A bioisosteric approach to the discovery of indole carbinol androgen receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2006.08.036

Two potential bioisosteres of the nonsteroidal antiandrogen bicalutamide, an imidazolidinone and an indole, were synthesized and tested for their androgen receptor binding. Indole was discovered to be a suitable bioisostere for the acyl anilide moiety in the parent compound. Several analogs in the indole series were found to be 10-fold better than bicalutamide in binding to the recombinant androgen receptor binding domain.

Full text of DOI:10.1016/j.bmcl.2006.08.036

Bioorganic & Medicinal Chemistry Letters 16 (2006) 5646–5649
A bioisosteric approach to the discovery of indole carbinol
androgen receptor ligands
James C. Lanter,* James J. Fiordeliso, George F. Allan, Amy Musto,
Do Won Hahn and Zhihua Sui
Johnson and Johnson Pharmaceutical Research and Development L.L.C., 1000 Route 202 S, Raritan, NJ 08869, USA
Received 13 May 2006; revised 1 August 2006; accepted 2 August 2006
Available online 30 August 2006
Abstract—Two potential bioisosteres of the nonsteroidal antiandrogen bicalutamide, an imidazolidinone and an indole, were syn-
thesized and tested for their androgen receptor binding. Indole was discovered to be a suitable bioisostere for the acyl anilide moiety
in the parent compound. Several analogs in the indole series were found to be 10-fold better than bicalutamide in binding to the
recombinant androgen receptor binding domain.
Ó 2006 Elsevier Ltd. All rights reserved.
The androgen receptor (AR) belongs to the nuclear hor-
mone receptor superfamily and is responsible for a range
of biological functions including sexual differentiation
and function as well as various anabolic effects.1 These
effects are mediated2 by the action of the natural ligand
testosterone (T) and its more potent tissue metabolite
dihydrotestosterone (DHT). The SAR of steroidal3 ana-
logs of these natural ligands has been extensively stud-
ied. Recently, there has been a surge in interest in
both agonist and antagonist structures of a nonsteroi-
dal4 nature. As part of our interest5 in the field, we
looked into bioisosteric6 replacement of portions of
the nonsteroidal antiandrogen bicalutamide (Fig. 1).
Replacement of the b-hydroxy aryl sulfone portion with
a heterocycle incorporating both a hydrogen bond
donating NH group and a pendant aromatic ring (path-
way a) led to a series of imidazolidinones 2. Alternative-
ly, tying the amide carbonyl back onto the anilide
aromatic ring (pathway b) resulted in the indole carbinol
scaffold 3 with a variety of linking groups (Z) for the
aryl ring.
b
NC
F
O
H
F3C
N
H
S
O
O
O
a
1
NC
NC
O
Ar
n
Ar
F3C
N
H
N
F3C
N
H
Z
N
O
H
H
O
2
3
Figure 1. Bicalutamide bioisosteres.
cyclization provided key intermediate imidazolidinone
5. Ester hydrolysis, acid chloride generation, and
coupling to the appropriate aniline afforded the target
structures 2.
To prepare the imidazolidinone, we started with the
known7 serine-derived aldehyde 4 (Scheme 1). Stepwise
reductive amination of the aldehyde with the appropri-
ate amine followed by Boc deprotection and triphosgene
The synthesis of the indole isosteres started with com-
mercially available anilines that are transformed via acid
catalyzed iodination,8 bis-sulfonylation, and hydrolysis
to the corresponding ortho iodo sulfonamides 6 (Scheme
2). These key intermediates were then reacted under
Sonogashira9a coupling-cyclization9b conditions with
propargyl alcohols 7 or 11 to afford the b-siloxy (8) or
b-chloro (12) indolyl carbinols, respectively. To avoid
cleavage of the ester linkage, elaboration of 8 required
Keywords: Androgen receptor; Bioisostere.
*
Corresponding author. Tel.: +1 6104584133; fax: +1 6104588249;
e-mail: jlanter@prdus.jnj.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.08.036

J. C. Lanter et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5646–5649
5647
series. To evaluate the binding affinity of the com-
pounds, we selected a commercially recombinant AR li-
gand binding domain kit.10 The use of this kit, in
contrast to isolation of AR from rat prostate cytosol,
had two key advantages. First, the ease of use and ready
availability of a validated receptor allowed this assay to
be utilized in a high throughput screening program that
was run in parallel with our directed synthesis efforts.
Second, the higher concentration of the AR leads to a
more robust response and a lower signal-to-noise ratio.
One caveat to bear in mind when using this system is
that, in general, non-steroidal AR ligands tended to
show a lower AR binding affinity than that reported
using prostate cytosol-derived receptor. To compensate
for this discrepancy, we used bicalutamide as a compar-
ator in this screening protocol.
Ar
MeO2C CHO
a,b
c,d
MeO2C
N
H
n
HN
Boc
NH2
4
Ar
e
HO2C
N
n
2
HN
O
Scheme 1. Synthesis of imidazolidinone isosteres. Reagents: (a)
Ar(CH2)nNH2, pTsOH (cat)/toluene, Dean–Stark; (b) NaBH4/MeOH
then HCl/dioxane; (c) (Cl3CO)2CO, Et3N/THF; (d) NaOH/MeOH—
H2O; (e) (COCl)2, DMF (cat)/CH2Cl2, then 4-amino-2-(trifluorometh-
yl)benzonitrile, Et3N/THF.
concurrent cleavage of the sulfonamide and the silyl
groups followed by mono-acylation of the resulting diol
9 with a single equivalent of the corresponding acid
chloride to afford the target benzoates 10 as racemic
mixtures. Treatment of 12 with two equivalents of the
appropriate phenyl thiol in the presence of sodium
methoxide effected the displacement and indole depro-
tection in one step to yield the targets 13 also as racemic
mixtures. These b-arylthio carbinols could be oxidized
to sulfoxides or sulfones 14 by treatment with 1–3 equiv-
alents of OxoneÒ in a bisphasic mixture of ethyl acetate
and water assisted by phase transfer catalysis.
Using this assay, we first examined the SAR of the imi-
dazolidinone series to determine how well it mimicked
the b-hydroxy arylsulfone segment of bicalutamide.
The closest analog we prepared (Compound 2, Fig. 1;
Ar = 4-fluorophenyl, n = 0) had an IC50 of only 30 lM
in the binding assay. Inserting a methylene spacer be-
tween the aryl ring and the imidazolinone (Ar = 4-fluor-
ophenyl, n = 1) nitrogen atom led to further erosion in
binding affinity (<50% inhibition at 30 lM concentra-
tion). We explored a range of substituents on both aro-
matic rings but unfortunately none these analogs
improved on the potency of the initial compounds. We
speculate that the rigidity of the imidazolidinone ring
With the chemical synthesis worked out and our
compounds in hand, we examined the SAR of all three
R6
O
R5
TBSO
HO
R2
R1
O
R2
R1
R2
R1
R3 R4
g
f
N
H
OH
10a-p
N
N
H
OH
OH
Ms
8
9
OH
e
d
O
O
OTBS
Ms
7
OTBS
R2
R1
R2
R1
I
R4 R5
a-c
N
H
R3
NH2
6
R2
R1
S(O)n
OH
e
OH
N
H
Cl
d
i
R5
Cl
14a-c
11
R4
Cl
R2
R1
R2
R1
S
R3
h
N
N
H
OH
OH
Ms
12
13a-m
Scheme 2. Synthesis of indole sulfide, sulfone and benzoate isosteres. Reagents and condition: (a) NIS, pTsOH (cat)/MeOH–THF; (b) excess MsCl/
pyridine, heat; (c) NaOH/MeOH–H2O; (d) (PPh3)2PdCl2 (cat), CuI (cat), Et3N/THF; (e) HCCMgBr/THF; (f) NaOH/THF–H2O; (g) ArCOCl, Et3N/
THF; (h) NaOMe, ArSH/THF; (i) oxoneÒ, TBAHS (cat), EtOAc–H2O.

5648
J. C. Lanter et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5646–5649
may prevent these structures from fitting into the bind-
ing pocket normally accommodating the more flexible
aryl methylenesulfonyl moiety of bicalutamide.11 We
did not further investigate this scaffold in vitro, turning
our attention instead to the indole bioisosteres.
active. Removal of the fluorine atom on the sulfide
(13b) or replacement with a chlorine atom (13c) did
not lead to any improvement in activity so we exam-
ined the indole ring. Removal of all indole substituents
(13d) from 13c as well as introduction of a carboxy-
methyl group at the 5-position (13e) led to inactive
compounds, while introduction of a chlorine at the
same position (13f) increased potency. Addition of a
trifluoromethyl group (13g) to the indole 6-position
of 13f further increased potency. Returning to investi-
gate the sulfide ring SAR, we found that a fluorine
replacement (13h) of the chlorine atom of 13g was well
tolerated. We oxidized the sulfur atom to two epimeric
sulfoxides (14b and epi-14b) that were both substantial-
ly less potent than the parent structure. Further oxida-
tion of the linker to the sulfone oxidation state (14c)
led to a 10-fold recovery of the activity. Introduction
of a strongly electron donating group (13i) in place
of the chlorine atom in 13g diminished activity some-
what, while replacement with a hydrogen atom (13j)
was well tolerated. Moving the chlorine atom from
the para position (13g) to the ortho position (13k)
was also well tolerated, while the addition of a second
chlorine to the 3-position (13l) provided a compound
that was more than 10-fold more potent than bicaluta-
mide. Comparison of this structure with the 13m indi-
cated that, of the compounds surveyed, the 5-chloro-6-
trifluoromethyl indole substitution pattern provided the
best activity.
In the benzoate series we first synthesized the exact ana-
log (10a) of bicalutamide and were pleased to note that
its binding affinity was roughly equivalent (Table 1).
Removal of the 4-fluoro substituent (10b) led to an
increase in binding affinity; replacement of the nitrile
in this analog with chlorine (10c) maintained the poten-
cy and replacement with a nitro group (10d) marginally
decreased potency. We further explored the SAR of 10d,
substituting the para position of benzoate ring with
strongly electron withdrawing (10e) and donating (10f
and 10g) substituents but these led to a decrease in activ-
ity. Substitution of that position with alkyl groups (10h
and 10i) also decreased activity, while introduction of
both phenyl (10j) and chloro (10k) moieties maintained
the potency. Moving the chlorine atom from the para to
the meta (10l) or ortho (10m) positions led to an increase
in potency. Replacement of the chlorine atom in 10m by
the isosteric trifluoromethyl group (10n) produced a
compound that was 10-fold more potent than bicaluta-
mide. Reduction of the nitro group (10o) caused a signif-
icant loss in activity much of which was recovered by
capping of the aniline with trifluoroacetyl group (10p).
We then turned our attention to the SAR of the b-aryl-
thio carbinol substituted indoles (Table 2). As with the
benzoate series we first prepared the exact analogs of
bicalutamide. Although the sulfide linked carbinol
(13a) was only 2-fold less potent than bicalutamide,
the sulfone linked structure (14a) was significantly less
In summary, of the two bicalutamide bioisosteres we
investigated, the indole series showed great promise,
while the imidazolidinone series did not warrant further
investigation. Within the indolyl carbinol scaffold, both
b-benzoyloxy and thioaryl groups yielded compounds
Table 1. Androgen receptor binding for benzoates 10
6
R
O
O
5
R
2
R
3
4
R
R
1
N
H
OH
R
Compound
R1
R2
R3
R4
R5
R6
AR bindinga (IC50, nM)
Bical
10a
10b
10c
10d
10e
10f
—
—
CN
—
H
—
H
H
H
H
H
H
H
H
H
H
H
Cl
H
H
H
H
—
F
—
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
1300 ( 106)
1200 ( 450)
480 ( 115)
440 ( 49)
820 ( 110)
1600 ( 289)
6400 ( 572)
2600 ( 572)
3000 ( 491)
3600 ( 1386)
940 ( 127)
720 ( 225)
460b
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CN
Cl
H
H
H
H
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NH2
NHCOCF3
H
H
H
NO2
OMe
NMe2
Me
tBu
Ph
Cl
H
10g
10h
10i
H
H
H
10j
H
10k
10l
H
H
H
10m
10n
10o
10p
Cl
CF3
CF3
CF3
H
460 ( 37)
140b
4100b
150b
H
H
H
a Values are means of three experiments, standard error of measurement (SEM) is given in parentheses (na, not active: <50% inhibition at 30 lM).
b Values are means of two experiments.

J. C. Lanter et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5646–5649
5649
Table 2. Androgen receptor binding for b-thiocarbinols 13 and 14
4
5
R
R
3
R
2
R
S(O)
n
1
N
H
OH
R
Compound
R1
R2
R3
R4
R5
n
AR bindinga (IC50, nM)
Bical
13a
14a
13b
13c
—
—
—
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
H
H
—
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
—
F
1300 ( 106)
2600 ( 288)
> 6900 ( >1559)
3000 ( 329)
1700 ( 866)
na
CF3
CF3
CF3
CF3
H
CN
CN
CN
CN
H
0
2
0
0
0
0
0
0
0
1
1
2
0
0
0
0
0
F
H
Cl
Cl
Cl
Cl
Cl
F
13d
13e
H
CO2Me
Cl
Cl
na
13f
H
660 ( 41)
180 ( 41)
160 ( 21)
5100 ( 1328)
4900b
13g
13h
14b
epi-14b
14c
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
Cl
Cl
F
Cl
Cl
F
F
460b
13i
13j
Cl
Cl
NH2
H
930 ( 139)
220 ( 37)
180b
13k
13l
Cl
Cl
H
Cl
Cl
100 ( 20)
1000 ( 508)
13m
NO2
a Values are means of three experiments, standard error of measurement (SEM) is given in parentheses (na, not active: <50% inhibition at 30 lM).
b Values are means of two experiments.
1, 138; (c) Wermuth, C. G. In The Practice of Medicinal
Chemistry; Wermuth, C. G., Ed., 2nd ed.; Academic Press:
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LaVoie, E. J. Chem. Rev. 1996, 96, 3147.
with 10-fold better binding affinity than the parent struc-
ture. These results provided an impetus for further eval-
uation of this novel series and will be discussed in due
course.
7. Gladiali, S.; Pinna, L. Tetrahedron: Asymmetry 1991, 2,
623.
8. Boger, D. L.; McKie, J. A. J. Org. Chem. 1995, 60, 1271.
9. (a) Campbell, I. B. In Organocopper Reagents; Taylor, R.
J. K., Ed.; IRL Press: Oxford, UK, 1994; pp 217–235; (b)
Sakamoto, Takao; Kondo, Yoshinori; Iwashita, Shigeki;
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Acknowledgment
The authors thank Professor Dale Boger for his sugges-
tions regarding the iodination of electron-deficient
anilines.
10. A typical procedure: five picomoles of the rat androgen
receptor ligand binding domain (Invitrogen, Carlsbad,
CA) was incubated with test compound (dissolved in
DMSO) and 0.5 nM tritium-labeled R1881(Perkin-Elmer,
Boston, MA) in 10 mM Tris–HCl, pH 7.4, 1.5 mM
EDTA, 1 mM dithiothreitol and 10% (v/v) glycerol.
Incubation was performed in a 0.15 mL volume in a 96-
well plate at 4 °C overnight. The next day, 20 lL of 25 mg/
mL human c-globulin (MP Biomedicals, Irvine, CA) and
55 lL of 40% (w/v) polyethylene glycol (PEG) 8000 (JT
Baker, Phillipsburg, NJ) were added to each well. After
one more hour at 4 °C, the binding reaction mixtures were
filtered through a FilterMate 196 (Perkin-Elmer) onto a
GF/C UniFilter-96, using 10% (w/v) PEG for filtration
and washing. The filter was dried, sealed on one side, and
25 lL of MicroScint-20 (Perkin-Elmer) was added to each
well. The other side was sealed and the plate was read on a
TopCount (Perkin-Elmer). Counts per minute from each
well were converted to percentage of inhibition using
compound-free and 1 lM R1881-containing wells as zero
and 100% inhibition controls, respectively.
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