Primaquine and Chloroquine Fumardiamides as Promising Antiplasmodial Agents

This paper describes a continuation of our efforts in the pursuit of novel antiplasmodial agents with optimized properties. Following our previous discovery of biologically potent asymmetric primaquine (PQ) and halogenaniline fumardiamides (1–6), we now report their significant in vitro activity against the hepatic stages of Plasmodium parasites. Furthermore, we successfully prepared chloroquine (CQ) analogue derivatives (11–16) and evaluated their activity against both the hepatic and erythrocytic stages of Plasmodium. Our results have shown that PQ fumardiamides (1–6) exert both higher activity against P. berghei hepatic stages and lower toxicity against human hepatoma cells than the parent drug and CQ derivatives (11–16). The favourable cytotoxicity profile of the most active compounds, 5 and 6, was corroborated by assays performed on human cells (human breast adenocarcinoma (MCF-7) and non-tumour embryonic kidney cells (HEK293T)), even when glucose-6-phosphate dehydrogenase (G6PD) was inhibited. The activity of CQ fumardiamides on P. falciparum erythrocytic stages was higher than that of PQ derivatives, comparable to CQ against CQ-resistant strain PfDd2, but lower than CQ when tested on the CQ-sensitive strain Pf3D7. In addition, both sets of compounds showed favourable drug-like properties. Hence, quinoline fumardiamides could serve as a starting point towards the development of safer and more effective antiplasmodial agents.


Introduction
Despite enormous efforts made by the scientific community, malaria remains a deadly disease, especially among pregnant women and children. According to the 2018 World Malaria Report, malaria is responsible for 266,000 deaths of children aged under five years worldwide [1]. In order to achieve the 2020 milestones of the WHO Global Technical Strategy for Malaria 2016-2030, which include a global reduction of at least 40% in malaria incidence and mortality rates, and disease elimination in at least 10 countries, substantial amounts of funding and research are required [2].
Malaria is caused by Plasmodium parasites, which infect their mammalian host upon the bite of an infected female Anopheles mosquito. Injected sporozoites travel to the liver, where they undergo an asymptomatic but obligatory phase of intra-hepatic replication. The liver stage of infection ends

Chemistry
Analogous to PQ-based hybrids 1-6, in fumardiamides 11-16, one amide bond was achieved with CQ, and the other one with halogenanilines (m-and p-fluoroaniline, mand p-chloroaniline, mand p-trifluoromethylaniline). A synthetic route developed earlier for the preparation of fumardiamides 1-6 was successfully applied here (Scheme 1). In short, the microwave-assisted method was applied for the first step to yield the primary amine 7 with the CQ core. Mono-ethyl fumarate was converted to the corresponding carboxylic acid chloride 8, which reacted with the primary amino group of 7 to give the amidoester 9. Basic hydrolysis of the product 9 by lithium hydroxide yielded the carboxylic acid 10. In the last step, the coupling between the acid 10 and halogenanilines was accomplished by means of (1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo [4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU) and N,N-diisopropylethylamine (DIEA). Purification of these compounds was carried out using crystallization methods and/or column chromatography. Yields for the first three reaction steps were high (73%−100%), but the yields varied from 25% to 91% for the last step. Taken together, our method offered title compounds 11-16 in good to moderate yields. All synthesized compounds were checked for purity and identified using elemental analysis, IR, 1 H and 13 C-NMR spectroscopy, and MS. The data obtained were in agreement with the proposed structures and are given in detail in the Supplementary Materials.

Erythrocytic Stages
We conducted the in vitro screening of the activity of PQ (1-6) and CQ (11)(12)(13)(14)(15)(16) fumardiamides against the erythrocytic stages of the CQ-sensitive (Pf 3D7) and CQ-resistant (Pf Dd2) P. falciparum strains. Both PQ and CQ were included as positive controls, and DMSO served as a negative control in these assays. PQ derivatives were poorly active, as only compound 2 displayed an IC 50 value against Pf 3D7 below 10 µM (IC 50 = 7.74 µM). Such findings were not surprising, as PQ itself possesses only modest erythrocytic stage activity [21]. Moreover, the basic aliphatic amine required for such a type of activity is masked through the amide bond with fumaric acid. In contrast, the Pf 3D7 strain was susceptible to CQ hybrids 11-16 in nanomolar concentrations. However, the most active compounds 14 and 16 were still less active than CQ (IC 50 = 0.035 µM vs. 0.0037 µM). On the other hand, their activity against the Pf Dd2 strain was similar to that of CQ (Table 1). It is worth noting that m-derivatives 11, 13, and 15 have consistently shown higher activity than their p-substituted counterparts.

Hepatic Stages
Next, we examined the antiplasmodial activity of PQ (1-6) and CQ (11)(12)(13)(14)(15)(16) derivatives against the hepatic stages of P. berghei, as well as toxicity to human hepatoma cells (Huh7) (Figure 2). PQ was included as a positive control, whereas DMSO served as a negative control in these assays. The results showed that PQ fumardiamides were markedly more active than the parent drug and their CQ counterparts. In addition, cell confluency measurements indicated that all PQ fumardiamides were non-toxic to human hepatoma cells. Such findings are in accordance with previous cytotoxicity studies of PQ fumardiamides on a panel of human cancer cell lines [20]. Conversely, CQ analogues displayed a higher level of toxicity than the PQ counterparts 1-6. Additionally, IC 50 values were determined for the most potent and promising PQ derivatives 3-6. All four compounds exerted marked activities against hepatic parasites, with IC 50 values ranging between 0.11 and 0.39 µM ( Figure 3 and Table 1), which are at least an order of magnitude lower than the parent drug (IC 50 = 8.4 ± 3.4 µM).   Table 1.  50 , the concentration of the tested compound necessary for 50% growth inhibition. 2 n.d., not determined. 3 Results represent mean ± SD, n = 1. 4 PQ, primaquine. 5 CQ, chloroquine.

Cytotoxicity Assay in Human Cell Lines
Additional evaluation of the cytotoxicity of the most active PQ derivatives 5 and 6 was performed on two human cell lines from different tissues, MCF-7 (breast adenocarcinoma) and HEK293T (non-tumour embryonic kidney). Neither of the compounds displayed toxicity towards the tested cell lines (Table 2). Further, we tested whether the favourable cytotoxicity profile of PQ derivatives 5 and 6 was maintained in the glucose-6-phosphate dehydrogenase (G6PD)-deficient conditions, which were induced by the addition of the G6PD inhibitor 6-aminonicotinamide (6-AN) at a non-toxic concentration (c = 1 µM) to the cells [18]. No differences between 6-AN-treated and non-treated cell lines were observed in terms of growth inhibition (Table 2, Figure 4). Thus, these data suggest that derivatives 5 and 6 are promising lead compounds for the development of hepatic stage antiplasmodial compounds with an improved safety profile in G6PD-deficient patients as well. Table 2. IC 50 values for PQ fumardiamides 5 and 6 tested in vitro on MCF-7 and HEK293T cell lines, alone or in combination with 6-aminonicotinamide.

Evaluation of Drug-Like Properties
The evaluation of the PQ fumardiamides' drug-likeness was reported earlier [20]. Thus, here we evaluated the drug-likeness of CQ derivatives 11-16, by employing the Chemicalize.org software to calculate an array of physicochemical parameters, including the topological polar surface area (TPSA), the number of atoms, molecular weight (MW), the partition coefficient (log P), the H-bond donor (HBD), the H-bond acceptor (HBA), and molecular refractivity (MR) ( Table 3) [22]. Our results show that all hybrids 11-16 fall within the Lipinski's and Gelovani's rules for prospective small molecular drugs (MW ≤ 500, log P ≤ 5, number of H-bond donors ≤ 5, number of H-bond acceptors ≤ 10, TPSA < 140 Å 2 , MR within the range of 40 and 130 cm 3 /mol, the number of atoms 20-70).
Both PQ and CQ hybrids were further analyzed through available filters for pan assay interference compounds (PAINS), which identify a substructure with an ability to interfere in any biological assay, based on various mechanisms [23]. Three computer programs, ZINC (http://zinc15.docking.org/ PAINS/), PAINS Remover (http://www.cbligand.org/PAINS/), and SwissADME (http://www.swissadme. ch/), were employed for compounds 1-6 and 11-16. In addition, we carried out the in silico identification of potential aggregators (Aggregator Advisor, http://advisor.docking.org). All tested compounds returned no PAINS and no aggregator alerts, thus showing that the selected hybrid compounds passed all filters. SwissADME bioavailability radars [24] of the most active compounds of both series, namely compounds 6 and 16, showed that all analyzed parameters are in the optimal range, with the exception of flexibility, which falls out of the set borders ( Figure 5).

Materials and Methods
Melting points were determined on the SMP3 apparatus (Barloworld Scientific, Stone, UK) in open capillaries and are presented uncorrected. A CEM Discover microwave reactor was used for microwave reactions (CEM GmbH, Kamp-Lintfort, Germany). IR spectra were recorded on Spectrum One (Perkin-Elmer, Waltham, MS, USA) and UV-Vis spectra on Lambda 20 double beam spectrophotometers (Perkin-Elmer, UK). NMR 1 H and 13 C spectra were recorded at 25 • C on the NMR Avance 600 spectrometer (Bruker, Leipzig, Germany) at 300.13 or 600.13 and 75.47 or 150.9 MHz for 1 H and 13 C nuclei, respectively. Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane in the 1 H and the dimethyl sulfoxide (DMSO) residual peak as a reference in the 13 C spectra (39.51 ppm). Coupling constants (J) are reported in hertz (Hz). Mass spectra were collected on an HPLC-MS/MS instrument (HPLC, Agilent Technologies 1200 Series; MS, Agilent Technologies 6410 Triple Quad, Santa Clara, CA, USA) using electrospray ionization in the positive mode. Elemental analyses were performed on a CHNS LECO analyzer (LECO Corporation, St. Joseph, MI, USA). All compounds were routinely checked by TLC with Merck silica gel 60F-254 glass plates using dichloromethane/methanol 9.5:0.5, 9:1, 8.5:1.5, and cyclohexane/ethyl acetate/methanol 1:1:0.5 as the solvent systems. Spots were visualized by short-wave UV light and iodine vapour. Column chromatography was performed on silica gel at 0.063 to 0.200 mm (Sigma-Aldrich, St. Louis, Missouri, United States, USA) with the same eluents used in TLC.

In Vitro Activity Against P. berghei Hepatic Stages
In vitro activity of the tested compounds against the liver stage of P. berghei infection was assessed as previously described [28,29]. Briefly, Huh7 cells, a human hepatoma cell line, were routinely cultured in 1640 Roswell Park Memorial Institute (RPMI) medium supplemented with 10% (v/v) fetal bovine serum, 1% (v/v) glutamine, 1% (v/v) penicillin/streptomycin, 1% non-essential amino acids, and 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). For drug screening experiments, Huh7 cells were seeded at 1 × 10 4 cell/well of a 96-well plate and incubated overnight at 37 • C with 5% CO 2 . Next, 10 mM stock solutions of test compounds were prepared in DMSO and were serially diluted in infection medium, i.e., culture medium supplemented with gentamicin (50 µg/mL) and amphotericin B (0.8 µg/mL), in order to obtain the test concentrations. On the day of the infection, the culture medium was replaced with serial dilutions of the test compounds and incubated for 1 h at 37 • C with 5% CO 2 . Next, 1 × 10 4 firefly luciferase-expressing P. berghei sporozoites, freshly isolated from the salivary glands of female infected Anopheles stephensi mosquitoes, were added to the cultures, plates were centrifuged at 1800× g for 5 min at room temperature, and incubated at 37 • C with 5% CO 2 .
To assess the effect of each compound concentration in cell viability, at 46 hours post-infection (hpi), cultures were incubated with Alamar Blue (Thermo Fisher Scientific, Waltham, MA, USA), according to the manufacturer's recommendations. Parasite load was then assessed by the bioluminescence assay (Biotium, Fremont, CA, USA), using the multi-plate reader Infinite M200 (Tecan, Männedorf, Switzerland). Nonlinear regression analysis was employed to fit the normalized results of the dose-response curves, and IC 50 values were determined using GraphPad Prism 6.0 (GraphPad Software, La Jolla California USA).

Cytotoxicity Assay in Human Cell Lines
The experiments were carried out on 2 human cell lines: MCF-7 (breast carcinoma) and HEK293T (embryonic kidney). MCF-7 and HEK293T cells were cultured as monolayers and maintained in Dulbecco's modified Eagle medium (DMEM), supplemented with 10% fetal bovine serum (FBS), 2 mM L-glutamine, 100 U/mL penicillin, and 100 µg/mL streptomycin in a humidified atmosphere with 5% CO 2 at 37 • C. The panel cell lines were inoculated in parallel onto a series of standard 96-well microtiter plates on day 0, at 1 × 10 4 to 3 × 10 4 cells/mL, depending on the doubling time of a specific cell line. Test compounds were then added in five 10-fold dilutions (10 −8 to 10 −4 M) alone or in combination with 6-AN (c = 1 µM) and incubated for a further 72 hours. Working dilutions were freshly prepared on the day of the testing.
After 72 hours of incubation, the cell growth rate was evaluated by performing the MTT assay [18]. The absorbance was directly proportional to cell viability. Each test point was performed in quadruplicate in three individual experiments. The IC 50 values were calculated from the dose-response curves using linear regression analysis. Each result is a mean value from at least two separate experiments.

Conclusions
The antiplasmodial potential of previously prepared PQ fumardiamides 1-6 and novel CQ analogues 11-16 was evaluated against the hepatic and erythrocytic stages of Plasmodium parasites. The evaluation of the compounds' activity against P. berghei hepatic stages in vitro revealed that: i) PQ hybrids show markedly higher activities than PQ itself, ii) PQ derivatives display higher activity than CQ fumardiamides, and iii) PQ fumardiamides are non-toxic to human hepatoma cells, whereas their CQ analogues exert higher levels of toxicity. The most active PQ derivatives 5 and 6 showed low cytotoxicity towards human cells (MCF-7 and HEK293T), which was also founnd when G6PD was inhibited. CQ fumardiamides displayed higher activity than PQ derivatives against P. falciparum erythrocytic stages in vitro. It is encouraging that the activity of the most active CQ derivatives 14 and 16 against the CQ-resistant strain, Pf Dd2, was comparable to that of the parent drug. Taken together, our results indicate that the quinoline core influences the profile of activity/toxicity for PQ and CQ fumardiamides. Due to their biological profile and favourable drug-like properties, quinoline fumardiamides could provide a strong basis for further optimisation towards the development of novel and safer antiplasmodial drugs.