Identification of potent and selective oxytocin antagonists. Part 1: Indole and benzofuran derivatives

Article abstract of DOI:10.1016/S0960-894X(02)00159-2

Studies to discover novel, potent and selective oxytocin antagonists are reported. Combinatorial libraries designed to find novel replacements of fragments of oxytocin antagonist L-371,257, identified pyrimidine, thiazole, indole and benzofuran as potential alternatives to the benzoic acid moiety of L-371,257. Additional investigations identified indole and benzofuran derivatives with potent oxytocin antagonist activity.

Full text of DOI:10.1016/S0960-894X(02)00159-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1399–1404
Identification of Potent and Selective Oxytocin Antagonists.
Part 1: Indole and Benzofuran Derivatives
Paul G. Wyatt,a,y Michael J. Allen,b Josie Chilcott,c Alison Foster,a
a
David G. Livermore,a Jackie E. Mordaunt,a,*Jan Scicinski and Patrick M. Woollardc
aDepartment of Medicinal Chemistry, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage,
Herts SG1 2NY, UK
bDepartment of Receptor Pharmacology, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage,
Herts SG1 2NY, UK
cDepartment of Research Biometabolism, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage,
Herts SG1 2NY, UK
Received 22 November 2001; revised 11 February 2002; accepted 1 March 2002
Abstract—Studies to discover novel, potent and selective oxytocin antagonists are reported. Combinatorial libraries designed to
find novel replacements of fragments of oxytocin antagonist L-371,257, identified pyrimidine, thiazole, indole and benzofuran as
potential alternatives to the benzoic acid moiety of L-371,257. Additional investigations identified indole and benzofuran deriva-
tives with potent oxytocin antagonist activity. # 2002 Elsevier Science Ltd. All rights reserved.
Introduction
prematurely labouring women when compared with
non-labouring women of a similar stage of pregnancy.4
OT antagonists have been investigated in the search for
effective therapies with alternative modes of action.
Although the peptide OT/vasopressin 1a antagonist
atosiban1 (Tractocile) has recently been approved for
use in Europe, it is not suitable for long-term main-
tenance treatment, as it is not orally bioavailable.5 In
the search for orally active agents suitable for main-
tenance treatment, potent and selective non-peptide OT
antagonists have been identified, but none has been
extensively investigated in the clinic.6
Although preterm birth accounts for 66% of all infant
mortality and morbidity, no safe and effective therapy
for maintenance of gestation is available.1 Limited effi-
cacy and/or maternal or fetal safety restrict the use of
current therapies.2 Oxytocin (OT), a neurophyseal hor-
mone, is a potent contractor of the uterus and is used
(as Syntocinin1) for the induction or augmentation of
labour.3 Also the density of myometrial oxytocin recep-
tors increases by 100-fold in women at term when com-
pared with non pregnant women and is elevated in
*Corresponding author. Fax: +44-1438-763624; e-mail: jem8704@gsk.com
yCurrent address: Astex-Technology, 250 Cambridge Science Park, Milton Road, Cambridge CB4 0WE, UK.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00159-2

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Merck Research Laboratories have developed potent
and selective OT antagonists, for example L-371,257 2
from Otsuka’s OPC 21268 1,7 but no compounds are
reported to be in clinical development.8 We have
recently reported a potent benzodiazepine OT antago-
nist.9 As an alternative strategy, 2 was used as a starting
point for a chemistry programme. The amine 3 R=H
and acid 4, precursors to 2, were used as cores in com-
binatorial libraries to identify replacements for the other
halves of the molecule. This paper reports on initial
investigations to discover novel potent and selective OT
antagonists.
were tested without purification, but no significant OT
binding antagonism was observed (data not shown).
Replacements of Acid 4
To identify alternatives to the benzoic acid moiety 4,
structurally diverse acids were coupled to amine 3
R=H. Of particular interest were heteroaromatic rings
such as pyrimidine, thiazole, indole and benzofuran,
examples of which were included in the acid sets. In the
first iteration 500 compounds were made and tested as
individual unpurified samples. Examples of the above
ring systems gave moderate levels of OT binding
inhibition. Additional libraries were synthesised to fur-
ther investigate the thiazole and pyrimidine leads.
Thiazole libraries of general structure 6 were synthesised
using a-bromo-ketone 5 and a diverse set of thioamides
(Scheme 1).10 Although a number of compounds had
moderate activity e.g., 3a (Table 1), none was suffi-
ciently active to progress. The pyrimidine derivatives
Replacements of Amine 3
Replacements of the amine moiety 3 R=H of 2 were
sought by coupling acid 4 with sets of amines, some of
which were selected to contain the pharmacophoric
characteristics of 3, that is, an aromatic ring and a
hydrogen-bond acceptor. The resulting discrete libraries
Scheme 1. (a) CH3C(O)CH2CO2CH3, DMAP, iPrOH, Na2CO3, reflux, 55%; (b) 4-(dimethylamino)pyridinium bromide perbromide, AcOH, 74%;
(c) RC(S)NH2, DMF, 70 ꢀC; (d) (CH3O)2CHNH(CH3)2, PhCH3, reflux, 81%; (e) RC(¼NH)NH2, NaOEt, EtOH, reflux.
Table 1. Inhibition of the binding of OT with human OT receptor (hOT)a
R
pKi hOT
R
pKi hOT
R
pKi hOT
2
8.0
3d
6.7
3h
6.4
3a
3b
3c
6.9
6.8
3e
3f
6.1
6.1
6.6
3i
3j
6.9
6.9
6.1
R1=H 6.7
3g
3k
aDisplacement of 3[H] oxytocin from hOT by the test compound.9

P. G. Wyatt et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1399–1404
1401
were investigated using solution chemistry to synthesise
6,5-Fused Ring Systems
compounds of structure 8 and solid-phase chemistry to
obtain 2-amino-pyrimidines 9. A library of pyrimidines
8 was obtained from the clean reaction of enamine 7
with a diverse set of amidines,11 but none of the com-
pounds had significant activity, for example 3b (Table 1).
A library of 2-amino-pyrimidines 9 was synthesised on
solid phase (Scheme 1),12 but the most potent
compounds were at best only moderately active.
As the initial library identified interesting activity for
some indole and benzofuran derivatives a number of 6,5
fused aromatic systems were investigated (Table 1). The
indole isomers 3c (R1=H) and 3d were equally potent,
but increasing the number of heteroatoms e.g., 3e,f gave
a significant loss of activity. The benzofuran isomers 3g,h
had similar activity to the indoles 3c (R1=H) and 3d.
Table 2. Inhibition of the binding of OT with human OT receptor (hOT)a
3c R1=
pKi hOT
3 R=
pKi hOT
3ca
3cb
7.4
3l
8.0
7.2
6.5
6.4
6.8
7.7
6.9
3cc
3cd
3ce
3cf
3cg
3m
6.4
6.4
6.7
3n
3o
3ch
6.8
aDisplacement of 3[H] oxytocin from hOT by the test compound.9
Scheme 2. (a) 3. HCl salt, 1-ethyl-3-(30-dimethylaminopropyl)carbodiimide HCl, N-hydroxybenzotriazole, iPr2EtN, DMF; (b) CH3SO2Cl, Et3N,
CH2Cl2; (c) NaH, DMF; (d) Pd(OAc)2, Bu4NCl, NaO2CH, Na2CO3, DMF; (e) TFA, CH2Cl2.

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P. G. Wyatt et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1399–1404
Indoles
acetamide 3cb had activity comparable to 3ca, so a ser-
ies of amide and sulphonamide libraries were made.
Although a number of compounds had activity equal to
3ca and 3cb none was significantly more potent.
Removal of the N-methyl group of the sulphonamide
3cc or amide 3cd or lengthening the chain 3ce caused a
4- to 8-fold loss of activity. The activity difference
between 3ca,cb and 3cc,cd suggested that the side chains
bound in a cis-orientation, so a number of cyclic com-
pounds were made to mimic the conformation of 3ca
and 3cb. Of the rings synthesised, the R-pyrrolidine sul-
phonamide derivative 3cf gave a 2-fold increase in
activity, whereas the corresponding S-isomer 3cg had a
4-fold loss of activity. Incorporation of a 7-methyl group
3l to further conformationally constrain the sulphonamide
moiety resulted in a further 2-fold increase in activity.
To investigate indole 3c for potential positions for sub-
stitution, the methyl-substituted indoles 3i–k were syn-
thesised. The results indicated that substitution is
tolerated in the 1- and 2- positions, 3i and 3j, respec-
tively, but not the 3-position 3k of the indole ring. The
latter finding was confirmed by the results from a lim-
ited number of additional 3-substituted indoles (data
not shown). The significant activity of 3i led to the
synthesis of an array of 1-substituted indole derivatives
by the sodium hydride mediated alkylation of 3c
(R1=H) (Table 2).
Most of the resulting compounds did not have
improved activity over 3i although, the sulphonamide
3ca was significantly more potent, possibly indicating
that the sulphonamide moiety was forming a hydrogen
bond with the receptor. Further compounds related to
3ca were synthesised to exploit this finding. The
Although compounds in this series were beginning to
show potent in vitro activity, all the compounds had high
clearances in rats after iv administration, equivalent to
Table 3. Inhibition of the binding of OT with human OT receptor (hOT)a and dog IV clearance (mL/min/kg) and bioavailability (%F)
R1
pKi hOT
Cl
F%
R2
pKi hOT
Cl
F%
13a
13b
7.4
9
27
13j
7.6
34
6.7
7.2
7.1
13k
8.2
8.2
8.5
34
17
20
13c
13d
6
9
49
69
13l
8
13m
13e
13f
13g
7.1
7.2
7.2
11
7
44
13n
13o
13p
7.8
8.4
7.6
20
27
100
8
13h
13i
7.6
7.8
49
18
13q
13r
7.6
7.6
31
15
16
12
aDisplacement of 3[H] oxytocin from hOT by the test compound.9

P. G. Wyatt et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1399–1404
1403
Scheme 3. (a) 3. HCl salt, 1-ethyl-3-(30-dimethylaminopropyl)carbodiimide HCl, N-hydroxybenzotriazole, iPr2EtN, DMF, 71%; (b) RCꢁCH,
PdCl2(PPh3)2, Et3N, CuI, DMF or for alkynes containing basic groups, Pd(OAc)2, nBuNH2, PPh3, CuI, THF; (c) tBuOCONHCH2CꢁCH,
PdCl2(PPh3)2, Et3N, CuI, DMF; (d) TFA, H2O, CH2Cl2, 100%; (e) R1COCl or R1SO2Cl, Et3N, CH2Cl2; or R1CO2H, 1-ethyl-3-(30-dimethylami-
nopropyl)carbodiimide HCl, N-hydroxybenzotriazole, iPr2EtN, DMF.
approximately two-thirds liver blood flow. Investigation
of the metabolism of this series indicated that the major
metabolites were associated with oxidation of the indole
2,3-double bond. As data from other derivatives (not
shown) indicated that it could be blocked by substitu-
tion of the 2- and 3-position, the 2-methyl and 2,3-
dimethyl derivatives of 3ce were synthesised. Inclusion
of the 2-methyl group (data not shown) did not affect
the activity or clearance on the compounds, whereas the
further substitution to give 3m, as predicted from pre-
vious results, gave a significant loss of activity. This
data confirmed that within this series, potent activity
and good pharmacokinetics could not be achieved
simultaneously. To circumvent the metabolic instability
alternatives to the indole ring were investigated. How-
ever, the systems investigated, for example 3n and 3o
(the latter synthesised from 1013 and 1114 Scheme 2),
resulted in less potent compounds compared with the
corresponding indole derivatives.
from 14 (Scheme 3) suggested improved activity could
be achieved in this series. On testing purified samples
from the library, a moderate increase in activity was
seen for the pyridyl derivative 13h. Compounds con-
taining other nitrogen heterocycles, for example imida-
zoles and tetrazoles were synthesised to expand on this
finding, but no significant increase in activity was
observed (data not shown). It was then postulated that
the pyridyl moiety was acting as a weak hydrogen bond
acceptor, so a range of compounds containing a hydrogen
bond acceptor, for example 13i–k was synthesised.
The increased activity of compounds such as 13i–k sug-
gested that the theory was correct. Elaboration of 13j to
the hydantoin 13l gave potent activity, although the
pharmacokinetic parameters were disappointing when
compared to 13c–f. Further substitution of the hydan-
toin, for example 13m,n and ring expansion 13o was
explored in an attempt to decrease plasma clearance.
These modifications gave compounds with potent
activity but not improved clearances. Alternative satu-
rated heterocycles such as the carbamate 13p either gave
less potent compounds or no improvement in plasma
clearances. Attempts to improve activity or reduce
plasma clearance by modifying the linking amide chain
to the hydrogen bond acceptor, for example 13q and
13r, again failed to identify compounds with improved
activity or pharmacokinetic parameters.
Benzofurans
The benzofuran core 3g (pKi 6.6) was elaborated by
substitution of the 2- or 3-positions. Derivatives with 3-
substituents gave no compounds of interest (data not
shown). However, the 2-acetamidoethyl derivative 13a
(Table 3; synthesised from acid 12,15 Scheme 3) proved
to have promising activity and pharmacokinetic para-
meters, so a number of amide, sulphonamide or urea
derivatives of 13 (R=(CH2)2NH2) were synthesised.
Investigation of heteroaromatic templates derived from
13k gave compounds with potent activity and promising
pharmacokinetic parameters; the results will be described
in a subsequent paper.
Some amides and ureas showed activity comparable
with 13a, but none was more potent, whereas the sul-
phonamides were less potent (data not shown). Further
compounds with the amide functionality constrained in
a ring 13b–f failed to give an improvement in activity
over 13a. This data suggests that, even though 13a is
significantly more active than the unsubstituted benzo-
furan core 3g, the additional amide of 13a does not
form a hydrogen bond with the receptor. Despite com-
pounds such as 13c–f having moderate levels of activity,
their good pharmacokinetic parameters in dogs gave an
impetus for an extensive investigation of 2-substituted
benzofurans. Whereas the shortened amide 13g gave
similar levels of activity to 13a, amide libraries derived
Conclusion
Combinatorial chemistry has been used extensively to
identify novel oxytocin antagonists. Libraries to find
replacements of amine 3 failed to give compounds of
interest. However, libraries to discover replacements of
the benzoic acid moiety 4 identified pyrimidine, thi-
azole, indole and benzofuran as possible alternatives.
Further focused libraries failed to identify compounds
with potent activity in the cases of pyrimidine and thi-
azole. Elaboration of the indole template identified

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P. G. Wyatt et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1399–1404
potent compounds, but further studies demonstrated
that the poor pharmacokinetic properties of the series
could not be improved on whilst retaining potent activ-
ity. The benzofuran template identified compounds with
potent activity and a range of pharmacokinetic proper-
ties; these compounds have been used as leads for
further chemical programmes.
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