Inhibition of monoamine oxidase by phthalide analogues

Article abstract of DOI:10.1016/j.bmcl.2013.01.003

Based on recent reports that the small molecules, isatin and phthalimide, are suitable scaffolds for the design of high potency monoamine oxidase (MAO) inhibitors, the present study examines the MAO inhibitory properties of a series of phthalide [2-benzofuran-1(3H)-one] analogues. Phthalide is structurally related to isatin and phthalimide and it is demonstrated here that substitution at C6 of the phthalide moiety yields compounds endowed with high binding affinities to both human MAO isoforms. Among the nineteen homologues evaluated, the lowest IC₅₀ values recorded for the inhibition of MAO-A and -B were 0.096 and 0.0014 μM, respectively. In most instances, C6-substituted phthalides exhibit MAO-B specific inhibition. Among a series of 6-benzyloxyphthalides bearing substituents on the para position of the phenyl ring the general order of potency was CF₃ > I > Br > Cl > F > CH₃ > H. The results also show that the binding modes of representative phthalides are reversible and competitive at both MAO isoforms. Based on these data, C6-substituted phthalides may serve as leads for the development of therapies for neurodegenerative disorders such as Parkinson's disease.

Full text of DOI:10.1016/j.bmcl.2013.01.003

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1269–1273
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Inhibition of monoamine oxidase by phthalide analogues
⇑
Belinda Strydom, Jacobus J. Bergh, Jacobus P. Petzer
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:
Based on recent reports that the small molecules, isatin and phthalimide, are suitable scaffolds for the
design of high potency monoamine oxidase (MAO) inhibitors, the present study examines the MAO inhib-
itory properties of a series of phthalide [2-benzofuran-1(3H)-one] analogues. Phthalide is structurally
related to isatin and phthalimide and it is demonstrated here that substitution at C6 of the phthalide moi-
ety yields compounds endowed with high binding affinities to both human MAO isoforms. Among the
nineteen homologues evaluated, the lowest IC50 values recorded for the inhibition of MAO-A and -B were
Received 24 October 2012
Revised 18 December 2012
Accepted 2 January 2013
Available online 11 January 2013
Keywords:
Phthalide
0.096 and 0.0014 lM, respectively. In most instances, C6-substituted phthalides exhibit MAO-B specific
inhibition. Among a series of 6-benzyloxyphthalides bearing substituents on the para position of the phe-
nyl ring the general order of potency was CF3 > I > Br > Cl > F > CH3 > H. The results also show that the
binding modes of representative phthalides are reversible and competitive at both MAO isoforms. Based
on these data, C6-substituted phthalides may serve as leads for the development of therapies for neuro-
degenerative disorders such as Parkinson’s disease.
2-Benzofuran-1(3H)-one
Monoamine oxidase
MAO-B
Reversible inhibition
Structure–activity relationship
Ó 2013 Elsevier Ltd. All rights reserved.
The monoamine oxidases (MAOs) are flavine adenine dinucleo-
tide (FAD) containing enzymes which catalyze the -carbon oxida-
the inhibition of MAO-A may also alleviate depression which is fre-
quently associated with Parkinson’s disease.1,11 MAO-A inhibitors
may, however, lead to serious adverse effects when combined with
dietary tyramine. Inhibition of the MAO-A-catalyzed metabolism of
tyramine in the intestinal endothelial cells results in excessive
amounts of tyramine entering the systemic circulation. Tyramine,
an indirectly acting sympathomimetic amine, initiates the release
of noradrenaline from peripheral adrenergic neurons and as a con-
sequence a severe hypertensive response, which may be fatal, may
occur.1,12 Because of this, the clinical use of MAO-A inhibitors has
been limited. Recently developed reversible inhibitors of MAO-A,
such as moclobemide, however, are considered safer than irrevers-
ible MAO-A inhibitors. For example, moclobemide is essentially free
from the tyramine reaction while retaining its antidepressant
efficacy.13 This suggests that non-selective MAO inhibitors for
Parkinson’s disease therapy should preferably act reversibly.
Based on these considerations, the present study examines the
possibility that a series of phthalide [2-benzofuran-1(3H)-one]
analogues may act as reversible inhibitors of MAO. Phthalide (1)
is structurally related to isatin (2) and phthalimide (3), small mol-
ecules which have been shown to be suitable scaffolds for the de-
sign of high potency MAO inhibitors (Fig. 1).14–16 Substitution of
isatin at both the C5 and C6 positions with the benzyloxy moiety
has yielded potent MAO inhibitors.15 In this regard, substitution
on the C5 position is more favourable for potent MAO-A and -B
inhibition compared to the C6 position. Similarly, substitution of
phthalimide at the C5 position with the benzyloxy moiety also
results in structures endowed with potent MAO inhibitory
activities.16 It is noteworthy that both substituted isatin and
a
tion of biogenic and xenobiotic amines.1 Since the principal
function of the MAOs in the central nervous system is to terminate
the actions of neurotransmitter amines, they are considered drug
targets for the treatment of psychiatric and neurological disor-
ders.2 MAO-A metabolizes serotonin and MAO-A inhibitors have
been used in the treatment of anxiety disorders and depressive ill-
ness.3 MAO-B is considered to be a major dopamine metabolizing
enzyme and MAO-B inhibitors are employed in the symptomatic
therapy of Parkinson’s disease.2,4,5 In Parkinson’s disease, MAO-B
inhibitors conserve depleted dopamine stores and prolong the
physiological action of dopamine. MAO-B inhibitors also may en-
hance dopamine levels derived from levodopa, the metabolic pre-
cursor of dopamine.6,7 In addition to a symptomatic benefit,
MAO-B inhibitors may also exert a neuroprotective effect by block-
ing the formation of H2O2 and aldehydic species, metabolic by-
products of MAO-B-catalyzed substrate metabolism.1 Considering
that central MAO-B activity increases with age,8–10 inhibition of
the MAO-B-catalyzed formation of these potentially harmful spe-
cies in the aged Parkinsonian brain is of particular significance.
It is noteworthy that dopamine is also metabolized by MAO-A in
the human brain, and MAO-A inhibitors have been reported to en-
hance central dopamine levels in primates.6,7 Non-selective MAO
inhibition may therefore represent an attractive strategy to en-
hance central dopamine levels in Parkinson’s disease. In addition,
⇑
Corresponding author. Tel.: +27 18 2992206; fax: +27 18 2994243.
E-mail address: jacques.petzer@nwu.ac.za (J.P. Petzer).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.01.003

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B. Strydom et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1269–1273
O
subsequently hydrolyzed in H2SO4 (50%) to yield 6-hydroxyphtha-
O
O
lide (9) in low yields (typically 10–20%).20,21 The target phthalide
analogues (6a–r) were obtained by reacting 9 with the appropri-
ately substituted alkyl bromide in the presence of K2CO3 (6–
83%).22 6-Benzylaminophthalide (6s) was synthesized according
to the same procedure from 6-aminophthalide (8) and benzyl bro-
mide (39%). In each instance, the structures and purities of the tar-
get compounds were verified by 1H NMR, 13C NMR, mass
spectrometry and HPLC analysis as cited in the Supplementary
data.
6
5
6
5
O
O
N
NH
N
H
1
2
3
O
O
N
O
N
N
N
Cl
N
N
8
8
O
O
N
(E)
O
To evaluate the MAO inhibitory properties of the phthalide ana-
logues, the recombinant human MAO-A and -B enzymes were em-
ployed.23 Kynuramine, a MAO-A/B mixed substrate served as
enzyme substrate. Kynuramine undergoes MAO-catalyzed oxida-
tion to yield 6-hydroxyquinoline, a compound which fluoresces
(kex = 310 nm; kem = 400 nm) in alkaline media.17 Concentration
measurements of 6-hydroxyquinoline can conveniently be made
via fluorescence spectrophotometry since both kynuramine and
the phthalide inhibitors are non-fluorescent under these assay con-
ditions. The inhibition potencies of the test compounds were calcu-
lated from the sigmoidal dose–response curves and are expressed
as the corresponding IC50 values. The IC50 values for the inhibition
of MAO by phthalide analogues 6a–s are given in Table 1. The re-
sults show that the phthalide analogues are highly potent inhibi-
tors of MAO-B, with all homologues, except 6s, exhibiting IC50
4
5
Figure 1. The structures of phthalide (1), isatin (2), phthalimide (3), 8-benzyloxy-
caffeine (4) and (E)-8-(3-chlorostyryl)caffeine (5).
phthalimide analogues interact reversibly with the MAO enzymes.
The benzyloxy moiety appears to be a particularly suitable side
chain for functionalizing MAO inhibitors, and several potent MAO
inhibitors possess this moiety. Among these is the non-selective
MAO inhibitor, 8-benzyloxycaffeine (4).17 The ability of 8-benzyl-
oxycaffeine to bind to both MAO-A and -B may depend, at least
in part, on the rotational freedom of the benzyloxy CH2–O ether
bond. More rigid structures, such as (E)-8-(3-chlorostyryl)caffeine
(5), are MAO-B specific inhibitors and do not bind to MAO-A. In
the present study we thus substituted phthalide on the C6 position
with the benzyloxy moiety and investigated the interactions of the
resulting structures with human MAO-A and -B. The C6 position of
phthalide is analogous to the C5 position of isatin. As mentioned
above, substitution on the C5 position of isatin is more favourable
for MAO inhibition than substitution on the C6 position. The pro-
posal that 6-benzyloxyphthalide may act as a human MAO inhibi-
tor is supported by a report, demonstrating that this compound
values in the low nanomolar range (IC50 <0.081 lM). For example,
the most potent MAO-B inhibitor among the test compounds is 6e,
the CF3 substituted benzyloxyphthalide homologue, with an IC50
value of 0.0014
lM. For comparison, the reversible MAO-B selec-
tive inhibitor, lazabemide, exhibits an IC50 value of 0.091
l
M under
the same conditions (unpublished data from our laboratory). Of
significance is the finding that a wide variety of substituents at
C6 of the phthalide moiety yield compounds with potent MAO-B
inhibition, with the different substituent groups yielding
similar potencies. The IC50 values recorded for the differently
substituted homologues were as follows: 6-benzyloxyphthalides
inhibits rat MAO-A and -B with IC50 values of 3.6 and 0.23 lM,
respectively.18 To establish structure–activity relationships (SARs)
a variety of substituents (Cl, Br, F, CF3, I, CH3) on the benzyloxy ring
were considered (compounds 6a–h). To investigate the importance
of the benzyloxy moiety for inhibitory activity, the effects of
phenylethoxy (6i–l), phenylpropoxy (6m), phenoxyethoxy (6n–
q), cyclohexylmethoxy (6r) and benzylamino (6s) substitution on
the C6 position of the phthalide ring were also examined.
(6a–h), IC50 = 0.0014–0.024
IC50 = 0.047–0.081 M; 6-(phenylpropoxy)phthalide (6m), IC50
0.0062 M; 6-(phenoxyethoxy)phthalides (6n–q), IC50 = 0.0074–
0.018 M; 6-(cyclohexylmethoxy)phthalide (6r), IC50 = 0.012 M. This
lM; 6-(phenylethoxy)phthalides (6i–l),
l
=
l
l
l
relatively large degree of tolerance for different substituents makes
C6-substituted phthalides good candidates for the design of MAO-
B inhibitors since structural modifications to improve drug proper-
ties may be made without the risk of loss of inhibition activity.
While the 6-benzyloxyphthalides (6a–h) were all found to be po-
tent MAO-B inhibitors, it is noteworthy that those homologues
containing substituents (Cl, Br, F, CF3, I, CH3) on the benzyloxy ring
were more potent than the unsubstituted homologue 6a. For
The C6 substituted phthalide analogues were synthesized
according to previously established procedures (Scheme 1). For
this purpose, commercially available 6-nitrophthalide (7) served
as key starting material. 6-Nitrophthalide was hydrogenated in
the presence of Pd/C (10%) to yield 6-aminophthalide (8) in moder-
ate yields (typically 54–73%).19 Treatment of 8 with NaNO2 in
H2SO4 (50%) gave the corresponding diazonium salt, which was
O
O
H
N
O
O2N
H2N
R
a
d
Br
O
O
O
R
7
8
6s
b, c
O
O
R
O
OH
d
Br
O
O
R
9
6a−r
Scheme 1. Synthetic route to the C6-substituted phthalide analogues 6a–s. Reagents and conditions: (a) H2, 10% Pd/C, 18 h; (b) 50% H2SO4, NaNO2, 2 °C; (c) 50% H2SO4,
125 °C; (d) DMF, K2CO3, 50–100 °C, 12 h.

B. Strydom et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1269–1273
1271
Table 1
MAO-A/B inhibitors are: 6b, 6m, 6o and 6r. These compounds pos-
sess IC50 values for the inhibition of both MAO isoforms smaller
than 0.2 lM. Compound 6j, also a potent dual inhibitor, was found
The IC50 values for the inhibition of recombinant human MAO-A and -B by phthalide
(1) and phthalide analogues 6a–s
O
to be the least selective among the phthalides evaluated (SI = 10).
Since potent MAO-A inhibitors were found among all classes of
C6 oxy phthalides examined (benzyloxy-, phenylethoxy-, phenyl-
propoxy-, phenoxyethoxy- and cyclohexylmethoxy-substituted
phthalides), it may be concluded that a wide variety of C6 substit-
uents are capable of MAO-A inhibition. As observed for the inhibi-
tion of MAO-B, 6-benzylaminophthalide (6s) was found, however,
R
6
O
R
IC50
(
l
M)a
SIb
MAO-A
MAO-B
1
H–
44.9 2.26
1.19 0.075
28.6 2.03
1.6
50
61
96
84
214
189
118
180
68
10
16
90
16
106
13
20
43
16
to be a weak inhibitor of MAO-A (IC50 = 59.9 lM). This suggests
that C6 amino substituents are also not suitable for MAO-A inhibi-
tion. It is interesting to note that 6-benzyloxyphthalide (6a) inhib-
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
6q
6r
6s
C6H5(CH2)O–
0.024 0.003
0.0028 0.0002
0.0024 0.0002
0.0064 0.0008
0.0014 0.0001
0.0018 0.0001
0.0090 0.0004
0.0035 0.0003
0.047 0.002
0.048 0.009
0.068 0.005
0.081 0.006
0.0062 0.0003
0.018 0.0003
0.011 0.002
0.012 0.0009
0.0074 0.0006
0.012 0.001
No inhibitionc
4-ClC6H4(CH2)O–
4-BrC6H4(CH2)O–
4-FC6H4(CH2)O–
4-CF3C6H4(CH2)O–
4-IC6H4(CH2)O–
4-CH3C6H4(CH2)O–
3-BrC6H4CH2O–
C6H5(CH2)2O–
4-ClC6H4(CH2)2O–
4-FC6H4(CH2)2O–
4-CH3C6H4(CH2)2O–
C6H5(CH2)3O–
C6H5O(CH2)2O–
4-ClC6H4O(CH2)2O–
4-BrC6H4O(CH2)2O–
4-FC6H4O(CH2)2O–
C6H11CH2O–
0.172 0.010
0.227 0.045
0.542 0.025
0.304 0.037
0.344 0.105
1.06 0.042
0.629 0.016
3.21 0.559
0.498 0.039
1.07 0.290
7.33 0.234
0.096 0.006
1.91 0.172
0.137 0.009
0.240 0.010
0.323 0.017
0.185 0.009
59.9 22.8
ited MAO-A and -B with IC50 values of 1.19
respectively. These IC50 values are threefold and tenfold more po-
tent than the previously reported values of 3.6 M and 0.23 M,
lM and 0.024 lM,
l
l
respectively, for the inhibition of rat brain MAO-A and -B.18 This re-
sult suggests that, while the rat enzymes may be useful for initial
screening, caution should be exercised since relatively large differ-
ences may exist between the inhibition potencies obtained with
the rat enzymes and those obtained with the human isoforms. As
observed for the MAO-B isoform, phthalide (1) also was a weak
MAO-A inhibitor compared to the C6 oxy-substituted phthalides
6a–r. Compared to 6a–r, phthalide (IC50 = 44.9 lM) is 6 to 467-fold
weaker as a MAO-A inhibitor. These data show that the C6 substi-
tuent is also an essential structural feature for potent MAO-A
inhibition.
C6H5CH2NH–
—
a
All values are expressed as the mean standard deviation (SD) of triplicate
To examine the reversibility of MAO inhibition by the phthalide
analogues, two representative compounds, 6m and 6e, were se-
lected for the evaluation of the reversibility of MAO-A and -B inhi-
bition, respectively. To evaluate the reversibility of inhibition, the
recovery of enzyme activities after the dilution of the enzyme-
inhibitor complexes was examined.24 MAO-A and -B were preincu-
bated for 30 min with the respective inhibitors at concentrations
equal to 10 Â IC50 and 100 Â IC50. The reactions were subsequently
diluted 100-fold to yield inhibitor concentrations of 0.1 Â IC50 and
1 Â IC50, and the residual enzyme activities were measured. The re-
sults show that, after dilution of 6m and 6e to 0.1 Â IC50, the activ-
ities of MAO-A and -B are recovered to levels of 86% and 68% of the
control levels, respectively (Fig. 2). After dilution of 6m and 6e to
determinations.
b
The selectivity index is the selectivity for the MAO-B isoform and is given as the
ratio of IC50(MAO-A)/IC50(MAO-B).
c
No inhibition at a maximum tested concentration of 100 lM.
example, the substituted homologues 6b–h displayed IC50 values
ranging from 0.0014 to 0.009 M, while the unsubstituted
homologue 6a exhibited an IC50 value of 0.024 M. Interestingly,
l
l
among the 6-benzyloxyphthalides bearing substituents on the para
position of the phenyl ring, compounds 6a–g, the general
order of potency was CF3 > I > Br > Cl > F > CH3 > H. Among the
6-(phenylethoxy)phthalides (6i–l) the general order of potency
was H > Cl > F > CH3. As mentioned above, 6-benzylaminophthalide
(6s) did not inhibit MAO-B which suggests that, in contrast to C6
oxy substituents, amino substituents at C6 of phthalide are not
suitable for MAO-B inhibition. Compared to the C6 oxy-substituted
phthalides 6a–r, phthalide (1) was found to be a weak MAO-B
MAO-A
MAO-B
100
75
50
25
0
100
75
50
25
0
inhibitor with an IC50 value of 28.6
1000-fold weaker than the MAO-B inhibition potency recorded
for 6a (IC50 = 0.024 M). This finding demonstrates the importance
lM. This is approximately
l
of the C6 substituent for MAO-B inhibition.
The results document that the phthalide analogues 6a–s also
are inhibitors of MAO-A. In fact twelve of the nineteen analogues
exhibited IC50 values in the submicromolar range (0.096–
0.629
oxy)phthalide (6m) with an IC50 value of 0.096
is also a highly potent MAO-B inhibitor (IC50 = 0.0062
on the selectivity index (SI) value, 6m is 16-fold more selective
for MAO-B than MAO-A. With the exception of 6-benzylaminoph-
thalide (6s), selective inhibition of MAO-B was also observed for
the other phthalide analogues examined (SI = 10–214). Although
the phthalides are MAO-B selective inhibitors, based on the potent
MAO-A inhibitory activities of most homologues, they may still be
deemed as suitable drug candidates where both MAO-A and -B
inhibition are required. Although many of the phthalide analogues
examined here act as potent inhibitors of MAO-A and -B, those
phthalides which may be considered as particularly potent dual
l
M). The most potent MAO-A inhibitor was 6-(phenylprop-
M. This compound
M). Based
l
l
50
50
50
50
Pargyline
No Inhibitor
No Inhibitor
[I] = 1 x IC
[I] = 1 x IC
(R)-Deprenyl
[I] = 0.1 x IC
[I] = 0.1 x IC
Figure 2. Reversibility of inhibition of MAO-A by 6m and MAO-B by 6e. MAO-A and
-B were preincubated with the test inhibitors 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, pargyline
and (R)-deprenyl, at 10 Â IC50 were similarly incubated with MAO-A and -B,
respectively, and diluted to 0.1 Â IC50
. The residual enzyme activities were
subsequently measured.

1272
B. Strydom et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1269–1273
100
75
50
25
0
100
MAO-A
MAO-B
75
50
25
0
-0.02
0.00
0.02
1/[S]
0.04
0.06
-0.02
0.00
0.02
1/[S]
0.04
0.06
Figure 3. Lineweaver–Burk plots of the oxidation of kynuramine by recombinant human MAO-A (left) and MAO-B (right) in the absence (filled squares) and presence of
various concentrations of 6m and 6e. For the studies with MAO-A the concentrations of 6m were: 0.024 M (open squares), 0.048 M (filled circles) and 0.096 M (open
circles). For the studies with MAO-B the concentrations of 6e were: 0.00035 M (open squares), 0.0007 M (filled circles) and 0.0014 M (open circles).
l
l
l
l
l
l
1 Â IC50, the activities of MAO-A and -B are recovered to levels of
45% and 14% of the control levels, respectively. Although this par-
tial recovery of enzyme activity is consistent with reversible inter-
actions of 6m and 6e with MAO-A and -B, respectively, inhibition
of MAO-B by 6e may not be fully reversible (see below). After sim-
ilar treatment of MAO-A with the irreversible inhibitor, pargyline,
and MAO-B with the irreversible inhibitor (R)-deprenyl at concen-
trations equal to 10 Â IC50, and dilution of the resulting complexes
to 0.1 Â IC50, the MAO activities are not recovered (1% and 2% of
control, respectively). Interestingly, after dilution of the 6e–MAO-
B complex to 0.1 Â IC50 and 1 Â IC50, the enzyme activities are
not recovered to 90% and 50%, respectively, as would be expected.
This result suggests that, for the inhibition of MAO-B, 6e may pos-
sess a quasi-reversible or tight-binding component.
The reversibility of inhibition was further examined by con-
structing sets of Lineweaver–Burk plots for the inhibition of
MAO-A and -B by 6m and 6e, respectively. The initial MAO cata-
lytic rates were recorded at 4 different substrate concentrations
in the absence of inhibitor, and presence of three different inhibitor
concentrations. The Lineweaver–Burk plots constructed from these
data are given in Figure 3. The graphs show that for the inhibition
of MAO-A by 6m, and the inhibition of MAO-B by 6e, the Linewe-
aver–Burk plots are linear and intersect on the y-axis. This indi-
cates that the modes of inhibition of both MAO-A and -B are
competitive and therefore reversible. From a replot of the slopes
of the Lineweaver–Burk plots versus inhibitor concentration, Ki
the analogues demonstrates that these compounds are dual
MAO-A/B inhibitors. The results further document that phthalides
interact reversibly with MAO-A and -B. Both reversibility and dual
MAO-A/B inhibition are attributes which are desirable when
designing antiparkinsonian therapies. It is noteworthy that a rela-
tively large variety of C6 substituents yield phthalides with potent
MAO-A and -B inhibitory activities. This is advantageous when
optimizing the properties of these structures since modifications
made to these structures, particularly to the C6 substituent, are
less likely to abolish MAO inhibition. It should, however, be noted
that, in contrast to oxy substituents, C6 amino substituents are not
suitable for MAO inhibition. This finding is in accordance to litera-
ture which reports that 8-aminocaffeines are weak MAO inhibitors
while 8-oxycaffeines are highly potent MAO inhibitors.17,27 Based
on this analysis it may be concluded that phthalides are suitable
lead compounds for the development of novel therapies for Parkin-
son’s disease.
Acknowledgements
We are grateful to André Joubert and Johan Jordaan of the SA-
SOL Centre for Chemistry, North-West University, for recording
the NMR and MS spectra. Financial support for this work was pro-
vided by the North-West University and the Medical Research
Council, South Africa. The financial assistance of the National Re-
search Foundation (DAAD-NRF) towards this research is hereby
acknowledged. Opinions expressed and conclusions arrived at,
are those of the authors and are not necessarily to be attributed
to the DAAD-NRF.
values of 0.062 lM and 0.0007 lM were obtained for the inhibition
of MAO-A and -B by 6m and 6e, respectively. These values are sim-
ilar to the Ki values calculated according to the equation by Cheng
and Prusoff25 of 0.025
lM and 0.0006 lM, for the inhibition of
Supplementary data
MAO-A and -B by 6m and 6e, respectively. The equation by Cheng
and Prusoff estimates Ki values from the experimentally deter-
mined IC50 values, and may be seen as an alternative to measuring
these values from replots of Lineweaver–Burk plots (see Supple-
mentary data). Since 6e may exhibit tight-binding to MAO-B, the
inhibition data were also analyzed according to the equation by
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.
01.003.
Morrison.26 This analysis gave a Ki value of 0.00046 0.00012
lM
References and notes
Analysis of the inhibition data by the Morrison equation may be
more appropriate for estimating a Ki value of 6e since this inhibitor
exhibits tight-binding characteristics to MAO-B. The Ki values ob-
tained for the inhibition of MAO-B by 6e, are, however, very simi-
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