DHA) treatment by undergoing a temporary growth arrest (17)(18)(19). Despite many unanswered questions about the role of dormancy, the importance of this phenomenon in the context of antimalarial chemotherapy and artemisinin resistance has become increasingly evident (17-23). Furthermore, the growth arrest of ring-stage parasites and their recovery rates observed in the in vitro ring survival assay (RSA) correlate strongly with parasite clearance half-lives after treatment of P. falciparum malaria with ACTs (22, 23), providing further support for the link between dormancy and treatment failures and possible resistance.Despite the concerns over emerging artemisinin resistance, ACTs still remain the treatment of choice for uncomplicated P. falciparum malaria (1). Therefore, to enhance or prolong the activity of ACTs against the emergence of resistance, it is important to d...

New developments in anti-malarial target candidate and product profiles

“…Finding other such mutually exclusive pairings in the portfolio will be important over the next period. An emerging area of parasitological understanding is the confirmation and relevance of asexual blood-stage dormancy or quiescence, and whether certain compounds promote dormancy or, in contrast, can kill or re-activate any putative dormant forms [164–166]. Such resistance mechanisms are extremely complicated and more basic science is required to understand the clinical significance of in vitro studies in this area.…”

Imidazolopiperazines Kill both Rings and Dormant Rings in Wild-Type and K13 Artemisinin-Resistant Plasmodium falciparum In Vitro

“…These DHA-pretreated rings (dormant rings) can resume growth a few days after ART drug removal ( 14 ). Our earlier experiments established that IPZ, similar to spiroindolones ( 30 ), do not seem to induce Plasmodium dormancy during the early ring stage ( Fig. 1C and D ).…”

Restructured mitochondrial-nuclear interaction inPlasmodium falciparumdormancy and persister survival after artemisinin exposure

“…Subsequent to early proposals that dormant erythrocytic stages are responsible for recrudescences of P. falciparum infection after treatment with artemisinin drugs [5], in vitro as well as in vivo studies continued to build evidence for the importance of these parasite forms in failures of the artemisinins to achieve complete cure [6-8, 23, 32, 36, 37]. Dormant parasites were found to be small and to have characteristic features that distinguish them from pyknotic or ringstage forms in erythrocytes: in Giemsa-stained thin blood smears, these features include a magenta nucleus bordered by a small region of blue cytoplasm [8,32,37]. Mitochondrial activity is vital to the survival of dormant forms and their return to active propagation; this activity has enabled the identification and separation of these forms by fluorescent markers specific for mitochondrial membrane potential [11].…”