2-Benzylidene-1-indanone derivatives as inhibitors of monoamine oxidase

Article abstract of DOI:10.1016/j.bmcl.2016.08.067

In the present study, a series of twenty-two 2-benzylidene-1-indanone derivatives were synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The 2-benzylidene-1-indanone derivatives are structurally related to a series of benzylideneindanone derivatives which has previously been found to be MAO-B inhibitors. This study finds that the 2-benzylidene-1-indanones are MAO-B specific inhibitors with IC₅₀values <2.74?μM. Among the compounds evaluated, twelve compounds exhibited IC₅₀?50?=?0.131?μM). An analysis of the structure–activity relationships for MAO-B inhibition show that substitution on the A-ring with a 5-hydroxy group and on the B-ring with halogens and the methyl group yield high potency inhibition. It may therefore be concluded that 2-benzylidene-1-indanone analogues are promising leads for design of therapies for disorders such as Parkinson's disease.

Full text of DOI:10.1016/j.bmcl.2016.08.067

Accepted Manuscript
2-Benzylidene-1-indanone derivatives as inhibitors of monoamine oxidase
Magdalena S. Nel, Anél Petzer, Jacobus P. Petzer, Lesetja J. Legoabe
PII:
S0960-894X(16)30901-5
DOI:
http://dx.doi.org/10.1016/j.bmcl.2016.08.067
BMCL 24193
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
11 July 2016
18 August 2016
20 August 2016
Please cite this article as: Nel, M.S., Petzer, A., Petzer, J.P., Legoabe, L.J., 2-Benzylidene-1-indanone derivatives
as inhibitors of monoamine oxidase, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://dx.doi.org/
10.1016/j.bmcl.2016.08.067
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2-Benzylidene-1-indanone derivatives as inhibitors of monoamine oxidase
Magdalena S. Nel,1,2 Anél Petzer,1,2 Jacobus P. Petzer,1,2 and Lesetja J. Legoabe 2,*
1.
Pharmaceutical Chemistry, School of Pharmacy, North-West University, Private Bag X6001,
Potchefstroom 2520, South Africa
2.
Centre of Excellence for Pharmaceutical Sciences, North-West University, Private Bag X6001,
Potchefstroom 2520, South Africa
*Corresponding author: Lesetja J. Legoabe, Tel.: +27 18 2992182, fax: +27 18 2994243
E-mail address: lesetja.legoabe@nwu.ac.za
Running title: 2-Benzylidene-1-indanones as MAO inhibitors
Keywords: monoamine oxidase, MAO, inhibition, reversible, 2-benzylidene-1-indanone
1

ABSTRACT
In the present study, a series of twenty-two 2-benzylidene-1-indanone derivatives were synthesised
and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The 2-
benzylidene-1-indanone derivatives are structurally related to a series of benzylideneindanone
derivatives which has previously been found to be MAO-B inhibitors. This study finds that the 2-
benzylidene-1-indanones are MAO-B specific inhibitors with IC50 values < 2.74 µM. Among the
compounds evaluated, twelve compounds exhibited IC50 < 0.1 µM and may be considered as high
potency inhibitors. The 2-benzylidene-1-indanone derivatives also inhibited MAO-A with the most
potent inhibition exhibited by 5g (IC50 = 0.131 µM). An analysis of the structure-activity relationships
for MAO-B inhibition show that substitution on the A-ring with a 5-hydroxy group and on the B-ring
with halogens and the methyl group yield high potency inhibition. It may therefore be concluded that
2-benzylidene-1-indanone analogues are promising leads for design of therapies for disorders such as
Parkinson’s disease.
2

The monoamine oxidases (MAOs) are mitochondrial bound enzymes which regulate the levels of
amine-containing compounds in the brain and peripheral tissues.1,2 The MAOs consist of two
isoforms, MAO-A and MAO-B. Both are widely expressed in mammalian tissues, but at different
levels. In humans MAO-A is the principal isoform in the intestines, placenta and heart while MAO-B
is the major isoform in platelets, glial cells in the brain and liver.1 The clinical importance of the
MAOs arises from their role in the oxidative deamination of neurotransmitter monoamines; MAO-A
catalyses the oxidation of 5-hydroxytryptamine (5-HT, serotonin) while the false neurotransmitter, β-
phenylethylamine, is a MAO-B specific substrate. The catecholamines, dopamine, noradrenaline and
adrenaline, as well as the dietary amines, tryptamine and tyramine, are oxidised by both MAO-A and
MAO-B.3 Inhibitors of MAO-A and MAO-B have thus been used for the treatment of diseases that
result from deficient neurotransmitter levels. For example, MAO-A inhibitors increase central
serotonin levels and are used for the treatment of major depression.4,5 MAO-B inhibitors are used in
the treatment of Parkinson’s disease where they reduce the MAO-catalysed breakdown of dopamine.3
This is expected to enhance striatal dopaminergic activity leading to the improvement of motor
symptoms.6 MAO-B inhibitors are often combined with L-Dopa, the metabolic precursor of
dopamine, in Parkinson’s disease therapy.7–9 Selective MAO-B inhibitors may also possess disease
modifying properties by protecting against neurodegeneration in Parkinson’s disease.10,11 The
neuroprotective effect of MAO-B inhibitors may, at least in part, be attributed to the reduction of the
central formation of toxic metabolic by-products (hydrogen peroxide and aldehydes) of the MAO
catalytic cycle.11
Although both MAO isoforms oxidise dopamine in the human brain, MAO-B specific inhibitors are
used for the treatment of Parkinson’s disease, principally because MAO-A inhibitors are used with
caution in the clinic. MAO-A inhibitors may lead to a potentially fatal hypertensive event when
combined with tyramine-containing food such as cheeses and fermented drinks (e.g. wine and
beer).12,13 Normally dietary tyramine (and other sympathomimetic amines) are extensively
metabolised by MAO-A in the intestinal wall and in the liver, thus preventing their entry into the
systemic circulation. When MAO-A is inhibited, tyramine and other sympathomimetic amines present
3

in food cannot be metabolised and thus reaches systemic levels high enough to induce a significant
release of noradrenaline from peripheral adrenergic neurons. This may lead to a potentially lethal
hypertensive crisis with cerebral haemorrhages.11,14 Selective MAO-B inhibitors do not have this
effect because there is little MAO-B in the intestine. The development of reversible MAO-A
inhibitors, however, avoids this problem because, with increasing substrate concentrations, the
reversible inhibitor is displaced from MAO-A, allowing metabolism to occur.15,16 As a result, the
focus now falls on the discovery and development of reversible MAO inhibitors for the treatment of
disorders such as depression and Parkinson’s disease.17,18
Figure 1. The structures of isoliquiritigenin (1), chalcone 2, aurone 3 and 2-benzylidene-1-indanone
4.
As early as 1987, chalcones (1,3-diphenyl-2-propen-1-ones) have been explored as potential MAO
inhibitors. Researchers have isolated isoliquiritigenin (1) from the roots of Glycyrrhiza uralensis and
carried out kinetic studies to determine the MAO inhibitory activity (Fig. 1).19 This compound
exhibited an IC50 value of 17.3 µM, but no distinction was made between MAO-A and MAO-B
inhibition.19 A number of studies have since shown that synthetic chalcones and chalcones from
natural sources are inhibitors of MAO.20–23 This is exemplified by synthetic chalcone 2 which inhibits
4

human MAO-B with an IC50 value of 0.0051 µM.20 This compound is a MAO-B specific inhibitor
since relatively weak inhibition of MAO-A was observed (IC50 = 4.95 µM).
In a recent study, Morales-Camilo and co-workers evaluated sixteen compounds, 8 chalcones and 8
aurones.24 Both the chalcones and aurones proved to be MAO-B specific inhibitors with compound 3
(IC50 = 11.6 µM) being the most potent inhibitor among the aurones. This was the first report that the
aurone class of compounds inhibits MAO. 2-Benzylidene-1-indanone may be considered to be the
cyclic analogue of chalcone. It may thus be postulated that 2-benzylidene-1-indanone derivatives
may, similar to chalcones, also possess MAO inhibition properties. Support for this viewpoint is the
observation that 2-benzylidene-1-indanone are structurally similar to aurones which, as mentioned
above, are compounds known to inhibit MAO. 2-Benzylidene-1-indanones have not been extensively
investigated as MAO inhibitors. In 2012, Huang and co-workers reported the MAO activities of a
series of seven 2-benzylidene-1-indanone derivatives.25 These compounds also are MAO-B specific
and exhibited IC50 values for the inhibition of MAO-B in the micromalor range (7.50–40.5 µM). The
most potent inhibitor, compound 4, exhibited an IC50 value of 7.5 µM for the inhibition of MAO-B.25
In this reported study ring A was disubstituted with hydroxy and methoxy substituents while the
benzylidene ring B was monosubstituted on the C4’ position with a dialkylamine. Based on the study
done by Huang and co-workers, the present study aims to expand on the structure-activity
relationships of MAO inhibition by 2-benzylidene-1-indanone derivatives. As secondary objective,
high potency MAO inhibitors may thus be discovered. For the purpose of this study, ring A will be
substituted on C5 and C6 with either a hydroxy or methoxy group, and ring B will be substituted on
C3’ and C4’ with halogens (F, Cl, Br), alkyl groups [CH3, CN, OCH3, CH(CH3)2], an amine
containing group [N(CH3)2] and the hydroxy group. For comparison, some 2-benzylidene-1-indanone
derivatives will not be substituted on the A- and/or B-rings (e.g. 5a, 5b, 6a–c). It is anticipated that
the halogen substituted derivatives may display potent MAO-B inhibition since the reported halogen
substituted chalcones (e.g. 2) possess high potency inhibition. In this respect, the halogen enhances
Van der Waals interactions with the entrance cavity of MAO-B, while the polar hydroxy group (and
5

possibly methoxy) establish polar contacts such as hydrogen bonding with residues and water
molecules in the MAO-B substrate cavity.20
In the present study twenty-two 2-benzylidene-1-indanone derivatives (5a–r and 6a–d) were
synthesised with the aim of examining their MAO inhibitory properties. The target 2-benzylidene-1-
indanone derivatives were synthesised employing either acidic (HCl) or basic (KOH) conditions.24
For the synthesis of 5a–r, 5-hydroxy- or 6-hydroxy-1-indanone was reacted with an appropriate
benzaldehyde in a reaction solvent consisting of a mixture of methanol and 32% HCl (1:1.5) (Fig. 2).
Compounds 6a–d, in turn, were synthesised by reaction of 1-indanone or 5-methoxy-1-indanone with
an appropriate benzaldehyde in methanol containing 4.7% KOH. These reactions gave the target 2-
benzylidene-1-indanone derivatives in yields of 8–83%. The structures of the target compounds were
verified by 1H NMR, 13C NMR and mass spectrometry as cited in the experimental section.
Figure 2. Synthetic pathway to 2-benzylidene-1-indanone derivatives (5a–r and 6a–d). Reagents and
conditions: (a) methanol/32% HCl (1:1.5), reflux; (b) KOH, methanol, rt.
The MAO inhibitory properties of the 2-benzylidene-1-indanone derivatives were examined using the
recombinant human MAO-A and MAO-B enzymes.26 The mixed MAO-A/B substrate, kynuramine,
was used as substrate for both enzyme isoforms. The enzyme reactions contained the enzyme,
substrate and test inhibitor and control reactions conducted in the absence of inhibitor were always
included. After incubation for 20 min at 37 °C, the reactions were terminated with the addition of
sodium hydroxide (2 N). The rate of oxidation of kynuramine by the MAOs was determined by
measuring (at endpoint) the concentration of 4-hydroxyquinoline, the metabolite of kynuramine
oxidation. Since 4-hydroxyquinoline fluoresces in alkaline media, concentration measurements were
6

carried out by fluorescence spectrophotometry. By thus measuring rates of oxidation of kynuramine
by the MAOs in the presence of various concentrations of the 2-benzylidene-1-indanones, sigmoidal
dose-response plots were constructed from which IC50 values were estimated. Examples of such
sigmoidal dose-response plots are shown in Fig. 3.
100
MAO-A
MAO-B
75
50
25
0
-3
-2
-1
0
1
2
3
Log[I]
Figure 3. Sigmoidal dose-response plots for the inhibition of MAO-A (open circles) and MAO-B
(filled circles) by 5e. The enzyme activities are given as mean ± SD.
The IC50 values for the inhibition of the human MAOs by the 2-benzylidene-1-indanone derivatives
are given in table 1. The IC50 values for the inhibition of MAO-B ranges from 0.0052–2.74 µM while
those for the inhibition of MAO-A ranges from 0.131 to >100 µM. From the selectivity index (SI)
values, it is clear that the 2-benzylidene-1-indanone derivatives are specific inhibitors of the MAO-B
isoform. Only 5b and 5o exhibit higher inhibition potencies for MAO-A compared to MAO-B. The
finding that the 2-benzylidene-1-indanones are mostly MAO-B specific, is similar to the MAO
isoform specificities of the 2-benzylidene-1-indanone derivatives previously reported.25 In general,
substitution on the A-ring with a 5-hydroxy group and on the B-ring with halogens and the methyl
group (5c–5j, 5m) yielded high potency MAO-B inhibition with IC50 values < 0.084 µM. 5-Hydroxy
substitution (5a, IC50 = 0.376 µM) appears to be more favourable than 6-hydroxy substitution (5b,
IC50 = 2.42 µM) for MAO-B inhibition. With 5-hydroxy substitution on the A-ring, polar (OH and
7

OCH3) and larger [N(CH3)2, CH(CH3)2] substituents on the B-ring yields comparatively lower MAO-
B inhibition (5n–r, IC50 = 0.215–2.74 µM). Interestingly, with either no substituent or a methoxy on
the A-ring, N(CH3)2 substitution on the B-ring yields good potency MAO-B inhibitors as shown with
6b (IC50 = 0.031 µM) and 6d (IC50 = 0.060 µM). An appropriate substitution pattern is, however,
required for potent MAO-B inhibition as evidenced by the finding that the unsubstituted derivative 6a
(IC50 = 2.57 µM) is a relatively weaker MAO-B inhibitor. Finally, 6c (IC50 = 0.0092 µM), substituted
with a methoxy on the A-ring is noteworthy as a very specific MAO-B inhibitor since it displays an
IC50 for MAO-A inhibition of > 100 µM. Derivative 6d, also substituted with a methoxy on the A-
ring, similarly displays weak MAO-A inhibition (IC50 = 11.5 µM) compared to the other 2-
benzylidene-1-indanones, which indicates that this structural feature reduces MAO-A inhibition, thus
enhancing specificity for MAO-B.
Table 1. The IC50 values for the inhibition of recombinant human MAO-A and MAO-B by 2-
benzylidene-1-indanone derivatives.
R
R’
IC50 (µM)a
SIb
MAO-A
MAO-B
5-OH
6-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
H
1.94 ± 0.201
0.440 ± 0.062
0.458 ± 0.049
0.679 ± 0.020
0.157 ± 0.015
0.741 ± 0.048
0.131 ± 0.008
1.05 ± 0.053
0.376 ± 0.101
2.42 ± 0.443
0.084 ± 0.018
0.047 ± 0.009
0.030 ± 0.002
0.013 ± 0.002
0.013 ± 0.002
0.0053 ± 0.0004
5.2
0.2
5.5
14
5a
5b
5c
5d
5e
5f
H
3’-F
4’-F
3’-Cl
4’-Cl
3’-Br
4’-Br
5.2
57
10
5g
5h
198
8

5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
5-OH
H
3’-CH3
4’-CH3
3’-CN
0.860 ± 0.076
3.22 ± 0.171
0.444 ± 0.043
2.08 ± 0.158
0.235 ± 0.010
1.20 ± 0.127
1.67 ± 0.033
1.19 ± 0.056
1.72 ± 0.114
0.961 ± 0.126
5.43 ± 0.754
1.52 ± 0.134
No inhc
0.052 ± 0.013
0.0052 ± 0.001
0.130 ± 0.029
0.316 ± 0.091
0.014 ± 0.007
0.966 ± 0.125
2.74 ± 0.668
0.471 ± 0.071
0.768 ± 0.044
0.215 ± 0.034
2.57 ± 0.972
0.031 ± 0.004
0.0092 ± 0.001
0.060 ± 0.007
17
5i
619
3.4
6.6
17
5j
5k
5l
4’-CN
3’,4’-Cl
3’-OH
5m
5n
5o
5p
5q
5r
6a
6b
6c
6d
1.2
0.6
2.5
2.2
4.5
2.1
49
4’-OH
3’-OCH3
4’-N(CH3)2
4’-CH(CH3)2
H
H
4’-N(CH3)2
H
5-OCH3
5-OCH3
>10870
192
4’-N(CH3)2
11.5 ± 0.798
a All values are expressed as the mean ± standard deviation (SD) of triplicate determinations.
b Selectivity index (SI) = IC50(MAO-A)/IC50(MAO-B).
c No inhibition observed at a maximal tested concentration of 100 µM.
While the 2-benzylidene-1-indanone derivatives are MAO-B specific inhibitors, good potency MAO-
A inhibitors also exist among the series. Derivative 5g (IC50 = 0.131 µM) possesses the highest
potency MAO-A inhibition among the series. Few clear structure-activity relationships for MAO-A
inhibition are apparent. As mentioned above, substitution with the methoxy group on the A-ring
reduces MAO-A inhibition. It is also noteworthy that the three most potent MAO-A inhibitors (5e, 5g,
5m) bear halogens on ring B. As for MAO-B, an appropriate substitution pattern is, however, required
for potent MAO-A inhibition since the unsubstituted derivative 6a (IC50 = 5.43 µM) is the third
weakest MAO-A inhibitor of the series. In contrast to MAO-B inhibition, 5-hydroxy substitution (5a,
IC50 = 1.94 µM) is less favourable than 6-hydroxy substitution (5b, IC50 = 0.440 µM) for MAO-A
inhibition. Interestingly, with an enhancement of MAO-A inhibition with 6-hydroxy substitution,
reversal of isoform selectivity occurs, and 5b is thus MAO-A specific (SI = 0.2).
9

As mentioned in the introduction, irreversible MAO-A inhibition may lead to a potentially fatal
hypertensive event when combined with certain food.12,13 Reversible MAO-A inhibitors, however, are
considered safer in this regard. We have therefore set out to investigate the reversibility of MAO-A
inhibition by 2-benzylidene-1-indanone derivative 5g, a compound that displays the highest potency
MAO-A inhibition of the present series. It is expected that this compound would act as a reversible
inhibitor since it does not possess functional groups associated with irreversible MAO inhibition.27 To
investigate the reversibility of inhibition dialysis was used. MAO-A and 5g (at a concentration of 4 ×
IC50) were combined and pre-incubated for 15 min and afterwards dialysed for 24 h. After the
addition of the substrate, kynuramine, the pre-incubations were diluted twofold to yield an inhibitor
concentration of 2 × IC50. The residual MAO-A activity was subsequently measured as described
above for the determination of IC50 values. As controls, this dialysis experiment was carried out in the
presence of the irreversible MAO-A inhibitor, pargyline, as well as in the absence of inhibitor. The
activities recorded in the absence of the inhibitor served as negative control and designate 100%
residual activity (Fig. 4). Also the residual MAO activities of undialysed mixtures of MAO-A and the
test inhibitor were measured for comparison. As evident form the results, 5g is a reversible MAO-A
inhibitor. Dialysis restores MAO-A activity to 99% of the negative control. For reversible inhibition it
is expected that dialysis of enzyme–inhibitor mixtures would restore activity to 100%. Inhibition,
however persists in undialysed mixtures of MAO-A and 5g with the residual activity at 50%. As
expected for the irreversible inhibitor, pargyline, dialysis does not restore activity with the residual
activity at 1.7%.
10

MAO-A
100
75
50
25
0
[I]=0
5g
dialysed
parg
5g
undialysed
Figure 4. A selected 2-benzylidene-1-indanone is a reversible MAO-A inhibitor. The test inhibitor 5g
(at 4 × IC50) was preincubated with MAO-A for 15 min, dialysed for 24 h and the residual enzyme
activity was measured (5g dialysed). Similar incubation and dialysis of MAO-A in the absence
inhibitor ([I] = 0, dialysed) and presence of the irreversible inhibitor, pargyline (parg, dialysed), were
also carried out. The residual activities of undialysed mixtures of MAO-A and the test inhibitor were
also recorded (5g undialysed).
Employing molecular docking studies, the binding modes and key interactions of chalcones to the
human MAOs have been proposed.20 For comparison with the chalcone class of compounds, the
present study docked selected 2-benzylidene-1-indanone derivatives, 5g (most potent MAO-A
inhibitor) and 5j (most potent MAO-B inhibitor) as well as chalcone 2 into the MAO active sites
using the CDOCKER docking algorithm of Discovery Studio 3.1 (Accelrys). A docking protocol
previously reported by us was followed for this purpose.26 As enzyme models, the crystal structures of
human MAO-A (PDB code: 2Z5X) and MAO-B (PDB code: 2V5Z) were employed.28,29 The binding
orientations and selected interactions of the inhibitors with the MAOs are given in Fig. 5 and 6. In the
MAO-B active site, chalcone 2 as well as 5g and 5j bind with ring A in the substrate cavity in
proximity to the FAD while ring B extends into the entrance cavity. The orientation of 2 is similar to
that reported in literature.20 In the substrate cavity, the polar functional groups of 2 establish hydrogen
bonding with Tyr-435 and a water molecule, while 5g and 5j forms π–π interactions with Tyr-398 and
11

Tyr-435. In the entrance cavity, which is reported to be a hydrophobic space, the B-rings of the
inhibitors are most likely stabilised by Van der Waals interactions. Interestingly, the B-ring of 2 may
undergo a π–σ interaction with Ile-199. It is also noteworthy that compared to 2, the carbonyl groups
of 5g and 5j are directed in the opposite direction, an orientation required for placement of the B-ring
in the entrance cavity. Since the carbonyl carbon of 2 is exocyclic and free to rotate, it may adopt the
observed orientation while still allowing for the side chain to extend into the entrance cavity.
12

Figure 5. The docked binding orientations of 2 (magenta), 5g (green) and 5j (yellow) in MAO-B.
In MAO-A, chalcone 2 also binds with ring A directed towards the rear of the active site where the
FAD is located. Here the polar functional groups establish hydrogen bonding with a water molecule
while π–π interaction occurs with Tyr-444. This orientation is also similar to that reported in
literature.20 Compound 5j binds with a similar orientation to 2 and forms π–π interactions with both
tyrosine residues. As seen with 5g, the reversed orientation is also a possibility for 2-benzylidene-1-
indanone derivatives. In this instance hydrogen bonding may occur with the main chain carbonyl
oxygen of Phe-208. Unfortunately, the docking studies do not provide an explanation for the observed
MAO-B specificities of chalcones and 2-benzylidene-1-indanones. From literature it is however clear
that, in MAO-A, the side chain of Phe-208 may restrict the binding of larger inhibitors. In MAO-B,
the residue that corresponds to Phe-208 is Ile-199. In contrast to Phe-208, the side chain of Ile-199
may rotate from the active site cavity, allowing for larger inhibitors to traverse both entrance and
substrate cavities. Some larger inhibitors are thus better accommodated in MAO-B than MAO-A
leading to specific inhibition of the MAO-B isoform. This analysis may explain the MAO-B
specificities of chalcones and 2-benzylidene-1-indanones.
13

Figure 6. The docked binding orientations of 2 (magenta), 5g (green) and 5j (yellow) in MAO-A.
In conclusion, based on a literature report that 2-benzylidene-1-indanone derivatives possess MAO
inhibitory properties, the present study synthesises a series twenty-two derivatives and evaluated their
human MAO inhibition potencies. The 2-benzylidene-1-indanone derivatives proved to be MAO-B
14

specific inhibitors with good potencies recorded for a number of compounds. Among the series,
twelve compounds exhibited IC50 < 0.1 µM and may be considered as high potency inhibitors. These
potencies are comparable to reference inhibitors such as lazabemide, which is reported to inhibit
MAO-B with an IC50 value of 0.091 µM under identical conditions.30 The most noteworthy structure-
activity relationship for MAO-B inhibition is that substitution on the A-ring with a 5-hydroxy group
and on the B-ring with halogens and the methyl group yield high potency inhibition. The 2-
benzylidene-1-indanone derivatives also inhibited MAO-A with 5g in particular displaying an IC50 of
0.131 µM. This is significantly more potent than the clinically used antidepressant, toloxatone, which
is reported to inhibit MAO-A reversibly with an IC50 value of 3.92 µM under identical conditions.30
The present study also confirms that a selected 2-benzylidene-1-indanone is a reversible MAO-A
inhibitor, a property which reduces the probability of adverse events arising from interaction with
certain food. The calculated logP values (ChemSketch) for the 2-benzylidene-1-indanone derivatives
ranges from 3.05 (5k, 5n)–4.97 (5r), indicating that these compounds are highly lipophilic. It may
thus be expected that the 2-benzylidene-1-indanone derivatives would exhibit limited aqueous
solubility and oral absorption. The ligand-lipophilicity efficiency (LLE) of the series ranges from 0.90
(6a)–3.3 (5k) for MAO-A and 1.22 (6a)–4.2 (5j) for MAO-B. Since the LLE for 5j is relatively large,
this compound may be suitable as a lead for MAO-B inhibitor design. In general a LLE >3 is
acceptable for a lead and indicates an acceptable pharmacological activity at the expense of higher
lipophilicity. It is, however, recommended that further structural modification should focus on
reducing lipophilicity, which represents a potential liability for bioavailability.
Based on this study, it may be concluded that 2-benzylidene-1-indanone derivatives are promising
MAO-B inhibitors for the treatment disorders such as Parkinson’s disease. Nonspecific MAO
inhibitors may also find application in the treatment of depression, particularly where depression is a
comorbidity of Parkinson’s disease.
Acknowledgements
15

The NMR and MS spectra were recorded by André Joubert and Johan Jordaan of the SASOL Centre
for Chemistry, North-West University. This work is based on the research supported in part by the
Medical Research Council and National Research Foundation of South Africa (Grant specific unique
reference numbers (UID) 85642, 96180, 96135). The Grantholders acknowledge that opinions,
findings and conclusions or recommendations expressed in any publication generated by the NRF
supported research are that of the authors, and that the NRF accepts no liability whatsoever in this
regard.
Conflict of Interest
The authors have no conflicts of interest to declare.
References
1. Ramsay, R. R. Curr. Top. Med. Chem. 2012, 12, 2189.
2. Ramsay, R R. Curr. Pharm. Des. 2013, 19, 2529.
3. Youdim, M. B.; Edmondson, D.; Tipton, K. F. Nat. Rev. Neurosci. 2006, 7, 295.
4. Schwartz, T. L. CNS Spectr. 2013, 18(Suppl 1), 25.
5. Lum, C. T.; Stahl, S. M. CNS Spectr. 2012, 17, 107.
6. Gershanik, O. S. Mov. Disord. 2015, 30, 103.
7. Shoulson, I.; Oakes, D.; Fahn, S.; Lang, A.; Langston, J. W.; LeWitt, P.; Olanow, C. W.; Penney, J. B.;
Tanner, C.; Kieburtz, K.; Rudolph, A. Ann. Neurol. 2002, 51, 604.
8. Fernandez, H. H.; Chen, J. J. Pharmacotherapy. 2007, 27, 174S.
9. Finberg, J. P.; Wang, J.; Bankiewicz, K.; Harvey-White, J.; Kopin, I. J.; Goldstein, D. S. J. Neural
Transm. Suppl. 1998, 52, 279.
10. LeWitt, P. A.; Taylor, D. C. Neurotherapeutics. 2008, 5, 210.
11. Youdim, M. B.; Bakhle, Y. S. Br. J. Pharmacol. 2006, 147(Suppl 1), S287.
12. Da Prada, M.; Zürcher, G.; Wüthrich, I.; Haefely, W. E. J. Neural Transm. Suppl. 1988, 26, 31.
13. Flockhart, D. A. J. Clin. Psychiatry. 2012, 73(Suppl 1), 17.
14. Bortolato, M.; Chen, K.; Shih, J. C. Adv. Drug Deliv. Rev. 2008, 60, 1527.
15. Bonnet, U. CNS Drug Rev. 2003, 9, 97.
16

16. Provost, J. C.; Funck-Brentano, C.; Rovei, V.; D'Estanque, J.; Ego, D.; Jaillon, P. Clin. Pharmacol.
Ther. 1992, 52, 384.
17. Carradori, S.; Silvestri, R. J. Med. Chem. 2015, 58, 6717.
18. Carradori, S.; Petzer, J. P. Expert Opin. Ther. Pat. 2015, 25, 91.
19. Tanaka, S.; Kuwai, Y.; Tabata, M. Planta Med. 1987, 53, 5.
20. Chimenti, F.; Fioravanti, R.; Bolasco, A.; Chimenti, P.; Secci, D.; Rossi, F.; Yáñez, M.; Orallo, F.;
Ortuso, F.; Alcaro, S. J. Med. Chem. 2009, 52, 2818.
21. Haraguchi, H.; Tanaka, Y.; Kabbash, A.; Fujioka, T.; Ishizu, T.; Yagi, A. Phytochemistry. 2004, 65,
2255.
22. Pan, X.; Kong, L. D.; Zhang, Y.; Cheng, C. H.; Tan, R. X. Acta Pharmacol. Sin. 2000, 21, 949.
23. Gao, G. Y.; Li, D. J.; Keungm W. M. J. Med. Chem. 2001, 44, 3320.
24. Morales-Camilo, N.; Salas, C. O.; Sanhueza, C.; Espinosa-Bustos, C.; Sepúlveda-Boza, S.; Reyes-
Parada, M.; Gonzalez-Nilo, F.; Caroli-Rezende, M.; Fierro, A. Chem. Biol. Drug Des. 2015, 85, 685.
25. Huang, L.; Lu, C.; Sun, Y.; Mao, F.; Luo, Z.; Su, T.; Jiang, H.; Shan, W.; Li, X. J. Med. Chem. 2012,
55, 8483.
26. Mostert, S.; Petzer, A.; Petzer, J. P. ChemMedChem. 2015, 10, 862.
27. Edmondson, D. E.; Mattevi, A.; Binda, C.; Li, M.; Hubálek, F. Curr. Med. Chem. 2004, 11, 1983.
28. Son, S. Y.; Ma, J.; Kondou, Y.; Yoshimura, M.; Yamashita, E.; Tsukihara, T. Proc. Natl. Acad. Sci.
USA. 2008, 105, 5739.
29. Binda, C.; Wang, J.; Pisani, L.; Caccia, C.; Carotti, A.; Salvati, P.; Edmondson, D. E.; Mattevi, A. J.
Med. Chem. 2007, 50, 5848.
30. Petzer, A.; Pienaar, A.; Petzer, J. P. Drug Res (Stuttg). 2013, 63, 462.
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