6,7-Dihydroxyisoquinoline-3-carboxylic acids are potent inhibitors on the binding of insulin-Like growth factor (IGF) to IGF-binding proteins: Optimization of the 1-position benzoyl side chain

Article abstract of DOI:10.1016/S0960-894X(03)00321-4

A series of 1-benzoyl isoquinolines, based on compound 1, was synthesized and evaluated for their ability to displace IGF-I from its complex with IGF-binding protein-3. Successful modifications of 1 included the replacement of the 3,4-dihydroxybenzoyl gro

Full text of DOI:10.1016/S0960-894X(03)00321-4

Bioorganic & Medicinal Chemistry Letters 13 (2003) 1927–1930
6,7-Dihydroxyisoquinoline-3-carboxylic Acids are
Potent Inhibitors on the Binding of Insulin-Like Growth Factor
(IGF) to IGF-Binding Proteins: Optimization of the
1-Position Benzoyl Side Chain
Yun-Fei Zhu, Keith Wilcoxen, Timothy Gross, Patrick Connors, Nathalie Strack,
Raymond Gross, Charles Q. Huang, James R. McCarthy, Qiu Xie,
Nicholas Ling and Chen Chen*
Department of Medicinal Chemistry and Department of Peptide Chemistry, Neurocrine Biosciences, Inc.,
10555 Science Center Drive, San Diego, CA 92121, USA
Received 29 October 2002; accepted 11 February 2003
Abstract—A series of 1-benzoyl isoquinolines, based on compound 1, was synthesized and evaluated for their ability to displace
IGF-I from its complex with IGF_binding protein-3. Successful modifications of 1 included the replacement of the 3,4-dihydroxy-
benzoyl group with a substituted benzyl group. These alternations culminated in the discovery of compounds such as 7o which had
excellent in vitro potency (Ki=9.4 nM) but with one less of the labile catechol functionality of 1.
# 2003 Elsevier Science Ltd. All rights reserved.
The insulin-like growth factors (IGF-I and II) are small
polypeptide hormones regulating cell proliferation, cell
differentiation, cell death, and cell metabolic activities.1
In blood and interstitial fluids, including the cerebro-
spinal fluid, the concentration of freely circulating IGF
is exceedingly low because most of the IGFs are bound
to one or more of six high affinity IGF-binding proteins
Figure 1. 1: Ki=5.6 nM.
(IGFBPs), which inhibit their interaction with the IGF-I
receptor.2 The major binding protein in circulation is
IGFBP-3, which together with an acid labile subunit
forms a trimolecular complex with IGF to sequester
most of the IGFs in blood.3
the potency enhancing 3-carboxylic group and the
1-(3,4-dihydroxybenzoyl) substituents. Methylation of
one or more of the four hydroxy groups of compound 1
as a replacement for the metabolically labile catechol
group resulted in less active analogues.5 Isoquinoline 1
was characterized as having excellent in vitro potency in
both binding and functional assays.
An orally active compound that can potentiate the
action of endogenous IGF-I could be the use of a small
molecule ligand that displaces the bound IGF-I from
the large pool of IGF/IGFBP complexes in the body
fluids. Recently we reported the discovery of isoquino-
line-3-carboxylic acids and 3-hydroxy isoquinolines as
the first potent non-peptidyl IGF-BP inhibitors (Fig. 1).4
Notable discoveries within the isoquinoline class included
Despite these favorable characteristics, the in vivo
duration of action and blood–brain barrier penetration
ability (<5%) of isoquinoline 1 was low. We attributed
this property in part to high polarity caused by the two
catechol groups and the carboxylic functionality. To
decrease the overall polarity of the molecule, we had
already discovered that the 3-carboxylate could be
replaced by a less polar hydroxy group with only 5-fold
*Corresponding author. Tel.:+1-858-658-7600; fax:+1-858-658-7619;
e-mail: cchen@neurocrine.com
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00321-4

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Y.-F. Zhu et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1927–1930
Scheme 1. Reagents and conditions: (a) POCl3/reflux; (b) S/heat; (c) 48% HBr/reflux; (d) SeO2/AcOH/reflux; (e) PdCl2H2/MeOH; (f) NaBH4/MeOH.
Scheme 2. Reagents and conditions: (a) Pd-C/H2/MeOH; (b) MeSO2Cl/Et3N/CH2Cl2.
loss in activity. To further improve this lead, we studied
the structure–activity relationships of the two catechol
sites. In this paper, we present the SAR results from a
series of replacement of the 1-(3,4-hydroxybenzoyl)
group for optimization of this class of compounds,
which were determined in a binding assay using
125I-IGF-I and IGF-BP3.4
as outlined in Scheme 3. The para-fluoride of 5a was
also replaced by thiophenol and alkylamine to give
compounds 7r and 7s, respectively.
The synthesis of 6-amino-7-hydroxyisoquinolines 14–16
is described in Scheme 4. Attempt to cyclize N-3-nitro-
Table 1. SAR at the 1-position of quinolines 6, 7 and 8
The isoquinoline core of 7 (Scheme 1) was synthesized
by cyclization of N-phenethyl amides 2 in the presence
of POCl3 at reflux, followed by dehydrogenation with
an oxidative procedure to give isoquinolines 4.6 When R
was a methyl group in 4, deprotection was accomplished
in 48% HBr at reflux to give benzyl compound 6a.
Oxidation of isoquinolines 4 with selenium dioxide in
refluxing acetic acid afforded the corresponding ketones
5 which were deprotected to give compounds 7. When
the catechol and acid groups were protected by a benzyl
moiety, compounds 5 were subjected to either a palla-
dium chloride catalyzed hydrogenation, or acid cata-
lyzed hydrolysis. Using hydrogenation the alcohol 8 was
also isolated in some cases, which could also be
obtained from the reduction of compounds 7 with
sodium borohydride (Scheme 1).
Compd
X
hIGFBP-3 Ki (nM)
1
3,4-(OH)2
3,4-OCH2O–
3,4-OCH2O–
2,4,6-Me3
2-Cl
3-F
4-CF3
3,4-F2
5.6Æ0.5
72Æ10
70Æ24
100Æ100
110Æ34
110Æ18
38Æ8
6a
7a
7b
7c
7d
7e
7f
51Æ18
29Æ5
7g
7h
7i
3,4-Cl2
4-NO2
4-NH2
4-NHSO2Me
4-PhO
2,4,6-Me3
2-Cl
3-F
32Æ1
130Æ59
110Æ77
32Æ22
11Æ6
The 4-nitrobenzoyl isoquinoline 7h was reduced to the
corresponding amine 7i with palladium catalyzed
hydrogenation, and mesylation of 7i afforded the sul-
phonamide 7j as showed in Scheme 2. A number of
aryloxy derivatives 7k-q were synthesized by a sub-
stitution reaction of 3,4-difluorobenzoyl compound 5a,
7j
8a
8b
8c
8d
8e
25Æ20
24Æ7
4-CF3
18Æ3

Y.-F. Zhu et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1927–1930
1929
Scheme 3. Reagents and conditions: (a) ArOH/tBuOK/THF; (b) 48% HBr/reflux; (c) ArSH/ACN/Á; (d) Me(CH2)3CH(Et)CH2NH/Á.
Scheme 4. Reagents and conditions: (a) Pd/C-H2/MeOH; (ii) Ac2O/heat; (b) POCl3/reflux; (c) S/heat; (d) SeO2/AcOH/reflux; (e) 48% HBr/reflux; (f)
MeSO2Cl/Et3N/CH2Cl2.
4-methoxylphenethylamide 9 was unsuccessful, so com-
pound 9 was first converted to the acetylamino analogue
10, which was then subjected to an acid catalyzed cycli-
zation, followed by oxidation to give compound 13.
Hydrolysis of 13 in 48% HBr at reflux gave the desired
amino compound 14 and a partially hydrolyzed by-
product 15. The later was converted to the corresponding
sulphonamide 16.
analogue 6a had the same potency (Ki=72 nM), indi-
cating the carbonyl group played a limited role in bind-
ing. While the 2,4,6-trimethyl-, 2-chloro- and
3-fluorobenzoyl analogues had no improvement in
potency (7b, 7c and 7d, Ki=100, 110 and 110 nM,
respectively), a strong electron-withdrawing group, such
as trifluoromethyl and nitro, at the para-position of the
benzoyl group increased activity 3-fold (7e and 7h,
Ki=38 and 32 nM, respectively). When the 4-nitro
group of 7h was reduced to the electron-donating amine
the potency of compound 7i decreased about 4-fold to
130 nM. Mesylation of compound 7i gave the corre-
sponding sulfonamide 7j, which did not increase activity
(Ki=110 nM). This result suggests that an acidic pro-
ton, such as phenol in 1, at this position may not be
needed for high potency (Table 1).
In our efforts to improve physicochemical properties of
this class of compounds for potential in vivo biological
activities we have prepared a series of substituted benzoyl
isoquinolines (7a–s) without one of the labile catechol
groups of 1. In our initial series, compound 7a, which
has the catechol masked by a methylene group, showed
good activity (Ki=70 nM). The corresponding benzyl

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Y.-F. Zhu et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1927–1930
Table 2. SAR of substituted 1-(3-fluorobenzoyl) isoquinolines and
related compounds 7k–s
Table 3. SAR of 6-aminoisoquinoline derivatives 14–16
Compd
Y
hIGFBP-3 Ki (nM)
Compd
X1
hIGFBP-3 Ki (nM)
14
15
16
NH2
NH2
NHSO2Me
60Æ29
850Æ790
>10,000
7k
7l
3-ClPhO
4-ClPhO
3,4-Cl2PhO
3,5-Cl2PhO
4-MePhO
11Æ9
20Æ0.4
12Æ6.6
9.0Æ5.6
9.4Æ8.0
32Æ24
32Æ8
7m
7n
7o
7p
7q
7r
4-CF3PhO
4-PhPhO
4-ClPhS
Me(CH2)3CH(Et)CH2NH
pound 16, which certainly was unexpected since we
designed the sulfonamide to mimic the hydroxy group.
One explanation for this result is that the sulfonamide
might be too bulky to mimic a much smaller hydroxy
group.
31Æ10
62Æ21
7s
Unexpectedly, the alcohol compounds 8 obtained from
reduction of the corresponding ketone (7b–e) showed
much improved activities. For example, compound 8b
(Ki=11 nM) had 10-fold increase in activity over 7b
(Ki=110 nM). Compounds 8c–d all showed good activ-
ities. Encouraged by these initial results we further
expanded the SAR to investigate more lipophilic sub-
stituents. Thus chlorine substituted phenoxyphenyl
derivatives 7k–7n had Ki values between 9 and 20 nM
(Table 2). The 4-methylphenoxy analogue 7o also
showed very good activity (Ki=9.4 nM), while the
4-trifluoromethyl and 4-pheny compound 7p and 7q
were about 3-fold less potent (both had Ki values of 32
nM). Finally, Replacement of the aryloxy group with
phenylthio (compound 7r) or alkylamino (compound
7s) also resulted in active compounds (Ki=31 and 62
nM, respectively). All these results demonstrated that a
lipophilic group at the 1-position of the isoquinoline
core is favored for high potency (Table 3).
In conclusion, we have demonstrated that the catechol
moiety at 1-position of the isoquinoline core 1 can be
replaced with a series of lipophilic benzoyl, benzyl and
a-hydroxybenzyl group while maintaining IGF-BP
inhibitory activity. In addition, an attempt to mimic the
6-hydroxy isoquinoline with sulfonamide was not suc-
cessful. In this study, 3,5-dichlorophenoxybenzoyl iso-
quinoline 7n was a potent IGF-BP inhibitor with much
less polar characteristics (clogP=6.0) than compound 1
(clogP=1.7).7 Further investigation on the optimization
of this series toward blood–brain barrier penetration
will be reported in due course.
References and Notes
1. (a) For recent reviews, see: Roith, D. L.; Bondy, C.; Yakar,
S.; Liu, J.; Butler, A. Endocr. Rev. 2001, 22, 53. (b) Jones, J. I.;
Clemmons, D. R. Endocr. Rev. 1995, 16, 3.
2. (a) Shimasaki, S.; Ling, N. Progress in Growth Factor Res.
1992, 3, 243. (b) Shimasaki, S.; Shimonaka, M.; Zhang, H.;
Ling, N. J. Biol. Chem. 1991, 266, 10646.
3. Leong, S. R.; Baxter, R. C.; Camerato, T.; Dai, J.; Wood,
W. I. Mol. Endocr. 1992, 6, 870.
4. Liu, X.-J.; Xie, Q.; Zhu, Y.-F.; Chen, C.; Ling, N. J. Biol.
Chem. 2001, 276, 32419.
5. Chen, C.; Zhu, Y.-F.; Liu, X.-J.; Lu, Z.-X.; Xie, Q.; Ling,
N. J. Med. Chem. 2001, 44, 4001.
6. Redel, J.; Bouteville, A. Bull. Soc. Chim. Fr. 1949, 443.
7. The program Pallas from CompuDrug Ltd was used to
calculate the logP value.
Based on the successful modifications of the 3,4-di-
hydroxybenzoyl group of 1, we next initiated the investi-
gation on the possibility of replacing the catechol
moiety on the isoquinoline ring. Compound 14, where
the 6-hydroxyl group of compound 1 was replaced by
an amino group, showed promising activity (Ki=60
nM), while the methylenedioxy derivative 15 was
10-fold less active in comparison with 7a (Ki=70 nM).
Further sulfonylation of 15 resulted in an inactive com-