Initial structure-activity relationship of a novel class of nonpeptidyl GnRH receptor antagonists: 2-arylindoles

Article abstract of DOI:10.1016/S0960-894X(00)00707-1

A nonpeptidyl GnRH receptor antagonist (1), with a unique 2-arylindole core, was identified through the Merck in-house screening for binding affinity on the rat GnRH receptor. SAR studies directed toward the alkoxy-ethanolamine and 2-aryl groups resulted in a simpler lead structure with improved activity. This compound 50 exhibits a 60-fold improvement in binding activity over our initial lead 1.

Full text of DOI:10.1016/S0960-894X(00)00707-1

Bioorganic & Medicinal Chemistry Letters 11 (2001) 509±513
Initial Structure±Activity Relationship of a Novel Class of
Nonpeptidyl GnRH Receptor Antagonists: 2-Arylindoles
Lin Chu,a,* Jennifer E. Hutchins,a Ann E. Weber,a Jane-Ling Lo,b
Yi-Tien Yang,b Kang Cheng,b Roy G. Smith,b Michael H. Fisher,a
Matthew J. Wyvratta and Mark T. Gouleta
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
bDepartment of Biochemistry and Physiology, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
Received 24 October 2000; accepted 6 December 2000
AbstractÐA nonpeptidyl GnRH receptor antagonist (1), with a unique 2-arylindole core, was identi®ed through the Merck in-
house screening for binding anity on the rat GnRH receptor. SAR studies directed toward the alkoxy-ethanolamine and 2-aryl
groups resulted in a simpler lead structure with improved activity. This compound 50 exhibits a 60-fold improvement in binding
activity over our initial lead 1. # 2001 Elsevier Science Ltd. All rights reserved.
Gonadotropin releasing hormone (GnRH, also known
as luteinizing hormone releasing hormone, LHRH),
identi®ed by Schally in 1971,1 is a decapeptide released
by the hypothalamus in a pulsatile fashion. It acts on
the pituitary gland to stimulate the biosynthesis and
secretion of luteinizing hormone (LH) and follicle-
stimulating hormone (FSH), which are responsible for
regulations of gonadal steroid production and repro-
ductive development. Several endocrine-based diseases,
such as endometriosis, prostate cancer, and uterine
®broids, can be treated with GnRH agonists,2 which
achieve the necessary suppression of the gonadal steroid
ter, we disclose a novel class of GnRH antagonists
along with initial structure±activity studies toward
improving the activity of this design.
In-house screening measuring binding anity on the rat
GnRH receptor10 identi®ed the 2-(3,4-dimethoxy-phe-
nyl)indole compound 1 (IC50=3 mM) as a GnRH
receptor antagonist. A medicinal chemistry e€ort was
initiated to determine the viability of this lead.
production through
a
receptor down-regulation
mechanism. One of the major drawbacks associated
with receptor down-regulation is that the initial over-
stimulation, the `¯are e€ect', tends to exacerbate the
symptoms of the disorders. There have been several
recent reports3,4 showing that peptidyl GnRH antago-
nists directly lowered the production of sex steroids
without the detrimental ¯are e€ect. These GnRH
antagonists, being peptidyl in nature, have not been
reported to exhibit useful oral activities, which could be
more easily obtained with nonpeptidyl, small molecules.
Recently, two di€erent classes of nonpeptidyl GnRH
receptor antagonists have been reported.5À9 In this let-
The lead compound 1 was prepared following the syn-
thetic sequence shown in Scheme 1.11 Commercially
available 4-benzyloxy-phenol (2) was alkylated with
(2S)-(+)-glycidyl 3-nitrobenzene-sulfonate to a€ord the
epoxide 3, which was reacted with 3-(2-aminoethyl)-2-
(3,4-dimethoxyphenyl)indole (4) in re¯uxing methanol
to form the epoxide-opened product 5 in good yields.
The ®nal deprotection was accomplished via palladium
hydroxide catalyzed hydrogenolysis. The corresponding
R enantiomer (6) was synthesized by substituting the
2S-glycidyl 3-nosylate with its R enantiomer.
*Corresponding author. Tel.: +1-732-594-1125; fax: +1-732-594-
5966; e-mail: lin_chu@merck.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00707-1

510
L. Chu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 509±513
Scheme 1. Reagents and conditions: (a) NaH, (2S)-(+)-glycidyl 3-
nitrobenzenesulfonate, DMF; (b) 3-(2-aminoethyl)-2-(3,4-dimethoxy-
phenyl)indole (4), MeOH, re¯ux; (c) Pd(OH)2, H2, MeOH.
Scheme 3. Reagents and conditions: (a) BBr3, CH2Cl2; (b) HOBT,
EDAC, 4, DMF/CH2Cl2; (c) BH3ꢀ THF, THF, re¯ux, 2 h; (d) excess
N,N-dimethylethanolamine, MeOH/THF, re¯ux, 3±5 h.
The synthesis of an analogue in which the oxygen of the
ether linkage in the lead was substituted with a methyl-
ene unit (10) is outlined in Scheme 2. 4-Benzyloxybenzyl
chloride (7) was converted to the corresponding
Grignard reagent, which was then reacted with (R)-gly-
cidyl tosylate to a€ord compound 8. Upon treatment
with potassium carbonate, compound 8 cyclized to the
epoxide 9, which was reacted with excess tryptamine 4
in re¯uxing methanol and deprotected with palladium
hydroxide catalyzed hydrogenolysis to reveal the target
compound 10.
acid (17), 5-(4-hydroxyphenyl)pentanoic acid (18), and
6-(4-hydroxyphenyl)hexanoic acid (19). The latter two
were readily obtainable from 2-carbon homologations
of acids 17 and 12, respectively, as shown in Scheme 4.
To introduce a wide range of aryl substituents to the 2-
position of the indole, a versatile and high-yielding
protocol employing a modi®ed Suzuki coupling reaction
as the key step was developed.12 Protection of the amino
group of tryptamine (24) with N-carbethoxyphthalimide
(Scheme 5) in re¯uxing THF for 1±2 days a€orded pure
phthalimide 25 in high yield after trituration with hex-
ane/CH2Cl2 (3:1). Selective bromination of phthalimide
25 on the 2-position of the indole was accomplished
with pyridine hydrobromide perbromide, and the pro-
duct bromide, compound 26, was subjected to modi®ed
Suzuki coupling conditions to a€ord the coupled com-
pound 27 in good yields. Deprotection of 27 with aqu-
eous hydrazine yielded tryptamine 28, which was
subsequently carried to the targeted analogue 29
through transformations described in Scheme 3.
To obtain a fully deoxygenated tether, 4-(4-methoxy-
phenyl)butyric acid (11, Scheme 3) was demethylated
with boron tribromide to give 4-(4-hydroxy-
phenyl)butyric acid (12), which was subsequently cou-
pled with tryptamine 4 to a€ord amide 13. Amide 13
.
was reduced to the dialkylamine with borane THF. It is
worth noting that the initial product from this borane
reduction was a very stable boron±amine complex,
which could be isolated by standard silica gel chroma-
tography. Due to stability and solubility issues, N,N-
dimethylethanolamine was used to break down the
boron±amine complex, and the amine product 14 could
be isolated in excellent overall yield.
All analogues prepared were tested in the rat GnRH
receptor binding assay for the ability to displace [125I]-
radio-labeled buserelin.11 Those with binding activities
The synthetic sequence outlined in Scheme 3 was also
applied toward the preparation of analogues of 14,
starting with 4-hydroxybenzoic acid (15), 2-(4-hydroxy-
phenyl)acetic acid (16), 3-(4-hydroxyphenyl)propionic
Scheme 4. Reagents and conditions: (a) NaH, BnBr, DMF; (b)
NaOH, MeOH/H2O; (c) HOBT, EDAC, N,O-dimethylhydroxylamine
hydrochloride, Et3N, DMF/CH2Cl2; (d) DIBAL, THF, À78 ꢁC; (e)
methyl (triphenylphosphoranylidene) acetate, CH2Cl2; (f) Pd(OH)2,
H2, MeOH/EtOAc; (g) NaOH, MeOH/H2O.
Scheme 2. Reagents and conditions: (a) Mg, Et2O; (b) (R)-glycidyl
tosylate, LiCl, CuCl2, THF; (c) K2CO3, MeOH, CH2Cl2; (d) 4,
MeOH, re¯ux; (e) Pd(OH)2, H2.

L. Chu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 509±513
511
Table 2.
X
Analogue IC50 (mM)
X
Analogue IC50 (mM)
ÀOH
14
30
32
34
2
30
5
ÀOH
ÀOBn
ÀNO2
ÀNH2
13
31
33
35
>3 0
Ða
>3 0
30
ÀOBn
ÀNO2
ÀNH2
10
a0% Inhibition at 10 mM.
Scheme 5. Reagents and conditions: (a) N-carbethoxyphthalimide,
THF, re¯ux; (b) pyridine hydrobromide perbromide, THF/CHCl3,
0 ꢁC; (c) boronic acid, Na2CO3, LiCl, Pd(PPh3)4, toluene/EtOH (1:1),
re¯ux, 3±5 h; (d) hydrazine in H2O, THF/EtOH, overnight; (e) 12,
HOBT, EDAC, DMF/CH2Cl2; (f) BH3ꢀ THF, THF, re¯ux; (g) N,N-
dimethylethanolamine, MeOH/THF, re¯ux, 3 h.
A series of analogues with various tether lengths was
synthesized in order to determine the optimal spacing
between the phenol and the central amine. As shown in
Table 3, analogues containing tether lengths of two to
®ve carbon atoms (14 and 37±39) had essentially equal
anity for the GnRH receptor.
better than 10 mM were titrated on a four-point curve,
and their IC50 values were calculated.
The investigation of the SARs in the indole region was
most rewarding. Table 4a shows some of the com-
pounds that we prepared with zero or one substituent
on the aryl group at the 2-position of the indole. para-
Substitutions were detrimental to binding activity; even
a small ¯uoro group decreased the binding anity sub-
stantially. From the study of disubstituted aryls (Table
4b), we discovered that the 3,5-dimethylphenyl group
provided a dramatic increase in binding activity (50).
Further investigation on bicyclic aryls (Table 4c) also
showed that several of these analogues bound better
than the 2-phenyl indole 41, but none was as active as
the 3,5-dimethylphenyl containing analogue 50.
To examine the importance of stereochemistry in the
lead compound 1, the corresponding R enantiomer 6
was synthesized and tested. Interestingly, the R isomer 6
was equipotent to the lead 1 (Table 1). We next sub-
stituted the oxygen of the ether linkage with a methyl-
ene unit (10) to evaluate the role of the oxygen on
receptor binding. Again, compound 10 exhibited similar
activity as the lead 1. Since neither the absolute stereo-
chemistry of the molecule nor the ether linkage
appeared signi®cant for GnRH receptor binding, sim-
pli®ed analogue 14 was prepared. We were pleased to
see that even with this simpli®ed tether connecting the
phenyl and the central amine, no loss of GnRH binding
activity was observed.
With the potency-enhancing, 3,5-dimethylphenyl group
on the 2-position of the indole, another tether length
study (Table 5) was conducted following the afore-
mentioned synthetic sequence. The result of this study
indicated that, in this context, the four-carbon tether
length is at least 4-fold more potent than all others tested.
Several analogues containing phenol replacements were
also prepared using the synthetic sequence outlined in
Scheme 3, and selected ones are listed in Table 2, toge-
ther with their amide counterparts. It is apparent that
the basic amine was superior to an amide linker for
maintaining anity for the GnRH receptor.
Table 3.
Table 1.
n
Analogue
IC50 (mM)
1
2
3
4
5
6
36
37
38
14
39
40
20
4
6
2
2
10
Analogue
ÀXÀ
ÀYÀ
IC50 (mM)
1
6
10
14
ÀOÀ
ÀOÀ
3
3
2
2
ÀCH2À
ÀCH2À

512
L. Chu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 509±513
Table 4a.
Table 5.
Analogue
Ar
IC50 (mM) Analogue
Ar
IC50 (mM)
n
Analogue
IC50 (mM)
1
2
3
4
5
6
61
62
63
50
64
65
9.0
14
41
42
43
2.0
1.3
44
45
46
47
0.5
0.2
0.9
0.6
0.20
0.25
0.05
0.20
0.60
11.0
9.0
potency-enhancing group. A subsequent tether length
study indicated that a four-carbon spacer between the
amine and phenol was optimal. A 60-fold improvement
in binding anity was accomplished through our initial
SAR studies. In conclusion, we have demonstrated the
viability of this novel class of 2-arylindoles as promising
nonpeptidyl GnRH receptor antagonists. Further pro-
gress in this structural class is described in the accom-
panying report.
Table 4b.
Analogue
Ar
IC50 (mM) Analogue
Ar
IC50 (mM)
48
49
50
51
1.5
52
53
54
1.7
0.17
0.05
0.7
0.4
0.9
References and Notes
1. Matsuo, H.; Baba, Y.; Nair, R. M. G.; Arimura, A.;
Schally, A. V. Biochem. Biophys. Res. Commun. 1971, 43,
1334.
2. Kardamakis, E.; Tzingounis, V. In Biomed. Health Res.;
IOSPress, Vol. 22 ( Bioactive Peptides in Drug Discovery and
Design: Medical Aspects), 1999; pp 275±285, and references
cited therein.
3. Kutscher, B.; Bernd, M.; Beckers, T.; Polymeropoulos,
E. E.; Engel, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 2149
and references cited therein.
Table 4c.
Analogue
Ar
IC50 (mM) Analogue
Ar
IC50 (mM)
4. Behre, H. M.; Nordho€, V.; Nieschlag, E. Ovul. Induct.
Update `98, Proc. World Conf., 2nd, 1997. Filicori, M.,
Flamigni, C., Eds.; Parthenon Publishing: Carnforth, UK
(English) 1998; pp 107±113.
5. Cho, N.; Harada, M.; Imaeda, T.; Imada, T.; Matsumoto,
H.; Hayase, Y.; Sasaki, S.; Furuya, S.; Suzuki, N.; Okubo, S.;
Ogi, K.; Endo, S.; Onda, H.; Fujino, M. J. Med. Chem. 1998,
41, 4190.
6. DeVita, R. J.; Hollings, D. D.; Goulet, M. T.; Wyvratt,
M. J.; Fisher, M. H.; Lo, J.-L.; Yang, Y.-T.; Cheng, K.;
Smith, R. G. Bioorg. Med. Chem. Lett. 1999, 9, 2615.
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55
56
57
0.13
2.5
58
59
60
0.30
2.5
2.8
1.9
8. Walsh, T. F.; Toupence, R. B.; Young, J. R.; Huang, S. X.;
Ujjainwalla, F.; DeVita, R. J.; Goulet, M. T.; Wyvratt, M. J.;
Fisher, M. H.; Lo, J.-L.; Ren, N.; Yudkovitz, J. B.; Yang, Y.-T.;
Cheng, K.; Smith, R. G. Bioorg. Med. Chem. Lett. 2000, 10,
443.
9. Young, J. R.; Huang, S. X.; Chen, I.; Walsh, T. F.; DeVita,
R. J.; Wyvratt, M. J.; Goulet, M. T.; Ren, N.; Lo, J.-L.; Yang,
Y.-T.; Yudkovitz, J. B.; Cheng, K.; Smith, R. G. Bioorg. Med.
Chem. Lett. 2000, 10, 1723.
The initial SAR study on the novel 2-arylindole GnRH
antagonist lead 1, simpli®ed the ether-ethanolamine lin-
ker to a four-carbon tether without any loss in binding
anity. It was also demonstrated that the basic amine
was important for maintaining anity for the GnRH
receptor. However, a variable distance between this
amine and the phenol pharmacophore was tolerated.
Furthermore, a wide range of aryl groups at the 2-posi-
tion of the indole were explored, which led to the dis-
covery of the 3,5-dimethylphenyl group being the most
10. Crude membranes prepared from rat pituitary glands were
the GnRH receptor source and [125I]buserelin (a peptidyl

L. Chu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 509±513
513
GnRH analogue) was used as the radio-labeled ligand. The
competitive binding was conducted in a Tris±HCl based bu€er
at 4 ꢁC for 90 min. The activities are presented as the IC50 for
the inhibition of [125I]buserelin binding to the receptors. A
similar assay is described in ref 5 using a di€erent radioligand.
11. Weber, A. E.; Mathvink, R. J.; Perkins, L.; Hutchins, J. E.;
Candelore, M. R.; Tota, L.; Strader, C. D.; Wyvratt, M. J.;
Fisher, M. H. Bioorg. Med. Chem. Lett. 1998, 8, 1101.
12. Chu, L.; Fisher, M. H.; Goulet, M. T.; Wyvratt, M. J.
Tetrahedron Lett. 1997, 38, 3871 and references cited therein.