In vitro Anti-Trypanosomal Activities of Indanone-Based Chalcones

Article abstract of DOI:10.1055/a-0775-0737

Human African trypanosomiasis is a neglected infectious disease that affects mostly people living in the rural areas of Africa. Current treatment options are limited to just four drugs that have been in use of four to nine decades. The life-threatening to

Full text of DOI:10.1055/a-0775-0737

Thieme
Original Article
Beteck RichardM et al. In vitro Anti-Trypanosomal Activi-
ties… Drug Res 2018; 00: 00–00
In vitro Anti-Trypanosomal Activities of Indanone-Based Chalcones
Authors
Richard M. Beteck1, Lesetje J. Legoabe1, Michelle Isaacs2, Heinrich C. Hoppe2,3
Affiliations
North-West University
Private Bag X6001
2520 Potchefstroom
South Africa
1
2
3
Centre of Excellence for Pharmaceutical Sciences,
North-West University, Potchefstroom, South Africa
Centre for Chemico- and Biomedicinal Research,
Rhodes University, Grahamstown, South Africa
Department of Biochemistry and Microbiology,
Rhodes University, Grahamstown, South Africa
Tel.: + 27/182/992 182
richmbi1@yahoo.com
ABstRACt
Key words
Human African trypanosomiasis is a neglected infectious dis-
ease that affects mostly people living in the rural areas of
Africa. Current treatment options are limited to just four drugs
that have been in use of four to nine decades. The life-threat-
ening toxic side-effects associated with the use of these drugs
are disconcerting. Poor efficacy, low oral bioavailability, and
high cost are other shortcomings of current HAT treatments.
Evaluating the potentials of known hits for other therapeutic
areas may be a fast and convenient method to discover new hit
compounds against alternative targets. A library of 34 known
indanone based chalcones was screened against T.b. brucei and
nine potent hits, having IC50 values between 0.5–8.9µM, were
found. The SAR studies of this series could provide useful infor-
mation in guiding future exploration of this class of compounds
in search of more potent, safe, and low cost anti-trypanosomal
agents. ▶Graphical Abstract.
indanones, chalcones, anti-trypanosomal, HTS, HAT
received 30.07.2018
accepted 17.10.2018
Bibliography
DOI https://doi.org/10.1055/a-0775-0737
Published online: 2018
Drug Res
© Georg Thieme Verlag KG Stuttgart · New York
ISSN 2194-9379
Correspondence
Richard M. Beteck
Centre of Excellence for Pharmaceutical Sciences,
School of Pharmacy
▶Graphical Abstract
and pathogenicity. T.b. gambiense is solely common in west Africa
and causes a chronic form of HAT [4, 5], while T.b rhodesiense is
endemic to east Africa and causes an acute form of HAT [6,7]. Both
forms of the disease are fatal if not treated [8].
Although tremendous success against HAT has been achieved
so far, with fewer deaths attributed to it [1], it’s still a public health
concern with 2184 new cases reported in 2016 [9] and 60 million
Introduction
Human African trypanosomiasis (HAT), a neglected infectious dis-
ease that is endemic to sub-Saharan Africa [1], arises from the bite
of an infected tsetse fly [2]. It is caused by two sub-species of Tryp-
anosoma brucei, namely T.b. gambiense and T.b rhodesiense[3].
The foregoing sub-species differ in their geographical preference
Beteck RM et al. Trypanocidal Chalcones … Drug Res

Thieme
Original Article
people currently at risk of contracting the disease [10] if effective
control measures are lacking. It is important to note that being a
neglected disease consigned to the rural areas of sub-Saharan
Africa where disease surveillance is challenged by several factors
[11], there exists a possibility that the burden of HAT is under-re-
ported.
Control and treatment of HAT relies solely on four chemothe-
rapeutic agents discovered 40 or more years ago: pentamidine,
suramin, malarsorprol, and eflornithine [12]. The foregoing drugs
are expensive and all suffer from poor oral bioavailability and hence
must be administered intravenously [13]. This is not an ideal route
of administration for a neglected disease, considering the correlate
that it requires special facilities and expertise not common in dis-
ease stricken areas. Moreover, these drugs cause life threatening
side effects such as hyperglycaemia, bone marrow toxicity, hypo-
glycaemia, and in some cases death [14]. There have also been is-
sues of treatment failure due the emergence of parasites resistant
to current treatment [15].
Chalcones are notable in that they are easy to synthesise [16].
Moreover, the therapeutic potentials of chalcones cannot be over
emphasised as compounds containing this scaffold have been pub-
lished for their anti-bacterial, anti-viral and anti-malarial potentials
[17–19]. These observations suggest that the chalcone class of
compounds is a potential source for the discovery of cheap and po-
tent anti-infectives.
We previously synthesised a series of indanone based chalcone
analogues and evaluated their inhibitory activities against recom-
binant human monoamine oxidase [20]. We were further interest-
ed in evaluating the anti-trypanosomal activities of this series. We
herein report potent trypanocidal compounds of the indanone-
based chalcone class.
The synthesis and characterisation of compounds employed in
this study have been previously reported [19, 21] and is depicted
in ▶Fig. 1 below. Briefly, 6/5-hydroxyl-1-indanone, 5-methoxyl-1-in-
danone, and 1-indanone were reacted with appropriate aromatic al-
dehydes using acidic or basic methanol as solvent. The compounds
were precipitated out of water and recrystallised from ethanol.
Compounds (1–34) were subjected to in vitro screening against
the 427 strain of T.b. brucei to evaluate their anti-trypanosomal
activities. After an incubation period of 48 h, the numbers of para-
sites surviving drug exposure were determined by adding resazurin.
Conversion of resazurin to resorufin by living cells was quantified
in a multiwell fluorescence plate reader (Exc560/Em590). Compounds
were tested in duplicate wells, and a standard deviation (SD) cal-
culated. Results are expressed as percentage (%) viability – the re-
sorufin fluorescence in compound-treated wells relative to untreat-
ed controls. Screening was performed at 20 μM final concentration
of synthesized compounds and pentamidine as the reference drug.
At 20 μM, almost half of the series inhibited parasite viability to
below 25 % (▶Fig. 2).
The aforementioned drawbacks associated with current treat-
ment underscore the need for alternative chemotherapeutic agents
against HAT.
In search of cheap anti-trypanosomal agents, we decided to sub-
ject our library of synthetic cyclic chalcone analogues to screening
against T.b. brucei.
▶Fig. 1 Synthesis of target compounds a. aReagents and condi-
tions: NaOH, MeOH, rt, 12h.
▶Fig. 2 Growth inhibitory effect of cyclic chalcones analogues on trypanosome parasites at 20 μM. At this concentration, 15 compounds inhibited
parasite viability to below 25%, which translates to a hit rate of almost 50%.
Beteck RM et al. Trypanocidal Chalcones … Drug Res

▶Fig. 3 Cytotoxicity potential of compounds evaluated against HeLa cell lines at 20 μM.
Compounds were further screened in vitro against a human cer-
vix adenocarcinoma (HeLa) cell line to assess their potential cyto-
toxicities. Screening was performed in duplicate using 20μM stock
solutions of synthesised compounds and emetine as the reference
drug, employing resazurin as a cell viability indicator as was done
in the trypanosome assay. Compounds resulting in less than 50%
Hela cell viability at 20 μM were assumed to have potential cyto-
toxicities and hence were not taken further for IC50 determination
(▶Fig. 3).
The nature of ring B also influences anti-trypanosomal activi-
ties. Compounds wherein ring B is a pyridyl unit elicited potent
anti-trypanosomal activities (e. g.20; IC50, 0.55µM), which in most
cases were superior to those of phenyl analogues (e. g.5; IC50
,
2.6 µM). Comparing compound 18 (IC50, 3.2 µM) and 21
(IC50 ≥ 200µM) suggests that substituting a pyridyl, or phenyl unit
with a 5 membered heteroaromatic ring leads to compounds de-
void of anti-trypanosomal activities.
Based on Hela cell viability result obtained, 9 out of the 15 com-
pounds inhibiting parasite viability to below 25 % had little effect
on HeLa cell viability (˃ 50 % cell viability at 20 µM). The observa-
tion posits that these compounds possess intrinsic anti-trypano-
somal activities and pose reduced cytotoxicity risk. These promis-
ing compounds were subjected to IC50 determination against
T.b. brucei and the results are summarised in ▶table 1.
In comparison with compound 1 (IC50, 3.0µM), structure activ-
ities relationship (SAR) analysis suggest that the presence of an –
OH group at position -5 of ring A with concurrent attachment of a
halogen to ring B promotes anti-trypanosomal activities. This is ev-
ident in compound 3 for example, which has a lower IC50 value of
1.9 µM. We also observed that the substitution pattern on
ring B greatly influences activities. For instance, the substitution of
a bromine atom in compound 7 (IC50, 2.5µM) with a nitrile unit as
is the case in compound 11 leads to almost a three-fold loss in ac-
tivity (IC50, 8.9 µM). This suggests that electron donating groups
attached to ring B tend to favour activity over electron withdraw-
Conclusion
Human African trypanosomiasis is a neglected infectious disease
that affects mostly people living in the rural areas of Africa. Current
treatment options are limited to just four drugs that have been in
use for more than four decades. The life-threatening toxic side-ef-
fects associated with the use of these drugs are alarming. Other
shortcomings of current HAT treatments include poor efficacy, low
oral bioavailability and high cost. All of these factors necessitate
the search for new trypanocidal agents.
Being a neglected disease that affects mostly the poorest commu-
nities in sub-Saharan Africa, an ideal anti-trypanosomal agent should
be relatively cheap. To this end, we decided to investigate the anti-tryp-
anosomal potential of indanone based chalcone analogues. This com-
pound class is cheap to obtain as it is synthesised from readily available
starting materials in a simple single synthetictransformation. The anti-
trypanosomal activities of this series were established by screening
against T.b. brucei. Nine potent hits, all having no cytotoxic effect on a
HeLa cell line (˃50% cell viability at 20µM), were discovered.
ing groups. Also, SAR comparison between compound 5 (IC50
,
2.6µM) and compound 12 (IC50, 0.93µM) shows clearly that anti-
trypanosomal activities increase with increased halogenation of
ring B. This remark is also evident when comparing compounds 18
(IC50, 3.2µM), which lacks a chlorine atom on ring B and its analogue
compound 20 (IC50, 0.55µM) having a chlorine atom on ring B.
It is noteworthy to highlight that the physicohemical properties of a
hit compound such as molecular weight, lipophilicity, total polar sur-
faceareaincreaseduringhitoptimizationandoftenleadtocompounds
with improved activities against the target in question, but poor lead-
likeproperties. Inanattempttoavoidpoorlead-likepropertiesatalater
Beteck RM et al. Trypanocidal Chalcones … Drug Res

Thieme
Original Article
▶table 1 Structures, percent viability of parasite cells and IC50 values of the most active and non-toxic compounds in vitro.
Compounds
%viability (IC50 µM)
Entry
1
R
R′(B)
MW
220
236
254
270
270
315
315
250
250
261
261
305
252
252
266
279
278
221
221
255
210
226
209
226
263
250
293
256
251
251
240
256
254
319
–
ClogP
3.5
3.4
3.6
3.6
4.1
4.3
4.3
3.9
3.9
2.9
2.9
4.7
2.8
2.8
3.3
3.6
4.8
1.9
1.9
2.7
2.7
3.2
2.2
3.2
3.7
3.8
3.9
5.2
2.3
22.3
2.9
3.4
3.4
3.8
–
tPsA
17.1
37.3
37.3
37.3
37.3
37.3
37.3
37.3
37.3
61.1
61.1
37.3
57.5
57.5
46.5
40.5
37.3
29.4
29.4
29.4
26.3
17.1
29.1
17.1
20.3
26.3
29.5
26.3
38.6
38.6
35.5
26.3
35.5
35.5
–
T.b. brucei
−0.25(3.0)
1.5 (ND)
19.2(1.9)
68.6(ND)
3.6(2.6)
-
phenyl
2
6-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
-
phenyl
3
4-F-phenyl
4-Cl-phenyl
3-Cl-phenyl
4-Br-phenyl
3-Br-phenyl
4-CH3-phenyl
3-CH3-phenyl
4-CN-phenyl
3-CN-phenyl
3,4-di-Cl-phenyl
4-OH-phenyl
3-OH-phenyl
4-OCH3-phenyl
4-N(CH3)2-phenyl
4-CH(CH3)2-phenyl
3-pyridyl
4
5
6
97.6(ND)
18.9(2.5)
57.6(ND)
84.9(ND)
80.6(ND)
23.0(8.9)
2.0(0.93)
18.0(2.9)
16.9(ND)
77.4(ND)
62.4(ND)
66.8(ND)
16.0(3.2)
0.35(ND)
0.69(0.55)
72.2(ND)
97.8(ND)
95.3(ND)
12.1(ND)
84.5(ND)
96.6(ND)
97.3(ND)
99.1(ND)
0.31(ND)
0.48(ND)
49.1(ND)
86.9(ND)
84.2(ND)
97.0(ND)
(0.0095)
7
8
9
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
PE
-
2-pyridyl
-
2-Cl-3-pyridyl
2-furanyl
-
-
2-thiophenyl
2-pyrroyl
-
-
3-thiophenyl
4-N(CH3)2-phenyl
phenyl
-
5-OCH3
5-OCH3
5-OCH3
5-OCH3
5-OCH3
5-OCH3
5-OCH3
5-OCH3
5-OCH3
-
4-N(CH3)2-phenyl
cyclohexyl
2-pyridyl
3-pyridyl
2-furanyl
2-thiophenyl
5-CH3-2-furanyl
5-Br-2-furanyl
-
ND, not determined; PE, pentamidine
Acknowledgements
stage of drug discovery, it has been widely recommended that lead-like
hitsbeprioritisedasstartingpointforhitoptimization. Lead-likehitsare
compounds having molecular weight ≤350, ClogP≤4.2 and tPSA≤90
Å2. Property profiling of our hit series (▶table 1) shows that they
meet the criteria set out for lead-like hits, making them worth con-
sidering for further hit optimization studies.
The authors acknowledge the financial support by North-West Uni-
versity, Potchefsoomcampus towards this research. The bioassaycom-
ponent of the project was funded by the South African Medical Re-
search Council (MRC) with funds from National Treasury under its Eco-
nomic Competitiveness and Support Package awardedto Prof. Hoppe.
Beteck RM et al. Trypanocidal Chalcones … Drug Res

Conflict of Interest
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chemotherapy strategies for the treatment of human African
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