Synthesis and inhibition study of monoamine oxidase, indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase by 3,8-substituted 5H-indeno[1,2-c]pyridazin-5-one derivatives

Article abstract of DOI:10.1016/j.ejmech.2011.09.042

Previous studies on 5H-indeno[1,2-c]pyridazin-5-one derivatives as inhibitors of MAO-B revealed that it was possible to increase the MAO-B inhibitory potency of 5H-indeno[1,2-c]pyridazin-5-ones by substituting the central heterocycle in the 3-position or

Full text of DOI:10.1016/j.ejmech.2011.09.042

European Journal of Medicinal Chemistry 46 (2011) 6104e6111
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European Journal of Medicinal Chemistry
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Short communication
Synthesis and inhibition study of monoamine oxidase, indoleamine 2,
3-dioxygenase and tryptophan 2,3-dioxygenase by 3,8-substituted
5H-indeno[1,2-c]pyridazin-5-one derivatives
,
a
a
b
c
a
,
,
**
J. Reniers a , C. Meinguet , L. Moineaux , B. Masereel , S.P. Vincent , R. Frederick b, J. Wouters a
*
a Drug Design and Discovery Center, Laboratoire de Chimie Biologique Structurale, Member of NARILIS, Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium
b Drug Design and Discovery Center, Département de Pharmacie, Member of NARILIS, Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium
c Laboratoire de Chimie Bio-Organique, Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Previous studies on 5H-indeno[1,2-c]pyridazin-5-one derivatives as inhibitors of MAO-B revealed that it
was possible to increase the MAO-B inhibitory potency of 5H-indeno[1,2-c]pyridazin-5-ones by
substituting the central heterocycle in the 3-position or C-8 with lipophilic groups which occupy the
substrate cavity or the entrance of the binding site, respectively. Here, four new 5H-indeno[1,2-c]pyr-
idazin-5-one derivatives containing lipophilic groups at both positions were synthesized and their
inhibitory potency against human monoamine oxidase A and B were evaluated. Selectivity of these
compounds against IDO and TDO, two enzymes sharing substrate similarity with MAO and involved in
the serotonergic and kynurenine pathways was also studied. All compounds showed higher activity and
selectivity against MAO-B, the most effective one being 3-methyl-8-meta-chlorobenzyloxy-5H-indeno
Received 21 February 2011
Received in revised form
16 September 2011
Accepted 22 September 2011
Available online 8 October 2011
Keywords:
5H-indeno[1,2-c]pyridazin-5-one
Monoamine oxidase (MAO)
Indoleamine 2,3-dioxygenase (IDO)
Tryptophan 2,3-dioxygenase (TDO)
Docking
[1,2-c]pyridazin-5-one (9a) which was shown to be a competitive inhibitor with a Ki value of 0.11 mM.
Replacing the methyl group in the 3-position with a metaeCF3ephenyl group (7a, 7b and 7c) abolished
the inhibitory potency against MAO-B. Indeed, the substitution of the 5H-indeno[1,2-c]pyridazin-5-one
core in the 3-position dramatically influences the MAO-inhibiting properties of these compounds.
Molecular docking studies of 9a within MAO-B suggest that the 5H-indeno[1,2-c]pyridazin-5-one scaf-
fold is well stabilized into the substrate cavity with the meta-chlorobenzyloxy side chain extending
towards a rather hydrophobic pocket at the entrance cavity.
X-ray
Ó 2011 Elsevier Masson SAS. All rights reserved.
1. Introduction
However, their distribution within the body and their substrate/
inhibitor specificity are significantly different [3e5]. MAO-A pref-
Monoamine oxidase (MAO) is a flavoenzyme with the flavin
adenine dinucleotide (FAD) covalently bound to a cysteine residue
by a 8a-(S-cysteinyl)-thioether linkage. The enzyme is anchored to
erentially catalyzes the deamination of serotonin, adrenaline, and
noradrenaline, and is selectively inhibited by clorgyline and
moclobemide; MAO-B preferentially catalyzes the deamination of
the mitochondrial outer membrane of neuronal, glial and several
other cell types. It catalyzes the oxidative deamination of biogenic
and xenobiotic amines to the corresponding aldehyde and
ammonia in the periphery as well as in the central nervous system
[1]. In mammals, MAO exists in two isoforms, MAO-A and MAO-B.
Both enzymes are dimeric in their membrane bound forms. Human
MAO-A (hMAO-A) and MAO-B (hMAO-B) are encoded by separate
genes and their amino acid sequences are w70% identical [2].
b-phenylethylamine and benzylamine, and is irreversibly inhibited
by selegiline. In vitro, dopamine and tyramine are deaminated by
both isoforms, but dopamine in vivo is preferentially metabolized
by MAO-B.
MAO-A and MAO-B are attractive targets for therapeutic inter-
vention. MAO-A inhibitors are used for the treatment of mental
depression and anxiety [6] whereas MAO-B inhibitors are used
with L-DOPA and/or dopamine agonists in the symptomatic treat-
ment of Parkinson’s disease [7,8]. However, most of the current
monoamine oxidase inhibitors lead to side effects by a lack of
affinity and selectivity towards one of the isoforms. Therefore, the
finding of reversible and selective inhibitors of MAO-A and MAO-B
remains an important problem. In this context, we addressed this
question and designed novel potent MAO inhibitors.
* Corresponding author. Tel.: þ32 81 724569; fax: þ32 81 725440.
** Corresponding author. Tel.: þ32 81 724550; fax: þ32 81 725440.
E-mail addresses: jeremy.reniers@fundp.ac.be (J. Reniers), johan.wouters@
fundp.ac.be (J. Wouters).
0223-5234/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2011.09.042

J. Reniers et al. / European Journal of Medicinal Chemistry 46 (2011) 6104e6111
6105
Tryptophane
IDO/TDO
Quinolinic acid
Nicotinamide
Proteins
5-Hydroxytryptophane
Kynurenine
MAO
IDO
5-Hydroxyindole
acetaldehyde
Serotonin
(5-HT)
N-formyl-5-
hydroxykynurenamine
Scheme 2. Pathways of tryptophan metabolism. Of the dietary tryptophan that is not
used in protein synthesis, 99% is metabolized along the kynurenine pathway (red
arrows). Alternative pathway is the conversion of tryptophan to serotonin (5-HT)
(serotonergic pathway, blue arrows) [14]. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article).
bMAO-B ¼ 0.10 nM, Scheme 1) bearing metaeCF3ephenyl and
methoxy groups in the 3- and 8-positions, respectively [11] leads us
to synthesize new 5H-indeno[1,2-c]pyridazin-5-one derivatives
containing lipophilic and bulky groups at both positions (7aeb,
Scheme 1) including 7c as a reference drug. In addition, the
synthesis of two new compounds (9aeb) structurally related to
compound 2 was performed (Scheme 4). Their human monoamine
oxidase A and B inhibitory potency was investigated. The IC50
values of the most potent compounds were evaluated and Ki values
Scheme 1. Rational design of new 5H-indeno[1,2-c]pyridazin-5-one analogues.
Previous studies on 5H-indeno[1,2-c]pyridazin-5-one deriva-
tives as inhibitors of MAO-B revealed that the substitution of 5H-
indeno[1,2-c]pyridazin-5-one scaffold in the 3-position with lipo-
philic and bulky groups such metaeCF3ephenyl (1, IC50 hMAO-
B ¼ 8.5 nM [9], Scheme 1) increases the MAO-B inhibitory potency
of this series [9,10]. Molecular docking studies suggest that the
metaeCF3ephenyl occupies the substrate cavity (Fig. 1) [9]. Further
studies also revealed that the substitution in the 8-position of the
central heterocycle with lipophilic and bulky groups such tri-
fluorobutyloxy (2, IC50 baboon MAO-B (bMAO-B) ¼ 14.0 nM [11],
Scheme 1) increases the MAO-B inhibitory potency of this series
[11,12]. Docking studies suggest that this trifluorobutyloxy side
chain occupies the entrance cavity (Fig. 1) [12]. Interestingly, the
inhibitory activity reported by Frederick et al. on derivative 7c (IC50
were estimated using the ChengePrusoff equation [13].
A
competitive-type inhibition for the most potent inhibitor was
confirmed from the LineweavereBurk plots. The Ki value was in
good agreement with the value deduced from the IC50
.
In this study, we also appraised the selectivity of the 5H-indeno
[1,2-c]pyridazin-5-ones against human indoleamine 2,3-
dioxygenase (hIDO) and ralstonia metallidurans tryptophan 2,3-
dioxygenase (rmTDO), two enzymes involved, like MAO, in the
metabolism of L-tryptophan (Scheme 2) [14]. It is known that IDO is
overexpressed in a variety of diseases, including cancer and
neurodegenerative disorders (e.g., Alzheimer’s disease) [15e17].
Recently, it was shown that TDO is overexpressed in cancer [18].
IDO degrades indoleamines such as L-tryptophan, D-tryptophan,
Fig. 1. Docking solutions of compounds 1 (magenta [9]) and 2 (cyan [12]) in the active site of hMAO-B (2V5Z.pdb). Only amino acids directly implicated in the active site are
displayed and labelled in green. FAD is in yellow and the water molecules are displayed as red spheres.

6106
J. Reniers et al. / European Journal of Medicinal Chemistry 46 (2011) 6104e6111
O
O
O
O
7
4
RX (X = Br, OTs), K2CO3
1
SeO2, dioxane, reflux
3 h
OH
OH
6
2
RO
RO
5
HO
3
CH3CN (90°C)/DMF,
4-5 h
3
4a R = (CH2)3CF3
4b R = CH2C6H5
4c R = CH3
5a R = (CH2)3CF3
5b R = CH2C6H5
5c R = CH3
O
F3C
AcOH, reflux, 3 h
1
9
2
CF3
RO
8
O
N
N
NH2-NH2.H2O, AcOH
r.t., overnight
OH
7
3
6
5
4
RO
O
O
O
7a R = (CH2)3CF3
7b R = CH2C6H5
7c R = CH3
6a R = (CH2)3CF3
6b R = CH2C6H5
6c R = CH3
CF3
Scheme 3. Synthetic pathway to 5H-indeno[1,2-c]pyridazin-5-one analogues 7aec.
serotonin (MAO-A substrate) and tryptamine [19,20]. Of the dietary
tryptophan that is not used in protein synthesis, most is metabo-
lized by IDO/TDO through the kynurenine pathway, and a small
amount of it is used to synthesize the neurotransmitter serotonin
[14] (Scheme 2). Consequently, IDO and TDO play a critical role in
determining the relative tryptophan flux in the serotonergic and
kynurenine pathways [21]. Previous studies have shown the
interest to use TDO inhibitors as serotonergic antidepressants [22].
So, this similarity in substrate between IDO and MAO, and the
implication of IDO/TDO and MAO in the serotonergic and kynur-
enine pathways lead us to study the selectivity of our compounds
against the three enzymes.
commercially available. The alkoxyninhydrins (5aec) were
synthesized by selenium dioxide oxidation of 5-alkoxy-1-
indanones (4aec) [11,12].
A dioxane solution of 5-alkoxy-1-
indanone was mixed with selenium dioxide and refluxed for 3 h.
This gave the 5-alkoxyninhydrins 5aec in 73e81% yield. A solution
of 5-alkoxyninhydrin (5aec) and 30-(trifluoromethyl)acetophe-
none in acetic acid was refluxed for 3 h to give the intermediate
aldol adduct (6aec) [11,12,23]. After cooling to room temperature,
the mixture reacted with hydrazine overnight. After purification on
a silica gel column (dichloromethane 100% v), the 8-(alkoxy)-3-(30-
(trifluoromethyl)phenyl)-5H-indeno[1,2-c]pyridazin-5-one (7aec)
isomer was obtained in 39e46% yield. X-ray diffraction was used to
unambiguously establish the position of the alkoxy group in the 8-
position of the 5H-indeno[1,2-c]pyridazin-5-one moiety. ORTEP
views of the conformations of 7a and 7b, with their atomic
numbering scheme are depicted in Fig. 2. The synthesis of 9aeb
started from the phenol 8 (Scheme 4). Benzylation of 8 was ach-
ieved upon reaction with 1-(bromomethyl)-3-chlorobenzene or 1-
(bromomethyl)-3-methylbenzene in the presence of silver oxide in
DMF at room temperature for 4 h [11]. This provided 9aeb in
modest yields (6e15% yield). Again, X-ray diffraction was used to
confirm the structure of 9a (Fig. 2).
2. Results and discussion
2.1. Synthesis of 5H-indeno[1,2-c]pyridazin-5-one analogues
The strategy used to synthesize 8-(alkoxy)-3-(30-(tri-
fluoromethyl)phenyl)-5H-indeno[1,2-c]pyridazin-5-one
deriva-
tives (7aec) is depicted in Scheme 3. This synthetic pathway
follows a four steps procedure reported by our group from the
common 5-hydroxy-1-indanone (3) intermediate [11,12].
The structures of the synthesized compounds were determined
by 1H NMR, 13C NMR, mass spectrometry (MS) and their purity was
assessed by elemental analyses.
5-Hydroxy-1-indanone (3) reacted with 1-tosyl-4,4,4-
trifluorobutane in the presence of K2CO3 in acetonitrile for 4 h at
90 ꢀC to give the 5-(4,4,4-trifluoromethylbutyloxy)-1-indanone
(4a) in almost quantitative (96%) yield [11,12]. 5-Benzyloxy-1-
indanone (4b) was obtained by reaction between the indanone
(3), K2CO3, benzyl bromide in DMF for 5 h at room temperature in
almost quantitative (90%) yield. 5-(Methoxy)-1-indanone (4c) is
2.2. Inhibitory potency of 5H-indeno[1,2-c]pyridazin-5-one
analogues against MAO-A, MAO-B, IDO and TDO
First, the hMAO-A and hMAO-B inhibitory potency of the
R
synthesized 5H-indeno[1,2-c]pyridazin-5-ones were assayed
in vitro in triplicate at 10 mM using recombinant hMAO isoforms
expressed in BTI (Bacillus thuringiensis israelensis) insect cells
infected with baculovirus. The activity of MAO-A and MAO-B was
determined by a luminescent method, according to a procedure
developed by Valley et al. [24]. The Ki values of the most potent
compounds were then estimated from the IC50 values using the
ChengePrusoff equation [13]. The results of MAO inhibition studies
are reported in Tables 1 and 2. Compounds 7c and 2 are known
inhibitors of MAO-B [11,23] and were used as references. All the 5H-
R
O
Br
HO
R = Cl, CH3
N
N
N
N
Ag2O, DMF, r.t., 4 h
O
O
8
9a R = Cl
9b R = CH3
Scheme 4. Synthetic pathway to 5H-indeno[1,2-c]pyridazin-5-one analogues 9aeb.

J. Reniers et al. / European Journal of Medicinal Chemistry 46 (2011) 6104e6111
6107
Fig. 2. The molecular structure of compounds 7a, 7b and 9a showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
indeno[1,2-c]pyridazin-5-one analogues except 7b present a higher
inhibitory potency against MAO-B than MAO-A. The MAO inhibi-
tory potency of these compounds proved to be highly dependent on
the alkyl/aryl-substituent in the 3-position. Analogues with a met-
aeCF3ephenyl group and a lipophilic group (methoxy, benzyloxy
or trifluorobutyloxy) in the 3 and 8-positions respectively (7aec)
These results show that the substitution at both positions
by lipophilic and bulky groups (7aec) is not tolerated compared
to analogues 9a, 9b and 2. With the aim of understanding the role
of an aryl group in the 3-position, molecular docking studies
of compounds 9a were performed and discussed in the next
section.
show moderate or no inhibition of MAO-B at 10
mM. Thus, the
The IDO and TDO inhibitory potencies of 5H-indeno[1,2-c]pyr-
inhibitory activity of 7c is not in agreement with the data reported
by Frederick et al. [11]. However, it was evidenced in the literature
that MAO affinity depends on species studied. In particular,
Novaroli et al. have evidenced important species-dependent
differences in MAO-B inhibitor specificity between human and rat
for 5H-indeno[1,2-c]pyridazin-5-one derivatives [9]. Indeed, these
derivatives show a greater inhibitory potency toward hMAO-B than
toward rMAO-B (e.g., 1, IC50 rMAO-B ¼ 280 nM and IC50 hMAO-
B ¼ 8.5 nM) [9]. So, a difference of affinity between baboon and
human enzymes can be conceived. In contrast, the substitution of
metaeCF3ephenyl group in the 3-position with a methyl group (9a,
9b and 2) leads to better inhibition (in the submicromolar range) of
hMAO-B (Tables 1 and 2, Fig. 3). Compound 9a, with a chlorine
meta-substituted benzyloxy group, is the most active and selective
idazin-5-one analogues were assayed in vitro at 25 mM. The activity
of IDO and TDO was determined by a colourimetric and fluoro-
metric method respectively [25,26]. The results reveal that the
synthesized 5H-indeno[1,2-c]pyridazin-5-one analogues display no
inhibition or a moderate inhibition against IDO and TDO. The most
effective one is 3-methyl-8-meta-methylbenzyloxy-5H-indeno[1,2-
c]pyridazin-5-one (9b) which displays a 66% inhibition of TDO. So,
5H-indeno[1,2-c]pyridazin-5-one derivatives display
a higher
activity and selectivity against hMAO-B.
2.3. Molecular docking studies of compound 9a against MAO-B
While the 5H-indeno[1,2-c]pyridazin-5-one derivatives differ-
ently substituted in the 3- and 8-positions investigated here were
weak MAO-A, IDO and TDO inhibitors, some were found to be
potent inhibitors of MAO-B. Among these, compound 9a, was the
most potent and selective MAO-B inhibitor. To gain insight into the
potential binding mode of 9a in the active site of MAO-B, molecular
docking studies were performed with the crystallographic struc-
ture of hMAO-B in complex with safinamide (2V5Z.pdb) as receptor
model [27]. With the exception of three highly conserved water
molecules all buried near the flavin adenine dinucleotide cofactor
(FAD) in the active site, the other crystallized water molecules were
deleted from the protein model [9]. In MAO-B, two distinct cavities
within this series, with a Ki against MAO-B of 0.16 mM (Fig. 3).
To confirm the inhibition mode of MAO-B by 9a, the Line-
weavereBurk plots were obtained from incubations at four
different substrate concentrations with and without three different
inhibitor concentrations. LineweavereBurk representation for 9a
on MAO-B demonstrates that its mechanism of inhibition is
competitive (Fig. 4). The Ki value for the inhibition of MAO-B by 9a
was determined to be 0.11 mM what is in agreement with the Ki
value estimated from the IC50 value using the ChengePrusoff
equation (Table 2) [13].

Table 1
Structure and inhibitory potency of synthesized 5H-indeno[1,2-c]pyridazin-5-one derivatives against hMAO-A, hMAO-B, hIDO and rmTDO.
Compound
Structure
Inhibition percentagea
hMAO-A at 10
m
M
hMAO-B at 10
mM
hIDO at 25
m
M
rmTDO at 25 mM
F3C
O
CF3
N
N
7a
NIb
57% ꢁ 10
NI
20% ꢁ 3
O
O
N
CF3
N
7b
10% ꢁ 10
7% ꢁ 3
17% ꢁ 1
NI
O
O
N
CF3
N
7c
9a
NI
36% ꢁ 2
93% ꢁ 4
NI
18% ꢁ 3
19% ꢁ 6
O
Cl
O
N
N
15% ꢁ 5
25% ꢁ 3
O
O
N
N
9b
17% ꢁ 1
90% ꢁ 0
14% ꢁ 2
66% ꢁ 4
O
F3C
O
N
N
2
NI
92% ꢁ 0
19% ꢁ 2
31% ꢁ 4
O
a
Inhibition percentages are expressed as mean with ꢁSD in brackets (n ¼ 3).
b
NI: no inhibition at 10 mM and 25 mM on MAO-A/MAO-B and IDO/TDO respectively.
100
80
60
40
20
0
9a
9b
2
Table 2
Ki values deduced from IC50/measured for hMAO-B for compounds 9a, 9b and 2.
Compound
Ki (mM)
a
Deduced from IC50
Measuredb
9a
9b
2
0.16 (0.14e0.19)
0.48 (0.42e0.55)
0.28 (0.26e0.30)
0.11 ꢁ 0.01
-9
-8
-7
-6
-5
-4
a
Ki values are estimated from the experimentally measured IC50 values using the
ChengePrusoff equation and are expressed as mean within brackets 95% confidence
intervals (n ¼ 3).
Log ([compound] (M))
b
Ki value is measured from the LineweavereBurk representation and is
Fig. 3. Doseeresponse curve of compounds 9a (-), 9b (:) and 2 ( ) on hMAO-B.
;
expressed as the mean ꢁ SD (n ¼ 3).
Inhibition percentages are shown as mean ꢁ SD with n ¼ 3.

J. Reniers et al. / European Journal of Medicinal Chemistry 46 (2011) 6104e6111
6109
can be found within the binding site; an “entrance cavity” and
a “substrate cavity” (Fig. 5a). The entrance cavity is connected to the
outside of the protein whereas the substrate cavity is located in the
vicinity of FAD. Both cavities are separated by ILE199 and TYR326
acting as a gate, protecting the catalytic region from the outside.
The ILE199 side chain is the latch separating both cavities and
displayed in Fig. 5a in the “open” position, allowing the 5H-indeno
[1,2-c]pyridazin-5-one derivatives to reach both cavities.
The docking solutions were ranked according to their respective
Goldscore values. Analysis of the binding mode for compound 9a
(Fig. 5a) revealed that 5H-indeno[1,2-c]pyridazin-5-one core is
located in the vicinity of the FAD cofactor with the C-8 side chain
projecting towards the entrance cavity of MAO-B. The substrate
cavity is mainly hydrophobic, the only hydrophilic region being
located betweenTYR398, TYR435 and flavinwhich form an aromatic
cage. The examination of the molecular electrostatic potential (MEP)
Fig. 4. LineweavereBurk plots of the inhibition of recombinant human MAO-B by 9a.
The lines were constructed in the absence (A) and presence of 2 M (-), 5 M (:)
and 15
M (C) of 9a with n ¼ 3.
m
m
m
Fig. 5. a) Simulated binding mode of 9a in the active site of hMAO-B (2V5Z.pdb). Only amino acids directly implicated in the active site are displayed and labelled (in green).
Compound 9a is in cyan. FAD is in yellow. The water molecules in the active site are displayed as red spheres. (b) Attractive molecular electrostatic potential (isovalue for
surfaces ¼ 10 kcal/mol) calculated around compound 9a. (PBE1PBE, 6-311G**).(For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)

6110
J. Reniers et al. / European Journal of Medicinal Chemistry 46 (2011) 6104e6111
distribution for compound 9a shows the presence of two large
attractive zones around the 5H-indeno[1,2-c]pyridazin-5-one ring
(Fig. 5b). The first one is centred on the carbonyl and the second one
is centred on the endocyclic hydrazine. So, it is not surprising to
observe that both regions are located within the hydrophilic region
of the MAO-B substrate cavity. Furthermore, the 5H-indeno[1,2-c]
Norberg for her assistance with the XRD analysis and Ms. Jenny
Pouyez for her assistance with the IDO inhibition assay.
Appendix. Supplementary material available
Chemicals and instrumentation; Synthesis of 5H-indeno[1,2-c]
pyridazin-5-ones; Single crystal X-ray crystallographic data of 5H-
indeno[1,2-c]pyridazin-5-one derivatives (7a, 7b and 9a); Enzy-
matic assays (MAO-A and eB, IDO and TDO); Molecular docking;
Molecular electrostatic potential (MEP) calculation.
CCDC-813566, CCDC-813564 and CCDC-813565 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
pyridazin-5-one ring is stabilized by p-p interactions with TYR398
and TYR435. The binding mode adopted by compound 9a allows the
meta-chlorobenzyloxy side chain tosettle within the entrance cavity
lined with hydrophobic amino acid residues. This hydrophobic
pocket is coated by PHE103, TRP119, LEU164, LEU167, PHE168, and
ILE316. Interestingly, the introduction of a chlorine atom on the
benzyl ring (9a, Ki ¼ 0.16
against MAO-B compared to the derivative bearing a methyl group
M). Previous studies have shown that addition of
mM) increases the inhibitory potency
(9b, Ki ¼ 0.48
m
a chlorine substituent on the phenyl group side chain enhance the
lipophilicity of the inhibitor, and therefore its affinity and its selec-
tivity against MAO-B establishing Van der Waals interactions in the
Appendix. Supplementary material
Supplementary data related to this article can be found online at
doi:10.1016/j.ejmech.2011.09.042.
hydrophobic entrance cavity [28]. Compound 2 (Ki ¼ 0.28
mM) with
the trifluorobutyloxy group displays a similar inhibition compared
to 9a. The methyl group in the 3-position is stabilized within the
hydrophobic cage formed by TYR398, TYR435 and FAD. The
substrate cavity being more sterically constrained than the entrance
cavity, the metaeCF3ephenyl group of compounds 7aec cannot
accommodate into the substrate cavity without modifying the
binding mode of 5H-indeno[1,2-c]pyridazin-5-one ring. So, these
observations may explain the reduced inhibition of compounds
7aec compared to 9aeb and 2.
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In order to design new, selective, and more potent MAO-B
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