sp2-Bridged diaryl retinoids: Effects of bridge-region substitution on retinoid X receptor (RXR) selectivity

Article abstract of DOI:10.1016/S0960-894X(00)00243-2

RXR class selectivity and RXR transcriptional activation activity compared to those for the retinoic acid receptor subtypes were enhanced on the 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylethenyl)benzoic acid scaffold and its 3-methyl analogue by replacing their 1,1-ethenyl bridge by a 1,1-(2-methylpropenyl) or cyclopropylidenylmethylene group. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00243-2

Bioorganic & Medicinal Chemistry Letters 10 (2000) 1307±1310
sp2-Bridged Diaryl Retinoids: E€ects of Bridge-Region
Substitution on Retinoid X Receptor (RXR) Selectivity
Marcia I. Dawson,a,* Peter D. Hobbs,b Ling Jong,b Dongmei Xiao,c
Wan-ru Chao,b Chin Pana and Xiao-kun Zhangc
aMedicinal Chemistry Department, Molecular Medicine Research Institute, 325 East Middle®eld Road, Mountain View, CA 94043, USA
bRetinoid Program, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025, USA
cThe Burnham Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA
Received 14 January 2000; accepted 15 March 2000
AbstractÐRXR class selectivity and RXR transcriptional activation activity compared to those for the retinoic acid receptor sub-
types were enhanced on the 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylethenyl)benzoic acid sca€old and its 3-methyl
analogue by replacing their 1,1-ethenyl bridge by a 1,1-(2-methylpropenyl) or cyclopropylidenylmethylene group. # 2000 Elsevier
Science Ltd. All rights reserved.
The RXRs are unique members of the nuclear receptor
superfamily of ligand-inducible transcription factors
because they heterodimerize with the retinoic acid recep-
tors (RARs), vitamin D receptor, thyroid hormone recep-
tor, and orphan receptors, such as the peroxisome
proliferator-activated receptors a and g, and TR3/nur77
(reviewed in ref 1). Increasing evidence indicates that the
RXR heterodimeric partner, its ligand, and the response
element to which the retinoid-heterodimer complex
binds in¯uence whether RXR will be occupied by ligand
to further modulate gene transcription.2 9 The ability of
RXR-selective retinoids to enhance transactivation by
RAR-selective retinoids2 9 suggests that they may also
enhance the therapeutic ecacy of these and other ligands
that activate RXR dimeric partners.
one of the RAR subtypes a, b, and g, or RXRa indi-
cated that (1) decreasing the distance between the
hydrophobic and polar termini in 2 by replacing its 2,6-
disubstituted naphthalenyl moiety by a 1,4-disubstituted
phenyl ring, (2) increasing the hydrophobicity of the sp2
bridge linking the aryl rings of 1, and (3) including a 3-
methyl group on the tetrahydronaphthalene ring of 4
enhanced RXRa transactivation, as demonstrated by 1,
4, and 5, respectively.10,15,18
RXR-selective 5 (LGD1069)19 prevented mammary
tumor formation in rats treated with the carcinogen N-
methyl-N-nitrosourea20 and showed anticancer ecacy in
clinical trials.21 The potent, more RXR-selective 2-[1-(5,6,
7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphthalenyl)
cyclopropyl]-5-pyridinecarboxylic acid (LGD100268)22
was reported to lack cancer preventive ecacy when
administered alone under these conditions.23 These and
other results8,17,24,25 suggest that the cancer preventive
and therapeutic e€ects of RXR-selective retinoids
require concomitant activation of RARs, as occurs by
using the panagonist 9-cis-RA alone or the combination
of an RXR-selective retinoid with an RAR-selective
retinoid24 or with another agent, such as interferon a.26
4-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl-
carbonyl)benzoic acid (SR11004, 1 in Fig. 1),10 an ana-
logue of the RAR-selective 6-substituted 2-naphthalene-
carboxylic acid CD567 (2),11 and 4-[2-(5,6,7,8-tetra-
hydro-3,5,5,8,8 - pentamethyl - 2 - naphthalenyl)propenyl]
benzoic acid (3-methyl-TTNPB, 3)12 provided the leads
to RXR-selective synthetic analogues of RAR/RXR
panagonist 9-cis-retinoic acid (9-cis-RA, 6 in Table
1).13,14 Retinoid receptor transcriptional activation assays
using the (TREpal)2-tk-CAT reporter construct15 17 in
CV-1 cells cotransfected with an expression vector for
Enhancing alkyl bulk on the bridge between the aryl rings
in sp3-carbon-bridged RXR analogues improved RXR
selectivity.10,15 Here, we report the e€ects on RXRa acti-
vation and selectivity on the (TREpal)2-tk-CAT reporter
construct, compared to those of the RAR subtypes, by
replacing the hydrogens on the ethenyl bridges of 4 and
*Corresponding author. Tel.: +1-650±237-7456; fax: +1-650±237-
7455; e-mail: mdawson@mmrx.org
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00243-2

1308
M. I. Dawson et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1307±1310
5 by alkyl substitutents. Substitution with methyl groups
produced 4-[1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-
naphthalenyl)-2-methylpropenyl]benzoic acid (7)10,15
and its more potent 3-methyl homologue (8)17,24 (see Table
1). However, introducing a sp2 cyclopropylidenylmethy-
lene bridge between the aryl rings provides selective
RXR/RAR-subtype panagonists, in which further mod-
i®cations of the aryl rings may modulate RAR-subtype
activity. For example, 9 activates both RARb and
RXRa on the (TREpal)2-tk-CAT, whereas at 10 nM 10
is more selective and more potent than 9 in activating
RXRa (108% compared to 2%).
In contrast, analogue 11, in which the bonds corre-
sponding to the ethenyl bridge and the 4-substituted
benzoic acid ring of 4 are coplanar by inclusion in a 5-
substituted 2-naphthalenecarboxylic acid ring, and the
2-methylpropylidene analogue 12 were inactive. We
previously found that removal of the Z-methyl group of
7 only slightly decreased RXRa activity.1 The loss of
activity by replacing the remaining E-methyl group by
an E-i-propyl group (12) suggests that further steric
constraints are present in the region of the RXRa
Figure 1. Structures of SR11004 (1), CD567 (2), 3-MeTTNPB (3),
MM11225 (4), and MM11247 (5), which were initial leads to more
potent RXR-selective retinoids.
Table 1. Retinoic acid receptor (RAR) subtype a, b, and g and retinoid X receptor (RXR) a 50% transcriptional activation (AC50) on the (TRE-
pal)2-tk-CAT by retinoids 7 to 12 compared to 1 mM trans-RA for the RARs and 1 mM 9-cis-RA (6) for RXRaa,b
50% Activation values (AC50, nM) (% activation at 1 mM compared to 1 mM trans-RA
for RARs or 1 mM 9-cis-RA for RXRa)
Structure
Name or code number
RARa
RARb
RARg
RXRa
6
9-cis-RA
19 (105)
2 (112)
3 (126)
7 (100)
7 R1=H, R2=Me
MM11217
MM11345
MM11346
MM11173
>1000 (0)
>1000 (0)
>1000 (19)
>1000 ( 23)
>1000 (21)
>1000 ( 6)
17 (85)
>1000 (0)
160 (71)
22 (107)
5 (118)
8 R1=Me, R2=Me
9 R1=H, R2=(CH2)
10 R1=Me, R2=(CH2)
>1000 ( 12)
>1000 (16)
>1000 (12)
>1000 ( 19)
<10 (120)
11
MM11258
>1000 (0)
>1000 (1)
>1000 (18)
> 1000 (12)
12
MM11344
>1000 (0)
>1000 ( 6)
>1000 ( 10)
> 1000 ( 1)
aNew compounds were fully characterized (IR, 1H NMR, mp) and passed analysis (elemental or HRMS).
bActivation (50%) for RARa, b, g, and RXRa on the (TREpal)2-tk-CAT by retinoids in monkey kidney CV-1 cells transfected with expression vectors for
each of these receptors compared to that of 1 mM trans-RA for the RARs and 1 mM 9-cis-RA for RXRa as 100%. Two copies of the TREpal response
element, which is activated by RARs and RXRs, were linked to the chloramphenicol acetyl transferase reporter (CAT) containing the thymidine kinase
promoter (tk).10 The b-galactosidase expression vector was used to normalize for transfection ecacy. Data points are the means of triplicate experiments.
AC50 values were calculated by interpolation of concentration-response curves. Assays were conducted at the Burnham Institute under license from Ligand
Pharmaceuticals for use of this patented technology.

M. I. Dawson et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1307±1310
1309
ligand-binding domain that interacts with the retinoid
bridge.
the far more RXR-selective 8 requires the presence of an
RAR agonist suggests that activation of RXRa alone is
not sucient for inhibiting the growth of these cells. These
data also suggest that RXR/RARb panagonists, such as
9, alone or in combination with another therapeutic agent,
may be useful in cancer therapy, whereas more RXR-
selective retinoids, such as 8 and 10, may only be useful in
combination with an RAR-selective retinoid or another
agent. Interestingly, we found that 9 inhibited MDA-MB-
231 cell growth after seven days in culture with an IC50
value of 1.1 mM, whereas 12.5 mM 9-cis-RA gave only
45% inhibition.
Previously, we demonstrated that RXR-selective reti-
noids inhibit the growth of retinoid-resistant MDA-MB-
231 human breast cancer cells in part by activating
RXRa-nur77 heterodimers on the bRARE to induce the
expression of RARb,25 which is lost in many cancers. We
found that trans-RA had no e€ect on the growth of
MDA-MB-231 cells, largely due to their lack of RARb,25
and 8 alone did not inhibit growth. However, trans-RA
plus 8 produced signi®cant inhibition, as did the RAR/
RXR panagonist 9 alone (Fig. 2).
The synthesis of 7 from diaryl ketone 1 was reported.10
Retinoids 9 and 10 were similarly prepared in high
yields from ketones 1 and 13 as outlined in Scheme 1,
whereas the more sterically hindered 8 required a step-
wise procedure involving a vinyl bromide-aryl bromide
coupling.
Inhibition by 9 is likely due to its induction of RARb
expression through the RXR-nur77,25 followed by its
activation of the synthesized RARb protein that forms
the RXR-RARb. Our ®nding that growth inhibition by
Acknowledgements
Support by the USPHS NCI Program Project Grant
CA51993 (to M.I.D. and X.Z.) is gratefully acknowl-
edged. Ms. Genet Zemede assisted in preparing this
manuscript.
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