Novel retinoic acid receptor α agonists: Syntheses and evaluation of pyrazole derivatives

Article abstract of DOI:10.1016/S0960-894X(00)00066-4

We have designed and synthesized a series of pyrazole derivatives as candidate retinoic acid receptor (RAR) agonists. One of them, 4-[5-(1, 5-diisopropyl-1H-3-pyrazolyl)-1H-2-pyrrolyl]benzoic acid (11b), which possesses a 2,5-disubstituted pyrrole moiety, showed selective transactivation activity for the RARα receptor, and had highly potent cell-differentiating activity on HL-60 cells. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00066-4

Bioorganic & Medicinal Chemistry Letters 10 (2000) 619±622
Novel Retinoic Acid Receptor ꢀ Agonists: Syntheses and
Evaluation of Pyrazole Derivatives
Kouichi Kikuchi, Shigeki Hibi, Hiroyuki Yoshimura, Kenji Tai, Takayuki Hida,
Naoki Tokuhara, Toshihiko Yamauchi and Mitsuo Nagai*
Tsukuba Basic Research Laboratories for Drug Discovery, Eisai Co. Ltd., 1-3, Tokodai 5-chome, Tsukuba-shi, Ibaraki, 300-2635, Japan
Received 16 December 1999; accepted 20 January 2000
AbstractÐWe have designed and synthesized a series of pyrazole derivatives as candidate retinoic acid receptor (RAR) agonists.
One of them, 4-[5-(1, 5-diisopropyl-1H-3-pyrazolyl)-1H-2-pyrrolyl]benzoic acid (11b), which possesses a 2,5-disubstituted pyrrole
moiety, showed selective transactivation activity for the RARa receptor, and had highly potent cell-di€erentiating activity on HL-
60 cells. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
patients.8 Moreover, this compound inhibited rat CII
arthritis.9 Therefore, RARa agonists appear to be
promising compounds for the treatment of cancer,
dermatological diseases, and immunological disorders.
Retinoids, natural and synthetic analogues of all-trans
retinoic acid (ATRA), play important roles in many
biological functions, including induction of cellular
proliferation, di€erentiation and death, as well as
developmental changes.1 It has been shown that reti-
noids exert their function through nuclear receptors
(RARa, b, and g).2
In the course of our studies aimed at synthesizing novel
retinoids,10 we have already reported that ER-34617,
which possesses both a quinoxaline and a 2,5-di-
substituted pyrrole moiety, showed selectivity for the
RARa receptor, as well as highly potent cell-di€er-
entiating activity on HL-60 cells.10c Many synthetic
retinoids, such as Am80 and ER-34617 (Chart 2), possess
a bicyclic hydrophobic part, except for Etretinate and
Barrero's monocyclic retinoids, in which the phenyl
hydrophobic part of Etretinate was replaced with
monocyclic pyrrole and imidazole.11 We ®rst introduced
a monocyclic pyrazole moiety into the hydrophobic part
of ER-34617. We hoped that such simple retinoid
derivatives might be selective RARa agonists. Here we
discuss the synthesis and structure±activity relationships
(SAR) of retinoids which possess a pyrazole and a 2,5-
disubstituted pyrrole as the hydrophobic part and linker,
respectively.
Although retinoids are thought to have great ther-
apeutic potential, the clinical use of retinoids is so far
limited mainly to dermatological diseases3 and also
some cancers, for which retinoids may have both
chemotherapeutic and chemopreventive applications.4
For example, ATRA and Etretinate are prescribed for
acute promyelocytic leukemia (APL) and psoriasis
patients, respectively (Chart 1). The main reason for the
limited use of retinoids so far is the wide range of toxic
e€ects of retinoids.5 Recent research has focused on the
synthesis and development of subtype-selective retinoids
in order to reduce the toxicity.6
Only a few of RARa agonists have been reported so far.
These include Am80, Am5806a and AGN193836.6b
Am80 is more potent than ATRA (all-trans retinoic
acid) as an in vitro di€erentiation inducer, and clinically
Am80 was e€ective in leukemia patients7 and psoriasis
Chemistry
A number of known active retinoids possess bulky
structure at the hydrophobic part. Therefore, we intro-
duced an isopropyl group at the 5-position of pyrazole
in order to enhance the anity for RARs. The synthesis
of the pyrazole derivatives is shown in Scheme 1.
*Corresponding author. Tel.: +81-298-47-5868; fax: +81-298-47-
2037; e-mail: m2-nagai@hhc.eisai.co.jp
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00066-4

620
K. Kikuchi et al. / Bioorg. Med. Chem. Lett. 10 (2000) 619±622
Chart 1.
Chart 2.
Scheme 1. Reagents and conditions: (a) methyl 2-methoxyacetate (2), Na, toluene, 70 ꢀC, 2 h; (b) NH2NH2, EtOH, re¯ux, 2 h; (c) i-Prl, NaH, DMF,
.
RT, 30 min; (d-i) CF3CH2NHNH2, EtOH; (d-ii) t-BuNHNH2 HCl, EtOH, re¯ux, 2 h; (d-iii) 2,5-dimethylphenylhydrazine hydrochloride, EtOH,
re¯ux, 2 h.
Condensation of 3-methyl-2-butanone (1) with methyl
2-methoxyacetate (2) in the presence of sodium gave the
diketone (3). The pyrazole derivatives (5a±d) were pre-
pared by two methods. Treatment of the diketone with
hydrazine, followed by alkylation of the pyrazole deri-
vative (4) with isopropyl iodide in the presence of
sodium hydride a€orded the 1,5-diisopropylpyrazole
(5b) as a minor isomer.12 A more direct method to
synthesize the N-alkylated pyrazole was performed by
treatment of the diketone (3) with substituted hydra-
zine. In this procedure, we obtained the pyrazole
derivatives (5a, 5c, 5d) as the main product isomer
(Scheme 1).
followed by hydrolysis to a€ord the pyrrole derivatives
(11a±d) (Scheme 2).
Results and Discussion
The above retinoids were synthesized and evaluated in
vitro for the ability to bind to the individual RARs and
to induce gene transcription in a co-transfection assay.
Co-transfection assays were performed as reported,10c,13
and results are reported as relative EC30 values (see
footnotes to Table 1). Binding assays for RAR receptor
subtypes were performed in a similar manner to that
described by Boehm et al.14 using [3H] ATRA. HL-60
di€erentiation activity was measured using CD11b as a
marker of di€erentiation. The results are summarized in
the table. RXRa transactivation was also studied, but none
of these compounds activated RXR (data not shown).
The formyl derivatives were obtained in three steps.
Treatment of the methyl ethers (5) with boron tri-
bromide followed by hydrolysis gave the alcohol, which
was oxidized with manganese dioxide to a€ord the for-
myl derivatives (7a±d). These were treated with vinyl
Grignard reagent followed by manganese dioxide
oxidation to give the enones (8a±d). Synthesis of the
1,4-diketones (10a±d) was achieved by means of a
thiazolium salt-catalyzed benzoin condensation type
reaction. The enones (8a±d) were treated with a com-
mercially available benzaldehyde derivative in the pre-
sence of 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium
chloride and triethylamine to a€ord the 1,4-diketones
(10a±d). These were treated with ammonium acetate
These four pyrazole derivatives showed weak anity
for RARs, and even the anity at RARa was 10- to
100-fold less potent than that of ATRA. However, in
the co-transfection assay, the 1-tri¯uoroethyl-5-iso-
propylpyrazole derivative (11a) possessed comparable
activity and maximum response to ATRA at RARa,
but was less potent at RARb and RARg, respectively.
Moreover, 11a showed comparable HL-60 di€erentiation-
inducing activity to ATRA. Introduction of a bulkier

K. Kikuchi et al. / Bioorg. Med. Chem. Lett. 10 (2000) 619±622
621
Scheme 2. Reagents and conditions: (a-i) BBr3, CH2Cl2, 0 ꢀC, 1 h; (a-ii) Na2CO3, dioxane, H2O re¯ux, 3 h; (a-iii) MnO2, CH2Cl2, RT, 13±16 h;
(b-i) vinylmagnesium bromide, THF, 0 ꢀC, 10 min; (b-ii) MnO2, CH2Cl2, RT, 13±16 h; (c) methyl 4-formylbenzoate (9), 3-benzyl-5-(2-hydroxyethyl)-
4-methylthiozolium chloride, Et3N, DMF, 80 ꢀC, 1±2 h; (d-i) AcONH4, MeOH, re¯ux, 3±5 h; (d-ii) NaOH, aqeous MeOH, re¯ux, 1 h.
Table 1. Competitive binding, transactivation and induction of HL-60 di€erentiation by the pyrazole derivatives
Binding anitya
Subtype-speci®c transactivatione
HL-60 di€erentiation-inducing
activity
b
f
Relative IC50
Relative EC30
No.
R
RARa
98
13
7.6
RARb
RARg
RARa
1.0
RARb
15
1.8
0.63
Ðg
RARg
88
14
2.3
Ðg
ED30 (nM)
11a
11b
11c
11d
CF3CH2
i-Pr
t-Bu
Ðc
660
110
Ðc
Ðc
0.62
0.27
0.050
0.19
0.22
70
2,5-dimethylphenyl 39
Ðc
Ðc
Ðg
>10000
ATRA
1.0
1.0
0.78 nMd
1.0
0.90 nMd
1.0
1.0
1.0
0.93 nMh
1.0
0.22 nMh
1.5
0.50 nMd 0.99 nMh
aSpeci®c binding anity was de®ned as the total binding minus the nonspeci®c binding, and the 50% inhibitory dose (IC50) values were obtained
from logarithmic plots. In some cases, Scatchard plot analysis was performed. The selectivity of test compounds for each receptor is indicated as
relative IC50, where the IC50 value for each receptor was divided by that of the natural ligand (ATRA).
bIC50/ATRA IC50
c Ð: not detectable (relative IC50 >1000).
dATRA IC50
.
.
eEC30 values were determined from full dose-response curves ranging from 0.1 nM to 3 mM. Retinoid activity is expressed in terms of relative EC30
which is the concentration of retinoid required to produce 30% of the maximal observed response, normalized relative to that of ATRA.
,
fEC30/ATRA EC30
g Ð: not detectable (relative EC30 >10000).
hATRA EC30
.
.
substituent at the 1-N-position of the pyrazole (11b,c)
resulted in signi®cant activation. Compound 11b acti-
vated RARa, having a transcription activity more
potent than that of ATRA. Although the 1-N-tert-
butyl-5-isopropylpyrazole derivative (11c) possesses low
selectivity for RARs, this compound is 30-fold more
potent in HL-60 di€erentiation-inducing activity than
ATRA. Introduction of a large aromatic substituent at
the 1-N-position of the pyrazole (11d) did not activate
any RARs, and this compound did not show HL-60
di€erentiation-inducing activity.
In conclusion, we have described the synthesis and eva-
luation of a new series of pyrazolylpyrrole derivatives.

622
K. Kikuchi et al. / Bioorg. Med. Chem. Lett. 10 (2000) 619±622
We found that the pyrazole derivatives have strong
transactivation activities. One of them, 4-[5-(1,5-di-
isopropyl-1H-3-pyrazolyl)-1H-2-pyrrolyl]benzoic acid
(11b; ER-38930) has selectivity for the RARa receptor,
and shows potent cell di€erentiation-inducing activity.
Thus, ER-38930 should be useful as a tool to study the
physiological role of RARa. It may also be useful as a
lead compound for RARa-selective retinoids, which
might have fewer toxic e€ects and better therapeutic
indices than non selective retinoid drugs.
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We thank Dr. Naoki Asai and Ms. Seiko Higashi for
the 2D-NOESY experiments and the technical assistance.
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