Biological evaluation of hydroxynaphthoquinones as anti-malarials

Background The hydroxynaphthoquinones have been extensively investigated over the past 50 years for their anti-malarial activity. One member of this class, atovaquone, is combined with proguanil in Malarone®, an important drug for the treatment and prevention of malaria. Methods Anti-malarial activity was assessed in vitro for a series of 3-alkyl-2-hydroxy-1,4-naphthoquinones (N1-N5) evaluating the parasitaemia after 48 hours of incubation. Potential cytotoxicity in HEK293T cells was assessed using the MTT assay. Changes in mitochondrial membrane potential of Plasmodium were measured using the fluorescent dye Mitrotracker Red CMXROS. Results Four compounds demonstrated IC50s in the mid-micromolar range, and the most active compound, N3, had an IC50 of 443 nM. N3 disrupted mitochondrial membrane potential, and after 1 hour presented an IC50ΔΨmit of 16 μM. In an in vitro cytotoxicity assay using HEK 293T cells N3 demonstrated no cytotoxicity at concentrations up to 16 μM. Conclusions N3 was a potent inhibitor of mitochondrial electron transport, had nanomolar activity against cultured Plasmodium falciparum and showed minimal cytotoxicity. N3 may serve as a starting point for the design of new hydroxynaphthoquinone anti-malarials.


Background
Despite the worldwide effort to understand molecular and cellular features of Plasmodium falciparum, the main aetiological agent of human malaria, the disease is still devastating. Parasite resistance to older anti-malarials raises the need for the development of new drugs [1,2]. The anti-malarials currently used stem from six drug classes: aminoquinolines, arylaminoalcohols, artemisinins, antifolates, antibiotics and inhibitors of the respiratory chain [3,4]. The last class is the subject of this report.
The hydroxynaphthoquinones have been extensively investigated over the past 50 years for their anti-malarial activity [5]. Hydrolapachol was the first hydroxynaphthoquinone discovered that possessed anti-malarial activity [6]. This discovery, which emerged at a time of great interest in the study of hydroxynaphthoquinone derivatives as potential new anti-malarials, resulted in the synthesis of a large family of different hydrolapachol analogs [7]. Work on the antimalarial properties of hydroxynaphthoquinones was revived when chloroquine resistance emerged, and it was discovered that atovaquone effectively inhibits plasmodial electron transport at the ubiquinone (coenzyme Q, 2) site [8].
Atovaquone is a hydroxynaphthoquinone that is used in combination with proguanil (Malarone®) for prophylaxis and therapy of uncomplicated malaria [9]. Atovaquone has excellent anti-malarial activity but exhibits poor pharmaceutical properties, such as low bioavailability and high plasma protein binding [10]. To improve drug bioavailability, several atovaquone analogs were prepared and changes were made to the naphthoquinone moiety, especially the alkyl side chain, because it is known that modifying this chain can alter drug activity [7] and counteract drug resistance [11][12][13]. Recently, it was demonstrated that 2methyl-heptyl or 2-methyl-heptyl-trifluoromethyl 2-hydroxy-1,4-naphthoquinones were highly effective against atovaquone-resistant P. falciparum [14].
The aim of this work was to test the activity of a new series of hydroxynaphthoquinones [15] against P. falciparum. One of these compounds, N3, had an IC 50 against cultured P. falciparum in the nM range, disrupted mitochondrial membrane potential, and had low toxicity against human cells suggesting potential as a lead compound for the development of new anti-malarial agents.

Changes in mitochondrial membrane potential (ΨΔmit)
Loss of parasite mitochondrial membrane potential (ΨΔmit) was determined using 5 μM Mitrotracker Red CMXROS as described previously [18]. Cultures were incubated for 30 min at 37°C with the dye and then for 1 h with 10-fold serial dilutions (0.001-100 μM) of N3 and atovaquone. As a control, 5 μM cyanide mchlorophenylhydrazone (CCCP), a protonophore that dissipate the membrane potential, was used. Results were analysed by flow cytometry as described above.

Statistical analyses
Analyses of parasitaemia were performed by a one-way analysis of variance test followed by post hoc analysis by the Dunnett's Multiple Comparison Test using GraphPad Prism software. IC 50 values were produced using sigmoid dose-response curves on GraphPad software. At least three independent experiments were performed for each assay.

In vitro activity of new hydroxynaphthoquinones
The ability of compounds N1-N5 to disrupt the in vitro growth of P. falciparum was tested. The naphthoquinones showed activity against P. falciparum, with IC 50 s of 0.4-89 μM (Figure 1 and Additional file 1: Figure S1). Only N3 had an IC 50 in the nM range (443 nM; Figure 2).

Cytotoxicity effects on cells HEK293T
Cytotoxic activity against HEK293T cells was assessed with a tetrazolium-based colorimetric assay. No significant cytotoxicity was observed at concentrations below 16 μM. For N3, the concentration leading to 50% cell death (CC 50 ) was 54.6 ± 0.23 μM (Figure 1). For atovaquone the CC 50 was 49 ± 0.45 μM.

Effect of N3 on ΔΨ mit
It was also verified the effects of compound N3 on P. falciparum mitochondrial membrane potential (ΔΨmit). Compound N3 showed an IC 50ΔΨmit = 16 μM and atovaquone an IC 50ΔΨmit = 4.4 μM (Figure 3).

Discussion
In an attempt to identify improved anti-malarials, the antiparasitic activities of synthetic hydroxynaphthoquinones using in vitro assays was evaluated. It was identified one compound, N3, with nanomolar activity against P. falciparum, confirmed activity against mitochondrial electron transport, and showed limited cytotoxicity against human cells.
The cytochrome bc1 complex catalyses transfer of electrons to maintain the membrane potential of mitochondria, and it is a validated target for anti-malarial drugs. Atovaquone is the only hydroxynaphthoquinone and inhibitor of the bc1 complex currently used to treat malaria. It is generally efficacious, but suffers from irregular absorption (improved with fatty food), limited drug resistance, and high cost of production [8,9,20]. Work to counteract atovaquone limitations has identified other hydroxynaphthoquinones with anti-malarial activity [8]. One series contained an ester at the 3-hydroxy group of atovaquone, with nanomolar anti-malarial activity; addition of long side chains decreased activity [11]. A series of 26 compounds based on the structure of rhinacanthin, a naphthoquinone with anticancer properties, was synthesized [21]; two of these had nanomolar activity and inhibited the cytochrome bc1 complex of P. falciparum. Another four hydroxynaphthoquinones were synthesized in an attempt to circumvent resistance to atovaquone, which is mediated by mutations in the mitochondrial cytochrome b gene [13]. The addition of a methyl radical on the naphthoquinone ring provided excellent activity against atovaquone-resistant strains of P. falciparum, with documentation of inhibition of the cytochrome bc1 complex [13]. It was recently screened 36 new anti-malarial  phenylsulfanylmethyl naphthoquinones structurally related to lapachol [22]. The compounds had moderate in vitro activity against P. falciparum.
Comparing the structures of atovaquone, N3 and BW58-C (an atovaquone precursor), these three structures are very similar in molecular volume, though N3 is much simpler to prepare and has no chiral centers ( Figure 4) and, therefore, it can serve as a starting point for a new series of hydroxynaphthoquinone antimalarials. The results indicate that the cyclohexane ring of atovaquone is not essential for antimalarial activity, since its replacement by a CH2 group in N3 only slightly decreased activity, and N3 was capable of inhibiting mitochondrial activity efficiently (Figures 2  and 3). Considering BW58-C, this molecule showed excellent results against murine malaria [23] and good  activity against respiration of mitochondria [24], but it was rapidly metabolized and eliminated in humans [25]. Interaction with cytochrome P450 enzymes and other aspects of metabolism are important components of drug design, and evaluation of the metabolism of N3 is needed.
Screening of a library of 2-hydroxy-naphthoquinones found compounds with alkyl side-chains that effectively inhibited the yeast bc1 complex [26]. In the present study, was evaluated 5 additional hydroxynaphthoquinones, and demonstrated that one of these, N3, was a potent inhibitor of mitochondrial electron transport, had nanomolar activity against cultured P. falciparum, and showed minimal cytotoxicity. Optimization of N3 thus offers potential for new candidate compounds to treat and prevent malaria.

Additional file
Additional file 1: Figure S1. Structures of other hydroxynaphthoquinones and effects on P. falciparum growth. Different concentrations of compounds were incubated for 48 h with P. falciparum. Results are shown as a dose response curve for compound N1, N2, N4 and N5 incubated for 48 h. Error bars represent standard error of the mean.

Competing interests
The authors declare that they have no competing interests.
Authors' contributions DCS carried out the in vitro assays and drafted the manuscript. SBF and DRR carried out the chemical synthesis and collaborated in the elaboration of the manuscript. LNC and MN carried out the cytotoxicity test and collaborated in the elaboration of the manuscript. MN carried out the Plasmodium culture and collaborated in the elaboration of the manuscript. CRSG and VFF conceived of the study, and participated in its design and coordination and collaborated in the elaboration of the manuscript. All authors read and approved the final manuscript.