Synthesis of carboxyimidamide-substituted benzo[c][1,2,5]oxadiazoles and their analogs, and evaluation of biological activity against Leishmania donovani

Abstract

A facile synthesis route to carboxyimidamide-substituted benzoxadiazoles and related derivatives was developed. A total of 25 derivatives were synthesized. They were evaluated for antileishmanial activity by inhibition of Leishmania donovani axenic amastigote growth using a fluorescent viability microplate assay. The most promising derivative (14) demonstrated an antileishmanial EC₅₀ of 4.0 μM, and it also showed activity in infected macrophages (EC₅₀ 5.92 μM) without signs of cytotoxicity.

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1. Introduction

Leishmaniasis is a spectrum of human diseases caused by at least 20 different species of protozoan parasites. These parasites are responsible for three major diseases: cutaneous, visceral, and mucocutaneous. Over 300 million people are at risk.¹ Visceral leishmaniasis (VL), caused primarily by L. donovani (anthroponotic) or L. infantum/L. chagasi (zoonotic), is fatal if left untreated. There are an estimated 300 000 cases of VL per year, and 20 000 to 40 000 deaths. Over 90% of new cases occur in six countries: Bangladesh, Brazil, Ethiopia, India, South Sudan, and Sudan. Although two new drugs, miltefosine and paromomycin, are currently in use and combination therapy is becoming more common,² an urgent need for new antileishmanial chemotypes persists. The major issues to be confronted in Leishmania treatment are the toxicity of primary medicines and the development of parasite resistance.

In addition, poor efficacy in the case of visceral leishmaniasis/HIV coinfection has raised the need for new antileishmanial treatments.³

In our previous study, 2-arylbenzimidazole and benzo[c][1,2,5]oxadiazole derivatives showed activity against the intracellular Gram-negative bacterium Chlamydia pneumoniae.⁴ In that study, a similarity-based model of C. pneumoniae dimethyladenosine transferase was used to virtually screen compounds from commercially available databases against C. pneumoniae. The 2-arylbenzimidazoles found were then tested and proven active against L. donovani.⁵ This prompted us to consider benzo[c][1,2,5]oxadiazole-derived compounds as a new family of antileishmanial agents. In addition, benzoxadiazoles and their N-oxides have been previously studied as antibiotic and antiparasitic agents.⁶ In this study, a facile synthesis route to carboxyimidamide-substituted benzoxadiazoles was developed, a set of 25 benzo[c][1,2,5]oxadiazole derivatives and other structurally related compounds was synthesized and evaluated as antileishmanial agents.

2. Results and discussion

2.1 Chemistry

Initially, benzo[c][1,2,5]oxadiazole-5-carbonitrile 4 was to be synthesized from 5-chloro- or 5-bromobenzo[c][1,2,5]oxadiazole in a microwave-assisted reaction using NaCN, CuCN, or K₄[Fe(CN)₆],⁷ but these syntheses failed to produce the target nitrile. Instead the synthesis of benzo[c][1,2,5]oxadiazole derivatives was started from the commercially available 4-amino-3-nitrobenzoic acid (Scheme 1). It was converted to benzo[c][1,2,5]oxadiazole-5-carboxylic acid (1) by using a two-step procedure via the N-oxide intermediate that was produced in the presence of sodium hypochlorite in an alkaline EtOH–H₂O solution and subsequently reduced to the carboxylic acid 1.⁸ Next, compound 1 was converted to the primary amide 3via the acyl chloride 2 in the presence of aqueous ammonia in 1,4-dioxane. The reaction of the obtained primary amide 3 with trifluoroacetic anhydride and Et₃N in THF⁹ gave the corresponding nitrile 4, which was converted to the amidoxime 5 by using hydroxylamine hydrochloride¹⁰ in the presence of Et₃N in EtOH. The final step to obtain the derivatives (6–18, 24) was carried out in the presence of the substituted phenyl isocyanates in THF or CHCl₃. The products 21–23 were synthesized in a similar manner by using a solution of Et₃N in THF. Compound 20 was synthesized from the acyl chloride 2via the hydroxamic acid 19. The final products 33–39 were synthesized from the commercially available nitriles by using the methods described above (Scheme 2). Finally, the related benzimidazole derivative 25 was formed from 3,4-diaminobenzonitrile in refluxing formic acid.¹¹

Scheme 1 Synthetic procedures and chemical structures of the products 2–24. Reagents and conditions: a) KOH, EtOH, H₂O, 70 °C, 2 h, then NaOCl, 0 °C → rt, 21 h, 87%; b) P(OEt)₃, EtOH, 70 °C, 17 h, 67%; c) SOCl₂, reflux, 4 h, 97%; d) NH₃ aq., 1,4-dioxane, 0 °C, 1.5 h, 90%; e) TFAA, Et₃N, THF, rt, 3 h, 79%; f) H₂NOH·HCl, Et₃N, EtOH, rt, 2 h, 94%; g) substituted phenyl isocyanate, THF or CHCl₃, rt, 3–21 or 96–120 h, 27–70%; h) MeOH, 0 °C, 0.5 h then H₂NOH·HCl, MeOH, rt, 1.5 h, 57%; i) THF, phenyl isocyanate, 57%; j) PhCOCl/PhOCOCl/PhCH₂COCl, Et₃N, THF, 70 °C, 2–4 h, 14–63%.

Scheme 2 Synthetic procedures and chemical structures of the products 25–39. Reagents and conditions: a) H₂NOH·HCl, Et₃N or Na₂CO₃, EtOH or H₂O, rt/100 °C, 3.5–24 h, 38–98%; b), phenyl isocyanate, THF, CHCl₃ or 1,4-dioxane, rt, 2–5 h or 72 h, 6–85%.

2.2 Evaluation of antileishmanial activity

The activity of the new compounds was determined by using a fluorescent viability microplate assay with L. donovani axenic amastigotes and alamarBlue.¹² All compounds were initially screened at 50 μM, and the most active derivatives sequentially tested at lower concentrations (15 and 5 μM).

First, a set of benzo[c][1,2,5]oxadiazole derivatives 6–18, with different substitution patterns in the “Eastern” phenyl ring of the structure, were synthesized and assayed. The derivative with no additional substituents (compound 6) showed 70% and 24% inhibition of L. donovani proliferation at 50 μM and 15 μM, respectively, whereas meta-Cl substitution (compound 14) increased growth inhibition to 95% and 87% at 50 μM and 15 μM, respectively (Table 1). Overall, meta substitution including Cl-, OMe-, Me-, and CF₃-substituted derivatives seems to be beneficial for antileishmanial activity.

Table 1 Inhibition of Leishmania donovani axenic amastigote growth

	% inhibition^a
Compound	50 μM	15 μM	5 μM
a ±SEM: at 50 μM 0.1–2.3%, 15 μM 0.2–1.9%, and 5 μM 2.4–5.9%. b Precipitates at 50 μM. c Amphotericin B is positive control and was tested at 1 μM.
6	70.1	24.2	—
7	42.0	—	—
8	67.2	44.6	—
9	64.1	—	—
10	38.0^b	—	—
11	71.4	31.8	—
12	46.0	—	—
13	29.6^b	—	—
14	94.8	87.2	43.4
15	44.0	—	—
16	42.1	—	—
17	51.4	—	—
18	51.7	—	—
20	96.2	68.2	54.6
21	51.1	—	—
22	26.5	—	—
23	22.4	—	—
24	43.9^b	—	—
33	78.4	5.4	—
34	98.8	24.9	—
35	72.0	—	—
36	97.9	35.2	—
37	95.2	48.0	—
38	91.9	21.6	—
39	93.0	33.9	—
Amphotericin B	—	—	99.7^c

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Replacing HN of the heteroatom containing chain with oxygen or carbon atom (compounds 22 and 23) significantly reduced antileishmanial activity to 27 and 22%, respectively, at 50 μM. Similarly, removal of the aniline NH (compound 21) reduced activity, but this compound still showed more activity (51% growth inhibition) than carboxy 22 or benzylic 23 derivatives. Moreover, the cyclohexane derivative 24 displays leishmanial inhibition in the absence of an aniline moiety. Replacement of the amino group in the imidamide 20 with a carbonyl group increased inhibition to 96% and 68% at concentrations of 50 μM and 15 μM, respectively.

Finally, a second set of related heterocyclic compounds was synthesized with replacements of the benzo[c][1,2,5]oxadiazole moiety. These heterocycles (benzothiophene 36, benzofuran 37, indole 38, and 1,3-benzodioxole 39) showed an increase in antileishmanial activity at 50 μM compared to the benzoxazole derivative 6. This suggests that the benzo[c][1,2,5]oxadiazole ring is not crucial to antileishmanial activity. In addition, the phenyl and naphthalene derivatives 33 and 34 significantly inhibited growth of L. donovani amastigotes (78% and 5% for 33, and 99% and 25% for 34, at concentrations of 50 μM and 15 μM, respectively). Good inhibition of compound 34 can be hypothesized to result from increased lipophilicity of the compound. Moreover, the phenyl derivative 33 is a very interesting compound, because the shorter synthetic route is likely to increase the prospects for further preparation of new antileishmanial derivatives of this chemotype.

Further evaluation of the most promising derivatives 14 and 20 revealed low EC₅₀ values on axenic amastigotes, 4.2 and 8.1 μM respectively; moderate cytotoxicity on the human THP-1 macrophage cell line, 79.9 and 33.7 μM respectively; and no cytotoxicity against murine fibroblasts at the highest concentration tested, 300 μM (Table 2). Values for amphotericin B, a reference compound used to treat leishmaniasis, are given for comparison. When tested at 5 μM on infected macrophages, a concentration non-toxic for the macrophage cell line, both compounds, 14 and 20, were still active, 33.1 and 19.9% amastigote growth inhibition respectively, albeit lower than that observed with axenic amastigotes. The EC₅₀ for 14, the most active and least cytotoxic derivative, on intracellular amastigotes in infected macrophages was 5.92 μM similar to that observed using axenic amastigotes.

Table 2 EC₅₀ of the selected derivatives on Leishmania donovani axenic amastigotes and intracellular amastigotes in L. donovani-infected macrophages; cytotoxicity on THP-1 macrophages and NIH/3T3 fibroblasts; and respective selectivity indices [SI]

Compound	Axenic amastigotes EC50 ± SEM (n = 2) (μM)	Intracellular amastigotes EC50 ± SEM (n = 3) (μM)	THP-1 EC50 (μM) (n = 2) [SI]	Fibroblasts EC50 (μM) (n = 2) [SI]
14	4.2 ± 0.2	5.92 ± 1.7	79.9 ± 5.4 [19.0]	>300 [>71.4]
20	8.1 ± 0.9	—	33.7 ± 5.0 [4.2]	>300 [>37.0]
Amphotericin B	0.09–0.7 (ref. 12, 14–16)	0.11–0.26 (ref. 17)	2.1–7.6 (ref. 15 and 16)	2.2 (ref. 18)

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3. Conclusion

The developed synthesis route to carboxyimidamide-substituted benzo[c][1,2,5]oxadiazoles and related derivatives facilitates access to a compound library and determination of the structure-activity relationships for this antileishmanial heterocyclic chemotype. Carboxyimidamide-substituted benzoxadiazole derivative 14 is the most promising compound of this study, demonstrating good antileishmanial inhibition activity in infected macrophages and, remarkably, no signs of cytotoxicity. Although mechanism of action of the compounds is not known yet, it is worth continuing the development of these compounds to explore further prospects of these compounds.

4. Experimental section

4.1 Chemistry

Unless otherwise stated, reactions were carried out in oven-dried glassware under an argon atmosphere. All reagents were commercially available and were acquired from Fluka (Buchs, Switzerland), Aldrich (Schnelldorf, Germany), Riedel-de Haën (Seelze, Germany), and Alfa Aesar (Karlsruhe, Germany). THF was distilled over sodium/benzophenone ketyl. The progress of chemical reactions was monitored by thin-layer chromatography on silica gel 60-F₂₅₄ plates acquired from E. Merck (Darmstadt, Germany). The eluent consisted of EtOAc and n-hexane, or EtOAc and MeOH, and detection was conducted at 254 or 366 nm. The products were purified by flash chromatography on silica gel with a Biotage SP1 purification system (Uppsala, Sweden) using 25+M cartridges or SNAP 10 g, 25 g, or 50 g cartridges, detection at 254 nm. Melting points were measured using an IA9100 digital melting point apparatus (Electrothermal Engineering, Essex, UK) and are uncorrected. IR spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer (Ettlingen, Germany) with ATR technique. The synthesized compounds were analyzed by NMR on a Varian Mercury 300 MHz spectrometer (Palo Alto, CA, USA). ¹H and ¹³C NMR were recorded as solutions in DMSO-d₆ (Aldrich). Chemical shifts (δ) are given in parts per million (ppm) relative to the NMR solvent signals (DMSO-d₆ 2.50 and 39.51 ppm for ¹H and ¹³C NMR, respectively). LC-MS analyses were performed with a HP1100 instrument (Agilent, Palo Alto, USA) with UV detector (λ 210 nm) and an Esquire LC spectrometer (Bruker Daltonik, Bremen, Germany) with ESI ion source. Signal separation was carried out by the use of a Waters XBridge C18 column (2.1 mm × 50 mm, 2.1 μm) with a Waters XBridge C18 guard column (Milford, MA, USA) (2.1 mm × 10 mm, 2.5 μm). The eluent consisted of water (+0.1% HCO₂H) and acetonitrile (+0.1% HCO₂H) (gradient run 80 : 20 → 5 : 95). Purity of all tested compounds was >95%. High resolution mass spectra (HRMS) were measured on a Synapt G2 HDMS Q-TOF-instrument (Milford, MA, USA) with positive mode ESI.

Benzo[c][1,2,5]oxadiazole-5-carboxylic acid (1). Potassium hydroxide (2.02 g, 36.0 mmol, 2.4 equiv.) was dissolved in equimixture of EtOH and H₂O (40 mL). 4-Amino-3-nitrobenzoic acid (2.73 g, 15.0 mmol) was added to this solution, and the resulting mixture was heated at 70 °C for 2 h and cooled to 0 °C. A 13% aqueous solution of sodium hypochlorite (80 mL) was added dropwise to this solution during 30 min. The resulting mixture was slowly allowed to warm to rt and stirred for 21 h. H₂O (35 mL) was added at 0 °C, and the mixture was acidified (pH < 1) with a 6 M aqueous solution of HCl resulting in the formation of a yellow precipitate. Sodium chloride (1.9 g) was added, and the mixture was further stirred for 1.5 h, filtered, and dried in vacuo to yield benzo[c][1,2,5]oxadiazole-5-carboxylic acid 1-oxide (2.4 g, 87%) as a yellow solid. ¹H NMR (300 MHz, DMSO-d₆) δ 8.06 (br s, 1H), 7.84 (br s, 1H), 7.73 (br s, 1H). A mixture of benzo[c][1,2,5]oxadiazole-5-carboxylic acid 1-oxide (1.90 g, 10.5 mmol) in absolute EtOH (25 mL) was heated to 70 °C, and triethyl phosphite (2.80 mL, 16.0 mmol, 1.5 equiv.) was added dropwise to the mixture. The resulting mixture was heated to reflux and the reflux was maintained for 17 h. The solvent was evaporated, and the residual black oil was treated with H₂O and extracted twice with EtOAc. The organic phase was extracted with a 2 M aqueous solution of NaOH, and the aqueous phase was washed three times with EtOAc. The aqueous phase was cooled to 0 °C and acidifed (pH < 1) with a 6 M aqueous solution of HCl resulting in the formation of a light brown precipitate, which was filtered and dried in vacuo to yield the compound 1 (1.10 g, 67%) as a brown solid. ¹H NMR (300 MHz, DMSO-d₆) δ 13.7 (br s, 1H), 8.63 (t, J = 1.1 Hz, 1H), 8.13 (dd, J = 9.4, 1.1 Hz, 1H), 7.97 (dd, J = 9.4, 1.1 Hz, 1H). ¹³C NMR (75 Hz, DMSO-d₆) δ 165.6, 149.3, 148.9, 134.7, 131.4, 119.4, 116.6. ¹H spectral data is consistent with those reported earlier.¹⁴

Benzo[c][1,2,5]oxadiazole-5-carbonyl chloride (2). A mixture of benzo[c][1,2,5]oxadiazole-5-carboxylic acid 1 (1.82 g, 11.1 mmol) and thionyl chloride (18 mL) was refluxed for 4 h. The resulting mixture was concentrated and dried in vacuo to yield 2 (2.0 g, 97%) as a brown solid. ¹H NMR (300 MHz, DMSO-d₆) δ 8.62 (t, J = 1.1 Hz, 1H), 8.12 (dd, J = 9.4, 1.1 Hz, 1H), 7.96 (dd, J = 9.4, 1.1 Hz, 1H). ¹³C NMR (75 Hz, DMSO-d₆) δ 165.4, 149.3, 148.9, 134.6, 131.3, 119.4, 116.6.

Benzo[c][1,2,5]oxadiazole-5-carboxamide (3). A 25% aqueous solution of ammonia (4.8 mL) was cooled in an ice bath. To this solution, compound 2 (0.700 g, 3.80 mmol) in 1,4-dioxane (8 mL) was added dropwise. The resulting mixture was stirred at the same temperature for 1.5 h and then evaporated in vacuo. The residue was dissolved in EtOAc and washed with H₂O. The aqueous phase was extracted twice with EtOAc. The combined organic phases were dried over anhydrous Na₂SO₄, evaporated, and dried in vacuo to yield 3 (0.56 g, 90%) as a light orange solid. ¹H NMR (300 MHz, DMSO-d₆) δ 8.55 (t, J = 1.2 Hz, 1H), 8.32 (br s, 1H), 8.12 (dd, J = 9.4, 1.2 Hz, 1H), 7.97 (dd, J = 9.4, 1.2 Hz, 1H), 7.79 (br s, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 166.0, 149.1, 148.9, 137.8, 131.4, 116.2, 115.8.

Benzo[c][1,2,5]oxadiazole-5-carbonitrile (4). Compound 3 (1.50 g, 9.20 mmol) was dissolved in anhydrous THF (30 mL), and distilled Et₃N (2.80 mL, 20.2 mmol, 2.2 equiv.) was added to this solution. The solution was cooled in an ice bath, and trifluoroacetic anhydride (1.50 mL, 11.0 mmol, 1.2 equiv.) in THF (8 mL) was added dropwise. The resulting mixture was stirred at rt for 3 h. H₂O was added and the volatiles were removed in vacuo. The residue was dissolved in CH₂Cl₂ and washed twice successively with a 0.1 M aqueous solution of HCl and a 0.1 M aqueous solution of NaOH. The organic phase was dried over anhydrous Na₂SO₄, evaporated, and dried in vacuo. The crude product was purified by flash column chromatography (EtOAc/n-hexane 1 : 2) to give 4 (1.10 g, 79%) as a pink solid. ¹H NMR (300 MHz, DMSO-d₆) δ 8.98 (t, J = 1.2 Hz, 1H), 8.28 (dd, J = 9.3, 1.2 Hz, 1H), 7.82 (dd, J = 9.3, 1.2 Hz, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 148.4, 147.9, 131.8, 125.7, 118.3, 117.2, 115.4.

N′-Hydroxybenzo[c][1,2,5]oxadiazole-5-carboximidamide (5). A mixture of compound 4 (1.05 g, 7.20 mmol), H₂NOH·HCl (750 mg, 10.8 mmol, 1.5 equiv.) and distilled Et₃N (1.50 mL, 10.8 mmol) in EtOH (45 mL) was stirred at rt for 2 h. The solvent was evaporated and the residue was dissolved in EtOAc and brine. The organic phase was washed with brine, dried over anhydrous Na₂SO₄, evaporated, and dried in vacuo to yield 5 (1.2 g, 94%) as a yellowish solid. ¹H NMR (300 MHz, DMSO-d₆) δ 10.38 (s, 1H), 8.29 (t, J = 1.1 Hz, 1H), 8.03 (dd, J = 9.6, 1.1 Hz, 1H), 7.96 (dd, J = 9.6, 1.1 Hz, 1H), 6.13 (s, 2H). ¹³C NMR (75 MHz, DMSO-d₆) δ 149.2, 149.0, 148.7, 136.4, 130.9, 115.2, 110.7.

N′-[[(3-Chlorophenyl)carbamoyl]oxy]benzo[c][1,2,5]oxadiazole-5-carboximidamide (14). A mixture of 5 (89.0 mg, 0.500 mmol), and isocyanate (0.550 mmol, 1.1 equiv.) in dry THF (5 mL) was stirred at rt for 4 h. The solvent was evaporated. The crude product was purified by flash SiO₂ column chromatography (EtOAc/n-hexane 1 : 2) to give 14 (0.10 g, 60%) as a white powder. ¹H NMR (300 MHz, DMSO-d₆) δ 9.68 (s, 1H), 8.57 (s, 1H), 8.14–8.13 (m, 2H), 7.73–7.71 (m, 1H), 7.52 (dd, J = 8.4, 1.2 Hz), 7.36 (t, J = 8.4 Hz, 1H), 7.18 (s, 2H), 7,12 (dd, J = 8.4, 1.2 Hz, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 153.6, 152.1, 148.9, 148.8, 140.0, 134.9, 133.1, 131.4, 130.4, 122.8, 118.8, 117.8, 116.0, 114.6. LC-MS: [M + H]⁺ 332.0 m/z (t_r = 6.9 min). FT-IR (ATR, cm⁻¹): 3440, 3349, 1739, 1647, 1274. HRMS (ESI): m/z calcd. for C₁₄H₁₀ClN₅O₃ [M + H]⁺ 332.0550, found 332.0551.

N-Hydroxybenzo[c][1,2,5]oxadiazole-5-carboxamide (19). Compound 2 (183 mg, 1.00 mmol) was stirred in MeOH (5 mL) in an ice bath for 30 min. The solvents were evaporated in vacuo to give methyl benzo[c][1,2,5]oxadiazole-5-carboxylate. A solution of hydroxylamine in MeOH was prepared by dissolving H₂NOH·HCl (542 mg, 7.80 mmol, 7.8 equiv.) in MeOH (5 mL) in a water–ice bath. KOH (645 mg, 11.5 mmol, 11.5 equiv.) in MeOH (10 mL) was added, resulting in formation of a precipitate. The resulting mixture was stirred in an ice–water bath for 30 min, and the precipitate was filtered off to yield a solution of N-hydroxycarboxamide in MeOH. The methyl ester (vide supra) was dissolved in this solution, stirred for 1.5 h, neutralized with acetic acid, and evaporated. The resulting residue was dissolved in EtOAc and washed with brine. The aqueous phase was further extracted with EtOAc. The combined organic phases were dried over with anhydrous Na₂SO₄, evaporated, and dried in vacuo. The crude product was purified by flash SiO₂ column chromatography (CH₂Cl₂/MeOH 9 : 1) to yield 19 (0.10 g, 57%) as a light solid. ¹H NMR (300 MHz, DMSO-d₆) δ 11.56 (br s, 1H), 9.40 (br s, 1H), 8.37 (s, 1H), 8.14 (dd, J = 9.4, 1.0 Hz, 1H), 7.88 (dd, J = 9.4, 1.0 Hz, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 161.7, 149.0, 148.7, 139.6, 131.0, 116.5, 114.8.

N-[(Phenylcarbamoyl)oxy]benzo[c][1,2,5]oxadiazole-5-carboxamide (20). A mixture of 19 (128 mg, 0.70 mmol) and phenyl isocyanate (87 μL, 0.77 mmol, 1.1 equiv.) in dry THF (5 mL) was stirred at ice–water bath temperature for 30 min. The resulting mixture was let warm to rt, stirred for 3 h, evaporated, and dried in vacuo. The crude product was purified by flash SiO₂ column chromatography (CH₂Cl₂/MeOH 19 : 1) to yield 20 (0.12 g, 57%) as a light brownish solid. ¹H NMR (300 MHz, DMSO-d₆) δ 12.79 (br s, 1H), 10.38 (br s, 1H), 8.55 (t, J = 1.2 Hz, 1H), 8.22 (dd, J = 9.5, 1.2 Hz, 1H), 7.92 (dd, J = 9.5, 1.2 Hz, 1H), 7.52–7.49 (m, 2H), 7.37–7.32 (m, 2H), 7.11–7.05 (m, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 163.0, 152.0, 149.1, 148.6, 138.0, 134.9, 130.6, 129.0, 123.3, 118.5, 117.1, 116.7. LC-MS: [M + H]⁺ 299.2 m/z (t_r = 5.2 min). FT-IR (ATR, cm⁻¹): 3321, 3224, 1759, 1663, 1535, 1203, 888. HRMS (ESI): m/z calcd. for C₁₄H₁₁N₄O₄ [M + H]⁺ 299.0780, found 299.0779.

4.2 Biology

Antileishmanial evaluation using axenic amastigotes. L. donovani (MHOM/SD/1962/1S-Cl2d) was used in all bioassays. Screening of the compounds for antileishmanial activity using axenic amastigotes was carried out using the alamarBlue (AbD Serotec, Oxford, UK) viability assay, as previously described, similar to that reported for leishmanial promastigotes.¹² Axenic amastigotes were grown at 37 °C in a 5% CO₂ incubator in complete RPMI 1640 containing 20% fetal calf serum, pH 5.5.¹³ Compounds to be assayed were diluted in the complete amastigote medium containing 1% DMSO to twice the final concentration used in the assays, and were aliquoted in triplicate (125 μL per well) into 96-well flat-bottom plates (Nunc, Roskilde, Denmark). For determination of the EC₅₀, final concentrations in triplicate spanning 300 to 0.14 μM were used. Amastigotes (5.0 × 10⁵ cells per mL; 125 μL per well) were added to each well and incubated for 24 h at 37 °C in a 5% CO₂ incubator. The alamarBlue viability indicator was added (25 μL per well) and the plates incubated for an additional 24 h at which time the fluorescence (λ_ex = 544 nm; λ_em = 590 nm) was measured in a microplate reader (Fluoroskan Ascent FL, Finland). Complete medium, both with and without DMSO, was used as negative controls (0% inhibition of amastigote growth). Amphotericin B (Sigma-Aldrich, St. Louis MO), a drug used to treat VL, was included as a positive control on each plate and gave >90% inhibition of parasite growth at 1.5 μM.

Cytotoxicity. EC₅₀ was determined on NIH/3T3 fibroblasts or THP-1 monocyte cells using the alamarBlue viability indicator as follows. Cells in DMEM medium plus 10% fetal calf serum and antibiotics were aliquoted in triplicate (fibroblast or THP-1 at 8 × 10⁵ or 6.4 × 10⁴ cells per well respectively; 125 μL per well) into 96-well plates (Nunc, Roskilde, Denmark). Compounds to be assayed were diluted in the same medium containing 1% DMSO at twice the final concentration used in the assays (300 to 0.14 μM for fibroblasts or 150 to 0.10 μM for THP-1), added to the cells (125 μL per well), and the plates incubated for 48 h at 37 °C in a 5% CO₂ incubator. The alamarBlue viability indicator was added (25 μL per well) and the plates incubated for an additional 3–5 h at which time the fluorescence (λ_ex = 544 nm; λ_em = 590 nm) was measured. Complete medium, both with and without DMSO, was used as negative controls (0% inhibition).

Activity on intracellular amastigotes. Inhibition of amastigote growth in infected THP-1 cells was measured as previously described.⁵ In brief, THP-1 cells in complete RPMI-1640 containing 10% fetal calf serum and antibiotics were treated for 24 h with 25 ng mL⁻¹ phorbol 12-myristate-13-acetate (PMA, Sigma-Aldrich, St. Louis, MO). PMA and non-adherent cells were removed with excess warm medium, and stationary-phase Ld:pSSU-int/LUC promastigotes expressing luciferase (5 : 1 parasite/macrophage ratio) used to infect the macrophages. Twenty-four hours later the adherent cells were washed 4–5 times with warm RPMI-1640 alone, and infected macrophages detached by treating with Trypsin EDTA. Infected macrophages (5 × 10⁴ cells/100 μL per well) were dispensed in triplicate into white 96-well flat bottom plates (NUNC, Denmark) and the compounds diluted in complete RPMI-1640 containing 1% DMSO final concentration and added in triplicate (50 μL per well). Cultures were incubated for a further 48 h (37 °C, 5% CO₂), lysed by the addition of Bright-Glo Luciferase Assay substrate (100 μL per well, Promega, MT, U.S.A.), and chemiluminescence measured using a microplate reader (Fluoroskan Ascent, Thermo Scientific). Amphotericin B (1 μM, >90% inhibition of parasite growth) was included as a positive control. Complete medium, both with and without DMSO, was used as negative controls. Experiments were repeated three times. Calculation of the EC₅₀'s and statistical analysis were carried out using GraphPad Prism Version 6.0b (GraphPad Software, Inc. San Diego, CA).

Acknowledgements

We thank Ms. Anna Keltikangas née Takala for the synthesis assistance. This study was supported by the Academy of Finland (projects 264020 and 265481 to JYK). CLJ holds the Michael and Penny Feiwel Professorial Chair of Dermatology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures for compounds 6–13, 15–18, 21–39. See DOI: 10.1039/c5md00119f

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