Chromone 3-phenylcarboxamides as potent and selective MAO-B inhibitors

Article abstract of DOI:10.1016/j.bmcl.2010.11.128

Monoamine oxidase (MAO) is an enzyme, present in mammals in two isoforms MAO-A and MAO-B. These isoforms have a crucial role in neurotransmitters metabolism, representing an attractive drug target in the therapy of neurodegenerative diseases (MAO-B) and d

Full text of DOI:10.1016/j.bmcl.2010.11.128

Bioorganic & Medicinal Chemistry Letters 21 (2011) 707–709
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Chromone 3-phenylcarboxamides as potent and selective MAO-B inhibitors
Alexandra Gaspar a, Joana Reis a, André Fonseca a, Nuno Milhazes a,b, Dolores Viña c,d, Eugenio Uriarte c,d
,
Fernanda Borges a,
⇑
a CIQ/Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
b Instituto Superior de Ciências da Saúde, Norte, Rua Central de Gandra, 1317. 4585-116 Gandra PRD, Portugal
c Departamento de Farmacología, Facultad de Farmacia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
d Departamento de Química Orgánica, Facultad de Farmacia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Monoamine oxidase (MAO) is an enzyme, present in mammals in two isoforms MAO-A and MAO-B. These
isoforms have a crucial role in neurotransmitters metabolism, representing an attractive drug target in
the therapy of neurodegenerative diseases (MAO-B) and depression (MAO-A). In this context, our work
has been focused on the discovery of new chemical entities (NCEs) for MAO inhibition, based on the
development of chromone carboxamides. Chromone derivatives with a carboxamide function located
Received 8 November 2010
Revised 26 November 2010
Accepted 30 November 2010
Available online 5 December 2010
in position 2- and 3- of the benzo-c-pyrone core, (compounds 2–6 and 8–12) were synthesized, with
Keywords:
Chromone
IMAO-B
Neurodegenerative diseases
moderate/good yields, by a one-pot condensation reaction using phosphonium salts as coupling reagents.
The synthetic compounds were screened towards human MAO isoforms (hMAO) to evaluate their
potency and selectivity. The chromone-3-carboxamides show high selectivity to hMAO-B, with com-
pounds 9 and 12 displaying IC50 values at nanomolar range.
Ó 2010 Published by Elsevier Ltd.
Parkinson’s disease (PD) is a neurodegenerative disorder charac-
terized by a myriad of symptoms that gradually decreases the
quality of life of the patient. The first line of treatment is a dopamine
replacement therapy with Levodopa.1 Among other therapeutic
strategies monoamine oxidase B (MAO-B) inhibitors have also been
extensively used in PD. In fact, selective MAO-B inhibitors (i.e.,
deprenyl and rasagiline) are currently used, alone or in combination
with Levodopa, in the symptomatic treatment of Parkinson’s dis-
ease. The side effects associated with the use of deprenyl and, to a
lesser extent, rasagiline, likely due to their irreversible mechanism
of inhibition, and the potential application of MAO-B inhibitors as
anti-Alzheimer’s agents are at the moment the driving forces for
the discovery of novel potent and selective MAO-B inhibitors.2,3
Privileged structures, such as benzopyranes, are currently
ascribed as supportive approaches in drug discovery. In fact, differ-
ent families of natural nitrogen and oxygen heterocycles, such as
xanthones and coumarins, have also been used as scaffolds in medic-
inal chemistry programs for searching novel MAO-B inhibitors.4,5
Chromones (benzopyran-4-one) are one of the most abundant
groups of naturally occurring heterocyclic compounds.4 Because
of their structural features they occupy an important place in the
realm of natural products and synthetic organic chemistry. In addi-
tion, remarkable antioxidant, anticancer and enzymatic inhibition
properties have been ascribed to these systems.5
Noteworthy evidences have been already acquired to reinforce
the interest of simple coumarins (benzopyran-2-one) as potent
and selective monoamine oxidase inhibitors (IMAO) and that their
affinity and selectivity can be efficiently modulated by appropriate
substitution patterns in the heterocyclic moiety.6 However, in
which concerns the benzopyran-4-one scaffold, only few works
were found in the literature about their putative potential as
IMAO.7
Accordingly, ourprojecthas beenfocusedonthediscoveryofnew
chemical entities for MAO inhibition with benzopyran-4-one sub-
structure (Scheme 1). Preliminary studies performed with
chromones (1) and (7) allow disclosing a significant chemical fea-
ture: the importance of the location of a carboxylic moiety in the
c
-pyrone nucleus. In fact, when the –COOH substituent is in position
3 of the heterocyclic scaffold (7) binds to hMAO-B, exerting a selec-
tive inhibition with respect to isoform (IC50 hMAO-B
A
0.048 0.0026 nM; SI >2083). The inhibition is of irreversible type
(data not shown). Its isomer (1), with the carboxylic function in po-
sition 2, doesn’t present activity for both MAO isoforms.8 Molecular
modeling studies performed with the chromone carboxylic acids re-
veal a crucial, undisclosed role of the presence of an hydrogen donor
group in position 3 of the pyrone ring that could be able to establish
hydrogen bond interactions with active site residues.8
In this context, and in an attempt to develop novel reversible
and selective MAO-B inhibitors, the synthesis of 2- and 3-carbox-
amide chromone derivatives capable of establishing hydrogen
interactions with the enzyme was performed. In this work, several
2- and 3-phenylcarboxamide chromones with or without different
⇑
Corresponding author.
E-mail address: fborges@fc.up.pt (F. Borges).
0960-894X/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2010.11.128

708
A. Gaspar et al. / Bioorg. Med. Chem. Lett. 21 (2011) 707–709
Table 1
O
O
COOH
O
CONHR
hMAO-A and hMAO-B inhibitory activity results for compounds 2–6, 8–12 and
reference compounds
a)
Compound
hMAO-A IC50
(
lM)
hMAO-B IC50
(
lM)
SI
O
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
2 R = Ph
—
—
—
—
3 R = (40-I-Ph)
4 R = (40-OCH3-Ph)
5 R = (40-SCH3-Ph)
6 R = (40-CH3-Ph)
8 R = Ph
(1)
2
3
4
5
6
R= Ph
R=(4'-I-Ph)
R=(4'-OCH3-Ph)
R=(4'-SCH3-Ph)
R=(4'-CH3-Ph)
—
0.40 0.022
0.069 0.003
0.45 0.029
0.12 0.0080
0.068 0.003
0.017 0.002
7.54 0.36
>250c
>1449c
>222c
>833c
>1471c
4043
0.87
9 R = (40-I-Ph)
10 R = (40OCH3-Ph)
11 R = (40SCH3-Ph)
12 R = (40-CH3-Ph)
Deprenyl
O
O
68.73 4.21b
6.56 0.76
a)
Iproniazide
CONHR
COOH
a
Inactive at 100 lM (highest concentration tested). At higher concentrations the
O
O
compounds precipitate.
b
P <0.01 versus the corresponding IC50 values obtained against hMAO-B, as
(7)
8
9
R= Ph
R (4'-I-Ph)
determined by ANOVA/Dunnett’s.
=
c
Values obtained under the assumption that the corresponding IC50 against
10 R=(4'-OCH3-Ph)
11 R=(4'-SCH3-Ph)
12 R=(4'-CH3-Ph)
hMAO-A is the highest concentration tested (100 lM).
to compounds 8 and 10. From the overall data it was concluded that
the positive hydrophobicity (+ ) of the substituent, besides induc-
tive and mesomeric effects, located on the phenyl exocyclic moiety
markedly influence the potency and selectivity of the chromone
carboxamides.
Noteworthy to mark out that the chromones bearing the same
type of substituents in position 2 of c-pyrone nucleus (chromones
Scheme 1. Structure of the chromones understudy. Reagents and conditions: (a) BOP
or PyBOP, R-C6H4–NH2, CH2Cl2, DMF, DIPEA.
p
type of substituents in para-position of the exocyclic aromatic
nucleus (Scheme 1) were obtained by an expedite synthetic strat-
egy and screened towards human MAO isoforms (hMAO) to evalu-
ate their potency/selectivity ratio.
2–6) present a total loss of MAO activity.
Preliminary studies on the type of hMAO inhibition were per-
formed revealing that chromone 3-phenylcarboxamides behave
as quasi-reversible MAO-B inhibitors (data not shown).
The chromone derivatives 2–6 and 8–12 were efficiently syn-
thesized according to the synthetic protocol outlined in Scheme
1.9 Chromone carboxamide derivatives were synthesized straight-
forward by a one-pot condensation reaction that occurs, in equi-
molar amounts, between the corresponding chromone carboxylic
acid (compound 1 or 7) and aniline or its ring-substituted deriva-
tives. The reaction was clean and the compounds were obtained
with moderate/high yields (50%–80%). After the reaction, the crude
products were purified by flash column chromatography and
crystallization.
The MAO inhibitory activity of compounds 2–6 and 8–12 was
evaluated in vitro by the measurement of the enzymatic activity
of human recombinant MAO isoforms in BTI insect cells infected
with baculovirus.10 Then, the IC50 values and MAO-B selectivity ra-
tios (SI) [IC50 (MAO-A)]/[IC50 (MAO-B)] for inhibitory effects of
both new compounds and reference compounds (R-(ꢀ)-deprenyl
and iproniazide) were calculated.
In the present Letter, the effect of the introduction of a methyl or
iodo substituent in para-position of the exocyclic aromatic ring of
chromone 3-phenylcarboxamides was outlined. In fact, the intro-
duction of this type of groups improves the pharmacologic potential
of chromone 3-phenylcarboxamides confirming that this lead could
be effectively optimized in a candidate for the treatment of neurode-
generative diseases. These findings have encouraged us to continue
the efforts towards the optimization of the lead compound.
In conclusion chromone appears to be an interesting scaffold for
the design of selective IMAO. The easy synthetic accessibility and
especially the versatile binding properties of chromones make
them as ‘privileged’ scaffolds. These discoveries open a new avenue
to obtain highly potent and selective MAO-B inhibitors structurally
based on chromone scaffold.
The results of the inhibitory potencies and selectivities of the
chromones under study towards MAO isoforms, and reference com-
pounds, are depicted in Table 1. From the data one can conclude that
Acknowledgements
The authors acknowledge Portuguese Fundação para a Ciência e
a Tecnologia (FCT) for financial support (project PTDC/QUI/70359/
2006) and AG doctoral grant (SFRH/BD/43531/2008).
‘chromones bearing carboxamide substituents in position 3 of the c-
pyrone nucleus’ act preferably as MAO-B inhibitors (IMAO-B) with
IC50 values in micro to nanomolar range (8–12). The same tendency
was found with the chromone carboxylic acids.8 In addition, one can
conclude that the type of substituents in the para-position of the
exocyclic aromatic ring can modulate the affinity and selectivity of
the chromones-3-carboxamides as IMAO-B. The introduction of a
methoxyl group (10) seems to have no effect in IMAO-B potency
whencomparedwith theactivityfound forcompound(8). Neverthe-
less it is important to point out that the introduction of a thiomethyl
group (11), a bioisostere of the methoxyl function, has improved the
potency of the compound 3–4-fold relatively to the compounds 8
and 10. The most promissory compounds as IMAO-B, with an IC50
<75 nm and a SI of >1440, are the compounds 9 and 12, which are
substituted in para-position by iodo and methyl groups, respec-
tively, representing an improvement of potency of six fold relatively
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A. Gaspar et al. / Bioorg. Med. Chem. Lett. 21 (2011) 707–709
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sodium phosphate buffer (0.05 M, pH 7.4) containing the test drugs in various
concentrations and adequate amounts of recombinant hMAO-A or hMAO-B
required and adjusted to obtain in our experimental conditions the same
reaction velocity. The reaction was started by adding (final concentrations)
General procedure for amide obtention. 2-Carboxychromone (1) or 3-
carboxychromone (7) (2.63 mmol) was dissolved in 6 mL of DMF and 0.37 mL
of diisopropylethylamine (DIPEA). The solution was then cooled to 0 °C and a BOP
(2.63 mmol) or PyBOP (2.63 mmol) solution in CH2Cl2 (6 mL) was added and the
mixture was stirred for 30 min. After, the corresponding amine was added in
equimolar amount and the temperature was let to gradually increase to room
temperature. The reaction was stirred for additional 4–6 h.
200 l
M AmplexÒ Red reagent, 1 U/mL horseradish peroxidase and 1 mM p-
tyramine. The production of H2O2 and, consequently, of resorufin was
quantified at 37 °C in a microplate fluorescence reader (excitation: 545 nm,
emission: 590 nm) over a 15 min period, in which the fluorescence increased
linearly. Control experiments were carried out simultaneously. The control
activity of hMAO-A and hMAO-B (using p-tyramine as a common substrate for
both isoforms) was 165 2 pmol of p-tyramine oxidized to p-
hydroxyphenylacetaldehyde/min (n = 20).
10. Evaluation of human monoamine oxidase (hMAO) isoform activity. The effects of
the tested compounds on hMAO isoform enzymatic activity were evaluated by
All IC50 values shown in the table are expressed as means SEM from five
experiments.
a
fluorimetric method following the experimental protocol previously