Synthesis and structure-activity relationships of 4-oxo-1-phenyl- 3,4,6,7-tetrahydro-[1,4]diazepino[6,7,1-hi]indoles: Novel PDE4 inhibitors

Article abstract of DOI:10.1016/S0960-894X(99)00573-9

A novel series of benzodiazepine derivatives have been discovered as inhibitors of PDE4 enzymes. We have found that our compounds are selective versus other PDE enzymes, and that the activity can be modulated by specific structural modifications. One compound exhibited a strong eosinophilic infiltration inhibiting action on sensitized Brown-Norway rats (compound 9, 5.1 mg/kg p.o.), moreover this compound is not emetic at 3 mg/kg i.v.

Full text of DOI:10.1016/S0960-894X(99)00573-9

Bioorganic & Medicinal Chemistry Letters 10 (2000) 35±38
Synthesis and Structure±Activity Relationships of
4-Oxo-1-phenyl-3,4,6,7-tetrahydro-[1,4]diazepino[6,7,1-hi]indoles:
Novel PDE4 Inhibitors
Yves Pascal, a,* Charles R. Andrianjara, a Eric Auclair, a Nadine Avenel, a
Bernadette Bertin, a Alain Calvet, d Frederic Feru, a Sophie Lardon, a
Indres Moodley, b Malika Ouagued, a Adrian Payne, c Marie-Pierre Pruniaux a
and Corinne Szilagyi a
aInstitut de Recherche Jouveinal-Parke Davis, 3, rue de la Loge, BP 100, 94265 Fresnes, France
bDepartment of Pharmacy, Faculty of Medicine, University of Witwatersrand, 7 York Road Parktown, Johannesburg, South Africa
cBritish American Tobacco, Millbank, Knowle Green, Staines TW18 1DY, UK
dParke Davis Research Center, 2800 Plymouth Road, Ann Arbor, 48105 Michigan, USA
Received 11 May 1999; accepted 14 October 1999
AbstractÐA novel series of benzodiazepine derivatives have been discovered as inhibitors of PDE4 enzymes. We have found that
our compounds are selective versus other PDE enzymes, and that the activity can be modulated by speci®c structural modi®cations.
One compound exhibited a strong eosinophilic in®ltration inhibiting action on sensitized Brown-Norway rats (compound 9, 5.1 mg/
kg p.o.), moreover this compound is not emetic at 3 mg/kg i.v. # 1999 Elsevier Science Ltd. All rights reserved.
Phosphodiesterases (PDE) are important enzymes, par-
ticipating in cellular regulation of the second messengers
cAMP or cGMP. Following the classi®cation proposed
by Beavo,1 the di€erent PDE isoforms can be grouped
in seven di€erent families.
compound 1. Several of these compounds exhibit potent
phosphodiesterase inhibiting properties with high selec-
tivity towards PDE4 and low anity for the HARBS.
In this paper we report the structure±activity relation-
ships and pharmacological activities of a novel series of
benzodiazepine PDE4 inhibitors.
PDE4 enzymes are cAMP speci®c, the inhibition of
which has attracted increasing interest as a possible
treatment of an allergic disease such as asthma.2,3
Among the PDE4 inhibitors that have been synthesised,
most compounds are derivatives of Rolipram (Fig. 1).4
Others are derivatives of xanthines, or pyridopyr-
imidines.5 Rolipram is a selective inhibitor at the cata-
lytic site, but also binds stereoselectively (R-Rolipram)
to a separate site called high anity Rolipram binding
site (HARBS). The IC50 of Rolipram on HARBS is
0.005 mM. This site is thought to be responsible for the
CNS in vivo activity of Rolipram6 and has been impli-
cated in the side e€ects observed, such as emesis.7,8
Chemistry
Several years ago, we studied cholecystokinin receptor
antagonists and synthesised a series of derivatives of
aminobenzodiazepine. By testing this library against
PDE4 enzymes extracted from human U937 cells, we
found that a compound of this series (1) had good
inhibitory activity.We subsequently carried out a struc-
ture±activity study around this compound in e€orts to
increase activity.
All compounds in this series were synthesised according
to Scheme 1, where a representative example (1) is shown.
Recently we have discovered a new series of PDE4
inhibitors in the benzodiazepine family, represented by
Starting aminobenzodiazepines,9 aminopyrrolobenzo-
diazepines10 or aminoquinolodiazepines11 have been
described previously. Enantiomeric separation of the
*Corresponding author. Fax: +33-146-835884; e-mail: yves.pascal
@wl.com
0960-894X/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(99)00573-9

36
Y. Pascal et al. / Bioorg. Med. Chem. Lett. 10 (2000) 35±38
One of the most potent compounds (8) displayed an
IC50 on PDE4 of 0.71‹0.166 mM and showed selectivity
for PDE4 against PDE3 (IC50 ꢀ 100 mM) and PDE1/5
(IC50 ꢀ 100 mM). Binding to the HARBS has been
determined by displacing radioactive [3H]-Rolipram
from brain membrane suspensions from Wistar rats.
In vivo activity has been assessed by measuring inhibition
of antigen-induced lung eosinophilia in ovalbumin-
sensitized Brown-Norway rats after oral administration.13
In this model, the active compounds 9 and 10 have
ED50s of 5.1 mg/kg p.o. [3.2±6.9] and 10.7 mg/kg p.o.
[3.44±33.26], respectively.
Figure 1. Rolipram.
aminopyrrolobenzodiazepines was carried out by crys-
tallisation of their salts with N-acetylphenylalanine in
n-propanol or ethylacetate. Condensations between
amines and di€erent acids were carried out by classical
methods: acid chloride and amine in an inert solvent
in presence of a base, dicyclohexylcarbodiimide and
hydroxybenzotriazole, penta¯uorophenyl ester of the
acid with the amine in an inert solvent, and O-[(ethoxy-
carbonyl)-cyanomethylene-amino]-N,N,N0,N0-tetramethyl-
uronium tetra¯uoroborate (TOTU) with the acid and
the amine in an inert solvent in presence of diisopropyl-
ethylamine.
Furthermore, these two compounds have been tested
for their emetic potential in dogs in comparison with
Rolipram. In this model, both compounds did not show
any emetic e€ect up to 3 mg/kg i.v. although rolipram is
strongly14 emetic at the dose of 0.03 mg/kg i.v.
Results and Discussion
We have systematically modi®ed several regions of the
parent compound, (3R)-3-(2-indolyl)-carboxylamino-4-
oxo-1-phenyl-3,4,6,7-tetrahydro-[1,4]diazepino[6,7,1-hi]-
indole (1), to elucidate important features for activity.
Pharmacology
The inhibitory activities of these compounds have been
tested (Table 1) on di€erent PDE isoenzymes extracted
from human U937 cells (PDE4),12 dog aorta smooth
muscle cells (PDE3) and guinea-pig trachea smooth
muscle cells (PDE1/5). IC50's have been determined by
assaying the cleavage of [3H]cAMP or [3H]cGMP to its
corresponding radiolabeled nucleoside 50-monophosphate.
Size of the fused ring C (Fig. 2)
We explored the importance of the ring structure fused to
the benzodiazepine nucleus by synthesis of compounds 3
and 4 (Table 1). Both the non-cyclic benzodiazepine 3 and
Scheme 1. Synthesis of benzodiazepine derivatives.

Y. Pascal et al. / Bioorg. Med. Chem. Lett. 10 (2000) 35±38
37
Table 1. Phosphodiesterase inhibitory activity and HARBS binding of benzodiazepine derivatives
No
n
Sructure
I
Con®guration
R
R1
H
R2
H
R3
R4 R5
Or Heterocycle
R6
PDE4
4
PDE3
a
PDE1/5
HARBS
Binding IC50a, mM
IC50 ,mM
1
1
1
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ꢀ100
NDc
2
3
4
5
I
I
I
I
R
R,S
R,S
S
H
H
H
H
F
H
H
H
ꢀ100
ꢀ100
ꢀ100
100
ND
ND
ND
ND
ND
ND
b
>
2
ꢀ100
100
1
6
1
1
1
1
1
1
I
I
I
I
I
I
R,S
R
Cl
MeO
Me
Me
Me
H
F
H
H
H
H
H
ꢀ100
0.65
0.71
1.14
2.6
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ND
100
ND
ND
1.09
ꢀ3
7
8
R
ꢀ100
ꢀ100
ꢀ100
ꢀ100
9
R
10
11
R
ꢀ3
R
4.2
ꢀ3
12
13
1
1
I
I
R
R
MeO
OH
H
H
1.62
0.41
82
65.3
>3
ND
ꢀ3.5
ꢀ100
14
15
16
17
18
19
20
21
1
1
1
1
1
1
1
1
II
II
II
II
II
II
II
II
R
R
R
R
R
R
R
R
Me
MeO
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
Cl
MeO
MeO
H
H
H
Cl
H
H
H
Cl
Cl
Cl
Cl
H
Cl
H
H
Cl
H
H
H
Cl
Cl
1.25
1.8
27
62
>100
40
>100
>100
71.7
ND
>100
>100
ꢀ3
ꢀ3
ND
ꢀ3
ꢀ3
ꢀ3
ꢀ2
ND
1.15
0.76
0.76
0.96
0.63
0.33
43.7
17.5
>100
7.85
>100
17.5
H
MeO
NH2
NH2
Me
H
aIC50 values are average of three independent determinations.
bCompound without C ring.
cNot determined.
the compound 4 (n ˆ 2) having a 6 membered ring
are not active (Table 1). Only compounds originally
selected, with a ®ve membered ring have the desired
activity. This is of great importance for the conformation
of the molecule because the ®ve membered ring impo-
ses a considerable rigidity to the structure, and only
two conformations are possible for the multi-ring
nucleus.

38
Y. Pascal et al. / Bioorg. Med. Chem. Lett. 10 (2000) 35±38
Figure 2. Structures of benzodiazepine derivatives in Table 1.
Stereochemistry
Brown-Norway rat. In addition, compound 9 shows no
emetic activity at therapeutic doses unlike Rolipram and
numerous inhibitors related to Rolipram. Since many of
the ®rst generation inhibitors have limited potential due
to side e€ects, this series of compounds o€ers a new
class with signi®cant potential for the treatment of
in¯ammatory diseases. More detailed SAR and phar-
macological activity will be communicated at a later
date.
The active isomer is of R con®guration, the S isomer
being completely devoid of inhibitory activity against
PDE4 (5 versus 1).
Derivatives of the phenyl ring D in position 1 of the main
ring B (Fig. 2)
The ortho ¯uoro analogue 2 showed that a slight mod-
i®cation of the D phenyl ring induces a total withdrawal
of the PDE4 inhibiting activity.
Acknowledgements
We thanks Drs. Annette Doherty and Roger Wriggles-
worth for their invaluable information and discussion
and proofreading of the manuscript.
Ring
Substitutions of ring A showed an in¯uence on activity.
Thus, the introduction of a methoxy or a methyl group
at the 9-position increases activity (compounds 7 versus
1, 12 versus 11, 10 versus 11, and 21 versus 20). The
PDE4 potency is further enhanced by the introduction
of a hydroxyl group at the same position (compound
13).
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Side chain modi®cations
The nature and substitution of the aromatic ring on
the side chain modulate the inhibitory activity of the
compounds. Among the derivatives of benzoic acid,
the 4-chloro substituted analogue 14 is active, and the
dichloro compounds are equivalent 16 or better 17
than 14. The monomethoxy substituent in the ortho
position (18) shows among the best activities, but the
dimethoxy substitution gives no improvement (19). The
best derivative is the tri-substituted 4-amino-3,5-
dichlorobenzoic analogue 21.
Compounds substituted with heterocyclic moieties
(Table 1) show di€erent activities according to the nat-
ure and substitution of the heterocycle. Among hetero-
cycles, a pyrazolotriazine is the best compound in vitro
(8). The 4-pyridine derivative 9 was found very promis-
ing in further pharmacological studies and was selected
for clinical investigations. In summary, a novel series of
benzodiazepine derivatives has been discovered as inhi-
bitors of the enzyme PDE4. We have shown that our
compounds are selective versus other PDE isoenzymes.
Compound 9 was selected for further biological evalua-
tion. This compound shows signi®cant in vivo activity
in a model of antigen induced eosinophilia in the
12. Smith, B. J.; Wales, M. R.; Jappy, J. W. G.; Perry, M. J.
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