Repurposing Anticancer Drugs To Tackle Malaria
Yohann Le Govic,[a, b] Sandrine Houzé,[c] and Nicolas Papon*[d]
Despite considerable efforts, malaria remains one of the most devastating infectious disease worldwide. In the absence of an effective vaccine, the prophylaxis and management of Plasmo- dium infections still rely on the therapeutic use of antimalarial agents. However, the emergence of resistant parasites has jeopardized the efficiency of virtually all antimalarial drugs, including artemisinin combination therapies (ACTs). Thus, there is an urgent need for innovative treatments with novel targets to avoid or overcome drug resistance. In this context, Huang & colleagues prioritized compounds that can block the activity of epigenetic enzymes, and described the discovery of a selective
P. falciparum histone deacetylase (HDAC) inhibitor with high activity against various stages of the parasite. These findings may pave the way toward developing new lead compounds with broad-spectrum activity, thus facilitating malaria treatment and elimination.
Malaria is one of the most devastating parasitic infectious diseases in the tropics. In 2019, 229 million malaria cases were reported worldwide, causing a total of 409 000 deaths, mainly affecting young children in Africa.[1] The unprecedented scale- up of malaria interventions over the past two decades has led to a substantial drop in disease incidence and mortality. Despite these efforts, the global malaria burden remains unacceptably high and the perspective on malaria eradication is currently being challenged by the emergence and spread of antimalarial drug resistance. Indeed, in the absence of an effective vaccine, the therapeutic use of antimalarial drugs is the cornerstone of malaria control in highly endemic areas. Alarmingly, antimalarial resistance has been described for nearly all currently available options, including first-line artemisinin-based combination therapies (ACTs).[2,3] Innovative antimalarial agents are therefore urgently needed, particularly those with new mechanisms of action.
In addition to overcoming drug resistance, current malaria research programs aim at blocking the Plasmodium life cycle. Indeed, the majority of the existing antimalarial drugs only target the symptom-causing asexual erythrocytic stages of the parasite (i. e. trophozoites and schizonts), which is insufficient to stop transmission and prevent relapses. Henceforth, the challenge relies on the identification of new compounds that could also act against pre-erythrocytic liver-stage and game- tocyte-stage parasites, the latter being capable of onward transmission to the mosquito vector. Although promising targets that may fulfill these criteria have been uncovered in recent years, a single molecule that inhibit both sexual and asexual stages is still lacking in the human pharmacopoeia. Furthermore, the search for new therapeutic agents with high safety profiles is of paramount importance.
[a] Dr. Y. Le Govic
Laboratoire de Parasitologie-Mycologie Centre de Biologie Humaine
CHU Amiens Picardie – site Sud Amiens (France)
[b] Dr. Y. Le Govic
Agents Infectieux, Résistance et Chimiothérapie (AGIR), UR 4294 Université de Picardie Jules Verne
UFR de Pharmacie Amiens (France)
[c] Prof. S. Houzé
CNR du Paludisme, AP-HP, Hôpital Bichat – Claude-Bernard Laboratoire de Parasitologie-Mycologie, UMR261 Merit Université de Paris
Paris (France)
[d] Prof. N. Papon
Host-Pathogen Interaction Study Group (GEIHP, EA 3142) UNIV Angers, UNIV Brest
SFR 4208 ICAT
Angers (France)
E-mail: [email protected]
The traditional drug discovery usually derives from several approaches, including modifications of existing agents, optimi- zation of current drug regimens (e. g. combination therapies), screening of natural products and chemical libraries, and repositioning drugs that are already used to treat other diseases. For instance, epigenetic modulators (‘epi-drugs’), which gather several anticancer drugs approved for clinical use, also hold tremendous promise as new antimalarial therapies.[4] Indeed, previous studies have shown that P. falciparum epigenome regulates key processes governing life-cycle pro- gression and enabling adaptation of the parasite.[5] Notably, the parasites possess a large array of histone-modifying enzymes, many of them being essential for asexual intra-erythrocytic development and gametocyte conversion.[6,7] Of note, several anticancer ‘epi-drugs’ demonstrated high anti-malarial activity in vitro and in vivo,[8] but none of them is currently undergoing clinical trials for malaria, which may be attributed to poor pharmacokinetics, bioavailability and selectivity. Recently, Huang and colleagues described the discovery of a novel, ‘epi- drug’ derivative, multi-stage antiplasmodial inhibitor that may be a suitable candidate for the treatment and control of malaria.[9] As a starting point, a collection of epigenetic inhibitors (n = 64) were screened for antimalarial activity against the labo- ratory-adapted strain 3D7, of which quisinostat (JNJ-26481585) represented the most potent candidate. Indeed, this compound inhibited the growth of P. falciparum in the nanomolar range (EC50 ~ 5 nM). This observation was corroborated in vivo, as quisinostat was the only molecule capable of blocking the multiplication of P. yoelii asexual blood-stages in BALB/c mice.
Figure 1. Chemical evolution of JX21108 from quisinostat. A collection of epigenetic inhibitors (n = 64) were screened for antimalarial activity against the laboratory-adapted strain 3D7, of which quisinostat represented the most potent candidate. To further enhance the antimalarial effect and improve safety of quisinostat, 15 novel hydroxamate analogs were
synthesized through modification of the diamine linker, and their antima- larial efficacy and cytotoxicity were assessed. Among them, one compound bearing a 2,7-diazaspiro[4.4]nonane connecting unit (JX21011) exhibited high potency in vitro against P. falciparum, and a slightly reduced cytotoxicity compared to quisinostat. To reach a better safety profile, a series of JX21011 derivatives with various capping groups were synthesized, leading to JX21108, which not only exhibited broad activity in vitro against several multiresistant P. falciparum strains, but also a striking reduction in cytotoxicity.
Quisinostat is an anticancer drug candidate well documented for targeting both class I and II histone deacetylases (HDACs) in human cells.[10,11] Considering the potential toxicity of quisino- stat in liver HepG2 and kidney 293T cell lines (selectivity index of 8 and 9, respectively), subsequent investigations then focused on chemical optimization of this lead compound (Figure 1).Quisinostat is composed of three distinct moieties: a hydroxamate group serving as a Zn2+ chelating pharmacophore, an N-methylindole ring (capping group) that establishes non-polar interactions within the catalytic pocket, and a piperidine-pyrimidine linker that connects the two aforemen- tioned moieties. To further enhance the antimalarial effect and improve safety, 15 novel spirocyclic hydroxamate analogs were synthesized through modification of the diamine linker, and their antimalarial efficacy and cytotoxicity were assessed. Among them, one compound (JX21011) with diazaspirononane scaffold exhibited high potency in vitro against chloroquine- sensitive 3D7 and chloroquine-resistant Dd2 strains of P. falciparum (EC50 ~ 3 nM each), and a slightly reduced cytotoxicity compared to quisinostat (selectivity index of ~ 24-32). To reach a better safety profile, a series of JX21011 derivatives with various capping groups was synthesized, leading to JX21108, which not only exhibited broad activity in vitro against several multiresistant P. falciparum strains (including two artemisinin- and piperaquine-resistant clinical isolates), but also a striking reduction in cytotoxicity (selectivity index 14- to 28-fold higher than JX21011). JX21108 showed antimalarial activity against all blood stages with a mechanism associated probably with the schizont growth or the invasion of red blood cells.
Additional experiments showed that JX21108 could elimi- nate all morphological stages of rodent parasites in vivo. More importantly, the selected compound showed high potency against stage IV (and to a lesser extent, stage II) gametocytes, indicating a potential for transmission control (Figure 2). Importantly, the racemic and enantiopure forms displayed similar potency in a rodent malaria model, making racemic JX21108 highly attractive for further investigations. Finally, JX21108 possesses inhibitory activity against all life stages of malaria parasites.
To decipher the molecular and cellular mechanisms under- lying the parasiticidal activity of JX21108, the authors then studied the histone acetylation level following JX21108 treat- ment, and found that this compound strongly increased the pan-acetylation of Plasmodium histone H3, similarly to SAHA (a broad HDAC catalytic pocket inhibitor) and quisinostat. In addition, homology modeling and docking studies showed that the catalytic pocket of PfHDAC1 (the unique class I HDAC member in P. falciparum) could adequately accommodate small molecules like quisinostat and JX21108 (Figure 2). To confirm that PfHDAC1 catalytic pocket is actually targeted by JX21108, the authors generated two conditional knock-down mutant strains by inserting the glucosamine-inducible glms ribozyme sequence in the 3’ untranslated region of PfHDAC1 or PfHDA1. The latter encodes a class II HDAC member, whose human orthologs are less sensitive to quisinostat than class I HDACs.[11] Importantly, the silencing of PfHDA1 did not modify the sensitivity of parasites treated with JX21108, while PfHDAC1- disrupted parasites were significantly more sensitive to JX21108 compared to the control strain, arguing in favor of a JX21108- mediated PfHDAC1 catalytic pocket inhibition. Furthermore, RNA-seq data of schizont-stage parasites revealed that abroga- tion of PfHDAC1 lead to regulation changes of 1,430 transcripts (771 down-regulated and 659 up-regulated), which is remark- ably similar with the transcriptomic changes induced by JX21108 treatment (505 down-regulated and 837 up-regulated transcripts). Particularly, 167 transcripts were down-regulated in both conditions, most of them being involved in host-cell attachment and entry. Altogether, these results strongly indicate that PfHDAC1 catalytic pocket is a molecular target of JX21108, whose antiparasitic activity might result from repres- sion of genes involved in the infection process (Figure 2).
In summary, this enlightening article by Huang et al.described the discovery of new lead compounds and their subsequent optimization to obtain greater antimalarial potency and pharmacologically acceptable properties. Although the selected molecule displayed an apparently attenuated cytotox- icity, enzymatic inhibition assays showed that JX21108 remains a potent inhibitor of human HDAC1. Nevertheless, hydroxa- mate-based HDAC inhibitors could constitute excellent future weapons in our limited antimalarial armamentarium. Since PfHDAC1 remains a very attractive drug target, further chemical optimization of JX21108 should be undertaken. Most recently, while main problem of the series remains the potential toxicity, the same research group reported the synthesis of a novel quisinostat derivative combining enhanced pharmacokinetics, multistage killing activity, and high potency against P. falcipa- rum multiresistant strains.[12] Taken together, these data suggest that targeting the epigenome machinery may constitute a relevant alternative strategy to cure and prevent malaria, overcome antimalarial resistance, and control human-to-mos- quito transmission, opening a new avenue for reaching the ambitious but indispensable goal of malaria eradication.
Figure 2. JX21108 targets P. falciparum histone deacetylase (PfHDAC1). Experiments showed that JX21108 could eliminate various stages of the parasites in vivo. More importantly, the selected compound showed high potency against stage IV (and to a lesser extent, stage II) gametocytes, indicating a potential for transmission control. More specifically, JX21108 was shown to inhibit PfHDAC1, an epigenetic enzyme involved in many stages of the developmental cycle of P. falciparum.
Conflict of Interest
The authors declare no conflict of interest.
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HIGHLIGHTS
Pick & mix: Malaria is one of the most devastating infectious diseases worldwide. There is an urgent need for innovative treatments with novel targets. Huang & colleagues priori- tized compounds that can block the activity of epigenetic enzymes, and described the discovery of a selective
P. falciparum histone deacetylase (HDAC) inhibitor with high activity against various stages of the parasite. These findings may pave the way toward developing new lead compounds with broad-spectrum activity, thus facilitating malaria treatment and elimination.
Dr. Y. Le Govic, Prof. S. Houzé, Prof. N. Papon*.