Synthesis and structure - Activity studies of side-chain derivatized arylhydantoins for investigation as androgen receptor radioligands

Article abstract of DOI:10.1016/S0960-894X(01)00146-9

A series of arylhydantoin derivatives modeled after the antiandrogen RU 58841 was generated to identify potential candidates for development as androgen receptor (AR) radioligands. Side-chain modified derivatives of RU 58841, suitable for labeling with ei

Full text of DOI:10.1016/S0960-894X(01)00146-9

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1045–1047
Synthesis and Structure–Activity Studies of Side-Chain
Derivatized Arylhydantoins for Investigation as
Androgen Receptor Radioligands
Marcian E. Van Dort* and Yong-Woon Jung
Division of Nuclear Medicine, Department of Radiology, 3480 Kresge III Building,
The University of Michigan Medical School, Ann Arbor, MI 48109-0552, USA
Received 22 December 2000; accepted 20 February 2001
Abstract—A series of arylhydantoin derivatives modeled after the antiandrogen RU 58841was generated to identify potential
candidates for development as androgen receptor (AR) radioligands. Side-chain modified derivatives of RU 58841, suitable for
labeling with either carbon-11 or radiohalogens (fluorine-18, iodine-123), were synthesized and tested for their AR binding affini-
ties. The N-(iodopropenyl) derivative 13 (Ki=13 nM) is a potential candidate for development as a radioiodinated AR ligand.
# 2001Elsevier Science Ltd. All rights reserved.
The hormone dependency of prostate cancer is well
established and androgen receptor (AR) expression is
frequently observed in primary prostate tumors and
metastases.1 As a consequence, a variety of radiolabeled
AR ligands are under investigation for the noninvasive
imaging of tumor sites in prostate cancer using positron
emission tomography (PET) or single photon emission
computed tomography (SPECT).2ꢀ6 The majority of
these studies reported to date have focused on steroid-
based radioligands.
AR in radioligand binding studies. Encouraged by this
finding, we initiated a study of RU 58841derivatives to
identify high-affinity analogues for subsequent radio-
tracer development. Our radiosynthetic strategy envi-
saged introduction of the radiolabel at the hydantoin
N(3)-position via attachment to an appropriate spacer
group. Accordingly, a series of side-chain modified
derivatives of RU 58841(Table 1) suitable for labeling
with either PET (carbon-11, fluorine-18) or SPECT
(iodine-123) radioisotopes was synthesized for evalua-
tion of their in vitro AR binding affinities.
The recent emergence of high-affinity, nonsteroidal anti-
androgens offers a useful alternative approach towards
AR radioligand development.7ꢀ11 As an example, the
arylhydantoin derivative RU 58841(Chart 1) is repor-
ted to display high affinity (Ka=1.2 nM) and selectivity
for AR (>1000-fold selectivity for AR over progestin,
glucocorticoid, mineralocorticoid and estrogen recep-
tors).10 The excellent AR selectivity, lower lipophilicity
and ease of structural modification of RU 58841and its
derivatives as compared to steroid-based ligands make
these compounds attractive candidates for investigation
as AR radioligands.
RU 58841and its key intermediate 4-(4,4-dimethyl-2,5-
dioxo-1-imidazolidinyl)-2-trifluoromethyl-benzonitrile (1)
were synthesized as previously reported.10 Synthesis of
compounds 2–8 was conducted as shown in Scheme 1
via generation of the anion of 1 by treatment with NaH
in DMF and subsequent alkylation with the appropriate
commercially available alkyl bromide or iodide.12
In our investigations, RU 58841displayed nanomolar
equilibrium binding affinity (Ki=26 nM) towards the rat
*Corresponding author. Tel.:+1-734-763-4647; fax: +1-734-936-
1571; e-mail: mvandort@umich.edu
Chart 1.
0960-894X/01/$ - see front matter # 2001Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00146-9

1046
M. E. Van Dort, Y.-W. Jung / Bioorg. Med. Chem. Lett. 11 (2001) 1045–1047
Compounds 9–12 were synthesized in a similar fashion
from the appropriate iodophenylalkyl tosylate pre-
cursors.13 The E-isomer of the iodopropenyl derivative
(13) was synthesized by alkylation of 1 with the E-iso-
mer of 1-(tri-n-butylstannyl)-3-chloro-1-propene14 and
treatment of the resulting intermediate (13a) with iodine
in chloroform (Scheme 2). The pure products were iso-
lated in 50–95% yields after chromatography and
recrystallized from EtOAc/hexane mixtures. The
The N-methyl and N-(2-methoxyethyl) substituted
hydantoin derivatives (2 and 3) were synthesized as
potential leads for the development of carbon-11
labeled AR radioligands. The AR binding affinity of the
N-methyl analogue 2 (Ki=37 nM) was comparable to
that of the lead compound RU 58841( Ki=26 nM) sug-
gesting that the small-volume methyl group was well
tolerated at the N(3) position. However, the 2-
methoxyethyl substituent was not as well tolerated dis-
playing an almost 5-fold reduction in AR affinity. A
series of RU 58841derivatives bearing fluoroalkyl side
chains (4–6) was also prepared to identify candidates for
labeling with fluorine-18. Since fluorine can mimic a
hydroxyl group in some biologically-active com-
pounds,18 we were particularly interested in the effect of
this substitution in RU 58841. All of the fluoroalkyl
derivatives displayed weaker AR binding affinity than
RU 58841. Notably, replacement of the hydroxyl group
of RU 58841with fluorine ( 6) leads to an 8-fold reduc-
tion in its AR binding affinity. Since fluorine can only
function as a hydrogen bond acceptor,19 the enhanced
binding affinity of RU 58841over 6 may be due to a
hydrogen bond donor interaction of its hydroxyl group
with the receptor site. However, it is also possible that
the lower binding affinity of 6 may be due to an unfav-
orable hydrophobic interaction of fluorine with the AR
binding site.
1
compounds gave H NMR and elemental analysis or
mass spectrometry data consistent with the assigned
structures.
The binding affinities of new ligands and reference
compounds to the rat prostate cytosolic AR were
determined using a competitive binding assay in the
presence of the high-affinity AR radioligand,
[3H]mibolerone.15ꢀ17 These data (expressed as inhibition
constants, Ki) are presented in Table 1. All ligands
demonstrated monophasic radioligand displacement
curves (Hill coefficient close to unity) indicating inter-
action with a single class of binding sites. The lead
derivative RU 58841displayed a Ki value of 26 nM in
these assays.
Table 1. Inhibition constants (Ki) for ligands at the rat androgen
receptor
In our design of iodinated compounds, we focused
solely on derivatives having an sp2 carbon-bound iodine
functionality as these are shown to have a higher meta-
bolic stability than iodoalkyl derivatives.20,21 Accord-
ingly, a series of N-(iodophenylalkyl) derivatives (7–12)
and the N-(iodopropenyl) derivative 13 (E-isomer) were
synthesized for AR binding studies. A broad spread of
AR affinities was observed within the N-(iodophenyl-
alkyl) series. Thus, introduction of either a meta-iodo-
benzyl or para-iodobenzyl group at N(3) (7 and 8) led to
a 5- to 6-fold reduction in AR affinity, whereas increas-
ing the chain length by a single methylene unit (9 and
10) resulted in a dramatic (16- to 32-fold) loss in binding
affinity. Interestingly, in the iodophenylpropyl series,
the para-iodo substituted analogue 12 (Ki=65 nM)
showed a 6-fold improvement in AR affinity over the
corresponding meta-iodo substituted analogue (11).
Among the new compounds studied, the N-(iodoprope-
nyl) analogue 13 displayed the highest AR binding affi-
nity (Ki=13 nM), which is a 2-fold improvement over
the lead derivative RU 58841. High AR binding affinity
(relative binding affinity=46% of testosterone), has also
been reported for the N-(cyanomethyl) derivative RU
58642 (R=CH2CN).22 The enhanced binding affinity
displayed by RU 58642 and 13 may be due to a
Compound
R
Ki (nM)a mp (ꢁC)b Yield (%)c
2
3
4
5
6
7
8
9
10
CH3
37ꢂ5
153–154
Oil
95
89
59
90
86
91
86
50
89
56
78
91
(CH2)2OCH3
(CH2)2F
(CH2)3F
120ꢂ20
1
ꢂ31 41 105–106
321ꢂ40
21 ꢂ27
123ꢂ25
150ꢂ10
91–93
71–73
113–114
101–103
(CH2)4F
3
CH2(m-I-Ph)
CH2( p-I-Ph)
(CH2)2(m-I-Ph)
(CH2)2( p-I-Ph)
(CH2)3(m-I-Ph)
(CH2)3( p-I-Ph)
(E)-CH2CH¼CHI
(CH2)4OH
809ꢂ120 146–148
415ꢂ82
360ꢂ20
65ꢂ15
144–145
90–92
112–113
108–109
11
12
13
13ꢂ2
RU 58841
Testosterone
Mibolerone
26ꢂ5
4.9ꢂ1.8
0.75ꢂ0.08
aData are presented as meanꢂSEM of three independent determina-
tions each conducted in duplicate.
bSolids gave white crystals after recrystallization from EtOAc/hexane
(1:5).
cIsolated yields after chromatography.
Scheme 1.

M. E. Van Dort, Y.-W. Jung / Bioorg. Med. Chem. Lett. 11 (2001) 1045–1047
1047
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12. The preparation of 8 is illustrative. Dry NaH (12 mg,
0.4 mmol) was added to a solution of 1 (99 mg, 0.33 mmol) in
dry DMF (1mL) under argon. The mixture was stirred at
ambient temperature for 15 min and treated dropwise with a
solution of 4-iodobenzyl bromide (117 mg, 0.4 mmol) in dry
DMF (1mL). Stirring was continued at ambient temperature
for 3 h and the reaction monitored by TLC analysis (silica;
35% EtOAc in hexane). The mixture was poured into satu-
rated brine (25 mL) and extracted with ether (2ꢃ25 mL). The
combined ether extracts were washed successively with satu-
rated brine (25 mL), water (2ꢃ25 mL) and dried (Na2SO4).
The residue obtained after removal of solvent was purified by
flash chromatography (35% EtOAc in hexane) to afford
146 mg (86%) of 8 as a white solid: mp 101–103 ꢁC (EtOAc/
favorable p electronic interaction of the cyano and ole-
finic moieties, respectively, with the AR binding site.
Additional studies are planned to confirm this hypoth-
esis.
In summary, our studies suggest that the nature of the
side chain in RU 58841derivatives plays a major role in
its AR affinity. In general, introduction of fluoroalkyl or
iodophenylalkyl substituents at the N(3) position of RU
58841has a detrimental effect on AR affinity. Such
compounds are therefore unsuitable for development as
AR radioligands. The N-(iodopropenyl) derivative 13,
however, emerges as a new lead for the development of
high-affinity radioiodinated AR radioligands. Studies
are underway to synthesize [I-123]-labeled 13 for eval-
uation as a SPECT AR radioligand.
1
hexane (1:5)); H NMR (CDCl3) d 8.17 (s, 1H), 8.03 (d, 1H,
J=8.4 Hz), 7.93 (d, 1H, J=8.4 Hz), 7.69 (d, 2H, J=8.3 Hz),
7.13 (d, 2H, J=8.3 Hz), 4.56 (s, 2H), 1.43 (s, 6H).
13. Van Dort, M. E.; Kilbourn, M. R.; Chakraborty, P. K.;
Richfield, E. K.; Gildersleeve, D. L.; Wieland, D. M. Appl.
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Nucl. Med. 1996, 37, 1197.
15. Androgen receptor binding assays were conducted by a
commercial laboratory (MDS Panlabs, Bothell, WA, USA).
16. Schilling, K.; Liao, S. Prostate 1984, 5, 581.
17. Liao, S.; Witte, D.; Schilling, K.; Chang, C. J. Steroid
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Acknowledgements
We thank the staff of the Phoenix Memorial Labora-
tories for use of their facilities. This research was sup-
ported by grants from the National Institutes of Health
(CA 77287) and the SPORE in Prostate Cancer (P50
CA 69568).
18. Card, P. J. J. Carbohydr. Chem. 1985, 4, 451.
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