Synthesis and structure-activity investigation of iodinated arylhydantoins and arylthiohydantoins for development as androgen receptor radioligands

Article abstract of DOI:10.1016/j.bmcl.2004.08.031

A series of side-chain derivatives of the arylhydantoin RU 58841 and the arylthiohydantoin RU 59063, wherein the aromatic trifluoromethyl was replaced with iodine, was synthesized for development as radioiodinated androgen receptor (AR) ligands. Compounds having N-(methyl) and N-(hydroxybutyl) side-chains displayed high AR binding affinity (0.71 nM < K_i < 11 nM). The synthesis and SAR of these ligands are discussed. A series of side-chain derivatives of the arylhydantoin RU 58841 and the arylthiohydantoin RU 59063, wherein the aromatic trifluoromethyl group was replaced with iodine, was synthesized for possible development as radioiodinated androgen receptor (AR) ligands. Derivatives containing the cyanomethyl, methoxyethyl and propenyl side-chains displayed moderately high affinity (K_i = 20-59 nM) towards the rat AR. Side-chains containing bulky lipophilic groups such as, benzyl and phenylpropyl, were poorly tolerated (K_i > 219 nM). Superior AR binding affinities (0.71 nM < K_i < 11 nM) were displayed by arylhydantoins and arylthiohydantoin derivatives containing hydroxybutyl or methyl side-chains. The latter compounds are potential candidates for development as radioiodinated AR ligands.

Full text of DOI:10.1016/j.bmcl.2004.08.031

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5285–5288
Synthesis and structure–activity investigation of
iodinated arylhydantoins and arylthiohydantoins
for development as androgen receptor radioligands
Marcian E. Van Dort* and Yong-Woon Jung
Division of Nuclear Medicine, Department of Radiology, The University of Michigan Medical School,
Ann Arbor, MI 48109-0552, USA
Received 14 July 2004; accepted 13August 2004
Available online 15 September 2004
Abstract—A series of side-chain derivatives of the arylhydantoin RU 58841 and the arylthiohydantoin RU 59063, wherein the aro-
matic trifluoromethyl group was replaced with iodine, was synthesized for possible development as radioiodinated androgen recep-
tor (AR) ligands. Derivatives containing the cyanomethyl, methoxyethyl and propenyl side-chains displayed moderately high affinity
(Ki = 20–59nM) towards the rat AR. Side-chains containing bulky lipophilic groups such as, benzyl and phenylpropyl, were poorly
tolerated (Ki > 219nM). Superior AR binding affinities (0.71nM < Ki < 11nM) were displayed by arylhydantoins and arylthiohy-
dantoin derivatives containing hydroxybutyl or methyl side-chains. The latter compounds are potential candidates for development
as radioiodinated AR ligands.
Ó 2004 Elsevier Ltd. All rights reserved.
Prostate cancer (PC) is the most frequently diagnosed
cancer and the second leading cause of cancer death in
American men. The American Cancer Society estimates
that in 2004, 230,110 men will be diagnosed with PC and
an estimated 29,900 will die of the disease.1 The impor-
tance of circulating androgens for the growth of prostate
tumors is well established and androgen receptor (AR)
expression is frequently observed in primary and meta-
static PC.2,3 This finding has presented strong impetus
for the development of radiolabeled AR ligands for
external diagnostic imaging of tumor sites in PC using
positron emission tomography (PET) or single photon
emission computed tomography (SPECT).4 To date,
the majority of these studies have focused on steroid-
based radioligands and reports of several radiofluori-
nated androgen derivatives for PET imaging of PC have
appeared in the recent literature.5,6 In contrast, the
development of high-affinity, radioiodinated steroid-
based AR radioligands for SPECT imaging has had lim-
ited success. This may be attributable, in part, to the
sensitivity of the AR ligand binding domain to the in-
creased steric bulk of iodinated steroids.7
A suitable [123I]-labeled AR radioligand for the early
detection and diagnosis of PC with SPECT could have
broad clinical utility due to the wide availability of
SPECT instrumentation in nuclear medicine clinics. To
address this goal, we focused on RU 58841 and RU
59063, prototypes of a new class of selective, high-affin-
ity aryl(thio)hydantoin AR ligands8,9 (Chart 1), as
leads for development of a radioiodinated SPECT
O
NC
F3C
N
N
X
OH
LIGAND
X
Ki (nM)a
26 5b
RU 58841
O
RU 59063
S
2.23 0.50c
aMean SEM. reference 16. creference 10.
b
*
Corresponding author at present address: Girindus America Inc.,
8560 Reading Road, Cincinnati, OH 45215-0027, USA. Tel.: +1 513
679 3010; fax: +1 513 679 3053; e-mail: mvandort@girindus.com
Chart 1.
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.08.031

5286
M. E. Van Dort, Y.-W. Jung / Bioorg. Med. Chem. Lett. 14 (2004) 5285–5288
radioligand. We hypothesized that the increased con-
formational flexibility of these nonsteroidal ligands over
steroid-based ligands could favor the design of iodinated
ligands that are better accommodated at the AR ligand
binding domain leading to improved binding affinity. In
the initial phase of this work, we observed that replace-
ment of the trifluoromethyl group in RU 59063with the
similarly hydrophobic iodine atom resulted in a 3-fold
higher AR binding affinity (8e, Ki = 0.71nM).10 How-
ever, tissue distribution studies conducted in castrated
rats with the corresponding radioiodinated analog
([125I]8e) showed low accumulation of radioactivity in
AR-rich target tissues including prostate.11 The lack
of AR-mediated target uptake of [125I]8e was ascribed
to the metabolic conversion of its side-chain hydroxyl
to a carboxylic acid based on the results of metabolic
studies reported by Cousty-Berlin et al. for the structur-
ally similar analogs RU 58841 and RU 59063.12 Thus, it
was of interest to explore the effect of alternative side-
chains having improved metabolic stability on the AR
binding affinity of iodinated RU 58841 and RU 59063
derivatives. Accordingly, the short series of such deriva-
tives was chosen for development based on synthetic
convenience and availability of side-chain precursors.
We report herein, a limited SAR investigation of these
derivatives to identify suitable high-affinity analogs for
development as [123I]-labeled AR ligands for SPECT
imaging of PC.
via initial synthesis of the 4-(tert-butyldimethylsilyloxy)
derivative 5f, followed by deprotection of the silyl group
with acid hydrolysis.
Synthesis of the thiohydantoin derivatives 8a–e were
accomplished as shown in Scheme 2. The 2-alkyl-
amino-2-cyanopropane derivatives (6a–e) were readily
prepared by the treatment of acetone cyanohydrin with
the appropriate alkylamine using published procedures.9
Condensation of 2-iodo-4-isothiocyanatobenzonitrile10
with 6a–e gave the imino derivatives 7a–e, which were
subsequently converted to the thiohydantoin derivatives
8a–e by acid hydrolysis (2N HCl). The final products
were purified to homogeneity by flash chromatography
and solids were recrystallized from EtOAc/hexane mix-
tures. The compounds gave 1H NMR and elemental
analysis or mass spectral data consistent with the as-
signed structures.
The binding affinities of new ligands and reference com-
pounds to the rat prostate cytosolic AR were deter-
mined using a competitive binding assay in the
presence of the high-affinity AR radioligand, [3H]mibol-
erone.13–15 These data (expressed as inhibition con-
stants, Ki) are presented in Table 1. All ligands
demonstrated monophasic radioligand displacement
curves (Hill coefficient close to unity) indicating interac-
tion with a single class of binding sites.
The hydantoin derivatives 5a–g were synthesized as out-
lined in Scheme 1. Treatment of 4-cyano-3-iodoaniline
(1)10 with phosgene provided the isocyanate 2, which
was condensed with 2-amino-2-cyanopropane8 to give
the imino derivative 3. Acid hydrolysis of 3 provided 4
in 64% overall yield. Synthesis of compounds 5a–g were
achieved by the treatment of 4 with NaH in DMF and
subsequent alkylation with the appropriate commer-
cially available alkyl halide or tosylate. Preparation of
the N-(4-hydroxybutyl) derivative 5g was conducted
In the hydantoin series of ligands, the parent unsubsti-
tuted hydantoin (4) displayed weak binding affinity to-
wards AR (Ki = 400nM). However, introduction of
either a methyl (5a) or hydroxybutyl (5g) group at the
N(3) position resulted in a dramatic improvement in
affinity (36- and 200-fold, respectively, over compound
4). In contrast, introduction of a butyl group at this
position resulted in only a slight enhancement (5-fold)
in affinity suggesting the importance of the polar hydr-
oxyl group in 5g for high AR affinity. A dramatic loss
HN
c
a
b
NC
NH2
NC
NCO
NC
N
NH
O
I
I
I
2
3
1
5a: R = CH3
5b: R = (CH2)3CH3
5c: R = CH2CH=CH2
5d: R = CH2CN
O
N
O
N
O
d
NC
NC
N
NH
R
O
I
I
5e: R = (CH2)3Ph
5a - 5g
4
5f: R = (CH2)4OSi(CH3)2(t-butyl)
5g: R = (CH2)4OH
e
Scheme 1. Reagents: (a) COCl2, toluene, reflux; (b) 2-amino-2-cyanopropane, Et3N, 1,2-dichloroethane; (c) 2N HCl, CH3OH, reflux; (d) (1) NaH/
DMF, (2) RX; (e) 2N HCl, CH3OH.

M. E. Van Dort, Y.-W. Jung / Bioorg. Med. Chem. Lett. 14 (2004) 5285–5288
5287
HN
O
CN
a
b
NC
N
NC
N
N
N
NHR
R
R
S
I
S
I
6a: R = CH3
8a: R = CH3
8b: R = (CH2)3CH3
7a - 7e
6b: R = (CH2)3CH3
6c: R = (CH2)2OCH3
6d: R = CH2Ph
8c: R = (CH2)2OCH3
8d: R = CH2Ph
6e: R = (CH2)4OH
8e: R = (CH2)4OH
Scheme 2. Reagents: (a) 2-iodo-4-isothiocyanatobenzonitrile, Et3N, THF, reflux; (b) 2N HCl, CH3OH, reflux.
Table 1. Inhibition constants (Ki) for ligands at the rat androgen receptor
O
NC
N
N
R
X
I
Compd
X
R
Kia (nM)
Mpb (°C)
Yieldc (%)
4
5a
O
O
O
O
O
O
O
S
H
CH3
400 41
198–200
193–194
100–102
96–97
64d
61e
68e
11
77 17
59
5
5b
5c
(CH2)3CH3
CH2CH@CH2
CH2CN
1
92e
e
5d
5e
20 3158–159
639 141
77
(CH2)3Ph
(CH2)4OH
CH3
Oil
Oil
86e
37e
73f
56f
66f
49f
76f
5g
8a
2
1
2.5 0.7
117 19
183–184
Amorphous
Oil
8b
8c
S
(CH2)3CH3
(CH2)2OCH3
CH2Ph
S
37
219
3
8
8d
8e
S
Amorphous
Amorphous
S
(CH2)4OH
0.71 0.22
4.9 1.8
0.75 0.08
Testosterone
Mibolerone
a Data are presented as mean SEM of three independent determinations each conducted in duplicate.
b Solids were recrystallized from EtOAc/hexane mixtures.
c Isolated yields after chromatography.
d Yield based on 1.
e Yield based on 4.
f Yield based on 2-iodo-4-isothiocyanatobenzonitrile.
in AR affinity was observed for analog 5e, which bears
the sterically demanding 3-phenylpropyl side-chain, sug-
gesting that there is limited tolerance for increased bulk
at this position. Introduction of the propenyl (5c) or
cyanomethyl (5d) side-chain afforded moderately high
AR affinities of 59 and 20nM, respectively. The latter
compounds were chosen for investigation since a signif-
icant improvement in AR binding affinity was previ-
ously noted upon introduction of the iodopropenyl
and cyanomethyl side-chains in the RU 58841 series.16,17
The improved binding affinity of these derivatives over 4
could be due to a favorable p electronic interaction of
the olefinic and cyano moieties with the AR binding site.
Thiohydantoin derivatives containing a methyl (8a) or
hydroxybutyl (8e) side-chain displayed high AR binding
affinity similar to that observed in the hydantoin series.
Moreover, these derivatives showed a 3-fold improve-
ment in affinity over their hydantoin counterparts sup-
porting the previous observation of enhanced affinity
with the thio function.9,10 Bulky, hydrophobic side-
chains such as butyl (8b) and benzyl (8d) were poorly
tolerated as previously noted in the hydantoin series.
In contrast, the more polar methoxyethyl derivative 8c
displayed moderately high affinity (Ki = 37nM) similar
to that seen with 5c and 5d. Taken together, this data
suggests that electronic effects in the side-chains of these

5288
M. E. Van Dort, Y.-W. Jung / Bioorg. Med. Chem. Lett. 14 (2004) 5285–5288
3. Hobisch, A.; Culig, Z.; Radmayr, C.; Bartsch, G.;
Klocker, H.; Hittmair, A. Prostate 1996, 28, 129.
4. Katzenellenbogen, J. A. J. Nucl. Med. 1995, 36, 8S.
5. Cutler, P. D.; Dehdashti, F.; Siegel, B. A.; Downer, J. B.;
Welch, M. J. J. Nucl. Med. 1996, 37, 87P.
6. Garg, P. K.; Labaree, D. C.; Hoyte, R. M.; Hochberg,
R. B. Nucl. Med. Biol. 2001, 28, 85–90.
7. Welch, M. J.; Downer, J. B.; Katzenellenbogen, J. A. In
Current Directions in Radiopharmaceutical Research and
Development; Mather, S. J., Ed.; Kluwer: Dordrecht, 1996;
pp 137–156.
8. Battmann, T.; Bonfils, A.; Branche, C.; Humbert, J.;
Goubet, F.; Teutsch, G.; Philibert, D. J. Steroid Biochem.
Mol. Biol. 1994, 48, 55.
9. Teutsch, G.; Goubet, F.; Battmann, T.; Bonfils, A.;
Bouchoux, F.; Cerede, E.; Gofflo, D.; Gaillard-Kelly,
M.; Philibert, D. J. Steroid Biochem. Mol. Biol. 1994, 48,
111.
ligands could play an important role in enhancing AR
binding affinity.
In summary, a limited series of aryl(thio)hydantoin
derivatives were synthesized and evaluated for their
AR binding affinity to identify suitable alternative can-
didates to the metabolically labile analog 8e for develop-
ment as AR radioligands. Our SAR studies suggest that
the high-affinity, N-methyl hydantoin and thiohydant-
oin derivatives (5a,8a) are promising candidates for
radiolabeling and further biological evaluation. The
binding affinity of these ligands towards the human
AR will also need to be evaluated to address the possi-
bility of cross-species differences. These investigations
will be the subject of future reports.
10. Van Dort, M. E.; Robins, D. M.; Wayburn, B. J. Med.
Chem. 2000, 43, 3344.
Acknowledgements
11. Van Dort, M. E.; Sherman, P. S., unpublished findings.
12. Cousty-Berlin, D.; Bergaud, B.; Bruyant, M. C.; Batt-
mann, T.; Branche, C.; Philibert, D. J. Steroid Biochem.
Mol. Biol. 1994, 51, 47–55.
13. Androgen receptor binding assays were conducted by a
commercial laboratory (MDS Panlabs, Bothell, WA,
USA).
We thank Dr. Craig Marhefka of Vertex Pharmaceuti-
cals Inc., Cambridge, MA for helpful discussions. This
research was supported by grants from the National
Institutes of Health (CA 77287) and the SPORE in Pros-
tate Cancer (P50 CA 69568).
14. Schilling, K.; Liao, S. Prostate 1984, 5, 581.
15. Liao, S.; Witte, D.; Schilling, K.; Chang, C. J. Steroid
Biochem. 1984, 20, 11.
References and notes
1. Jemal, A.; Tiwari, R. C.; Murray, T.; Gafoor, A.;
Samuels, A.; Ward, E.; Feuer, E. J.; Thun, M. J. CA
Cancer J. Clin. 2004, 54, 8–29.
2. Heinlein, C. A.; Chang, C. Endocrine Rev. 2004, 25(2),
276–308.
16. Van Dort, M. E.; Jung, Y.-W. Bioorg. Med. Chem. Lett.
2001, 11, 1045.
17. Battmann, T.; Branche, C.; Bouchoux, F.; Cerede, E.;
Philibert, D.; Goubet, F.; Teutsch, G.; Gaillard-Kelly, M.
J. Steroid Biochem. Mol. Biol. 1998, 64, 103.