Anti-leishmanial and cytotoxic activities of a series of maleimides: Synthesis, biological evaluation and structure-activity relationship

Article abstract of DOI:10.3390/molecules23112878

In the present study, 45 maleimides have been synthesized and evaluated for anti-leishmanial activities against L. donovani in vitro and cytotoxicity toward THP1 cells. All compounds exhibited obvious anti-leishmanial activities. Among the tested compounds, there were 10 maleimides with superior anti-leishmanial activities to standard drug amphotericin B, and 32 maleimides with superior anti-leishmanial activities to standard drug pentamidine, especially compounds 16 (IC₅₀ < 0.0128 μg/mL) and 42 (IC₅₀ < 0.0128 μg/mL), which showed extraordinary efficacy in an in vitro test and low cytotoxicities (CC₅₀ > 10 μg/mL). The anti-leishmanial activities of 16 and 42 were 10 times better than that of amphotericin B. The structure and activity relationship (SAR) studies revealed that 3,4-non-substituted maleimides displayed the strongest anti-leishmanial activities compared to those for 3-methyl-maleimides and 3,4-dichloro-maleimides. 3,4-dichloro-maleimides were the least cytotoxic compared to 3-methyl-maleimides and 3,4-non-substituted maleimides. The results show that several of the reported compounds are promising leads for potential anti-leishmanial drug development.

Full text of DOI:10.3390/molecules23112878

molecules
Article
Anti-Leishmanial and Cytotoxic Activities of a Series
of Maleimides: Synthesis, Biological Evaluation and
Structure-Activity Relationship
Yongxian Fan 1, Yuele Lu 1, Xiaolong Chen 1,*, Babu Tekwani 2, Xing-Cong Li 2 and
Yinchu Shen 1
1
Institute of Fermentation Engineering, College of Biotechnology and Bioengineering, Zhejiang University of
Technology, 18# Chaowang Road, Hangzhou 310032, China; lilyfan@zjut.edu.cn (Y.F.);
luyuele@zjut.edu.cn (Y.L.); syc@zjut.edu.cn (Y.S.)
National Center for Natural Products Research, Research Institute of Pharmaceutical Sciences and
2
Department of BioMolecular Sciences, School of Pharmacy, The University of Mississippi,
University, MS 38677, USA; btekwani@olemiss.edu (B.T.); xcli7@olemiss.edu (X.-C.L.)
*
Correspondence: richard_chen@zjut.edu.cn; Tel.: +86-571-88320571
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Muhammad Ilias and Charles L. Cantrell
Received: 25 September 2018; Accepted: 31 October 2018; Published: 5 November 2018
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: In the present study, 45 maleimides have been synthesized and evaluated for anti-leishmanial
activities against L. donovani in vitro and cytotoxicity toward THP1 cells. All compounds exhibited
obvious anti-leishmanial activities. Among the tested compounds, there were 10 maleimides with
superior anti-leishmanial activities to standard drug amphotericin B, and 32 maleimides with superior
anti-leishmanial activities to standard drug pentamidine, especially compounds 16
and 42 (IC50 < 0.0128 g/mL), which showed extraordinary efficacy in an in vitro test and low
cytotoxicities (CC50 > 10 g/mL). The anti-leishmanial activities of 16 and 42 were 10 times better
(IC50 < 0.0128 µg/mL)
µ
µ
than that of amphotericin B. The structure and activity relationship (SAR) studies revealed that
3,4-non-substituted maleimides displayed the strongest anti-leishmanial activities compared to those
for 3-methyl-maleimides and 3,4-dichloro-maleimides. 3,4-dichloro-maleimides were the least cytotoxic
compared to 3-methyl-maleimides and 3,4-non-substituted maleimides. The results show that several of
the reported compounds are promising leads for potential anti-leishmanial drug development.
Keywords: anti-leishmanial activity; Leishmania donovani; maleimides; cytotoxicity; SAR
1. Introduction
Leishmaniasis, which is caused by several species of Leishmania, is one of the major tropical diseases
defined by the World Health Organization (WHO), affecting about 12 million people [1,2]. A wide range
of clinical manifestations are encompassed, including visceral leishmaniasis, cutaneous leishmaniasis,
and mucocutaneous leishmaniasis. Among them, visceral leishmaniasis is the most severe form of the
disease. Visceral leishmaniasis, also called black fever or Kala-azar, has a fatality rate as high as 100%
within two years if untreated, and spontaneous cure is extremely rare [3,4]. Visceral leishmaniasis is
found throughout the intertropical and temperate regions, and threatens around 350 million people in
88 countries. Up to now, few medicines have been available for leishmaniasis. Pentostam is the most
widely used drug, which contains the heavy metal antimony. Other medicines, such as amphotericin B
and its derivatives [5], liposomal amphotericin B [5], paromomycin [6], and miltefosine [7], have their
individual problems, such as toxicity, poor efficacy, or high cost. Meanwhile, the emerging drug-resistant
parasites have caused further problems for therapy of the disease. Therefore, the discovery of new types of
medicines with novel chemical structures is highly desirable.
Molecules 2018, 23, 2878; doi:10.3390/molecules23112878
www.mdpi.com/journal/molecules

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Recently, other candidate compounds, including buparvaquone [8], aminoquinolines [9], peptoids [10],
(4-arylpiperazin-1-yl)(1-(thiophen-2-yl)-9H-pyrido[3,4-b]indol-3-yl) methanone derivatives [11], amino
acid-triazole hybrids [12], biscoumarins [13], triazolyl quinoline derivatives [14], benzopiperidine,
benzopyridine and phenyl piperazine based compounds [15], 2,5-diarylidene cyclohexanones [16],
natamycin [17], piperazinyl-β-carboline derivatives [18], thiosemicarbazones [19], and so on, were
evaluated and investigated. But for most of them, the IC50s of anti-leishmanial activities still remained to
be in micromolar ranges except thiosemicarbazones (two compounds had the IC50s of 0.060 µg/mL and
0.068 µg/mL against promastigotes of L. major.) [19].
Maleimides, including natural products with maleimide core moiety, had excellent
biological activities, including antimicrobial [20 28] and enzyme inhibition activities [29 31].
–
–
N-(4-Fluorophenyl)-dichloro-maleimide, significantly inhibiting fungal growth, was first developed
and used to control the diseases of apple scab, rice blast and tomato late blight [32]. The MICs
of N-butyl-maleimide and N-(4-phenylbutyl)-maleimide against 10 fungi were in the range of
0.48–15.63 µg/mL, similar to that of ampicillin, and had little toxicity to humans [33]. The antimicrobial
mechanism was investigated and found that maleimides could interact preferably with the hydrophobic
domains of target enzymes resulting in inactivation of sulfhydryl groups [29], which were essential for their
catalytic activities. The activities were greatly affected by the structure of C=C double bond in the circle of
diimide. For instance, N-ethyl-maleimide (NEM) and N-tert-butyl-maleimide inhibited
β-(1,3)-glucan
synthase with IC50 values 8.5 1.1 g/mL, 13.7 2.3 g/mL, respectively, and then influenced microbial
±
µ
±
µ
growth [34]. In addition to great antimicrobial activities, maleimides had also been widely researched in
medicine as antianxiety [35], anti-inflammatory [36], anticancer [37,38] and neuroprotective agents.
However, it should be noted that the reported studies were mainly focused on antimicrobial
activity. No anti-leishmanial activity of maleimides has been reported before this. In the present
work, a series of N-substituted maleimides, methyl-maleimides, and dichloro-maleimides were
synthesized, and their anti-leishmanial effects in vitro and cytotoxicity toward THP1 cells were
investigated. The results suggest that some of the synthesized maleimides might be developed as the
anti-leishmanial drugs in the future.
2. Results and Discussion
2.1. Chemistry
All maleimides were synthesized employing two methods according to an improved procedure
based on the reported methods [22,32,33], using amines and maleic anhydrides as starting materials
(Scheme 1). Path A was a facile method by one-step reaction to prepare 3,4-dichloromaleimides and
3-methylmaleimides with shorter reaction time, especially for synthesis of the former. However, path B
was used to prepare N-alkylmaleimides (1–4) by two-step reactions, dehydration and ring-closing
reaction. Furthermore, path A had higher yields and an easier isolation method when compared to
path B. The desired compound could be easily synthesized in good yield (more than 70%) in one or
two steps, as either crystalline or oily compound.
.
Scheme 1. Synthesis of N-substituted maleimide derivatives. Path A: a. CH3COOH. Path B: b. toluene,
◦
◦
25–65 C, 2–8 h; c. CH3COONa, (Et)3N, 101 C, 10–24 h.

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2.2. Biological Evaluations
2.2.1. Anti-leishmanial Activity
45 maleimides [20,29,32,33] (Figure 1) and two drugs, pentamidine (Figure 2) and amphotericin
B (Figure 3), were evaluated against L. donovani, to identify the most active compounds that are
worthy of further investigation (Table 1). Anti-leishmanial activity in vitro was described in terms of
IC50, which is the effective concentration (
promastigotes in their exponential growth phase. Most of tested compounds had good anti-leishmanial
activities (IC50s for 41 compounds were less than 1 g/mL). Among them, the IC50s of 16 and 42
were less than 0.0128 g/mL, much less than those of amphotericin B (IC50 = 0.12 g/mL) and
pentamidine (IC50 = 0.64 µg/mL), which reached nanogram grade. These activities were stronger than
most compounds in literature [ 19]. There were another eight candidate compounds ( 6, 7 13
14, and 41) which had better activities than amphotericin B.
µg/mL) required to achieve 50% growth inhibition, with
µ
µ
µ
8–
1,
3,
,
8,
,
Figure 1. Chemical structures of the studied N-substituted maleimides.

Molecules 2018, 23, 2878
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Figure 2. The chemical structure of pentamidine.
Figure 3. The chemical structure of amphotericin B.
Table 1. Anti-leishmanial activities and cytotoxicity of maleimides 45, pentamidine and amphotericin B.
1–
Compound
L. donvani IC50 (µg/mL) THP1 CC50 (µg/mL)
SI
LogP
1
2
3
4
5
6
7
8
0.08
0.36
0.11
0.27
2.21
0.08
0.10
0.11
0.24
0.15
0.32
1.02
0.10
0.08
0.21
<0.0128
0.26
0.58
0.81
0.96
0.58
1.22
13.17
0.55
0.34
0.64
0.89
0.47
0.65
0.49
0.48
1.64
0.40
0.76
>10
>10
>10
>10
>10
>125.0
>27.8
>90.9
>37.0
>4.5
57.0
0.72
1.14
1.56
2.39
4.06
1.21
1.49
1.91
1.14
1.63
1.30
1.70
2.12
2.95
1.62
0.70
1.03
1.07
1.91
2.74
1.56
1.84
2.47
3.30
2.61
1.05
1.38
0.80
1.22
1.63
2.47
1.57
1.99
1.22
4.56
4.13
8.25
>10
>10
>10
>10
3.65
3.89
>10
>10
>10
7.64
5.89
8.10
7.94
5.99
>10
4.39
0.80
>10
7.15
>10
>10
>10
>10
>10
>10
>10
41.3
75.0
9
>41.7
>66.7
>31.3
>9.8
36.5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
48.6
>23.2
>781.3
>38.5
13.2
7.3
8.4
13.7
4.9
>0.8
8.0
2.4
>15.6
8.0
>21.3
>15.4
>20.4
>20.8
>6.1
>25.0
>13.2

Molecules 2018, 23, 2878
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Table 1. Cont.
Compound
L. donvani IC50 (µg/mL) THP1 CC50 (µg/mL)
SI
LogP
35
36
37
38
39
40
41
42
0.66
0.36
0.59
0.45
0.13
0.13
0.11
<0.0128
0.99
0.43
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
ND
ND
>15.2
>27.8
>16.9
>22.2
>76.9
>76.9
>90.9
>781.3
>10.1
>23.3
>38.5
-
1.71
1.38
1.78
2.19
3.03
2.27
2.27
1.70
2.34
0.78
1.10
2.84
2.30
43
44
45
0.26
0.64
0.12
pentamidine
amphotericin B
-
2.2.2. Cytotoxicity
In order to verify the safety of maleimides, they were tested for cytotoxicity against human
monocytic leukemia cells (THP1) 50% cytotoxic concentration values represent the concentration of
compound required to kill 50% of the THP1 cells were calculated (CC50). The selectivity indices were
calculated using the formula SI = CC50 /IC50, against promastigotes. Interestingly, CC50s of 31 out
of 45 maleimides were greater than 10 µg/mL, which highlighted their safety on mammalian cells
(Table 1). SIs of 16 and 42 were greater than 781.3.
2.3. Structure-Activity Relationships
2.3.1. Influences of Substituents at the 3- and 4-Positions of the Maleimide Ring
As shown in Table 1, the introduction of substituents at the 3- and 4-positions on the maleimide
ring had different influences on the anti-leishmanial activities against L. donovani, depending on
the type of introduced substituents. In general, 3,4-non-substituted maleimides (
very strong anti-leishmanial activities, with IC50 values ranging from 0.0128 to 2.21
0.336 on the average, comparing to 1.774 g/mL for 3-methyl-maleimides and 0.501 g/mL for
3,4-dichloro-maleimides. Especially, 6, 7 13 14, and 16 showed more interesting anti-leishmanial
activities than the corresponding 3-methyl-maleimides and 3,4-dichloro-maleimides, which were
superior or much superior to amphotericin B (IC50 = 0.12 g/mL). In them, 15 compounds
had better activities than pentamidine (IC50 = 0.64 g/mL) except (IC50 = 2.21 g/mL) and
12 IC50 = 1.02 µg/mL). Moreover, 16 and 42 displayed the strongest anti-leishmanial activity
IC50 < 0.0128 µg/mL). However, there was no apparent regularity for the influences of variation in
substituents at the 3- and 4-positions on the maleimide ring. As to cytotoxicity, 3,4-dichloro-maleimides
were least cytotoxic, whose CC50s were all higher than 10 g/mL. Furthermore, 3-methyl-maleimides
1
–
17) displayed
µg/mL,
µ
µ
1,
3,
,
8,
,
µ
µ
5
µ
(
(
µ
(18–27) were the most cytotoxic, whose CC50s were all less than 10 µg/mL.
2.3.2. Influences of the N-Substituents
Influences of variation in the alkyl chain length. Results in Table 1 show that the N-alkyl substituents
(
1
to
chain increased, the anti-leishmanial activity decreased significantly. However,
highest anti-leishmanial activity, with an IC50 of 0.08 g/mL, which was one of the compounds
5
,
18 to 20 and 28 to 31) had influences on anti-leishmanial activity. As the length of the N-alkyl
1
exhibited the
µ
with the best activity. It could possibly be explained that with the change in the polarity and
N-alkyl chain length, their ability to connect enzymes differs. Therefore, with variation in chain
length,
1
to
5
,
18 to 20 and 28 to 31 had different influences on anti-leishmanial activities. It was
to 21 to 22 and 32 and 33) showed a correlation with the
observed that N-phenylalkyl substituents (
6
8,

Molecules 2018, 23, 2878
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anti-leishmanial activity against L. donovani. As the alkyl chain length of N-phenylalkyl substituents
increased, the anti-leishmanial activities decreased gradually (except for 33), which might be explained
by the N-C distance between the two rings playing an important role in the anti-leishmanial activities
of these compounds.
Influences of the substituted benzene ring. As for compounds with a mono-substituent on position 4
of phenyl ring, such as 10 to 12, and 35 to 37, it was obvious that the different groups had different
influences on the inhibition. However, there was no apparent regularity for the effects of variation with
a mono-substituent on position 4 of phenyl ring. Furthermore, compounds 13 to 14, 24 to 25 and 38 to
43 with double substituents on benzene ring displayed much stronger anti-leishmanial activity against
L. donovani. However, positions of the substituents had apparent influences on the inhibition behavior.
For example, N-2-methyl-3-nitro-substituted compound (42) exhibited the highest anti-leishmanial
activity (IC50 < 0.0128
µg/mL). And 13, 14, 39, 40, and 41 also showed very strong anti-leishmanial
activity (IC50 ranged from 0.08–0.13
µg/mL). In spite of these factors, steric hindrances might affect
anti-leishmanial activity. From the results in Table 1, it can be concluded that the logP value (predicted
by Chemoffice 2014) is an important parameter influencing the anti-leishmanial activities against
L. donovani. However, there was no apparent regularity between the IC50 values and log P values on
the whole.
3. Experimental Section
3.1. Chemistry General Details
Starting materials were obtained from Aldrich and used as received. Melting points (MP)
1
were measured with a WRS-1A melting point apparatus, and were uncorrected. H-NMR spectra
were recorded on a Bruker AVANCE III 500 spectrometer (Bruker, London, UK) at 500 MHz
using tetramethylsilane (TMS) as an internal standard. Electrospray ionization-mass spectra (EIMS)
were measured on a mass spectrometer (Thermo Fisher Scientific, LCQ/ADVANTAGE, 81 Wyman
Street, Waltham, MA, USA). IR spectrum was recorded in KBr pellets on a Nicolet 6700 infrared
spectrophotometer (Thermo Fisher Scientific, 81 Wyman Street, Waltham, MA, USA).
1
The structures of all the compounds were determined by IR, EI–MS, H-NMR. Note that
compounds
1
–
12
,
17
,
21
,
22
,
28
–
37 had been previously reported [27
,
33
,
39
–
43], which were coincident
1
with the previous report by EI MS and H-NMR. Physical and spectroscopic data of the other
compounds were shown in the supplementary material.
3.2. Biology
In vitro anti-leishmanial assay. The antileishmanial activity of the compounds was tested in vitro
against a culture of L. donovani promastigotes [44]. The promastigotes were grown in RPMI 1640
medium supple◦mented with 10% fetal calf serum (Gibco Chem. Co., 81 Wyman Street, Waltham,
MA, USA) at 26 C. A three day old culture was diluted to 5
×
105 promastigotes/mL. Drug dilutions
were prepared directly in cell suspension in 96-well plates. Plates were incubated at 26 ◦C for 48 h
and the growth of Leishmania promastigotes was determined by the Alamar blue assay as described
earlier [45]. Standard fluorescence was measured on a Fluostar Galaxy plate reader (BMG Lab
Technologies, Offenburg, Germany) at an excitation wavelength of 544 nm and an emission wavelength
of 590 nm. Pentamidine and amphotericin B were used as the standard anti-leishmanial agents. IC50
values were computed from dose-response curves as above. The tested compounds were diluted
with six concentrations (from 40–0.0128 µg/mL). Cytotoxicity assay. The in vitro cytotoxicity was
determined against human monocytic leukemia cells (THP1) with a simple colorimetric method using
the dye Alamar Blue [45]. THP1 suspensions were grown in RPMI-1640 medium supplemented with
◦
10% FBS, 2 mM glutamine, 50
in a 5% CO2 atmosphere. Cells were grown to a density between 0.2 and 1
µ
g/mL gentamicin and 0.0025 mg/L of amphotericin B (Sigma) at 37 C
×
106 cells/mL. Culture

Molecules 2018, 23, 2878
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medium was replaced every 2–3 days with fresh growth medium. DMSO was used as the solvent, and
the test compounds was with six concentrations from 10–0.0032 µg/mL.
4. Conclusions
In summary, a series of maleimides have been synthesized and evaluated for inhibitory activities
against L. donovani. All compounds exhibited obvious anti-leishmanial activities, especially with
compounds 16 and 42 showing extraordinary potency in an in vitro test and low cytotoxicity.
The anti-leishmanial activities of the two compounds were 10 times better than that of amphotericin
B. Therefore, further preclinical studies of 16 and 42 aimed at leishmaniasis are important for the
therapy of this neglected disease. The cytotoxicity of these compounds was low, with nearly no
toxicity (>10 µg/mL). Thus, compounds 16 and 42 are promising candidates for visceral leishmaniasis.
Among the tested compounds, there were 10 maleimides with superior anti-leishmanial activities to
amphotericin B, and 32 maleimides with superior anti-leishmanial activities to pentamidine. The SAR
study showed that 3,4-non-substituted maleimides displayed the strongest anti-leishmanial activities
compared to those 3-methyl-maleimides and 3,4-dichloro-maleimides. When the length of the alkyl
side chain (N-alkyl and N-phenylalkyl) increased, the anti-leishmanial activities decreased significantly.
There was no obvious regularity for the influences of variation with a mono-substituent on phenyl ring
position 4. And the position of the substituent had an obvious influence on the inhibition behavior.
As to cytotoxicity, 3, 4-dichloro-maleimides were least cytotoxic compared to 3-methyl-maleimides
and 3,4-non-substituted maleimides.
Supplementary Materials: The following are available online, the structure data of all maleimides and the
biological methods in detail.
Author Contributions: Conceptualization, Y.X. and X.C.; Methodology, Y.X. and B.T.; Software, Y.L.; Validation,
X.C., Y.S., B.T. and X.-C.L.; Formal Analysis, Y.L.; Investigation, X.C.; Resources, X.C.; Data Curation, Y.X. and Y.F.;
Writing-Original Draft Preparation, Y.F., X.C., Y.L. and X.-C.L.; Writing-Review & Editing, Y.F., Y.L. And X.-C.L.;
Visualization, Y.L. and Y.X.; Supervision, Y.S. and X.C.; Project Administration, X.C.; Funding Acquisition, X.C.
Funding: This study received financial support from the National Natural Science Foundation of China (Grant No.
21572206 and No. 21172198).
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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