Effective Tetradentate Compound Complexes against Leishmania spp. that Act on Critical Enzymatic Pathways of These Parasites

molecules Article Effective Tetradentate Compound Complexes against Leishmania spp. that Act on Critical Enzymatic Pathways of These Parasites Kristína Urbanová1, Inmaculada Ramírez-Macías 1, Rubén Martín-Escolano 1, María JoséRosales 1, Olaf Cussó2, Joan Serrano 2, Anna Company 2, Manuel Sánchez-Moreno 1, Miquel Costas 2,* , Xavi Ribas 2,* and Clotilde Marín1,* 1 Department of Parasitology, Instituto de Investigación Biosanitaria (ibs. Granada), Hospitales Universitarios de Granada/University of Granada, Severo Ochoa s/n, E-18071 Granada, Spain; [email protected] (K.U.); [email protected] (I.R.-M.); [email protected] (R.M.-E.); [email protected] (M.J.R.); [email protected] (M.S.-M.) 2QBIS-CAT Research Group, Institut de Química Computacional i Catàlisi (IQCC), and Departament de Química, Universitat de Girona. Campus de Montilivi, E-17071 Girona, Spain; [email protected] (O.C.); [email protected] (J.S.); [email protected] (A.C.) *Correspondence: [email protected] (M.C.); [email protected] (X.R.); [email protected] (C.M.); Tel.: +34-972-418-333 (M.C.); +34-972-418-333 (X.R.); +34-958-248-886 (C.M.) Received: 23 November 2018; Accepted: 21 December 2018; Published: 31 December 2018   Abstract: The spectrum and efficacy of available antileishmanial drugs is limited. In the present work we evaluated in vitro the antiproliferative activity of 11 compounds based on tetradentate polyamines compounds against three Leishmania species (L. braziliensis,L. donovani and L. infantum) and the possible mechanism of action. We identified six compounds ( 3 , 5 , 6 , 7 , 8 and 10 ) effective against all three Leishmania spp both on extracellular and intracellular forms. These six most active leishmanicidal compounds also prevent the infection of host cells. Nevertheless, only compound 7 is targeted against the Leishmania SOD. Meanwhile, on the glucose metabolism the tested compounds have a species-specific effect on Leishmania spp.: L. braziliensis was affected mainly by 10 and 8 , L. donovani by 7 , and L. infantum by 5 and 3 . Finally, the cellular ultrastructure was mainly damaged by 11 in the three Leishmania spp. studied. These identified antileishmania candidates constitute a good alternative treatment and will be further studied. Keywords: amastigote; antileishmania; antiproliferative; Leishmania spp.; promastigote; SOD; tetradentate polyaminic compounds; ultrastructure 1. Introduction Leishmaniasis are a group of complex sandfly-transmitted diseases caused by a protozoa parasite from over 20 Leishmania species that affect mainly the poorest population of developing countries [1]. According to World Health Organization, leishmaniasis causes 20,000–30,000 deaths and 700,000–1 million new cases annually that impose a serious health problem worldwide. They are widely spread in east Africa, Latin America, eastern Mediterranean region, south-east Asia region, and European region by importing cases mainly from Africa and America. However, the principal etiological agents, phlebotomine vectors, animal reservoirs and clinical syndromes in each continent may be different [ 1 ]. There are three predominant clinical forms. Cutaneous leishmaniasis (CL) presents itself by self-healing erythematous papules and, therefore, is non-life-threatening. Mucosal or muco-cutaneous leishmaniasis (MCL) causes deforming inflammation of the mucosa with non-self-healing ulcerations, which can disseminate through the lymphatic system. In the last, visceral or kala-azar Leishmaniasis Molecules 2019,24, 134; doi:10.3390/molecules24010134 www.mdpi.com/journal/molecules Molecules 2019,24, 134 2 of 21 (VL), there are no apparent ulcerations, but the symptoms are much more severe. If left untreated, it is a fatal disease characterized by irregular fever, malaise, weight loss, hepatosplenomegaly and anaemia with or without lymphadenopathy, frequently associated with secondary infections [ 2 ]. Even after a treatment, the disease reemerges or reappears as post-kala-azar dermic leishmaniasis (PKDL), causing hypopigmentation, disperse dermic or epidermic inflammation and macular, popular or nodular rash. People with PKDL are a potential source of kala-azar infection [1]. Leishmania braziliensis is one of the most representative species that cause MCL, and is mainly active in Brazil, Colombia, Venezuela and the Andes range [ 3 ]. On the other hand, L. infantum is the primary agent responsible for CL in the Mediterranean Basin (Spain, Greece and northern Africa) [4]. VL and PKDL are caused by L. donovani–L. infantum complex in the Old World and by L. infantum (syn. L. chagasi) in the New World, although PKDL is extremely rare [ 5 ]. VL has by far the greatest impact, not only because of the prevalence and lethality which placed leishmaniasis ninth in a global analysis of infectious diseases. Also, there is a growing incidence of resistance against the first-line antimony drugs and drug combinations in endemic regions [6]. The treatment depends on several factors such as type of disease, parasite species, concomitant pathologies, and geographic location [ 1 ]. Apart from the increasing drug resistance, there are other drawbacks that make the current treatment difficult, such as the administration method, a long treatment period, high toxicity and cost and limited availability in endemic regions. The spectrum and efficacy of available antileishmanial drugs is limited. The front-line drugs are pentavalent antimonial compounds (sodium stibogluconate or Pentostam and meglumine antimoniate or Glucantime), while amphotericin B, pentamidine and paromomycin (aminosidine) are used as second-line chemotherapy. Other alternative antileishmanial medicines are miltefosine, azithromycin, allopurinol, dapsone, sitamaquine, rifampicin or azole medicines: ketoconazole, fluconazole, itraconazole, none of which were designed specifically as leishmaniasis treatment. Combining the drugs in order to create a multidrug therapy may bring significant advantages and better therapeutic effects than each of the components alone [ 7 , 8 ]. The treatment success depends of the clinical form of leishmaniasis, the Leishmania spp. and the geographical region [9]. Target Product Profile (TPP), a planning tool for promising therapeutic candidates, takes into account different factors (compound efficacy, delivery mode, dosage form, time of treatment, stability, tolerability, safety, contraindications, and cost) [ 10 ]. A broad-spectrum activity, in terms of distinct species and strains, should be considered as additional specificities for VL and CL [11,12]. Therefore, given the lack of appropriate treatments and the epidemiological impact of the disease, the development of new drug candidates, safer and more effective, remains a priority. In this respect, in the last decade our group has studied many potential leishmanicidal and trypanocidal compounds, comprising natural extracts and newly synthesized compounds [13–15]. In our research, we try to develop molecules that are able to inhibit parasite-specific enzymes such as the iron superoxide dismutase (Fe-SOD). The Fe-SOD plays a key role in the oxidative stress defence mechanism in the trypanosomatid family, and this isoenzyme is not present in mammalian cells. The typical mammalian superoxide dismutases are linked either to manganese or to copper/zinc atoms (Mn-SOD or Cu/Zn-SOD). Mehlotra [ 16 ] has demonstrated that the parasitic protozoan survival is closely related to the ability of Fe-SOD to decompose the superoxide radical originated by the host cell. In this line, some of us have studied in depth during the last decade the redox properties of polyaminic tetradentate compounds, including their corresponding Fe and Mn-based coordination complexes, and their ability to create powerful oxidizing species upon fast reaction with hydrogen peroxide [ 17 ]. Our starting point for the present study was to test the ability of this type of compounds to cause a disruption of Fe-SOD activity, and therefore, to affect the viability of the parasite. The major aim of the present work was to evaluate in vitro the antiproliferative activity of 11 tetradentate polyaminic compounds, fully stable in water and in cell cultures [ 17 ], against promastigote and amastigote forms of three Leishmania spp. (L. braziliensis,L. donovani and L. infantum) and their non-specific cytotoxicity against macrophages, as well as to perform infectivity assays on Molecules 2019,24, 134 3 of 21 macrophage cells. Furthermore, we have studied the effect of the compounds on the ultrastructure of Leishmania promastigotes by transmission electronic microscopy (TEM), the SOD-isoenzyme specificity and the caused metabolic alterations by 1 H-NMR analysis, specifically on the glycolytic pathway since it is the prime energy source of the parasites. 2. Results 2.1. Selection of the Compounds Under Study We selected two Fe-based complexes 1 and 2 , four Mn-based complexes 3 – 6 and five polyamine metal-free compounds 7 – 11 . It was envisioned that the selected metal based compounds could generate oxidizing species that may elude the protecting role of the Fe-SOD enzyme of the parasite. On the other hand, the metal-free compounds were selected to understand the effect of the metal in some cases (compounds 7 , 9 , 11 ) whereas 8 and 10 were tested for their known ability to strongly chelate transition metals, aiming at disrupting the parasite metabolism by sequestering transition metals essential for their viability, such as iron. 2.2. In Vitro Antileishmanial Activity We have tested the in vitro inhibitory properties of 11 polyaminic compound complexes (Figure 1) against the extra- and intracellular forms of three Leishmania spp., L.braziliensis,L.donovani and L.infantum. The IC 50 values for promastigote and amastigote forms were calculated by non-linear regression analysis from Kc values at concentrations ranging between 1 and 100 µ M and are shown in Table 1A, which also includes the IC 50 of Glucantime (Glu), a frequently used reference drug. Also, we have determined the toxicity values against J774.2 murine macrophages. The selectivity index values (SI, IC 50 macrophage cell toxicity/IC 50 of promastigote or amastigote forms of the parasite) and the times by which the tested compounds exceed the SI value of Glu are shown in Table 1B. Taking into consideration the fact that amastigote forms are the ones present in mammalian hosts, the IC 50 and SI achieved against this parasitic form were determining for us to carry on with subsequent assays. Figure 1. Chemical structure of the tested polyaminic compound complexes. Molecules 2019,24, 134 4 of 21 We found that six compounds ( 3 , 5 , 6 , 7 , 8 and 10 ) were effective against all three Leishmania spp. (Table 1). However, 3 remained slightly below the cut-off value (drug SI 20 or more times higher than that of the reference drug as hit activity criteria proposed by [ 18 ]) for L. donovani. Five compounds ( 3 , 5 , 6 , 7 , and 10 ) presented 2 to 30-fold lower IC 50 values against amastigotes than Glu in all three Leishmania spp. Still, we could observe some species-specificity in the in vitro effectiveness of the compounds. Compounds 6 , 7 and 8 were from 8 to 11-fold more effective against L. donovani amastigotes than against L. braziliensis and L. infantum amastigotes. On the other hand, 3 and 5 manifested an efficacy 2 and 4 times higher in L. infantum amastigotes compared to the other Leishmania spp. The last compound of this subseries, 10 , was equally effective in all three target species. All subsequent tests were carried out for only the most leishmanicidal compounds ( 3 , 5 , 6 , 7 , 8 and 10 for L. braziliensis and L. infantum, and 5,6,7,8and 10 for L. donovani). After the initial in vitro studies we were interested in finding out if the six most leishmanicidal compounds could also prevent the infection of host cells. For this reason, we conducted an assay in which the J774.2 macrophages were exposed to the metacyclic promastigotes and the compound at IC 25 concentration at the same time. The propagation of the parasitic infection was determined by measuring the infection rate and the average number of amastigotes per macrophage cell present during the 10-day treatment period. In addition, the endocytic (short-time of infection) and infection (long-time of infection) indexes were calculated. The control samples did not include any inhibitory compounds, so, the last day the infection rates in L. braziliensis,L. donovani and L. infatum reached 72, 63 and 62%, respectively (Figures 2A, 3A and 4A). In L. braziliensis, the two compounds that caused the highest infectivity inhibition were 5 and 6 (inhibition of 81 and 74%) that caused a decrease in amastigote count of 76 and 63%, respectively. As is shown in Figure 2, all the compounds were much more effective than the reference drug, Glu (infectivity reduction of 36%). These two compounds were the most effective against L. infantum, as well, originating an infectivity inhibition of 84 and 77%, and an average amastigote count reduction of 76 and 70% (Figure 4). The compound that manifested the highest inhibitory capacities was 7 against L. donovani (Figure 3). It confirmed its inhibitory capacities against L. braziliensis and L. infantum, but the effect was not so powerful, confirming the species-specificity we suspected. The infectivity reduction reached 88% and caused an amastigote per macrophage cell count reduction of 74%. In general, the onset of all compounds against the subgenus Viannia (L. braziliensis) was slower than against Leishmania (L. donovani and L. infantum). The decline in the infection rate of L. braziliensis started to appear between days 4 and 6 (Figure 2A,B). Molecules 2019,24, 134 5 of 21 Table 1. In vitro activity, toxicity ( A ) and selectivity index ( B ) found for the polyaminic compound complexes on extracellular and intracellular forms of Leishmania spp. Compounds IC50 µMa Toxicity IC50 Macrophage (µM) Leishmania infantum Leishmania braziliensis Leishmania donovani Promastigote Forms Amastigote Forms Promastigote Forms Amastigote Forms Promastigote Forms Amastigote Forms Glucantime®18.0 ±3.1 24.2 ±2.6 25.6 ±1.6 30.4 ±6.1 27.3 ±4.3 33.3 ±3.7 15.20 ±1.3 1 18.5 ±1.6 20.5 ±1.8 22.3 ±0.6 21.3 ±0.9 28.6 ±5.8 12.5 ±2.7 37.9 ±4.2 2 52.6 ±3.1 57.5 ±2.6 37.2 ±1.9 12.5 ±1.5 19.7 ±1.7 16.3 ±2.5 105.6 ±8.8 3 8.5 ±0.4 3.7 ±1.6 9.3 ±0.3 8.7 ±0.6 28.6 ±3.5 11.7 ±1.3 98.5 ±6.3 4 27.4 ±0.7 24.7 ±1.2 26.4 ±0.7 19.4 ±1.7 60.2 ±7.1 29.9 ±1.6 60.3 ±3.3 5 1.3 ±0.1 2.3 ±0.8 3.5 ±0.6 8.8 ±1.3 6.8 ±0.2 10.7 ±0.8 97.2 ±3.8 6 7.3 ±0.1 10.2 ±1.5 10.1 ±3.1 11.7 ±1.1 4.0 ±0.1 1.3 ±0.4 137.6 ±7.4 7 12.3 ±2.2 9.0 ±0.4 11.7 ±0.8 8.5 ±0.3 8.1 ±3.9 1.1 ±0.2 172.1 ±11.3 8 83.0 ±9.9 22.5 ±3.1 73.8 ±3.6 21.7 ±1.4 31.7 ±2.4 1.9 ±0.3 268.3 ±16.2 9 71.7 ±10.0 33.4 ±3.2 53.2 ±3.6 30.7 ±2.5 29.5 ±3.6 14.5 ±2.7 100.5 ±4.8 10 2.6 ±0.7 10.4 ±0.1 5.5 ±0.8 11.9 ±0.8 18.6 ±0.9 11.6 ±3.3 126.1 ±7.1 11 87.5 ±5.1 34.6 ±2.5 60.6 ±3.8 30.4 ±1.3 29.5 ±4.1 19.8 ±2.6 75.1 ±4.7 SI b Leishmania infantum Leishmania braziliensis Leishmania donovani Promastigote Forms Amastigote Forms Promastigote Forms Amastigote Forms Promastigote Forms Amastigote Forms Glucantime®0.8 0.6 0.6 0.5 0.6 0.5 1 2.1 (3) 1.8 (3) 1.7 (3) 1.8 (4) 1.3 (2) 3.0 (6) 2 2.0 (2) 1.8 (3) 2.8 (5) 8.4 (17) 5.4 (9) 6.5 (13) 3 11.6 (14) 26.6 (44) 10.6 (18) 11.3 (23) 3.4 (6) 8.4 (17) 4 2.2 (3) 2.4 (4) 2.3 (4) 3.1 (6) 1.0 (2) 2.0 (4) 5 74.8 (93) 42.3 (70) 27.8 (46) 11.0 (22) 14.3 (24) 9.1 (18) 6 18.8 (24) 13.5 (22) 13.6 (23) 11.8 (24) 34.4 (57) 105.8 (212) 7 14.0 (17) 19.1 (32) 14.7 (25) 20.2 (40) 21.2 (35) 156.5 (313) 8 3.2 (4) 11.9 (20) 3.6 (6) 12.4 (25) 8.5 (14) 141.2 (282) 9 1.4 (2) 3.0 (5) 1.9 (3) 3.3 (7) 3.4 (6) 6.9 (14) 10 48.5 (61) 12.1 (20) 22.9 (38) 10.6 (21) 6.8 (11) 10.9 (22) 11 0.9 (1) 2.2 (4) 1.2 (2) (5) 2.5 (4) 3.8 (8) Results are averages of four separate determinations. a IC 50 = the concentration required to give 50% inhibition, calculated by non-linear regression analysis from the Kc values at concentrations employed (from 100 µ M to 1 µ M). Each drug concentration was tested in triplicate. Values are the means of four separate determinations ± standard deviation. b Selectivity index = IC 50 Macrophage Cell/IC 50 extracellular and intracellular form of parasite. In brackets: number of times that compound exceeds the reference drug SI, on extracellular and intracellular forms of Leishmania spp. Molecules 2019,24, 134 6 of 21 However, the same reduction of subgenus Leishmania (Figure 3A,B and Figure 4A,B) appeared between day 2 and 4 for most compounds, with some noticeable exceptions (day 2 for 5, 6 and 7 against L. donovani, or 0.5 days for 8 against L. infantum). Evidently, in view of the previous results, the leishmanicidal activity of the compounds was also visible in endocytic and infective indexes (Figures 2E, 3E and 4E) from the beginning of the treatment, and maintained until the day 10 after treatment. Moreover, all compounds tested showed endocytic and infective indexes lower than those obtained for Glu for the three Leishmania spp. Because the inhibitory effect of all compounds was evident only 2 days after treatment, we can suggest that these leishmanicidal compounds are fast killing drugs. It is worth mentioning that all compounds tested against three Leishmania spp. in the present research study exhibited an infection rate capacity up to 3 times higher than Glu. Figure 2. Leishmania braziliensis infectivity assay. ( A , B ) Infection rate percentage and ( C , D ) amastigote count per macrophage cell. In brackets: percentage of decrease in infection rate/number of amastigotes per macrophage cell in comparison to the control measured on the last day of the experiment (peak of infection). ( E ) Effects of the compounds on the endocytic and infection indexes. Each drug at IC 25 concentration was tested in triplicate. Values are the means of four separate determinations ±standard deviation. Molecules 2019,24, 134 7 of 21 Figure 3. Leishmania donovani infectivity assay. ( A , B ) Infection rate percentage and ( C , D ) amastigote count per macrophage cell. In brackets: percentage of decrease in infection rate/number of amastigotes per macrophage cell in comparison to the control measured on the last day of the experiment (peak of infection). ( E ) Effects of the compounds on the endocytic and infection indexes. Each drug at IC 25 concentration was tested in triplicate. Values are the means of four separate determinations ±standard deviation. Molecules 2019,24, 134 8 of 21 Figure 4. Leishmania infantum infectivity assay. ( A , B ) Infection rate percentage and ( C , D ) amastigote count per macrophage cell. In brackets: percentage of decrease in infection rate/number of amastigotes per macrophage cell in comparison to the control measured on the last day of the experiment (peak of infection). ( E ) Effects of the compounds on the endocytic and infection indexes. Each drug at IC 25 concentration was tested in triplicate. Values are the means of four separate determinations ±standard deviation. 2.3. Studies on the Mechanism of Action We performed several experiments to elucidate a possible mechanism of action for the six most active tetradentate compound complexes, according to the SI. 2.3.1. SOD Inhibition The previous extraordinary results prompted us to evaluate the inhibitory effect of 3 , 5 – 8 and 10 on the SOD activity. We were looking for a compound that would selectively inhibit parasitic SODs and not their human equivalent. We used promastigotes of Leishmania spp. to obtain Fe-SOD by cellular disruption and precipitation and assayed the compounds at concentrations ranging from 100 µM to 0.1 µM in each case. The results are shown in Figure 5. The corresponding IC 50 values were calculated and are included in the same figure. Not all compounds were equally effective against the three Leishmania spp. As can Molecules 2019,24, 134 9 of 21 be concluded from the figure, 3 is not an SOD inhibitor, since its IC 50 values exceeded 100 µ M in both parasitic Fe-SOD and human CuZn-SOD, even causing a greater inhibition in the human isoform. 5 caused notable inhibition in L. donovani Fe-SOD (IC50 = 0.4 µ M) but not in other Leishmania spp. 6 is undoubtedly a potent SOD inhibitor and it showed preference for the parasitic isoforms like we were looking for. It produced an inhibition from 3 to 44-fold higher than the one of human SOD. The last three compounds (7, 8 and 10) did not inhibit the CuZn-SOD in the same extent as the Fe-SOD of Leishmania spp. As it is shown in Figure 5B,D,F, these compounds are very powerful Fe-SOD inhibitors: from 3 (8 against L. infantum Fe-SOD) to 5000 times ( 7 against L. donovani Fe-SOD) more effective than against CuZn-SOD. Figure 5. SOD inhibition assay. Representation of the inhibition of 3 , 5 – 8 and 10 against: ( A , B ) L. braziliensis Fe-SOD ( C , D )L. donovani Fe-SOD ( E , F )L. infantum Fe-SOD and ( G , H ) human CuZn-SOD. Each drug concentration was tested in triplicate. Values are the means of four separate determinations ±standard deviation. In brackets: IC50 value, calculated by non-linear regression analysis. The most effective compound against L. braziliensis Fe-SOD was 10 and against L. infantum Fe-SOD were 6 and 7 . 7 was also the overall most potent Fe-SOD inhibitor against all Leishmania spp., but particularly against L. donovani. Furthermore, it is the least toxic compound for the human isoform. 2.3.2. 1H-NMR Trypanosomatids catabolizes glucose at a high rate, excreting into the medium partly oxidized end products such as acetate, succinate, L-alanine or L-lactate [ 19 , 20 ]. Therefore, we cultivated the promastigotes of L. braziliensis,L. donovani and L. infantum in glucose-rich medium (MTL) for 96 h with Molecules 2019,24, 134 16 of 21 Synthesis of Compound 2 Compound 2 was synthesized as follows: TACN2 (labelled as 11 ; 47.1 mg, 0.14 mmol) was dissolved in acetonitrile (1 mL). A solution of FeCl 2 (36.2 mg, 0.28 mmol) in acetonitrile (2 mL) was prepared under inert atmosphere. The solution of FeCl 2 was added dropwise at room temperature to the solution of TACN2, the mixture turned pale yellow and was stirred at room temperature for 1 h. Then, AgClO 4 (114.7 mg, 0.55 mmol) was added and the formation of a white precipitate was observed. The mixture was stirred over 30 min. Afterwards a white powder and a purple solution were clearly distinguished. The purple solution was filtered through syringe filters (25 mm Ø) and the white precipitate was washed with CH 3 CN until the filtrates became colorless. 121 mg of purple crystals (0.11 mmol, 80%) were obtained after diffusion with diethyl ether. HRMS (CH 3 CN): calcd. for [(C 18 N 6 H 40 )Fe 2 (ClO 4 ) 3 ] + 749.0466, found 749.0461 (Supplementary data 1). 1 H-NMR (300 MHz, CD 3 CN): 107.9, 101.1, 63.5, 37.2, 35.0, 32.1, 4.5 ppm (Supplementary data 2). Anal Calcd. for [(C18H40N6)Fe(CH3CN)3](ClO4)4: C, 29.62%; N, 12.95%; H, 5.07%. Found: C, 28.9%; N, 12.95%; H, 5.08%. X-Ray structure: 2 has the formula: [C 30 H 58 Fe 2 N 12 ](ClO 4 ) 4· (CH 3 CN), with the following Unit Cell Parameters: a 11.570(5) b 14.555(6) c 17.298(7), space Group P-1. Crystal data has been deposited with the Cambridge Crystallographic Database with the CCDC reference number 1047857 (Supplementary Material 3). 4.2. Parasite Strain and Culture Promastigote forms of three different Leishmania spp., L. donovani (MHOM/PE/84/LC26), L. infantum (MCAN/ES/2001/UCM-10) and L. braziliensis (MHOM/BR/1975/M2904), were cultured in vitro in Medium Trypanosomes Liquid (MTL) supplemented with 10% inactivated foetal calf serum (FCS) in an air atmosphere at 28 ◦ C, in Roux flasks (Corning, Manassas, VA, USA) with a surface area of 75 cm2, according to [33]. 4.3. In Vitro Screening Assays The compounds and the reference drug Glu were dissolved in DMSO (Panreac, Barcelona, Spain) and tested from 100 µ M to 1 µ M with 0.1% (v/v) DMSO final concentration in plates. Therefore, controls were prepared including 0.1% (v/v) of this component, as previously described [33]. 4.3.1. Cell Culture and Cytotoxicity Tests The macrophage line J774.2 [European collection of cell cultures (ECACC) number 91051511] was derived in 1968 from a tumour in a female BALB/c mouse. Macrophages were grown in humidified 95% air, 5% CO 2 atmosphere at 37 ◦ C in Minimal Essential Medium (MEM) with 2 mM glutamine and 20% (v/v) inactivated FCS. The cytotoxicity test on macrophages was performed by Flow cytometry analysis according to a method previously described [ 34 ]. Briefly, cells were seeded at a density of 5 × 10 4 cells/well in 24-well microplates (Nunc, Naperville, IL, USA). After 48 h incubation, the medium was removed, and fresh medium was added together with each test compound at concentrations from 100 µ M to 1 µ M and cultured in 500 µ L/well volumes for 72 h. Thereafter, cells were recovered by centrifugation, and cell viability was determined by Flow cytometry (FACS Vantage flow cytometer, Becton Dickinson, San Jose, CA, USA) after incubating the cells with propidium iodide (100 mg/mL) and fluorescein diacetate (100 ng/mL). The percentage of viability was calculated in comparison with the control cultures. The toxicity is expressed as the IC 50 value and was calculated using linear regression analysis from the Kc values of the concentrations employed (100-1 µ M) [ 35 ]. Each drug concentration was tested in triplicate in four separate determinations. 4.3.2. Promastigote Tests: Extracellular Forms Promastigote forms were collected in the exponential growth phase and seeded at 1 × 10 5 parasites/mL in 24-well microplates (Nunc, Naperville, IL, USA), after adding each compound at Molecules 2019,24, 134 17 of 21 concentrations from 100 µ M to 1 µ M and cultured in 500 µ L/well volumes at 28 ◦ C. After 72 h incubation, the leishmanicidal activity was assessed using a Neubauer haemocytometric chamber in comparison with the control cultures. The leishmanicidal effect is expressed as the IC 50 value and was calculated using linear regression analysis from the Kc values of the concentrations employed (100-1 µM) [35]. Each drug concentration was tested in triplicate in four separate determinations. 4.3.3. Amastigote Tests: Intracellular Forms J774.2 macrophages were grown and seeded at a density of 1 × 10 4 cells/well in 24-well microplates (Nunc) with rounded coverslips on the bottom and cultured for 48 h in humidified 95% air, 5% CO 2 atmosphere at 37 ◦ C in Minimal Essential Medium (MEM) with 2 mM glutamine and 20% (v/v) inactivated FCS. Adherent macrophages were infected with metacyclic promastigotes in the stationary growth phase of L. donovani, L. infantum and L. braziliensis at a multiplicity of infection (MOI) ratio of 1:10 and maintained for 24 h. Non-phagocytosed parasites were removed by washing, and after addition of the testing compounds at concentrations from 100 µ M to 1 µ M, cultured in 500 µ L/well volumes. Control cultures were also included. After 72 h incubation, leishmanicidal activity was assessed based on the number of amastigotes in treated and untreated cultures in methanol-fixed and Giemsa-stained preparations by analyzing 500 host cells distributed in randomly chosen microscopic fields. The leishmanicidal effect was calculated as described to determine the leishmanicidal activity in the promastigote forms [ 35 ]. Each drug concentration was tested in triplicate in four separate determinations. 4.4. Infectivity Assay Adherent macrophages were grown, seeded and infected with metacyclic promastigotes as mentioned above. The compounds at IC 25 concentrations (calculated using linear regression analysis from the Kc values of the concentrations employed) were added immediately after infection and incubation was performed for 12 h at 37 ◦ C in humidified 95% air, 5% CO2 atmosphere. in humidified 95% air, 5% CO 2 atmosphere at 37 ◦ C Non-phagocytosed parasites and drugs were removed by washing, and then the infected cultures were grown for 10 days, adding fresh culture medium every 48 h, according to [ 35 ]. Amastigotes per macrophage count was determined in methanol-fixed and Giemsa-stained preparations as mentioned above every 48 h. The infection rate (defined as the percentage of infected cells), the average number of amastigotes per macrophage, and the endocytic and infection indexes (calculated by multiplying the infection rate by the average number of amastigotes per macrophage [ 36 ]) were determined. Each drug concentration was tested in triplicate in four separate determinations. 4.5. Ultrastructural Alterations Parasites were cultured at a density of 5 × 10 5 cells/mL with medium containing the tested compounds at IC 25 concentrations. After 72 h at 28 ◦ C, the cultures were processed using a technique previously described [33] and analysed by TEM (LIBRA 120 PLUS, Zeiss, Barcelona, Spain). 4.6. Metabolite Excretion Cultures of L. donovani, L. infantum and L. braziliensis promastigotes were seeded in culture flaks at 5 × 10 5 cells/mL, and after the addition of the compounds at IC 25 concentrations at 28 ◦ C. Control cultures were also included. After 72 h of incubation, parasites were centrifuged at 400 × g for 10 min, and supernatants were collected in order to determine the excreted metabolites by 1 H-NMR. Chemical shifts were expressed in parts per million (ppm) using sodium 2,2-dimethyl-2-silapentane-5-sulphonate as the reference signal. The One-dimensional 1 H-NMR spectra were acquired with a VARIAN DIRECT DRIVE 400 MHz Bruker spectrometer (Rheinstetten, Germany) with an AutoX probe using D 2 O as solvent. The chemical shifts used to identify the respective metabolites were consistent with those described previously by some of the authors [ 37 ]. The spectral region of 1.0–5.5 ppm was bucketed Molecules 2019,24, 134 18 of 21 into a frequency window of 0.1 ppm. The peak (2.6 ppm) corresponding to DMSO was removed before binning, and the regions corresponding to water (4.5–5.5 ppm) and glucose (3.4–3.8 ppm) were excluded during binning to avoid artefacts due to pre-saturation. The aromatic region was excluded because the signal to noise ratio in this region was poorer in comparison to the aliphatic region. The resulting integrals were normalised to the working region (1.0–3.4) ppm of the spectrum to correct for inter-sample differences in dilution. The binning and normalisations were achieved using Mestrenova 9.0 software (Santiago de Compostela, Spain). The matrix obtained in Mestrenova was imported into Microsoft Excel (Redmond, WA, USA) for further data analyses. 4.7. Fe-SOD Enzymatic Inhibition Parasite cells were collected in the exponential growth phase by centrifugation (400g for 10 min). The pellet obtained after centrifugation was lysated by sonication, processed according to [ 38 ] and obtained the homogenate fraction which contained the parasite Fe-SOD. The protein content was quantified using the Sigma Bradford test, which uses BSA as a standard (no trace ability was certified for the BSA standard) [ 39 ]. Superoxide dismutase activities were determined using a method previously described [ 40 ], which measures the reduction in nitrobluetetrazolium (NBT) by superoxide ions. Human CuZn-SOD. Substrates used in these assays were obtained from Sigma Chemical Co. (St. Louis, MO, USA). Resulting data were analysed using the Newman−Keuls test. 5. Conclusions In conclusion, we identified compounds 3 , 5 , 6 , 7 , 8 and 10 as potential fast killing drugs to fight against leishmaniasis. These tetradentate compounds exhibited a remarkable antileishmania effect against the two morphological forms of the parasite, even better than Glu, with a larger spectrum of action and lower toxicity. Our studies suggest the possible mechanism of action of some of them: 3 and 5 seem to act at the glucose metabolism level of L. infantum; 6 and 7 involve the inhibition of the parasite SOD and the alteration in the glucose metabolism of L. infantum and L. donovani, respectively; 8 affects the glucose metabolism of L. braziliensis; and 10 damages the cellular ultrastructure on the three Leishmania spp.; without forgetting the powerful oxidizing of these compounds. Based on their biological properties (leishmanicidal activity), 5 was the compound that demonstrated the best activity/selectivity profile against the two morphological forms of the three Leishmania spp. In general, they are promising molecules for the development of new leishmanicidal agents that could be implemented into a step further within the preclinical phase. Moreover, due to their mechanisms of action seems to be different, it merits mention that combined therapies (also using Glu) should be considered for obtaining improved efficacy. Therefore, we present drugs for the development of an easy-to-synthesise, water-soluble and water-stable antileishmanial agents as a promising therapeutic alternative to current treatments. Supplementary Materials: The following are available online, Figure S1: Crystal structure of [Fe II2 (TACN 2 )(CH 3 CN) 6 ](ClO 4 ) 4 ( 2 ). Perchlorate anions have been omitted for clarity. (Selected bond distances: av. Fe-NCCH 3 = 1.946 ± 0.004 Å, av. Fe-N TACN = 2.038 ± 0.014 Å. Figure S2: Additional ultrastructural images of L. braziliensis promastigotes. Figure S3: Additional ultrastructural images of L. donovani promastigotes. Figure S4: Additional ultrastructural images of L. infantum promastigotes. Figure S5: 1 H-NMR (400 MHz, CD 3 CN) of 2 . Figure S6: HRMS of 2. Author Contributions: K.U., I.R.-M. have contributed equally to this work. Conceptualization, M.S.-M.; Data curation, K.U., I.R.-M. and R.M.-E.; Formal analysis, R.M.-E., M.J.R., A.C., M.S.-M., X.R. and C.M.; Funding acquisition, M.S.-M., M.C. and X.R.; Investigation, K.U., I.R.-M., O.C. and J.S.; Methodology, M.C.; Supervision, M.J.R., A.C., M.C., X.R. and C.M.; Writing—Original draft, R.M.-E. and C.M.; Writing—Review & editing, R.M.-E., M.C., X.R. and C.M., M.S.-M., M.C., X.R. and C.M. are guarantors of the paper. Funding: This research was funded by THE SPANISH MINISTRY OF ECONOMY AND COMPETITIVENESS (MINECO), CONSOLIDER-INGENIO 2010, grant number CDS2010-00065; THE EUROPEAN RESEARCH FOUNDATION, grant number ERC-2009-StG-239910; THE MICINN, grant number CTQ2009-08464; and INNPLANTA, grant number INP-2011-0059-PCT-420000-ACT1. Molecules 2019,24, 134 19 of 21 Acknowledgments: We acknowledge Generalitat de Catalunya for an ICREA Academia Award (X.R. and M.C.). R.M.-E. is grateful for a FPU Grant (FPU14/01537) from the MINISTRY OF EDUCATION OF SPAIN. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results References 1. World Health Organization. Available online: http://www.who.int/news-room/fact-sheets/detail/ leishmaniasis (accessed on 11 October 2018). 2. 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