α-Tetralone derivatives as inhibitors of monoamine oxidase

Article abstract of DOI:10.1016/j.bmcl.2014.04.021

In the present study, a series of fifteen α-tetralone (3,4-dihydro-2H-naphthalen-1-one) derivatives were synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The α-tetralone derivatives examined are structurally related to a series of chromone (1-benzopyran-4-one) derivatives which has previously been shown to act as MAO-B inhibitors. The results document that the α-tetralones are highly potent MAO-B inhibitors with all compounds exhibiting IC₅₀ values in the nanomolar range (<78 nM). Although most compounds are selective inhibitors of MAO-B, the α-tetralones are also potent MAO-A inhibitors with ten compounds exhibiting IC₅₀ values in the nanomolar range (<792 nM). The most potent MAO-B inhibitor, 6-(3-iodobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one, exhibits an IC₅₀ value of 4.5 nM with a 287-fold selectivity for MAO-B over the MAO-A isoform, while the most potent MAO-A inhibitor, 6-(3-cyanobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one, exhibits an IC₅₀ value of 24 nM with a 3.25-fold selectivity for MAO-A. Analyses of the structure-activity relationships for MAO inhibition show that substitution on the C6 position of the α-tetralone moiety is a requirement for MAO-A and MAO-B inhibition, and that a benzyloxy substituent on this position is more favourable for MAO-A inhibition than phenylethoxy and phenylpropoxy substitution. For MAO-B inhibition, alkyl and halogen substituents on the meta and para positions of the benzyloxy ring enhance inhibitory potency. It may be concluded that α-tetralone derivatives are promising leads for design of therapies for Parkinson's disease and depression.

Full text of DOI:10.1016/j.bmcl.2014.04.021

Bioorganic & Medicinal Chemistry Letters 24 (2014) 2758–2763
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
a
-Tetralone derivatives as inhibitors of monoamine oxidase
a
b
Lesetja J. Legoabe a, , Anél Petzer , Jacobus P. Petzer
⇑
a Centre of Excellence for Pharmaceutical Sciences, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
b Pharmaceutical Chemistry, School of Pharmacy, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 February 2014
Revised 4 April 2014
Accepted 7 April 2014
Available online 16 April 2014
In the present study, a series of fifteen a-tetralone (3,4-dihydro-2H-naphthalen-1-one) derivatives were
synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The
-tetralone derivatives examined are structurally related to a series of chromone (1-benzopyran-4-one)
derivatives which has previously been shown to act as MAO-B inhibitors. The results document that the
-tetralones are highly potent MAO-B inhibitors with all compounds exhibiting IC50 values in the nano-
molar range (<78 nM). Although most compounds are selective inhibitors of MAO-B, the -tetralones are
a
a
a
Keywords:
Monoamine oxidase (MAO)
Inhibition
also potent MAO-A inhibitors with ten compounds exhibiting IC50 values in the nanomolar range
(<792 nM). The most potent MAO-B inhibitor, 6-(3-iodobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one,
exhibits an IC50 value of 4.5 nM with a 287-fold selectivity for MAO-B over the MAO-A isoform, while
the most potent MAO-A inhibitor, 6-(3-cyanobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one, exhibits
an IC50 value of 24 nM with a 3.25-fold selectivity for MAO-A. Analyses of the structure–activity relation-
a-Tetralone
Structure–activity relationship
ships for MAO inhibition show that substitution on the C6 position of the a-tetralone moiety is a require-
ment for MAO-A and MAO-B inhibition, and that a benzyloxy substituent on this position is more
favourable for MAO-A inhibition than phenylethoxy and phenylpropoxy substitution. For MAO-B inhibi-
tion, alkyl and halogen substituents on the meta and para positions of the benzyloxy ring enhance inhib-
itory potency. It may be concluded that
a-tetralone derivatives are promising leads for design of
therapies for Parkinson’s disease and depression.
Ó 2014 Elsevier Ltd. All rights reserved.
The flavin-containing monoamine oxidases (MAO-A and MAO-
B) are located on the outer membranes of mitochondria, and cata-
inhibitors such as moclobemide appears to be devoid of the so-
called ‘cheese effect’.6 Similarly, the reversible MAO-A inhibitor,
toloxatone, does not elicit tyramine-associated adverse effects in
healthy volunteers when combined with a dose of tyramine consis-
tent with normal food intake.7 To minimise the occurrence of side
effects, MAO-A inhibitors should therefore interact reversibly with
the enzyme. Since tyramine is largely metabolised by MAO-A in
the intestine, MAO-B-selective inhibitors do not elevate systemic
tyramine concentrations and thus do not cause tyramine-induced
hypertension.8
The principal indication for MAO-B inhibitors is for the treat-
ment of Parkinson’s disease since MAO-B represents a major
catabolic enzyme of dopamine in the central nervous system.1,8,9
MAO-B inhibitors reduce the MAO-B-catalysed depletion of central
dopamine stores and thus may provide relief of the motor symp-
toms in Parkinson’s disease.10,11 MAO-B inhibitors are also consid-
ered useful in combination with levodopa, the metabolic precursor
of dopamine.12 In combination with levodopa, MAO-B inhibition
may enhance the levodopa mediated elevation of dopamine levels.
Furthermore, the combined use of levodopa and MAO-B inhibitors
allows for a reduction of the dosage of levodopa that is necessary
for a therapeutic response which, in turn, leads to diminished
lyse the
a-carbon oxidation of neurotransmitter and dietary
amines in the brain and peripheral tissues.1,2 Because the MAOs
modulate the levels of key signalling molecules, inhibitors of these
enzymes have been developed as drugs for the treatment of neuro-
psychiatric and neurodegenerative disorders such as depression
and Parkinson’s disease.3 In the central nervous system, serotonin
is mostly metabolised by MAO-A, and MAO-A inhibitors are in cur-
rent use as effective antidepressants.3,4 The clinical use of MAO-A
inhibitors have, however, declined in recent years because of blood
pressure changes that may arise from the combination of MAO-A
inhibitors with the dietary amine, tyramine, present in wine,
cheese and fermented products.5 Intestinal (and peripheral)
MAO-A acts as a metabolic barrier for tyramine exposure, and
inhibitors of MAO-A lead to an increase in systemic tyramine con-
centrations, which results in a potentially severe hypertensive
response. This adverse effect is, however, mostly associated with
irreversible MAO-A inhibition, and the newer generation reversible
⇑
Corresponding author. Tel.: +27 18 2992182; fax: +27 18 2994243.
E-mail address: lesetja.legoabe@nwu.ac.za (L.J. Legoabe).
http://dx.doi.org/10.1016/j.bmcl.2014.04.021
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

L. J. Legoabe et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2758–2763
2759
levodopa associated side effects. MAO-B inhibition also may exert
neuroprotective effects in Parkinson’s disease. The MAO catalytic
cycle produces aldehydes and hydrogen peroxide as metabolic
by-products, both of which may contribute to the neurodegenera-
tive processes in Parkinson’s disease.1,13 Aldehydes may be
MAO-B isoform. Based on the structural similarity between the
chromone moiety and -tetralone (5), the present study examines
the possibility that a series of -tetralone derivatives (6a–o) may
act as reversible and selective inhibitors of recombinant human
MAO-A and MAO-B (Table 1). In the present study the -tetralone
a
a
a
involved in the aggregation of
a
-synuclein, a process which is
moiety was substituted on the C6 position with the benzyloxy,
phenylethoxy and phenylpropoxy substituents. The selection of
these substituents was based on the observation that substitution
of chromone with these arylalkyloxy groups results in potent MAO
inhibition.19,20 Furthermore, in a recent study it was shown that
substitution on the C6 and C7 positions of the 3,4-dihydro-2(1H)-
quinolinone moiety (7) with these arylalkyloxy groups yields high
potency MAO-B selective inhibitors.21 Since it is reported that alkyl
and halogen substitution on the benzyloxy ring significantly
enhance both MAO-A and MAO-B inhibitory potencies of benzyl-
oxy-substituted chromones and 3,4-dihydro-2(1H)-quinolinones,
we have therefore further explored the MAO inhibitory properties
associated with the pathogenesis of Parkinson’s disease,14 while
hydrogen peroxide may result in oxidative damage and the promo-
tion of apoptotic signalling events.14,15 The possibility that MAO-B
inhibitors may act as neuroprotectants is especially relevant in
elderly Parkinson’s disease patients. MAO-B activity in the brain
is reported to increase with age, and the resulting increase of
MAO-B-catalysed by-products may thus accelerate the neurode-
generative processes.16,17 It is noteworthy that MAO-A also metab-
olises dopamine in the primate and possibly human brain and,
similar to MAO-B inhibitors, MAO-A inhibitors also enhance the
elevation of dopamine levels derived from levodopa.11 By acting
synergistically, non-selective MAO inhibitors may thus be more
efficacious in the therapy of Parkinson’s disease compared to
MAO-B selective inhibitors.8 In addition, since a significant propor-
tion of Parkinson’s disease patients exhibit signs of depression, the
inhibition of MAO-A may also be beneficial in this regard.1 To min-
imise the possibility of tyramine-induced side effects, non-selec-
tive MAO inhibitors should, however, possess reversible modes of
action, particularly with respect to the inhibition of MAO-A.
Based on these considerations, the design of new MAO inhibi-
tors is pursued by several research groups.18 In this regard, MAO
inhibitors should act reversibly with the MAOs, particularly with
the MAO-A isoform. Although selective MAO-A inhibitors are usu-
ally employed in the therapy of depression, while MAO-B selective
inhibitors are used in the therapy of Parkinson’s disease, non-selec-
tive MAO inhibitors may have added therapeutic value, particu-
larly in Parkinson’s disease. We have recently shown that
chromone (1-benzopyran-4-one) (1) is a suitable scaffold for the
design of MAO inhibitors (Fig. 1).19,20 Substituted chromones have
been shown to act as reversible, competitive MAO inhibitors, with
substitution on C6 and C7 yielding particularly potent MAO-B
inhibitors. For example, benzyloxy substitution on C6 and C7 of
the chromone moiety yields compounds 2 and 3, which are
reported to inhibit human MAO-B with IC50 values of 0.053 and
of the benzyloxy-substituted
a-tetralones by substitution on the
benzyloxy phenyl ring with halogens (F, Cl, Br, I) and alkyl groups
(CH3, CN, CF3). The aim of this study is therefore to discover novel
highly potent MAO-A and MAO-B inhibitors, and to derive struc-
ture–activity relationships (SARs) for MAO inhibition by the
ralone class of compounds. As discussed, it would be preferable for
the -tetralones to interact reversibly with MAO-A. While selective
a-tet-
a
inhibition would be acceptable, non-selective inhibition of the
MAOs may have additional therapeutic value. Active MAO inhibi-
tors may thus act as leads for the design of new therapies for
depression and Parkinson’s disease.
The
a-tetralone derivatives 6a–o were synthesised in good
yields according to a two step procedure (Scheme 1). Commercially
available 6-methoxy-1-tetralone (8) was firstly hydrolysed in the
presence of anhydrous AlCl3 to yield the key reagent, 6-hydroxy-
1-tetralone (9). 6-Hydroxy-1-tetralone (9) was suspended in
Table 1
The IC50 values for the inhibition of recombinant human MAO-A and MAO-B by
a-
tetralone derivatives 6a–o,
a
-tetralone (5) and 6-hydroxy-1-tetralone (9)
O
0.085
were potent MAO-A inhibitors, with the most potent inhibitor,
compound 4, exhibiting an IC50 value of 0.095
M.19 All chromone
derivatives reported were, however, selective inhibitors of the
l
M, respectively.19,20 Selected chromone derivatives also
R
l
6
a
R
IC50
(l
M)
SIb
MAO-A
MAO-B
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
5
C6H5CH2O–
0.792 0.155
0.392 0.044
0.266 0.079
0.207 0.021
0.238 0.026
0.212 0.018
0.311 0.114
1.29 0.14
2.25 0.425
2.09 0.370
0.024 0.001
0.656 0.062
0.547 0.135
1.32 0.068
1.16 0.032
14.8 2.12
0.063 0.016
0.017 0.005
0.008 0.003
0.015 0.004
0.007 0.001
0.009 0.001
0.009 0.001
0.0045 0.0003
0.0065 0.0009
0.011 0.002
0.078 0.010
0.011 0.001
0.006 0.000
0.033 0.015
0.032 0.010
18.6 1.58
13
23
33
14
34
24
35
287
346
190
0.3
60
91
40
36
0.8
0.7
O
O
O
O
O
O
(3-FC6H4)CH2O–
(4-FC6H4)CH2O–
(3-ClC6H4)CH2O–
(4-ClC6H4)CH2O–
(3-BrC6H4)CH2O–
(4-BrC6H4)CH2O–
(3-IC6H4)CH2O–
(4-IC6H4)CH2O–
(4-CH3C6H4)CH2O–
(3-CNC6H4)CH2O–
(4-CNC6H4)CH2O–
(4-CF3C6H4)CH2O–
C6H5(CH2)2O–
C6H5(CH2)3O–
H–
O
6
7
O
1
2
3
O
O
O
O
O
R
6
O
4
5
6
9
HO–
43.8 3.34
66.4 13.6
0.091c
Br
6
Lazabemide
Toloxatone
Not determined
3.92c
Not determined
7
a
N
H
O
All values are expressed as the mean SD of triplicate determinations.
The selectivity index is the selectivity for the MAO-B isoform and is given as the
ratio of IC50(MAO-A)/IC50(MAO-B).
b
7
c
Figure 1. The structures of compounds discussed in the text.
Taken from Ref. 24.

2760
L. J. Legoabe et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2758–2763
inhibitor of MAO-B with an IC50 value of 0.078
to that recorded for lazabemide (IC50 = 0.091
l
M, a value similar
O
O
l
M).24 Based on this
observation, 6k represents a compound that potently inhibits both
MAO isoforms. As discussed above, inhibitors of both MAO iso-
forms may have additional therapeutic value, particularly in Par-
a
H3C
O
HO
8
9
kinson’s disease. Other a-tetralone derivatives were also found to
be relatively potent MAO-A inhibitors with, in addition to 6k, 9
O
compounds (6a–g, 6l–m) exhibiting IC50 values in the nanomolar
b
(<1
tuted
l
M) range. It may thus be concluded that, while the C6-substi-
-tetralones examined here are mostly MAO-B selective
R
Br
a
R
O
inhibitors, many homologues are also potent MAO-A inhibitors.
In fact 10 of the 15 homologues examined possess IC50 values for
the inhibition of both MAO-A and MAO-B in the nanomolar range.
An analysis of the SARs for the inhibition of the MAOs reveals
interesting trends. The results show that benzyloxy substitution
6a-o
Scheme 1. Synthetic route to the
conditions: (a) AlCl3, toluene, reflux; (b) acetone, K2CO3, reflux.
a-tetralone derivatives 6a–o. Reagents and
on the C6 position of the
a-tetralone moiety is more favourable
acetone and was treated with an appropriately substituted arylal-
kyl bromide in the presence of K2CO3. After heating the mixture at
reflux for 6 h, the reaction was cooled and filtered through a pad of
celite. The crude thus obtained was purified by recrystallization
and the structures of the target compounds were verified by 1H
NMR, 13C NMR and mass spectrometry as cited in the Supplemen-
tary material.
for MAO-A inhibition than phenylethoxy and phenylpropoxy sub-
stitution. In this regard, the benzyloxy-substituted homologue 6a
(IC50 = 0.792
the phenylethoxy- [6n, IC50 = 1.32
tuted [6o, IC50 = 1.16 M] homologues. For MAO-B inhibition, the
phenylethoxy- [6n, IC50 = 0.033 M] and phenylpropoxy-substi-
tuted [6o, IC50 = 0.032 M] homologues are, in turn, more potent
inhibitors than the benzyloxy-substituted homologue 6a
(IC50 = 0.063 M). These results indicate that increasing the
length/size of the C6 substituent may represent a strategy to the
reduce the MAO-A inhibition potency of -tetralones, while
lM) is a modestly more potent MAO-A inhibitor than
lM] and phenylpropoxy-substi-
l
l
l
The MAO inhibitory properties of the
a-tetralone derivatives
were evaluated using the recombinant human MAO-A and MAO-
B enzymes.22 The catalytic activities of the MAO enzymes were
measured by employing kynuramine as enzyme substrate for both
MAO-A and MAO-B. The MAOs oxidise kynuramine to ultimately
l
a
enhancing MAO-B inhibition potency. This would result in more
selective MAO-B inhibition. Among these 3 compounds 6a is thus
the least selective MAO-B inhibitor with a selectivity index (SI) of
13. Another interesting SAR is that, for the most part, alkyl and hal-
ogen substituents on the meta and para positions of the benzyloxy
ring enhance MAO-B inhibitory potency. With the exception of 6k,
all homologues with substituents on the benzyloxy ring (6b–j, 6l–
m) are more potent MAO-B inhibitors than the benzyloxy-substi-
tuted homologue 6a. In this regard, no clear correlation between
the position (meta or para) of the substituent and MAO-B inhibition
potency is apparent, and high potency inhibitors are represented
by compounds substituted on both the meta and para positions
of the benzyloxy ring. Definitive correlations between the nature
(alkyl and halogen) of the substituent and MAO-B inhibition
potency also are not apparent, and high potency inhibitors are rep-
resented by compounds substituted with halogens (F, Cl, Br, I) and
alkyl groups (CH3, CN, CF3) on the benzyloxy ring. Also of interest is
the observation that, with the exception of 6h–j, substitution on
the benzyloxy ring leads to enhanced MAO-A inhibition compared
the benzyloxy-substituted homologue 6a. For MAO-A inhibition,
clear correlations between the nature and positions of the substitu-
ent on the benzyloxy ring also do not exist. It is, however, notewor-
thy that, compared to substitution with other halogens (F, Cl, Br),
the iodo-substituted homologues, 6h and 6i, are relatively weaker
yield 4-hydroxyquinoline,
(kex = 310 nm; kem = 400 nm) in alkaline media.23 Since neither
kynuramine nor the -tetralone derivatives examined here fluo-
resce under the specific assay conditions, the MAO-catalysed gen-
eration of 4-hydroxyquinoline can be readily measured by
fluorescence spectrophotometry. By graphing the residual MAO
a
metabolite which fluoresces
a
activities recorded in the presence of the test
a-tetralones versus
the logarithm of inhibitor concentration, sigmoidal concentra-
tion–inhibition curves were obtained from which the inhibition
potencies, the corresponding IC50 values, were calculated.
The IC50 values for the inhibition of human MAO-A and MAO-B
by the
results show that the
of MAO-B with all compounds exhibiting IC50 values in the low
nanomolar range (IC50 values <0.078 M). The results further dem-
onstrate that, with the exception of 6k, all of the -tetralone deriv-
a-tetralone derivatives 6a–o, are given in Table 1. The
a-tetralone derivatives are potent inhibitors
l
a
atives are selective inhibitors of the MAO-B isoform. The most
potent MAO-B inhibitor, 6-(3-iodobenzyloxy)-3,4-dihydro-2H-
naphthalen-1-one (6h), exhibits an IC50 value of 0.0045 lM with
a 287-fold selectivity for MAO-B over the MAO-A isoform. Com-
pared to the reference MAO-B-selective inhibitor, lazabemide
(IC50 = 0.091
potent as a MAO-B inhibitor under identical conditions.24 A num-
ber of other -tetralones also are highly potent MAO-B inhibitors.
For example, in addition to 6h, 6 compounds (6c, 6e–g, 6i and 6m)
exhibited IC50 values <0.01 M. Among the -tetralones evaluated,
lM), compound 6h is approximately 20-fold more
a
MAO-A inhibitors with IC50 values >1
eral, iodo substitution on the benzyloxy ring may reduce the MAO-
A inhibition potency of -tetralones, while high potency MAO-B
lM. This suggest that, in gen-
l
a
a
compound 6i is the most selective MAO-B inhibitor with a 346-fold
selectivity for MAO-B over the MAO-A isoform. As discussed above,
MAO-B selective inhibitors are particularly useful in the therapy of
Parkinson’s disease.
inhibition is retained. As exemplified by 6k, substitution on the
meta position with the nitrile functional group results in a large
enhancement of MAO-A inhibition potency with only a slight
reduction of MAO-B inhibition potency. Compared to 6a, com-
pound 6k is 33-fold more potent as a MAO-A inhibitor, while being
approximately equipotent as a MAO-B inhibitor. Further investiga-
tion is necessary to establish the molecular basis of the effect of
meta nitrile substitution on MAO-A inhibition. A possible explana-
tion for the observation that alkyl and halogen substituents on the
benzyloxy ring enhance MAO inhibitory potency is that these sub-
stituents enhance the lipophilicities of the compounds, and thus
Van der Waals interactions with the MAO enzymes. The regions
It is interesting to note that one
a-tetralone derivative, com-
pound 6k, is a MAO-A selective inhibitor with a 3.25-fold selectiv-
ity for MAO-A over MAO-B. Compound 6k also is the most potent
MAO-A inhibitor among the
value of 0.024 M. For comparison, the reversible MAO-A inhibitor,
toloxatone, is reported to inhibit human MAO-A with an IC50 value
of 3.92
M under identical conditions.24 In spite of its selectivity
and high potency inhibition of MAO-A, 6k also is a high potency
a-tetralones evaluated with an IC50
l
l

L. J. Legoabe et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2758–2763
2761
towards the entrances of the active sites of MAO-A and MAO-B,
where the benzyloxy rings of these inhibitors are expected to bind,
100
75
50
25
0
are reported to be hydrophobic in nature.25,26
To evaluate the importance of the C6 substituent for the inhibi-
tion of the MAOs by the a-tetralone derivatives 6a–o, a-tetralone
(5) and 6-hydroxy-1-tetralone (9) were also evaluated as human
MAO inhibitors. The results document that 5 and 9 are compara-
tively weak MAO inhibitors. Compared to the benzyloxy-substi-
tuted derivative 6a,
IC50(MAO-B) = 18.6 M] is 19-fold and 295-fold weaker as an
inhibitor of MAO-A and MAO-B, respectively. 6-Hydroxy-1-tetra-
lone (9) was found to be an even weaker MAO inhibitor than -tet-
ralone. This result demonstrates that appropriate C6 substitution is
a-tetralone [IC50(MAO-A) = 14.8 lM;
l
a
a requirement for the MAO inhibitory activities of
derivatives.
a-tetralone
The reversibility of MAO-A and MAO-B inhibition by the
a-tet-
ralone derivatives was examined by employing compounds 6k and
6h are representative inhibitors. These compounds are respectively
the most potent MAO-A and MAO-B inhibitors discovered in this
study. The reversibility of MAO inhibition was investigated by
measuring the recoveries of MAO activities after dialysis of
enzyme-inhibitor mixtures.27 The MAO enzymes were incubated
in the presence of the representative inhibitors, at concentrations
equal to 4 Â IC50, for a period of 15 min and subsequently dialysed
for 24 h. As shown in Figure 2, MAO-A inhibition by 6k is com-
pletely reversed after 24 h of dialysis, and MAO-A catalytic activity
is recovered to a level of 108% of the control value (recorded in the
absence of inhibitor). In contrast, the MAO-A activity of undialysed
mixtures of MAO-A and 6k is 44% of the control value. These data
suggest that 6k is a reversible MAO-A inhibitor. After similar treat-
ment and dialysis of mixtures of MAO-A and the irreversible inhib-
itor, pargyline, the enzyme activity is not recovered, with the
residual enzyme activity at a level of only 1.6% of the control value.
The results given in Figure 3 show that MAO-B inhibition by 6h is
only partially reversed after 24 h of dialysis, with the MAO-B cata-
lytic activity recovering to a level of 59% of the control value. The
enzyme activity of undialysed mixtures of MAO-B and 6h is 23%
Figure 3. Reversibility of the inhibition of MAO-B by 6h. MAO-B and 6h (at
4 Â IC50) were preincubated for 15 min, dialysed for 24 h and the residual enzyme
activity was measured (6h–dialysed). MAO-B was similarly preincubated in the
absence (No inhibitor–dialysed) and presence of the irreversible inhibitor, (R)-
deprenyl (Depr–dialysed), and dialysed. For comparison, the residual MAO activity
of undialysed mixtures of MAO-B with 6h is also shown (6h–undialysed).
of the control value. For comparison, after similar preincubation
and dialysis of mixtures of MAO-B with the irreversible inhibitor,
(R)-deprenyl, the enzyme activity is not recovered, with only 7%
activity (of the control value) remaining. While an explanation
for the observation that MAO-B inhibition by 6h is only partially
reversed by dialysis, is not readily apparent, compound 6h may
act as a quasi-reversible or tight-binding inhibitor. The potential
tight-binding of inhibitors to MAO-B has previously been reported.
For example, chromone carboxamides display a quasi-reversible
binding to human MAO-B,28,29 while oxadiazolones are reported
to bind reversibly and time-dependently to rat brain MAO-B.30
Other reports also describe oxadiazolones as reversible and extre-
mely tight-binding MAO-B specific inhibitors.31 Further investiga-
tion is necessary to clarify this point.
100
75
50
25
0
The modes of MAO-A and MAO-B inhibition by the representa-
tive inhibitors, 6k and 6h, were further investigated by construct-
ing sets of Lineweaver–Burk plots for the inhibition of the MAOs.
Each plot was constructed by measuring the catalytic rates of the
MAOs at 8 different kynuramine concentrations (15–250 lM).
Lineweaver–Burk plots were thus constructed in the absence of
inhibitor, and presence of five different concentrations of the rep-
resentative inhibitors, 6k and 6h. The sets of Lineweaver–Burk
plots for the inhibition of MAO-A and MAO-B by 6k and 6h are
shown Figures 4 and 5, respectively. In both instances, the lines
of Lineweaver–Burk plots are linear and intersect on the y-axis.
This suggests that 6k is a competitive inhibitor of human MAO-A,
and 6h is a competitive inhibitor of human MAO-B. This is addi-
tional evidence that these inhibitors are reversible MAO inhibitors.
From graphs of the slopes of the Lineweaver–Burk plots versus the
concentrations of the representative inhibitors, a Ki value of
0.025 lM for the inhibition of MAO-A by 6k is estimated, while a
Ki value of 0.0071
estimated.
l
M for the inhibition of MAO-B by 6h is
Figure 2. Reversibility of the inhibition of MAO-A by 6k. MAO-A and 6k (at
4 Â IC50) were preincubated for 15 min, dialysed for 24 h and the residual enzyme
activity was measured (6k–dialysed). MAO-A was similarly preincubated in the
absence (No inhibitor–dialysed) and presence of the irreversible inhibitor, pargyline
(Parg–dialysed), and dialysed. For comparison, the residual MAO activity of
undialysed mixtures of MAO-A with 6k is also shown (6k–undialysed).
In conclusion, the present study shows that a series of
a-tetra-
lone derivatives are highly potent inhibitors of the human MAOs.
With the exception of 6k, all homologues examined are MAO-B
selective inhibitors. For example, the most potent MAO-B inhibitor,

2762
L. J. Legoabe et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2758–2763
compound 4 [IC50(MAO-A) = 0.095 l
M], reported in literature.19
1200
800
Since 6k also is a potent MAO-B inhibitor, this compound repre-
sents a non-selective MAO inhibitor. As discussed, by acting syner-
gistically such compounds may have enhanced therapeutic value,
particularly in Parkinson’s disease. It is important to note that 6k
is a reversible and competitive MAO-A inhibitor. Since reversible
inhibitors, in general, do not provoke tyramine-induced adverse
effects, 6k is expected to exhibit an acceptable safety profile in this
respect.7 Since clinically used irreversible MAO-B inhibitors are
reported to possess excellent safety profiles, the observation that
100
75
50
25
0
400
-0.02
0.00
[I], µM
0.02
MAO-B inhibition by
partially reversible is not of concern from a safety point of view.32
Further examination of the physicochemical properties of -tetral-
ones is necessary to determine if these promising inhibitors may
be acceptable as lead compounds for the treatment of depression
and Parkinson’s disease.
a-tetralones (as demonstrated with 6h) is
a
Acknowledgements
-0.02
0.00
0.02
0.04
0.06
1/[S]
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, 80647 and 80637). 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 whatso-
ever in this regard.
Figure 4. Lineweaver–Burk plots of human MAO-A activities in the absence (filled
squares) and presence of various concentrations of 6k (IC50 = 0.024 M). For these
studies the concentrations of 6k employed were 0.006–0.030 M. The inset is a
graph of the slopes of the Lineweaver–Burk plots versus inhibitor concentration
l
l
(Ki = 0.025 lM).
1200
800
Supplementary data
100
75
50
25
0
400
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.04.021.
-0.006 -0.003 0.000 0.003 0.006
[I], µM
References and notes
1. Youdim, M. B. H.; Bakhle, Y. S. Br. J. Pharmacol. 2006, 147, S287.
2. Ramsay, R. R. Curr. Top. Med. Chem. 2012, 12, 2189.
3. Youdim, M. B.; Edmondson, D.; Tipton, K. F. Nat. Rev. Neurosci. 2006, 7, 295.
4. Yamada, M.; Yasuhara, H. Neurotoxicology 2004, 25, 215.
5. Da Prada, M.; Zürcher, G.; Wüthrich, I.; Haefely, W. E. J. Neural Tansm. 1988, 26,
31.
6. Bonnet, U. CNS Drug Rev. 2003, 9, 97.
7. Provost, J. C.; Funck-Brentano, C.; Rovei, V.; D’Estanque, J.; Ego, D.; Jaillon, P.
Clin. Pharmacol. Ther. 1992, 52, 384.
-0.02
0.00
0.02
0.04
0.06
8. Youdim, M. B.; Weinstock, M. Neurotoxicology 2004, 25, 243.
9. Foley, P.; Gerlach, M.; Youdim, M. B.; Riederer, P. Parkinsonism Relat. Disord.
2000, 6, 25.
1/[S]
10. Di Monte, D. A.; DeLanney, L. E.; Irwin, I.; Royland, J. E.; Chan, P.; Jacowec, M.
W.; Langston, J. W. Brain Res. 1996, 738, 53.
11. Finberg, J. P.; Wang, J.; Bankiewich, K.; Harvey-White, J.; Kopin, I. J.; Goldstein,
D. S. J. Neural Transm. 1998, 52, 279.
12. Fernandez, H. H.; Chen, J. J. Pharmacotherapy 2007, 27, 174S.
13. Marchitti, S. A.; Deitrich, R. A.; Vasiliou, V. Pharmacol. Rev. 2007, 59, 125.
14. Burke, W. J.; Kumar, V. B.; Pandey, N.; Panneton, W. M.; Gan, Q.; Franko, M. W.;
O’Dell, M.; Li, S. W.; Pan, Y.; Chung, H. D.; Galvin, J. E. Acta Neuropathol. 2008,
115, 193.
15. Mallajosyula, J. K.; Kaur, D.; Chinta, S. J.; Rajagopalan, S.; Rane, A.; Nicholls, D.
G.; Di Monte, D. A.; Macarthur, H.; Andersen, J. K. PLoS One 2008, 3,
e1616.
16. Nicotra, A.; Pierucci, F.; Parvez, H.; Santori, O. Neurotoxicology 2004, 25, 155.
17. Fowler, J. S.; Volkow, N. D.; Wang, G. J.; Logan, J.; Pappas, N.; Shea, C.;
MacGregor, R. Neurobiol. Aging 1997, 18, 431.
18. Carradori, S.; Secci, D.; Bolasco, A.; Chimenti, P.; D’Ascenzio, M. Expert Opin.
Ther. Pat. 2012, 22, 759.
19. Legoabe, L. J.; Petzer, A.; Petzer, J. P. Eur. J. Med. Chem. 2012, 49, 343.
20. Legoabe, L. J.; Petzer, A.; Petzer, J. P. Bioorg. Chem. 2012, 45, 1.
21. Meiring, L.; Petzer, J. P.; Petzer, A. Bioorg. Med. Chem. Lett. 2013, 23, 5498.
22. Novaroli, L.; Reist, M.; Favre, E.; Carotti, A.; Catto, M.; Carrupt, P. A. Bioorg. Med.
Chem. 2005, 13, 6212.
Figure 5. Lineweaver–Burk plots of human MAO-B activities in the absence (filled
squares) and presence of various concentrations of 6h (IC50 = 0.0045 M). For these
studies the concentrations of 6h employed were 0.001–0.005 M. The inset is a
graph of the slopes of the Lineweaver–Burk plots versus inhibitor concentration
(Ki = 0.0071 M).
l
l
l
compound 6h, exhibits an IC50 value of 0.0045 lM with a 287-fold
selectivity for MAO-B over the MAO-A isoform. When compared to
the previously studied chromone derivatives,19,20 which served as
leads for the current study, the
inhibition potencies. For example, the MAO-B inhibition potencies
of the -tetralones ranged from 0.0045 to 0.078 M, while those of
the chromone derivatives ranged from 0.002 to 0.370
M.19,20
Most of the -tetralone derivatives also were potent MAO-A inhib-
itors. The most potent MAO-A inhibitor, compound 6k, possesses
an IC50 value of 0.024 M. While a number of the reported chro-
a-tetralones have similar MAO-B
a
l
l
a
l
mones also inhibited MAO-A, 6k is approximately a fourfold more
potent MAO-A inhibitor than the most potent chromone derivative,
23. Strydom, B.; Malan, S. F.; Castagnoli, N., Jr.; Bergh, J. J.; Petzer, J. P. Bioorg. Med.
Chem. 2010, 18, 1018.

L. J. Legoabe et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2758–2763
2763
24. Petzer, A.; Pienaar, A.; Petzer, J. P. Arzneimittel-Forsch 2013, 63, 462.
25. Binda, C.; Newton-Vinson, P.; Hubálek, F.; Edmondson, D. E.; Mattevi, A. Nat.
Struct. Biol. 2002, 9, 22.
26. Son, S.-Y.; Ma, J.; Kondou, Y.; Yoshimura, M.; Yamashita, E.; Tsukihara, T. Proc.
Natl. Acad. Sci. U.S.A. 2008, 105, 5739.
27. Petzer, A.; Pienaar, A.; Petzer, J. P. Life Sci. 2013, 93, 283.
28. Gaspar, A.; Silva, T.; Yáñez, M.; Viña, D.; Orallo, F.; Ortuso, F.; Uriarte, E.; Alcaro,
S.; Borges, F. J. Med. Chem. 2011, 54, 5165.
29. Gaspar, A.; Reis, J.; Fonseca, A.; Milhazes, N.; Viña, D.; Uriarte, E.; Borges, F.
Bioorg. Med. Chem. Lett. 2011, 21, 707.
30. Krueger, M. J.; Mazouz, F.; Ramsay, R. R.; Milcent, R.; Singer, T. P. Biochem.
Biophys. Res. Commun. 1995, 206, 556.
31. Mazouz, F.; Gueddari, S.; Burstein, C.; Mansuy, D.; Milcent, R. J. Med. Chem.
1993, 36, 1157.
32. Pae, C. U.; Bodkin, J. A.; Portland, K. B.; Thase, M. E.; Patkar, A. A. J. Clin.
Psychiatry 2012, 73, 661.