Inhibition of monoamine oxidase by 3,4-dihydro-2(1H)-quinolinone derivatives

Article abstract of DOI:10.1016/j.bmcl.2013.08.071

In the present study, a series of 3,4-dihydro-2(1H)-quinolinone derivatives were synthesized and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The 3,4-dihydro-2(1H)-quinolinone derivatives are structurally related to a series of coumarin (1-benzopyran-2-one) derivatives which have been reported to act as MAO-B inhibitors. The results document that the quinolinones are highly potent and selective MAO-B inhibitors with most homologues exhibiting IC₅₀ values in the nanomolar range. The most potent MAO-B inhibitor, 7-(3-bromobenzyloxy)-3,4-dihydro-2(1H)-quinolinone, exhibits an IC₅₀ value of 2.9 nM with a 2750-fold selectivity for MAO-B over the MAO-A isoform. An analysis of the structure-activity relationships for MAO-B inhibition shows that substitution on the C7 position of the 3,4-dihydro-2(1H)-quinolinone scaffold leads to significantly more potent inhibition compared to substitution on C6. In this regard, a benzyloxy substituent on C7 is more favourable than phenylethoxy and phenylpropoxy substitution on this position. It may be concluded that C7-substituted 3,4-dihydro-2(1H)-quinolinones are promising leads for the therapy of Parkinson's disease.

Full text of DOI:10.1016/j.bmcl.2013.08.071

Bioorganic & Medicinal Chemistry Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Inhibition of monoamine oxidase by 3,4-dihydro-2(1H)-quinolinone
derivatives
Letitia Meiring a, Jacobus P. Petzer a, Anél Petzer b,
⇑
a Pharmaceutical Chemistry, School of Pharmacy, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
b Centre of Excellence for Pharmaceutical Sciences, 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:
In the present study, a series of 3,4-dihydro-2(1H)-quinolinone derivatives were synthesized and evalu-
ated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The 3,4-dihydro-2(1H)-
quinolinone derivatives are structurally related to a series of coumarin (1-benzopyran-2-one) derivatives
which have been reported to act as MAO-B inhibitors. The results document that the quinolinones are
highly potent and selective MAO-B inhibitors with most homologues exhibiting IC50 values in the nano-
molar range. The most potent MAO-B inhibitor, 7-(3-bromobenzyloxy)-3,4-dihydro-2(1H)-quinolinone,
exhibits an IC50 value of 2.9 nM with a 2750-fold selectivity for MAO-B over the MAO-A isoform. An anal-
ysis of the structure–activity relationships for MAO-B inhibition shows that substitution on the C7 posi-
tion of the 3,4-dihydro-2(1H)-quinolinone scaffold leads to significantly more potent inhibition
compared to substitution on C6. In this regard, a benzyloxy substituent on C7 is more favourable than
phenylethoxy and phenylpropoxy substitution on this position. It may be concluded that C7-substituted
3,4-dihydro-2(1H)-quinolinones are promising leads for the therapy of Parkinson’s disease.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 28 June 2013
Revised 12 August 2013
Accepted 15 August 2013
Available online xxxx
Keywords:
Monoamine oxidase
Reversible inhibition
Selectivity
3,4-Dihydro-2(1H)-quinolinone
Structure–activity relationship
The monoamine oxidases (MAOs) are mitochondrial bound en-
zymes which metabolize neurotransmitter and dietary amines in
the brain and peripheral tissues. The MAOs have been drug targets
for numerous decades and inhibitors of these enzymes are used
primarily to treat neuropsychiatric syndromes.1,2 MAO-B inhibi-
tors, in particular, are considered useful in the therapy of Parkin-
of Parkinson’s disease.9 Hydrogen peroxide formed during MAO
catalysis may lead to oxidative damage and promotes apoptotic
signaling events.9,10 Considering that MAO-B activity increases in
the brain with age, the concomitant increase of these metabolic
by-products may be especially relevant to the pathogenesis of
Parkinson’s disease.11,12
son’s disease since oxidation by MAO-B represents
a
major
It should be noted that MAO-A also metabolizes dopamine in
the primate and possibly human brain and, similar to MAO-B
inhibitors, MAO-A inhibitors also enhance the elevation of dopa-
mine levels derived from levodopa.4 The clinical use of MAO-A
inhibitors have, however, declined in recent years because of side
effects that may arise from the combination of MAO-A inhibitors
with the dietary amine, tyramine. MAO-A inhibitors block the
peripheral metabolism of tyramine, and the subsequent increase
in systemic tyramine concentrations leads to the release of norepi-
nephrine from peripheral neurons, which results in a potentially
severe hypertensive response.13 Another adverse effect of MAO-A
inhibitors which limits their clinical use is serotonin toxicity, a
potentially fatal syndrome which develops when serotonergic
agents and MAO-A inhibitors are combined.14,15 The combination
of MAO-A inhibitors, which block the MAO-A-catalyzed metabo-
lism of serotonin, with selective serotonin reuptake inhibitors
(SSRIs) and serotonin-releasing agents leads to excessive extracel-
lular serotonin concentrations in the central nervous system and
hence serotonin toxicity. Based on these considerations, inhibitors
with a high degree of selectivity for MAO-B over the MAO-A iso-
form are generally more suitable for Parkinson’s disease therapy.
catabolic pathway of dopamine in the central nervous system.
MAO-B inhibitors conserve the depleted dopamine stores in the
parkinsonian brain and enhance the elevation of dopamine levels
after administration of levodopa, the metabolic precursor of dopa-
mine.3,4 The combined use of levodopa and MAO-B inhibitors al-
lows for a reduction of the dosage of levodopa that is necessary
for a therapeutic response which, in turn, leads to diminished levo-
dopa associated side effects.5 MAO-B may also indirectly increase
extracellular dopamine concentrations by blocking the metabolism
of b-phenethylamine. b-Phenethylamine is a false neurotransmit-
ter that mediates the release of neuronal dopamine and inhibits
its active uptake.6,7 Also of interest are reports that MAO-B inhib-
itors may exert neuroprotective effects in Parkinson’s disease by
reducing the formation of potentially harmful metabolic by-prod-
ucts of MAO catalysis.1,8 For example, aldehydes derived from
the MAO catalytic cycle has been implicated in the aggregation
of a-synuclein, a process which is associated with the pathogenesis
⇑
Corresponding author. Tel.: +27 18 2994464; fax: +27 18 2994243.
E-mail address: 12264954@nwu.ac.za (A. Petzer).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.08.071
Please cite this article in press as: Meiring, L.; et al. Bioorg. Med. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.bmcl.2013.08.071

2
L. Meiring et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
Table 1
In addition, MAO-B inhibitors with a reversible mode of action may
possess certain advantages over irreversible MAO-B inhibitors,
which are currently used in Parkinson’s disease therapy. The most
notable advantage is an immediate recovery of enzyme activity
when the inhibitor has been eliminated from the tissues. In con-
trast, after termination of treatment with irreversible inhibitors,
the rates of recovery of enzyme activity are slow and variable, in
part because the turnover rate for the biosynthesis of MAO-B in
the human brain may be as much as 40 days.16,17
The IC50 values for the inhibition of recombinant human MAO-A and MAO-B by
compounds 4 and 5
O
6
R
R
7
N
H
O
O
N
H
O
4
5
R
IC50
(
l
M)a
MAO-B
SIb
In the search for improved antiparkinsonian therapies, the de-
sign of new MAO-B inhibitors is pursued by several research
groups. Based on the above analyses these inhibitors should be
reversible and selective for the MAO-B isoform. In this regard, cou-
marin (1-benzopyran-2-one) (1) has emerged as a particularly
promising scaffold (Fig. 1).18 Substituted coumarins have been
shown to act as competitive MAO inhibitors, with substitution on
C7 of the coumarin ring yielding particularly potent MAO-B inhib-
itors. For example, 7-(3,4-difluorobenzyloxy)-3,4-dimethylcouma-
rin (2) was shown to inhibit rat brain MAO-B with an IC50 value of
1.14 nM and a 108-fold selectivity for MAO-B over the MAO-A iso-
form.18 Based on the structural similarity between the coumarin
moiety and 3,4-dihydro-2(1H)-quinolinone (3), the present study
examines the possibility that a series of 3,4-dihydro-2(1H)-quinoli-
none derivatives (4 and 5) may act as reversible and selective
inhibitors of recombinant human MAO-B. For this purpose substi-
tution on the C6 and C7 positions of the 3,4-dihydro-2(1H)-quin-
olinone moiety was considered. Since alkyloxy substituents on
C6 and C7 of the coumarin moiety yields compounds with good
MAO-B inhibitory potencies,18 in the present study alkyloxy sub-
stituents (benzyloxy, phenylethoxy and phenylpropoxy) were also
selected for substitution on C6 and C7 of 3,4-dihydro-2(1H)-quin-
olinone ring system (Table 1). Among these, the benzyloxy side
chain has been shown to be particularly suited for enhancing the
MAO inhibition potencies of coumarin.18 Furthermore, the pro-
posal that benzyloxy-substituted 3,4-dihydro-2(1H)-quinolinones
may act as MAO inhibitors is supported by a report, demonstrating
that 7-(benzyloxy)-3,4-dihydro-2(1H)-quinolinone (5a) inhibits
MAO-A
4a
4b
4c
4d
4e
5a
5b
5c
5d
5e
C6H5CH2–
25.3 15.2
50.6 10.3
12.4 1.93
22.5 9.69
19.7 12.8
90.4 47.1
No Inhc
7.98 1.09
53.7 12.0
22.5 4.08
4.01 1.196
0.620 0.148
0.086 0.029
2.33 1.33
0.284 0.049
0.038 0.013
6
3-ClC6H4CH2–
3-BrC6H4CH2–
C6H5(CH2)2–
C6H5(CH2)3–
C6H5CH2–
3-ClC6H4CH2–
3-BrC6H4CH2–
C6H5(CH2)2–
C6H5(CH2)3–
82
144
10
69
2379
–
2751
281
173
0.0062 0.00063
0.0029 0.0009
0.191 0.041
0.130 0.010
a
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
b
ratio of IC50(MAO-A)/IC50(MAO-B).
c
No inhibition at a maximum tested concentration of 100 lM.
procedure (Scheme 1).19 Commercially available 6-hydroxy-3,4-
dihydro-2(1H)-quinolinone (6) and 7-hydroxy-3,4-dihydro-2(1H)-
quinolinone (7), suspended in ethanol, were treated with an appro-
priately substituted alkyl bromide in the presence of KOH. After
heating the mixture at reflux for 5 h, the reaction was poured into
aqueous NaOH (1%). 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
Supplementary data.
To evaluate the MAO inhibitory properties of the 3,4-dihydro-
2(1H)-quinolinone derivatives (4 and 5), the recombinant human
MAO-A and MAO-B enzymes were used.20 To measure MAO activ-
ities, the MAO-A/B mixed substrate, kynuramine, was employed.
Kynuramine is oxidized by the MAOs to ultimately yield 6-
hydroxyquinoline, a metabolite which fluoresces (kex = 310 nm;
kem = 400 nm) in alkaline media.21 Using fluorescence spectropho-
tometry, the formation of 6-hydroxyquinoline can be readily mea-
sured in the presence of the test inhibitors since the 3,4-dihydro-
2(1H)-quinolinone derivatives do not fluoresce under these assay
conditions. From the MAO activity measurements in the presence
of the test inhibitors, sigmoidal concentration–inhibition curves
were constructed and the inhibition potencies, the corresponding
IC50 values, were calculated (Fig. 2).
rat MAO-A and MAO-B with IC50 values of 102 lM and 1.05 lM,
respectively.18 We have therefore further explored the MAO inhib-
itory properties of the benzyloxy-substituted 3,4-dihydro-2(1H)-
quinolinones by substitution on the benzyloxy phenyl ring with
halogens (Cl, Br). Halogen substitution on the benzyloxy ring has
previously been shown to significantly enhance the MAO-B inhib-
itory properties of 7-substituted benzyloxy-3,4-dimethylcouma-
rins as exemplified by structure 2.18 The aim of this study is
therefore to discover novel highly potent and selective MAO-B
inhibitors which may act as leads for the design of antiparkinso-
nian therapies.
The C6- and C7-substituted 3,4-dihydro-2(1H)-quinolinone
derivatives 4 and 5 were synthesized according to the literature
The IC50 values for the inhibition of human MAO-A and MAO-B
by the 3,4-dihydro-2(1H)-quinolinone derivatives, 4 and 5, are
given in Table 1. The results show that the 3,4-dihydro-2(1H)-
HO
6
O
F
6
R
R
O
O
a
O
R
R
Br
Br
+
+
O
O
N
H
O
O
1
3
2
N
H
O
F
6
4
5
O
6
R
a
R
7
N
H
N
H
O
O
N
H
O
O
HO
N
H
7
O
N
H
7
O
4
5
7
Figure 1. The structures of coumarin (1), 7-(3,4-difluorobenzyloxy)-3,4-dimethyl-
coumarin (2), 3,4-dihydro-2(1H)-quinolinone (3) and the 3,4-dihydro-2(1H)-quin-
olinone derivatives 4 and 5.
Scheme 1. Synthetic route to the 3,4-dihydro-2(1H)-quinolinone derivatives 4 and
5. Reagents: (a) KOH, ethanol, reflux.
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L. Meiring et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
3
bition than phenylethoxy and phenylpropoxy substitution on this
position. For example, the C7 benzyloxy substituted homologue
100
50
5a (IC50 = 0.038
phenylethoxy [(5d); IC50 = 0.191
IC50 = 0.130 M] substituted homologues. Interestingly, for the
C6-substituted 3,4-dihydro-2(1H)-quinolinones, the benzyloxy
substituted homologue 4a (IC50 = 4.01 M) was a weaker MAO-B
lM) is at least threefold more potent than the
l
M] and phenylpropoxy [(5e);
Laz
l
5c (MAO-A)
l
5c (MAO-B)
inhibitor than the C6 phenylethoxy (4d) and phenylpropoxy (4e)
substituted homologues. Reasons for the different trends observed
with the C7- and C6-substituted 3,4-dihydro-2(1H)-quinolinones
are not apparent. From a design point of view, it is noteworthy
that, for the C7 benzyloxy substituted 3,4-dihydro-2(1H)-quinoli-
nones, halogen (Cl, Br) substitution on the benzyloxy phenyl ring
further enhances MAO-B inhibition potency. In this regard, the
-4
-3
-2
-1
Log[I]
0
1
2
chlorine and bromine substituted homologues 5b (IC50
0.0062 M) and 5c (IC50 = 0.0029 M) are 6- to 13-fold more po-
tent than the unsubstituted compound 5a (IC50 = 0.038 M). For
=
l
l
Figure 2. The sigmoidal concentration–inhibition curves for the inhibition of
recombinant human MAO-A (filled circles) and MAO-B (open circles) by various
concentrations of 5c. For comparison, the sigmoidal concentration–inhibition curve
(squares) for the inhibition of MAO-B by lazabemide (Laz) is also provided.
l
those compounds with benzyloxy substituents on C6 of the 3,4-
dihydro-2(1H)-quinolinone moiety, a similar trend was observed
with the chlorine and bromine substituted homologues 4b (IC50
0.620 M) and 4c (IC50 = 0.086 M) exhibiting more potent
MAO-B inhibition than the unsubstituted 6-benzyloxy-3,4-dihy-
dro-2(1H)-quinolinone 4a (IC50 = 4.01 M). From these data it is
=
l
l
quinolinone derivatives are potent inhibitors of MAO-B with most
homologues (8 of 10) exhibiting IC50 values in the nanomolar
range. The results further demonstrate that all of the 3,4-dihy-
dro-2(1H)-quinolinone derivatives are selective MAO-B inhibitors.
The most potent MAO-B inhibitor, 7-(3-bromobenzyloxy)-3,4-
dihydro-2(1H)-quinolinone (5c), is an exceptionally potent MAO-
l
apparent that bromine substitution yields more potent MAO-B
inhibitors compared to chlorine substitution. Further investigation
is necessary to evaluate the effects on MAO-B inhibition of other
halogen and alkyl substituents on the benzyloxy phenyl ring. For
the inhibition of MAO-A, no clear SARs are apparent. As noted
above, the most potent MAO-B inhibitor of the series 5c also was
the most potent MAO-A inhibitor. Also, since 5c as well as 4c,
the second most potent MAO-A inhibitor of the series, contain bro-
mine on the benzyloxy phenyl ring, substitution with this halogen
also enhances MAO-A inhibitory potency. To evaluate the impor-
tance of the C6 and C7 substituent for the inhibition of the MAOs
by the the 3,4-dihydro-2(1H)-quinolinone derivatives, 6-hydroxy-
3,4-dihydro-2(1H)-quinolinone (6) and 7-hydroxy-3,4-dihydro-
2(1H)-quinolinone (7) were also evaluated as human MAO inhibi-
tors. The results are given in Table 2 and show that 6 and 7 are
B inhibitor with an IC50 value of 0.0029
lM. Even though 5c (IC50
= 7.98 M) also was the most potent MAO-A inhibitor of the series,
l
this compound is a highly selective inhibitor with a ꢀ2750-fold
selectivity for MAO-B over the MAO-A isoform. Another highly po-
tent MAO-B inhibitor among the compounds evaluated is com-
pound 5b (IC50 = 0.0062
any inhibitory activity towards MAO-A (up to a maximal tested
concentration of 100 M) it may also be considered as highly selec-
tive for MAO-B. Compared to the reversible MAO-B selective inhib-
itor, lazabemide (IC50 = 0.091 M), compounds 5b and 5c are
lM). Since compound 5b did not exhibit
l
l
approximately 14- and 31-fold, respectively, more potent as
MAO-B inhibitors under identical conditions.22 It is interesting to
note that 5a is a relatively potent MAO-B inhibitor with an IC50 va-
weak MAO inhibitors with IC50 values >161 lM. This result demon-
strates that appropriate C6 and C7 substitution is a requirement for
the MAO inhibitory activities of 3,4-dihydro-2(1H)-quinolinone
derivatives.
The reversibility of MAO-B inhibition by the most potent com-
pound of the series 5c was evaluated by examining the recovery
of enzyme activity after the dilution of the enzyme–inhibitor com-
plexes.23 None of the 3,4-dihydro-2(1H)-quinolinone derivatives
were potent MAO-A inhibitors. For this purpose, MAO-B and 5c
were combined and preincubated for 30 min at inhibitor concen-
trations equal to 10 Â IC50 and 100 Â IC50. The reactions were sub-
lue of 0.038
lM. This IC50 value is 27-fold more potent than the
previously reported value of 1.05
l
M for the inhibition of rat brain
MAO-B.18 This result suggests that relatively large differences may
exist between the inhibition potencies obtained with rat MAO-B
and those obtained with the human isoform. The potencies by
which 5a inhibits human (IC50
= 90.4 lM) and rat (IC50 =
102 M) MAO-A are, however, similar.
l
An analysis of the structure–activity relationships (SARs) for
MAO-B inhibition reveals interesting trends. Substitution on the
C7 position of the 3,4-dihydro-2(1H)-quinolinone moiety leads to
significantly more potent MAO-B inhibition compared to substitu-
Table 2
tion on C6. For example 5a (IC50 = 0.038
benzyloxy moiety on C7, is approximately 100-fold more potent
than 4a (IC50 = 4.01 M), the homologue bearing the benzyloxy
lM), substituted with the
The IC50 values for the inhibition of recombinant human MAO-A and MAO-B by 6-
hydroxy-3,4-dihydro-2(1H)-quinolinone (6) and 7-hydroxy-3,4-dihydro-2(1H)-quin-
olinone (7)
l
moiety at C6. In fact compounds 5a–e were in each instance more
potent MAO-B inhibitors than their corresponding C6 substituted
homologues 4a–e. It may thus be concluded that C7-substituted
3,4-dihydro-2(1H)-quinolinones are, in general, more suitable for
the design of exceptionally potent MAO-B inhibitors than C6-
substituted 3,4-dihydro-2(1H)-quinolinones. In spite of this, with
the appropriate substitution certain C6-substituted 3,4-dihydro-
HO
6
7
O
N
H
O
HO
N
H
6
7
IC50
(l
M)a
SIb
MAO-A
MAO-B
2(1H)-quinolinones such as 4c (IC50 = 0.086 lM) may still be
6
7
161 16.1
183 2.48
201 31.3
No inhc
0.8
—
viewed as potent MAO-B inhibitors. Another interesting SAR is
the finding that a benzyloxy substituent on C7 of the 3,4-dihy-
dro-2(1H)-quinolinone moiety is more favourable for MAO-B inhi-
a–c
See Table 1 for footnotes.
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4
L. Meiring et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
1200
800
100
75
50
25
0
100
75
400
-0.002
0.000
[I], µM
0.002
0.004
50
25
0
-0.02
0.00
0.02
0.04
0.06
r
yl
to
bi
50
C50
n
epre
i
1/[S]
h
x I
n
I
1 x IC
] =
1
No
= 0.
[I
]
(R)-D
I
[
Figure 4. Lineweaver–Burk plots for the inhibition of human MAO-B by 5c. The
plots were constructed in the absence (filled squares) and presence of various
concentrations of 5c. The inset is a plot of the slopes of the Lineweaver–Burk plots
versus inhibitor concentration.
Figure 3. The reversibility of inhibition of MAO-B by compound 5c. MAO-B was
preincubated with 5c at 10 Â IC50 and 100 Â IC50 for 30 min and then diluted to
0.1 Â IC50 and 1 Â IC50
, respectively. For comparison, the irreversible MAO-B
inhibitor, (R)-deprenyl, at 10 Â IC50, was similarly incubated with MAO-B and
diluted to 0.1 Â IC50. The residual activity of MAO-B was subsequently measured.
were employed. Based on its high MAO-B inhibitory potency and
selectivity over the MAO-A isoform, 5c represents a suitable lead
for the development of novel therapies for Parkinson’s disease. In
addition, 5c interacts reversibly with MAO-B, which is a desirable
property when designing antiparkinsonian therapies. The limited
SARs derived for the inhibition data show that substitution on
the C7 position of the 3,4-dihydro-2(1H)-quinolinone moiety, par-
ticularly with the benzyloxy substituent, is more favourable for
MAO-B inhibition than substitution on the C6 position. In addition,
halogen substituents on the benzyloxy phenyl ring further en-
hances MAO-B inhibition. Although a limited number of deriva-
tives were examined, this study provides ‘proof of concept’ for
the proposal that the 3,4-dihydro-2(1H)-quinolinone moiety is a
promising scaffold for the design of MAO-B inhibitors. Further
examination of the physicochemical properties of 3,4-dihydro-
2(1H)-quinolinones is necessary to determine if these promising
inhibitors may be acceptable as lead compounds for the treatment
of central nervous system disorders.
sequently diluted 100-fold to yield concentrations of 5c of 0.1 Â
IC50 and 1 Â IC50, and the residual enzyme activities were mea-
sured. Control reactions conducted in the absence of inhibitor were
also included in the study. The results are given in Figure 3 and
show that, after dilution of 5c to concentrations of 0.1 Â IC50 and
1 Â IC50, the catalytic activities of MAO-B are recovered to levels
of 70% and 36% of the control levels, respectively. This result sug-
gests that 5c acts as a reversible MAO-B inhibitor since, after sim-
ilar treatment of MAO-B with the irreversible inhibitor (R)-
deprenyl at concentrations equal to 10 Â IC50, and dilution of the
resulting reactions to 0.1 Â IC50, the MAO-B activities are not
recovered (0.7% of control). Interestingly, after dilution of 5c to
concentrations of 0.1 Â IC50 and 1 Â IC50, the MAO-B catalytic
activities are not recovered to 90% and 50%, respectively, as would
be expected for reversible inhibition. This result suggests that 5c
may possess
component.
a quasi-reversible interaction or tight-binding
To further examine the mode of MAO-B inhibition by the 3,4-
dihydro-2(1H)-quinolinone derivatives, a set of Lineweaver–Burk
plots for the inhibition of MAO-B by 5c was constructed. For this
purpose, the MAO-B catalytic rates were recorded at eight different
Acknowledgments
The NMR and MS spectra were recorded by André Joubert and
Johan Jordaan of the SASOL Centre for Chemistry, North-West Uni-
versity. 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 and 80647). 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.
kynuramine concentrations (15–250 lM) in the absence of inhibi-
tor, and presence of five different concentrations (1 Â IC50, ½ Â
4
3
IC50
,
 IC50, 1  IC50 and 11  IC50) of 5c. The set of Linewe-
4
4
aver–Burk plots is given in Figure 4. The observation that the lines
are linear and intersect on the y-axis suggests that 5c is a compet-
itive inhibitor of human MAO-B. This is further evidence that the
interaction of 5c with MAO-B is reversible. From a replot of the
slopes of the Lineweaver–Burk plots versus the concentration of
5c, a Ki value of 0.0027 lM for the inhibition of MAO-B is
estimated.
Supplementary data
In conclusion, the present study shows that a series of 3,4-dihy-
dro-2(1H)-quinolinone derivatives are highly potent and selective
MAO-B inhibitors, even when compared to the previously studied
coumarin derivatives.18 For example, the most potent inhibitor,
compound 5c (IC50 = 2.9 nM) is approximately equipotent to cou-
marin derivative 2, which represents the most active MAO-B inhib-
itor among a large series of coumarin derivatives previously
studied.18 It should be noted that 2 was evaluated as an inhibitor
of rat brain MAO while in the present study, the human enzymes
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.bmcl.
2013.08.071.
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