Evaluation of nitrocatechol chalcone and pyrazoline derivatives as inhibitors of catechol-O-methyltransferase and monoamine oxidase

Article abstract of DOI:10.1016/j.bmcl.2020.127188

Literature reports that chalcones inhibit the monoamine oxidase (MAO) enzymes, mostly with specificity for the MAO-B isoform, while nitrocatechol compounds are established inhibitors of catechol–O-methyltransferase (COMT). Based on this, nitrocatechol der

Full text of DOI:10.1016/j.bmcl.2020.127188

Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Evaluation of nitrocatechol chalcone and pyrazoline derivatives as inhibitors
of catechol-O-methyltransferase and monoamine oxidase
Rialette Hitge, Sharissa Smit, Anél Petzer, Jacobus P. Petzer⁎
Pharmaceutical Chemistry, School of Pharmacy and Centre of Excellence for Pharmaceutical Sciences, North-West University, Potchefstroom 2520, South Africa
A R T I C L E I N F O
A B S T R A C T
Keywords:
Literature reports that chalcones inhibit the monoamine oxidase (MAO) enzymes, mostly with specificity for the
MAO-B isoform, while nitrocatechol compounds are established inhibitors of catechol–O-methyltransferase
(COMT). Based on this, nitrocatechol derivatives of chalcone have been proposed to represent dual-target-di-
rected compounds that may inhibit both MAO-B and COMT. Both these enzymes play key roles in the meta-
bolism of dopamine and levodopa, and inhibitors are thus relevant to the treatment of Parkinson’s disease. The
present study expands on the discovery of dual MAO-B/COMT inhibitors by synthesising additional nitrocatechol
derivatives of chalcones which include heterocyclic derivatives, and converting them to the corresponding
pyrazoline derivatives. The newly synthesised chalcone and pyrazoline compounds were evaluated as inhibitors
of human MAO and rat COMT, and the inhibition potencies were expressed as IC50 values. A pyrazoline deri-
vative, compound 8b, was the most potent COMT inhibitor with an IC50 value of 0.048 μM. This is more potent
than the reference COMT inhibitor, entacapone, which has an IC50 value of 0.23 μM. The results indicated that
the pyrazoline derivatives (IC50 = 0.048–0.21 µM) are more potent COMT inhibitors than the chalcones
(IC50 = 0.14–0.29 µM). Unfortunately, the chalcone and pyrazoline derivatives were weak MAO inhibitors with
IC50 values > 41.4 µM. This study concludes that the nitrocatechol derivatives investigated here are promising
COMT inhibitors, while not being suitable as MAO inhibitors. Using molecular docking, potential binding modes
and interactions of selected inhibitors with COMT are proposed.
Monoamine oxidase
Catechol-O-methyltransferase
Chalcone
Pyrazoline
Multi-target-directed
Parkinson’s disease
Monoamine oxidase (MAO) metabolises biogenic amines in the
central and peripheral tissues, and thus plays an important role in the
inactivation of neurotransmitters.1–3 Two isoenzymes of MAO have
been identified, MAO-A and MAO-B, and inhibitors of these enzymes
have found application in the treatment of neuropsychiatric and neu-
rodegenerative disorders. In this regard, MAO-A degrades serotonin and
noradrenaline in the central nervous system (CNS), and MAO-A in-
hibitors have been useful for the treatment of psychiatric illnesses such
as depression.1,4,5 In the CNS, MAO-B is a major metabolic enzyme for
dopamine and β–phenylethylamine, and MAO-B inhibitors have thus
been used for the treatment of neurodegenerative disorders, particu-
larly Parkinson’s disease.2 The rationale for the use of specific MAO-B
inhibitors in Parkinson’s disease is based on the inhibition of dopamine
metabolism which results in the enhancement of striatal dopaminergic
activity.6 MAO-B inhibitors are often co-administered with levodopa in
an attempt to further enhance dopamine levels following levodopa
treatment.7,8 A more theoretical rationale for the use of MAO-B in-
hibitors in patients with Parkinson’s disease originates from the ob-
servation that hydrogen peroxide is produced as by-product of MAO
catalysis. Since hydrogen peroxide may lead to oxidative damage, it has
been postulated that hydrogen peroxide produced by MAO-B in the
brain may contribute to neurodegeneration in Parkinson’s disease.
MAO-B inhibitors have thus been advocated as potential neuroprotec-
tive agents.2
arboxylates levodopa in thLe-agmaisntrooianctiedstidneaclartrbaocxtylaansde l(iAvAerDCto), ydieelcd-
dopamine, and subsequently only a fraction a levodopa dose reaches
the systemic circulation.9 The combination of levodopa with an AADC
inhibitor such as carbidopa or benserazide, increases the fraction of
levodopa that reaches the systemic circulation, thus improving the
bioavailability of levodopa to the brain.10,11 Dopamine as well as le-
vodopa is also metabolised by catechol–O–methyltransferase (COMT)
in the central and peripheral tissues. In the periphery, COMT inhibitors
such as tolcapone and entacapone reduce the O–methylation of levo-
dopa, and thus enhance the fraction of levodopa that is available for
uptake into the brain.12–15 Central inhibitors of COMT (e.g. tolcapone)
may also reduce levodopa and dopamine metabolism in the brain,
which further may improve the therapeutic efficacy of levodopa.16
The enzyme, aromatic
⁎ Corresponding author.
E-mail addresses: 12264954@nwu.ac.za (A. Petzer), jacques.petzer@nwu.ac.za (J.P. Petzer).
https://doi.org/10.1016/j.bmcl.2020.127188
Received 14 February 2020; Received in revised form 8 April 2020; Accepted 9 April 2020
0960-894X/©2020ElsevierLtd.Allrightsreserved.
Pleasecitethisarticleas:RialetteHitge,etal.,Bioorganic&MedicinalChemistryLetters,https://doi.org/10.1016/j.bmcl.2020.127188

R. Hitge, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
Fig. 1. The structures of chalcone compounds discussed in the text.
COMT inhibitors are usually administered in conjunction with a com-
bination of carbidopa/levodopa. The inhibition of levodopa metabolism
by AADC and COMT inhibitors allows for a reduction in the levodopa
dose, which greatly reduces the potential of levodopa-associated ad-
verse effects.17,18 Levodopa continues to be the gold standard drug for
the symptomatic treatment of Parkinson’s disease, and because of this
much interest in the development of inhibitors of the COMT enzyme
exists.16,19
Table 1
The IC50 values for the inhibition of human MAO-A and MAO-B, and the IC50
values for the inhibition of rat liver COMT by the chalcone and pyrazoline
compounds and reference inhibitors.
Based on the roles of MAO-B and COMT in the metabolism of do-
pamine and levodopa, dual inhibitors of these enzymes may synergise
to enhance dopaminergic neurotransmission and provide an improved
therapeutic outcome in Parkinson’s disease. Chalcones are known to
inhibit the MAOs, mostly with specificity for the MAO-B isoform.20 For
example, chalcone 1 is a high potency MAO-B inhibitor with an IC50
value 0.0044 µM, while no inhibition of MAO–A is observed at con-
centrations of 50 µM (Fig. 1).20 Heterocyclic chalcone derivatives such
as 2 and 3 are also known to inhibit MAO-B. These compounds inhibit
MAO-B with IC50 values of 0.174 µM and 0.067 µM, respectively.21,22
The nitrocatechol moiety, in turn, is present in the clinically used
COMT inhibitors such as tolcapone and entacapone, and is considered
privileged for the inhibition of COMT. Based on this, it has been pro-
posed that nitrocatechol derivatives of chalcone may exhibit dual in-
hibition of COMT and MAO-B.23 A recent study thus showed that ni-
trocatechol derivatives of chalcone are high potency inhibitors of
COMT, although only moderate MAO-B inhibition was observed. For
example, chalcone 4 is a potent COMT inhibitor (IC50 = 0.29 µM) while
also acting as a moderately potent MAO-B inhibitor (IC50 = 13.9 µM).23
Chalcone 5 was the most potent COMT inhibitor reported in the lit-
erature study (IC50 = 0.07 µM).23
R
IC50 (μM)
MAO-A
SD
IC50 (μM)
COMT
SD
MAO-B
6a
6b
6c
3-CN-C6H4- 58.9
4-CN-C6H4- 45.3
2.45
2.52
4.69
60.6
56.6
55.8
5.17
7.36
5.00
0.24
0.18
0.14
0.07
0.06
0.04
3-OH-
43.9
C6H4-
7a
7b
7c
7d
7e
7f
2-thienyl
3-thienyl
2-thiazolyl
3-furyl
2-furyl
1-methyl-
1H-
49.9
41.4
69.6
52.9
60.5
51.5
4.53
3.09
5.27
2.34
2.99
4.89
62.9
42.1
59.2
71.4
68.2
66.2
7.58
4.14
9.75
2.99
0.19
0.23
0.24
0.29
0.05
0.05
0.03
0.03
0.03
0.03
12.49 0.23
3.57
0.29
pyrazol-4-
yl
8a
8b
8c
3-CN-C6H4- 70.6
4-CN-C6H4- 76. 6
7.47
7.43
2.98
55.4
83.2
85.2
3.21
14.5
7.48
0.079
0.048
0.075
0.009
0.03
0.03
To expand on the structure–activity relationships (SARs) for dual
MAO/COMT inhibition, this study investigated additional nitrocatechol
derivatives. This study included three nitrocatechol derivatives of
chalcone (6a–c) bearing polar functional groups on ring B since these
are expected to increase inhibition of MAO-B (Table 1). In particular,
the nitrile functional group was considered as polar substituent since
benzo- and phthalonitriles are well-known to potently inhibit MAO-B.24
Hydroxy substitution also is known to enhance MAO-B inhibition.25
This effect of polar substitution on MAO B inhibition is due to the in-
teraction of the polar groups with the polar region of the MAO-B active
site in proximity to the FAD. For the first time, heterocyclic chalcone
derivatives (7a–f) that incorporate the nitrocatechol moiety will be
investigated as potential dual MAO/COMT inhibitors. As mentioned
above, heterocyclic chalcone derivatives have been reported to act as
good potency MAO-B inhibitors. This study will also convert some of
the chalcone compounds to the corresponding pyrazoline derivatives
3-OH-
87.0
C6H4-
9a
2-thiazolyl
3-furyl
84.6
90.9
98.0
86.3
–
4.90
3.22
6.62
68.2
126
12.64 0.16
3.06 0.18
0.05
9b
0.05
0.05
0.05
0.05
9c
2-furyl
77.4
17.16 0.18
9d
2-thienyl
11.26 69.2
–
2.82
0.21
0.21
0.23
–
Entacapone
Curcumin
5.02
0.45
2.56
(8a–c; 9a–d) since this class of compounds have been reported to be
good potency MAO-B inhibitors.26,27 The possibility that pyrazolines
may inhibit COMT has not been investigated before.
The nitrocatechol derivatives of chalcone (6a–c; 7a–f) were syn-
thesised according to the reaction scheme given in Fig. 2. The synthetic
route consisted of three steps. Firstly, nitration of 4-hydroxy-3-meth-
oxyacetophenone (apocynin; 10) was carried out with nitric acid in the
2

R. Hitge, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
Fig. 2. The synthetic route for the synthesis of nitrocatechol derivatives of
chalcone and the pyrazoline derivatives. Reagents and conditions: (a) HNO3,
acetic acid, rt; (b) AlCl3, pyridine, ethyl acetate, 77 °C; (c) ethanol, KOH, RCHO,
rt; (d) acetic acid, hydrazine hydrate, 120 °C.
Fig. 3. Sigmoidal plots for the inhibition of COMT by 7f (open circles) and 9c
presence of acetic acid to yield the nitro derivative 11 (5–ni-
troapocynin). In the second step, demethylation of 11 was carried out
with AlCl3/pyridine to yield the 5-acetyl-3-nitrocatechol 12.23,28 The
target chalcones (6a–c; 7a–f) were obtained via the Claisen-Schmidt
condensation reaction between 12 and an appropriately substituted
aldehyde in ethanol. Potassium hydroxide served as the base.29 The
nitrocatechol derivatives of chalcone were purified by crystallisation
from an appropriate solvent (yields: 12–35%). The pyrazoline deriva-
tives (8a–c; 9a–d) were synthesised by reacting the corresponding
chalcones with hydrazine hydrate in the presence of acetic acid.26,30
The yields for these reactions ranged from 38 to 96%. The chalcone and
pyrazoline derivatives were characterised by NMR and MS, while the
purity was assessed by HPLC (see supplementary material).
(filled circles).
the nitrocatechol derivatives of chalcone (6a–c; 7a–f) and the pyrazo-
line derivatives (8a–c; 9a–d) are good potency inhibitors of COMT with
IC50 values < 0.29 μM. The pyrazoline derivatives are more potent
inhibitors than the chalcones, with 8b being the most potent inhibitor
with an IC50 value of 0.048 μM. COMT inhibitors containing the 3-ni-
trocatechol moiety that have been developed and introduced into the
market include entacapone (IC50 = 0.23 µM) that display similar po-
tency under these experimental conditions. Among the heterocyclic
derivatives, pyrazoline 9a was found to be the most potent inhibitor
with an IC50 value of 0.16 µM.
It is noteworthy that all nitrocatechol derivatives of chalcone and
the pyrazoline derivatives are good potency COMT inhibitors. This
shows that the B-ring does not have a significant effect on COMT in-
hibition. The B-ring can thus be explored to improve MAO-B inhibition
The MAO inhibitory properties of the nitrocatechol derivatives of
chalcone and the pyrazoline derivatives were investigated with the
recombinant human MAO enzymes. The MAO activity measurements
were carried out with kynuramine as substrate, which is converted by
MAO to yield the fluorescent compound, 4-hydroxyquinoline.31,32 After
measuring the catalytic activities of MAO-A and MAO-B in the absence
and presence (0.003–100 µM) of the test inhibitors, IC50 values were
as it is unlikely to affect COMT inhibition to
a large extent.
Nitrocatechol derivatives of chalcone and the corresponding pyrazoline
derivatives may serve as leads for the future design of potent COMT
inhibitors. This study is the first report of COMT inhibition by pyr-
azoline compounds and thus proposes the further development of these
nitrocatechol derivatives as potentially clinically useful COMT in-
hibitors. It should be kept in mind that the pyrazoline derivatives are
chiral and represent the racemates of two enantiomers. It is not clear if
both enantiomers contribute equally to COMT inhibition, but future
studies should separate the enantiomers of a representative inhibitor
(e.g. 8b) to determine the stereochemistry of the eutomer and distomer.
As mentioned, nitrocatechol derivatives of chalcone have previously
been investigated as COMT inhibitors.23 In accordance with the find-
ings of this study, the pyrazoline compounds are significantly more
potent COMT inhibitors than the chalcones investigated previously. The
most potent pyrazoline compound (8b, IC50 = 0.048 µM) is approxi-
mately 1.46-fold more potent that the most potent chalcone (5,
IC50 = 0.07 µM) reported in literature.23
estimated in triplicate and are reported here as the means
standard
deviation (SD). These IC50 values are given in Table 1 and show that the
nitrocatechol derivatives of chalcone (6a–c; 7a–f) and the pyrazoline
derivatives (8a–c; 9a–d) are relatively weak inhibitors of MAO-A and
MAO-B, with IC50 values > 41.4 μM. These values are significantly
higher than those of the reference inhibitor, curcumin, which inhibit
MAO-A and MAO-B with IC50 values of 5.02 µM and 2.56 µM, respec-
tively. It is well-known that benzo- and phthalonitriles inhibits MAO-B
potently, and hydroxy substitution is also known to enhance MAO-B
inhibition of chalcones.24,25 Since chalcones are in general good po-
tency inhibitors of MAO-B, particularly compounds containing polar
groups on the A-ring (e.g. OH), it may be concluded that nitro sub-
stitution on the A-ring greatly diminishes MAO-B inhibition.20 It is in-
teresting to note that the pyrazoline derivatives are in general weaker
MAO inhibitors than the corresponding chalcones. The only exception
is MAO-B inhibition by 8a versus 6a.
To obtain insight into potential binding orientations and interac-
tions of pyrazoline compound 8b with the COMT active site, molecular
docking was carried out. For comparison, the corresponding chalcone,
6b, was also investigated. For this study, rat COMT complexed with 3,5-
dinitrocatechol was used as protein model (PDB code: 1VID).36 Pre-
paration of the protein models and ligands were carried out according
to the previously reported protocol with Discovery Studio, while
docking was done with AutoDock Vina.31,37 The accuracy of the
docking procedure was assessed by redocking the co-crystallized ligand
into the COMT active site, which yielded a root mean square deviation
(RMSD) of 0.684 Å between the docked orientation and the position of
the co-crystallised ligand (Fig. 4). In the crystal structure, the Mg2+ co-
factor is octahedrally coordinated to the side chains of Asp141 and
Asp169, Asn170, the two hydroxy groups of the catechol substrate, and
To determine whether the synthesised compounds are inhibitors of
COMT, soluble fractions obtained from homogenates of rat liver tissue
were used as enzyme source.23,33,34 Ethical approval for the collection
and use of animal tissue was obtained from the Research Ethics Com-
mittee of the North-West University (ethics approval numbers: NWU-
00561-19-S5 and NWU-00562-19-S5). Esculetin (6,7–dihydrox-
ycoumarin) served as substrate and is methylated by COMT to yield
scopoletin, a reaction that may be monitored by fluorescence spectro-
photometry.35 COMT catalytic activity was thus measured in the ab-
sence and presence (0.003–80 µM) of the test inhibitors, and IC50 values
were estimated in triplicate and are given as the means
SD (Fig. 3).
The IC50 values that were recorded are given in table 1, and show that
3

R. Hitge, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
Fig. 4. The co-crystallised (teal) and docked (magenta) orientations of 3,5-dinitrocatechol in the active site of rat COMT (PDB code: 1VID). The orientation of
tolcapone (green) in the active site of rat COMT (PDB code: 3S68).
Fig. 5. The docked orientations of 6b (purple) and the R- (yellow) and S-enantiomers (orange) of 8b in the active site of rat COMT.
a water molecule.36,38 The 1-hydroxy group of 3,5-dinitrocatechol is
hydrogen bonded to Glu199 and Asn170, and the 2-hydroxy and 3-nitro
groups are hydrogen bonded to Lys144. It may be concluded that the
formation of a hydrogen bond network is crucial for the binding of
nitrocatechol compounds to COMT.
groups of the initrocatechol moiety undergoes similar coordination
with the Mg2+ as 3,5-dinitrocatechol above, are presented here as these
are most probable. The crystal structure of tolcapone in complex with
rat COMT also exhibit this orientation and interaction pattern.39 As
expected, chalcone 6b adopts a similar binding orientation to 3,5-di-
nitrocatechol and forms a similar interaction network (Fig. 5). In this
respect, the catechol OH groups coordinates with the magnesium ion,
Both enantiomers of 8b as well as 6b were docked into COMT, and
among the solutions generated only those where the catechol hydroxyl
4

R. Hitge, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
while hydrogen bonding occurs with Lys144, Asn170 and Glu199. The
B-ring of the chalcone projects out of the active site and undergoes very
limited interactions, mainly a pi-alkyl interaction with Pro174, while
the nitrocatechol ring forms a pi-alkyl interaction with Met40.
The results of the docking study further show that 3,5-dini-
trocatechol of 8b is correctly placed for coordination with the Mg2+
and the two enantiomers bind similar with respect to the 3,5-dini-
trocatechol moieties. The two enantiomers exhibit very similar binding
orientations and interactions. The catechol OH groups coordinates with
the magnesium ion, while hydrogen bonding occurs with Lys144,
Asn170 and Glu199. The nitrocatechol rings of both enantiomers form
pi-alkyl interactions with Pro174 and Met40, while the pyrazoline rings
form pi-alkyl interactions with Pro174. Interestingly, for the S-en-
antiomer, a hydrogen bond exists between the nitrile and Lys5.
Compared to chalcone 6b, the side chain of the S-enantiomer of 8b
undergoes more extensive interactions which may explain the better
inhibition potencies observed for the pyrazolines. However, the
AutoDock Vina binding affinity values (kcal/mol) for 6b and 8b are
very similar, which underscores the observation that the polar inter-
actions of the nitrocatechol moieties of these compounds are virtually
identical.
References
1. Youdim MB, Edmondson D, Tipton KF. The therapeutic potential of monoamine
oxidase inhibitors. Nat Rev Neurosci. 2006;7:295–309. https://doi.org/10.1038/
nrn1883.
2. Youdim MB, Bakhle YS. Monoamine oxidase: isoforms and inhibitors in Parkinson's
disease and depressive illness. Br J Pharmacol. 2006;147:S287–S296. https://doi.org/
10.1038/sj.bjp.0706464.
3. Tipton KF. 90 years of monoamine oxidase: some progress and some confusion. J
Neural Transm (Vienna). 2018;125:1519–1551. https://doi.org/10.1007/s00702-
018-1881-5.
4. Lum CT, Stahl SM. Opportunities for reversible inhibitors of monoamine oxidase-A
(RIMAs) in the treatment of depression. CNS Spectr. 2012;17:107–120.
5. Naoi M, Maruyama W, Shamoto-Nagai M. Type A monoamine oxidase and serotonin
are coordinately involved in depressive disorders: from neurotransmitter imbalance
to impaired neurogenesis. J Neural Transm (Vienna). 2018;125:53–66. https://doi.
org/10.1007/s00702-017-1709-8.
6. Fernandez HH, Chen JJ. Monoamine oxidase-B inhibition in the treatment of
Parkinson's disease. Pharmacotherapy. 2007;27:174s–185s. https://doi.org/10.1592/
phco.27.12part2.174S.
7. Birkmayer W, Knoll J, Riederer P, Youdim MB. (-)-Deprenyl leads to prolongation of
L-dopa efficacy in Parkinson's disease. Mod Probl Pharmacopsychiatry.
1983;19:170–176.
8. Riederer P, Muller T. Monoamine oxidase-B inhibitors in the treatment of Parkinson's
disease: clinical-pharmacological aspects. J Neural Transm. 2018;125:1751–1757.
https://doi.org/10.1007/s00702-018-1876-2.
9. Seeberger LC, Hauser RA. Carbidopa levodopa enteral suspension. Expert Opin
Pharmacother. 2015;16:2807–2817. https://doi.org/10.1517/14656566.2015.
1111336.
In this study, series of novel nitrocatechol derivatives of chalcone
and pyrazoline derivatives were investigated with the aim of dis-
covering compounds that exhibit dual inhibition of MAO-B and COMT.
The results show that the study compounds are weak MAO inhibitors. In
contrast to their MAO inhibition potencies, the nitrocatechol chalcone
and pyrazoline derivatives are potent inhibitors of COMT with IC50
values < 0.29 μM. The pyrazoline derivatives were found to be more
potent inhibitors than the chalcones. Chalcones contain the α,β-usatu-
rated carbonyl system which may be considered to be a Michael ac-
ceptor. Such compounds may act as electrophiles and could therefore
be a liability from a medicinal chemistry point of view.40,41 In this re-
gard, the pyrazolines are more desirable for the future development of
COMT inhibitors, and they are expected to be less reactive than the
corresponding chalcone compounds. A further point of interest is that
all nitrocatechol derivatives of chalcone and pyrazoline derivatives are
good potency COMT inhibitors, which may be attributed to the 3-ni-
trocatechol moiety (A-ring). The B-ring therefore does not affect COMT
inhibition to a large degree, and thus represents a site for future
structural modification to improve MAO-B inhibition potency. Even in
the absence of good potency MAO-B inhibition, nitrocatechol-con-
taining pyrazoline derivatives may represent useful leads for the future
design of COMT inhibitors.
10. Freitas ME, Ruiz-Lopez M, Fox SH. Novel Levodopa formulations for Parkinson's
disease. CNS Drugs. 2016;30:1079–1095. https://doi.org/10.1007/s40263-016-
0386-8.
11. Poewe W, Antonini A. Novel formulations and modes of delivery of levodopa. Mov
Disord. 2015;30:114–120. https://doi.org/10.1002/mds.26078.
12. Learmonth DA, Palma PN, Vieira-Coelho MA, Soares-da-Silva P. Synthesis, biological
evaluation, and molecular modeling studies of a novel, peripherally selective in-
hibitor of catechol-O-methyltransferase. J Med Chem. 2004;47:6207–6217. https://
doi.org/10.1021/jm040848o.
13. Nissinen E, Linden IB, Schultz E, Pohto P. Biochemical and pharmacological prop-
erties of a peripherally acting catechol-O-methyltransferase inhibitor entacapone.
Naunyn Schmiedebergs Arch Pharmacol. 1992;346:262–266.
14. Salamon A, Zadori D, Szpisjak L, Klivenyi P, Vecsei L. Opicapone for the treatment of
Parkinson's disease: an update. Expert Opin Pharmacother. 2019;20:2201–2207.
https://doi.org/10.1080/14656566.2019.1681971.
15. Akhtar MJ, Yar MS, Grover G, Nath R. Neurological and psychiatric management
using COMT inhibitors: a review. Bioorg Chem. 2020;94:103418https://doi.org/10.
1016/j.bioorg.2019.103418.
16. Silva T, Mohamed T, Shakeri A, et al. Development of blood-brain barrier permeable
nitrocatechol-based catechol O-methyltransferase inhibitors with reduced potential
for hepatotoxicity. J Med Chem. 2016;59:7584–7597. https://doi.org/10.1021/acs.
jmedchem.6b00666.
17. Guldberg HC, Marsden CA. Catechol-O-methyl transferase: pharmacological aspects
and physiological role. Pharmacol Rev. 1975;27:135–206.
18. Mannisto PT, Kaakkola S. Catechol-O-methyltransferase (COMT): biochemistry,
molecular biology, pharmacology, and clinical efficacy of the new selective COMT
inhibitors. Pharmacol Rev. 1999;51:593–628.
19. Learmonth DA, Vieira-Coelho MA, Benes J, Alves PC, Borges N, Freitas AP. da-Silva
PS. Synthesis of 1-(3,4-dihydroxy-5-nitrophenyl)-2-phenyl-ethanone and derivatives
as potent and long-acting peripheral inhibitors of catechol-O-methyltransferase. J
Med Chem. 2002;45:685–695.
Declaration of Competing Interest
20. Chimenti F, Fioravanti R, Bolasco A, et al. Chalcones: a valid scaffold for monoamine
oxidases inhibitors. J Med Chem. 2009;52:2818–2824. https://doi.org/10.1021/
jm801590u.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
21. Robinson SJ, Petzer JP, Petzer A, Bergh JJ, Lourens AC. Selected furanochalcones as
inhibitors of monoamine oxidase. Bioorg Med Chem Lett. 2013;23:4985–4989.
https://doi.org/10.1016/j.bmcl.2013.06.050.
22. Minders C, Petzer JP, Petzer A, Lourens AC. Monoamine oxidase inhibitory activities
of heterocyclic chalcones. Bioorg Med Chem Lett. 2015;25:5270–5276. https://doi.
org/10.1016/j.bmcl.2015.09.049.
Acknowledgements
23. Engelbrecht I, Petzer JP, Petzer A. Nitrocatechol derivatives of chalcone as inhibitors
of monoamine oxidase and catechol-O-methyltransferase. Cent Nerv Syst Agents Med
Chem. 2018;18:115–127. https://doi.org/10.2174/1871524918666180426125714.
24. Manley-King CI, Bergh JJ, Petzer JP. Monoamine oxidase inhibition by C4-sub-
stituted phthalonitriles. Bioorg Chem. 2012;40:114–124. https://doi.org/10.1016/j.
bioorg.2011.10.003.
This work was financially supported by the National Research
Foundation of South Africa [Grant specific unique reference numbers
(UID) 85642 and 96180]. The Grantholders acknowledge that opinions,
findings and conclusions or recommendations expressed in any pub-
lication generated by the NRF supported research are that of the au-
thors, and that the NRF accepts no liability whatsoever in this regard.
25. Legoabe L, Kruger J, Petzer A, Bergh JJ, Petzer JP. Monoamine oxidase inhibition by
selected anilide derivatives. Eur J Med Chem. 2011;46:5162–5174. https://doi.org/
10.1016/j.ejmech.2011.08.036.
26. Chimenti F, Bolasco A, Manna F, et al. Synthesis and selective inhibitory activity of 1-
acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives against monoamine oxi-
dase. J Med Chem. 2004;47:2071–2074. https://doi.org/10.1021/jm031042b.
27. Manna F, Chimenti F, Bolasco A, et al. Inhibition of amine oxidases activity by 1-
acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives. Bioorg Med Chem Lett.
2002;12(24):3629–3633. https://doi.org/Pii S0960-894x(02)00699-6.
28. Kiss LE, Ferreira HS, Torrao L, et al. Discovery of a long-acting, peripherally selective
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.bmcl.2020.127188.
5

R. Hitge, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
inhibitor of catechol-O-methyltransferase. J Med Chem. 2010;53:3396–3411. https://
screening assay to identify inhibitors of human catechol-O-methyltransferase. Anal
Biochem. 2004;331:198–200. https://doi.org/10.1016/j.ab.2004.04.026.
36. Vidgren J, Svensson LA, Liljas A. Crystal structure of catechol O-methyltransferase.
Nature. 1994;368:354–358. https://doi.org/10.1038/368354a0.
doi.org/10.1021/jm1001524.
29. Klinke P, Gibian H. Über chalkone. Eur J Inorg Chem. 1961;94:26–38.
30. Manna F, Chimenti F, Fioravanti R, et al. Synthesis of some pyrazole derivatives and
preliminary investigation of their affinity binding to P-glycoprotein. Bioorg Med Chem
Lett. 2005;15:4632–4635. https://doi.org/10.1016/j.bmcl.2005.05.067.
31. Mostert S, Petzer A, Petzer JP. Indanones as high-potency reversible inhibitors of
monoamine oxidase. ChemMedChem. 2015;10:862–873. https://doi.org/10.1002/
cmdc.201500059.
37. Trott O, Olson AJ. AutoDock Vina: improving the speed and accuracy of docking with
a new scoring function, efficient optimization, and multithreading. J Comput Chem.
2010;31:455–461. https://doi.org/10.1002/jcc.21334.
38. Rutherford K, Le Trong I, Stenkamp RE, Person VW. Crystal structures of human
108V and 108M catechol O-methyltransferase. J Mol Biol. 2008;380:120–130.
https://doi.org/10.1016/j.jmb.2008.04.040.
32. Novaroli L, Reist M, Favre E, Carotti A, Catto M, Carrupt PA. Human recombinant
monoamine oxidase B as reliable and efficient enzyme source for inhibitor screening.
Bioorg Med Chem. 2005;13:6212–6217. https://doi.org/10.1016/j.bmc.2005.06.043.
33. Hirano Y, Tsunoda M, Funatsu T, Imai K. Rapid assay for catechol-O-methyl-
transferase activity by high-performance liquid chromatography-fluorescence de-
tection. J Chromatogr B Analyt Technol Biomed Life Sci. 2005;819:41–46. https://doi.
org/10.1016/j.jchromb.2005.01.019.
39. Ellermann M, Lerner C, Burgy G, et al. Catechol-O-methyltransferase in complex with
substituted 3'-deoxyribose bisubstrate inhibitors. Acta Crystallogr D Biol Crystallogr.
2012;68:253–260. https://doi.org/10.1107/S0907444912001138.
40. Zhuang CL, Zhang W, Sheng CQ, Zhang WN, Xing CG, Miao ZY. Chalcone: a privi-
leged structure in medicinal chemistry. Chem Rev. 2017;117:7762–7810. https://doi.
org/10.1021/acs.chemrev.7b00020.
34. Zhu BT, Wang P, Nagai M, Wen Y, Bai HW. Inhibition of human catechol-O-me-
thyltransferase (COMT)-mediated O-methylation of catechol estrogens by major
polyphenolic components present in coffee. J Steroid Biochem Mol Biol.
2009;113:65–74. https://doi.org/10.1016/j.jsbmb.2008.11.011.
41. Srinivasan B, Johnson TE, Lad R, Xing CG. Structure-activity relationship studies of
chalcone leading to 3-hydroxy-4,3 ',4 ',5 '-tetramethoxychalcone and its analogues as
potent nuclear factor kappa B inhibitors and their anticancer activities. J Med Chem.
2009;52:7228–7235. https://doi.org/10.1021/jm901278z.
35. Kurkela M, Siiskonen A, Finel M, Tammela P, Taskinen J, Vuorela P. Microplate
6