Discovery of novel aldose reductase inhibitors characterized by an alkoxy-substituted phenylacetic acid core

Article abstract of DOI:10.1002/ardp.200600054

In continuation of our effort aimed towards the development of novel aldose reductase inhibitors, several phenylacetic acids bearing an alkoxy substituent in position 3 or 4, respectively, were prepared and screened. The latter represent formal ring openi

Full text of DOI:10.1002/ardp.200600054

Arch. Pharm. Chem. Life Sci. 2006, 339, 559–563
D. Rakowitz et al.
559
Full Paper
Discovery of Novel Aldose Reductase Inhibitors Characterized
by an Alkoxy-Substituted Phenylacetic Acid Core
Dietmar Rakowitz, Andreas Gmeiner, and Barbara Matuszczak
Institute of Pharmacy, University of Innsbruck, Innsbruck, Austria
In continuation of our effort aimed towards the development of novel aldose reductase inhibi-
tors, several phenylacetic acids bearing an alkoxy substituent in position 3 or 4, respectively,
were prepared and screened. The latter represent formal ring opening products of the cyclohe-
xylmethyloxyphenylacetic acids IIa and IIb, recently elaborated in our group. Out of these
series, compounds 4aa and 4ba characterized by an n-heptyloxy subunit turned out to be the
most potent inhibitors. Based on these unexpected results, we suggest that such an alkyl side
chain acts as a useful surrogate for the 4-bromo-2-fluorobenzyl residue often found in potent
aldose reductase inhibitors.
Keywords: Aldose reductase / Enzyme inhibitors / Inhibitor / Substituted phenylacetic acids /
Received: March 21, 2006; accepted: May 27, 2006
DOI 10.1002/ardp.200600054
pathway (i.e. aldose reductase, EC 1.1.1.21, ALR 2) has
been recognized as one possible target for preventing the
onset or progression of long-term complications. How-
ever, so far only a very small number of aldose reductase
inhibitors (ARIs) have met the criteria of sufficient
potency, oral activity, and an acceptable side-effect pro-
file. Therefore, the area still requires further efforts, in
particular with respect to the discovery of new lead struc-
tures.
Recently, we have designed several novel types of ARIs
which, in fact, turned out to exhibit enzyme inhibition
in the micromolar range [5–8]. Out of these series, com-
pounds of type I were identified as the most potent inhi-
bitors [7]. They appear to possess an acidic function and a
lipophilic substructure R (e.g. 4-bromo-2-fluorobenzyl)
which represent essential structural requisites for an
ALR 2 inhibitory effect, in accordance with known phar-
macophoric requirements [9–13]. On these considera-
tions in a search for new ARIs, we have synthesized deriv-
atives bearing different lipophilic moieties [8]. Surpris-
ingly, within these series, we have found that a cyclohex-
ylmethyl side chain seems to be a useful surrogate for the
4-bromo-2-fluorobenzyl residue. The latter, however, can
be often found in potent aldose reductase inhibitors (e.g.
zenarestat, ponalrestat, minalrestat, and AS-3201 [14]).
Based on these results, additional modifications charac-
Introduction
Changes in human behaviour and lifestyle over the last
century have resulted in a dramatic increase in the inci-
dence of diabetes worldwide [1]. The total number of peo-
ple with diabetes is projected to rise from 171 million in
the year 2000 to 366 million in 2030 [2]. All forms of dia-
betes mellitus are characterized by chronic hyperglycae-
mia and the development of diabetes-specific microvas-
cular pathology in the retina, renal glomerulus, and peri-
pheral nerve. As a consequence, diabetes mellitus is a
leading cause of blindness, end stage renal disease, and a
variety of debilitating neuropathies [3]. These long-term
complications can be explained by several biochemical
mechanisms, mainly enhanced formation of advanced
glycation end-products, activation of protein kinase C iso-
forms, increase in oxidative stress, and activation of the
polyol pathway [4]. This causal link between elevated glu-
cose levels and the above-mentioned pathways can pro-
vide the basis for the development of new pharmacologi-
cal agents. Inhibition of the key enzyme of the polyol
Correspondence: Dietmar Rakowitz, Leopold-Franzens-Universitꢀt
Institute of Pharmacy, Innrain 52a, Innsbruck A-6020, Austria.
E-mail: dietmar.rakowitz@uibk.ac.at
–
Fax: +43 512 507-2940
i 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

560
D. Rakowitz et al.
Arch. Pharm. Chem. Life Sci. 2006, 339, 559–563
Table 1. Aldose reductase inhibition data of compounds of type
4a, 4b, I, and II.
Figure 1. Structures of compounds of type I and II.
No
R
Position
of OR
IC50 (lmM)* or Inhi-
bition
terized by formal replacement of the cyclic substructure
by a chain were envisaged. In this context, ring-opened
derivatives of compounds of type IIa and IIb (Fig. 1) bear-
ing an n-heptyl or isobutyl residue instead of a cyclohex-
ylmethyl moiety became an object of our interest. Since
structure-activity relationships have shown that deriv-
atives of 3-hydroxy- and 4-hydroxyphenylacetic acids are
in general favourable to inhibition of the enzyme com-
pared to the corresponding ortho-substituted isomers [8],
2-alkoxyphenylacetic acid derivatives were not investi-
gated.
at 100 lM (%)
4aa
4ab
4ac
4ad
4ae
4af
Ia [7]
IIa [8]
4ba
4bb
4bc
4bd
4be
4bf
n-heptyl
isobutyl
n-pentyl
n-hexyl
n-octyl
n-nonyl
4-Br-2-F-benzyl
–CH2-cyclohexyl
n-heptyl
isobutyl
n-pentyl
n-hexyl
n-octyl
n-nonyl
4-Br-2-F-benzyl
–CH2-cyclohexyl
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
21.4 (19.0–24.1)
42%
69.4 (59.5–81.0)
56.1 (47.7–66.1)
37.1 (28.5–48.4)
29.1 (17.9–47.7)
20.9 (18.0–24.3)
32.1 (23.1–44.5)
34.7 (27.2–44.2)
86.5 (75.2–99.4)
81.6 (72.8–91.6)
45.7 (31.1–67.3)
38.6 (29.8–49.8)
42.9 (40.5–45.5)
40.2 (38.3–42.1)
45.0 (44.1–45.9)
Ib [7]
IIb [8]
Results and discussion
The target compounds 4aa, 4ab, 4ba, and 4bb were pre-
pared starting from the hydroxyphenylacetates of type I
by O-alkylation in the presence of potassium carbonate in
dry N,N-dimethylformamide followed by alkaline hydro-
lysis of the ester function. Inhibitory activities were eval-
uated in a spectrometric assay with D,L-glyceraldehyde as
the substrate and NADPH as the cofactor. The biological
results (IC50 values) are given in Table 1.
* IC50 values (95% CL).
Whereas formal replacement of cyclohexylmethyl by
isobutyl leads to a marked decrease in enzyme inhibi-
tion, interestingly, both O-heptyl substituted phenylace-
tic acids (4aa: IC50: 21.4 lM, 4ba: IC50: 34.7 lM) exhibit
higher activities than the parent compounds IIa (IC50:
32.1 lM) and IIb (IC50: 45.0 lM). Therefore, these results
indicate that, within these series the aromatic subunit R
in compounds of type I was found to be replaceable even
by an n-heptyl side chain. Since to our knowledge, no
comparable findings for aldose reductase inhibitors have
been published before, further studies concerning the
influence of the length of the side chain on the activity
were performed. Thus, the corresponding derivatives
bearing an alkyl substituent formally resulting from
introduction as well as removal of one or two methylene
units were of interest. These compounds became accessi-
ble by treatment of the hydroxy compound 2a and 1b,
respectively, with the appropriate alkylating agent and
subsequent hydrolysis of the ester function (Scheme 1).
The biological data revealed that elongation of the side
Scheme 1. Synthesis routes of compounds of type 4.
(i) 1) + R-I + K2CO3 in DMF, 2) alkaline hydrolysis.
chain (i.e. 4ae, 4af, 4be, and 4bf) does not result in a sig-
nificant change in inhibitory activity. On the contrary,
shortening of the alkyl substituent (i.e. 4ac, 4ad, 4bc,
and 4bd) exhibits an effect on enzyme inhibition.
Here, reduction of each methylene group decreases the
aldose reductase inhibitory effect (factor ranging from
1.3 to 2.6).
Conclusions
In continuation of our program aimed at the develop-
ment of O-substituted hydroxyphenylacetic acids as ARIs
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Arch. Pharm. Chem. Life Sci. 2006, 339, 559–563
Table 2. Data of compounds of type 3a and 3b.
Novel Aldose Reductase Inhibitors
561
No
R9
OR
Yield
Purification
properties
IR (cm– 1
MS (EI)
)
1H-NMR
Formula (MW)
3aa
C2H5
C17H26O3 (278.39)
3-OC7H15
58%
1737
278
7.19 (d, J = 8.4 Hz, 1H, phenyl-H), 6.84-6.76 (m,
3H, phenyl-H), 4.15 (q, J = 7.2 Hz, 2H, OCH2CH3),
3.94 (t, J = 6.4 Hz, 2H, OCH2CH2), 3.57 (s, 2H,
CH2CO), 1.84–1.70 (m, 2H, CH2), 1.52–1.21 (m,
11H, 46CH2, OCH2CH3), 0.89 (t, J = 6.4 Hz, 3H,
CH3)
cc with CH2Cl2/lp
(1/1) colourless oil
3ad
C2H5
C16H24O3 (264.37)
3-OC6H13
4-OC7H15
59%
1736
264
7.19 (d, J =8.4 Hz, 1H, phenyl-H), 6.88–6.81 (m,
3H, phenyl-H), 4.15 (q, J = 7.1 Hz, 2H, OCH2CH3),
3.94 (t, J = 6.6 Hz, 2H, OCH2CH2), 3.57 (s, 2H,
CH2CO), 1.83–1.70 (m, 2H, CH2), 1.52–1.21 (m,
9H, 36CH2, OCH2CH3), 0.90 (t, J = 6.6 Hz, 3H,
CH3)
7.21–7.14 (m, 2H, phenyl-H), 6.88–6.81 (m,
2H, phenyl-H), 3.93 (t, J = 6.4 Hz, 2H, OCH2),
3.68 (s, 3H, OCH3), 3.55 (s, 2H, CH2CO), 1.83–
1.70 (m, 2H, CH2), 1.52–1.26 (m, 8H, 4 x CH2),
0.89 (t, J =6.4 Hz, 3H, CH3)
7.21–7.14 (m, 2H, phenyl-H), 6.88–6.81 (m,
2H, phenyl-H), 3.93 (t, J = 6.6 Hz, 2H, OCH2),
3.68 (s, 3H, OCH3), 3.55 (s, 2H, CH2CO), 1.83–
1.69 (m, 2H, CH2), 1.52-1.26 (m, 6H, 36CH2),
0.90 (t, J = 6.6 Hz, 3H, CH3)
cc with CH2Cl2/lp
(1/1) colourless oil
3ba
3bd
CH3
CH3
61%
1740
264
C16H24O3 (264.37)
cc with CH2Cl2/lp
(9/1) colourless oil
4-OC6H13
63%
1741
250
C15H22O3 (250.34)
cc with CH2Cl2/lp (9/1)
colourless oil
we have recently reported that formal replacement of
the (substituted) benzyl moiety by cyclohexylmethyl
resulted in active compounds IIa and IIb [8]. In view of
this, we have now investigated whether a cyclic moiety is
an essential substructure for enzyme inhibition. Interest-
ingly, derivatives bearing the n-heptyl side chain which
represents a form of the ring opened analogue of cyclo-
hexylmethyl exhibit even higher activities (4aa:
IC50 = 21.4 lM, 4ba: IC50 = 34.7 lM). Moreover, it should
be noted that the inhibitory effect of these compounds is
comparable with those of the 4-bromo-2-fluoroben-
Experimental
Chemistry
Melting points were determined with a Kofler hot-stage micro-
scope (C. Reichert, Vienna, Austria) and are uncorrected. Infra-
red spectra (KBr pellets) were recorded on a Mattson Galaxy Ser-
ies FTIR 3000 spectrophotometer (Mattson Instruments, Inc.,
Madison, WI, USA). Mass spectra were obtained on a Finnigan
MAT SSQ 7000 spectrometer (EI, 70 eV or CI, 200 eV, reactant
gas: methane; Thermo Electron Corporation, Bremen, Ger-
many). The 1H-NMR spectra were recorded in CDCl3 solution in
5 mm tubes at 308C on a Varian Gemini 200 spectrometer
(199.98 MHz; Varian Inc., Palo Alto, CA, USA) with the deuterium
signal of the solvent as the lock and TMS as internal standard.
Chemical shifts are expressed in parts per million. Reactions
were monitored by TLC using Polygramm SIL G/UV254 (Macherey-
Nagel) plastic-backed plates (0.25 mm layer thickness). The yields
given are not optimized. Light petroleum refers to the fraction
of b.p. 40–608C. Elemental analyses were performed by Mag. J.
Theiner, Mikroanalytisches Laboratorium, Faculty of Chemistry,
University of Vienna, Austria and the data for C and H are within
l0.4% of the calculated values.
zylated congeners Ia (IC50 = 20.9 lM) and Ib (IC50
=
40.2 lM). To our knowledge, this structural modification
is unique in the class of ARIs. Therefore, we intend to
study the utility of an n-alkyl as a bioisoster for the 4-
bromo-2-fluorobenzyl moiety in highly potent ARIs like
zenarestat.
The authors wish to acknowledge Schlachthof Salzburg-Berg-
heim (Austria) (KR Ing. Sebastian Grießner and Mag. Verena
Klob) for providing calf lenses.
The following compounds are already known but not tested as
aldose reductase inhibitors: (4-heptyloxyphenyl)acetic acid 4ba
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562
D. Rakowitz et al.
Arch. Pharm. Chem. Life Sci. 2006, 339, 559–563
Table 3. Data of compounds of type 4a and 4b.
No
OR
Yield
Purification
properties
IR (cm– 1
MS (CI)
)
1H-NMR
Formula (MW)
4aa
3-OC7H15
C15H22O3 (250.34)
88%
1694
251
7.25–7.18 (m, 1H, phenyl-H), 6.86–6.78 (m,
3H, phenyl-H), 3.93 (t, J = 6.6 Hz, 2H, OCH2),
3.61 (s, 2H, CH2CO), 1.84–1.70 (m, 2H, CH2),
1.47-1.30 (m, 8H, 46CH2), 0.89 (t, J = 6.6 Hz,
3H, CH3)
recryst. from dipe/lp
colourless crystals
mp 82–858C
4ab
4ac
3-OCH2CH(CH3)2
C12H16O3 (208.26)
96%
1709
209
7.27–7.19 (m, 1H, phenyl-H), 6.87–6.79 (m,
3H, phenyl-H), 3.71 (d, J = 6.6 Hz, 2H, OCH2),
3.62 (s, 2H, CH2CO), 2.17–1.97 (m, 1H, CH),
1.02 (d, J = 7.0 Hz, 6H, 26CH3)
7.27–7.18 (m, 1H, phenyl-H), 6.86–6.78 (m,
3H, phenyl-H), 3.94 (t, J = 6.4 Hz, 2H, OCH2),
3.61 (s, 2H, CH2CO), 1.85–1.71 (m, 2H, CH2),
1.52–1.28 (m, 4H, 26CH2), 0.93 (t, J = 7.2 Hz,
3H, CH3)
recryst. from dipe/lp
colourless crystals
mp 41 –448C
86%
recryst. from lp
colourless crystals
mp 82–858C
3-OC5H11
C13H18O3 (222.29)
1699
223
4ad
4ae
4af
3-OC6H13
C14H20O3 (236.31)
71%
1698
237
9.74 (br s, 1H, COOH), 7.23–7.18 (m, 1H, phe-
nyl-H), 6.86–6.79 (m, 3H, phenyl-H), 3.94 (t, J =
6.6 Hz, 2H, OCH2), 3.61 (s, 2H, CH2CO), 1.84–
1.70 (m, 2H, CH2), 1.49–1.29 (m, 6H, 36CH2),
0.90 (t, J = 6.6 Hz, 3H, CH3)
7.27–7.19 (m, 1H, phenyl-H), 6.86–6.78 (m,
3H, phenyl-H), 3.94 (t, J = 6.6 Hz, 2H, OCH2),
3.61 (s, 2H, CH2CO), 1.84–1.70 (m, 2H, CH2),
1.48–1.24 (m, 10H, 56CH2), 0.89 (t, J = 6.6 Hz,
3H, CH3)
7.26–7.18 (m, 1H, phenyl-H), 6.86–6.78 (m,
3H, phenyl-H), 3.94 (t, J = 6.6 Hz, 2H, OCH2),
3.61 (s, 2H, CH2CO), 1.83–1.70 (m, 2H, CH2),
1.48–1.28 (m, 12H, 66CH2), 0.88 (t, J = 6.4 Hz,
3H, CH3)
recryst. from dipe/lp
colourless crystals
mp 80–838C
3-OC8H17
C16H24O3 (264.37)
80%
1698
265
recryst. from lp
colourless crystals
mp 83–848C
3-OC9H19
C17H26O3 (278.39)
82%
1698
279
recryst. from lp
colourless crystals
mp 83–858C
4ba
4-OC7H15
C15H22O3 (250.34)
76%
1701
251
9.74 (brs, 1H, COOH), 7.18 (d, J = 8.6 Hz, 2H,
phenyl-H), 6.85 (d, J = 8.6 Hz, 2H, phenyl-H),
3.93 (t, J = 6.6 Hz, 2H, OCH2), 3.57 (s, 2H,
CH2CO), 1.83–1.70 (m, 2H, CH2), 1.46–1.30 (m,
8H, 46CH2), 0.89 (t, J = 6.6 Hz, 3H, CH3)
7.18 (d, J = 8.6 Hz, 2H, phenyl-H), 6.85 (d, J = 8.6
Hz, 2H, phenyl-H), 3.70 (d, J = 6.2 Hz, 2H,
OCH2), 3.58 (s, 2H, CH2CO), 2.17–1.97 (m, 1H,
CH), 1.01 (d, J = 6.6 Hz, 6H, 26 CH3)
7.21–7.14 (m, 2H, phenyl-H), 6.89–6.81 (m,
2H, phenyl-H), 3.93 (t, J = 6.6 Hz, 2H, OCH2),
3.58 (s, 2H, CH2CO), 1.84–1.70 (m, 2H, CH2),
1.52-1.27 (m, 4H, 26CH2), 0.93 (t, J = 7.2 Hz,
3H, CH3)
recryst. from dip/lp
colourless crystals
mp 82–848C
4bb
4bc
4-OCH2CH(CH3)2
C12H16O3 (208.26)
82%
1709
209
recryst. from dipe/lp
colourless crystals
mp 79–848C
73%
recryst. from lp
colourless crystals
mp 79–818C
4-OC5H11
C13H18O3 (222.29)
1696
223
4be
4bf
4-OC8H17
C16H24O3 (264.37)
69%
1698
265
7.21–7.14 (m, 2H, phenyl-H), 6.89–6.81 (m,
2H, phenyl-H), 3.93 (t, J = 6.6 Hz, 2H, OCH2),
3.58 (s, 2H, CH2CO), 1.83–1.69 (m, 2H, CH2),
1.48–1.29 (m, 10H, 56CH2), 0.89 (t, J = 6.4 Hz,
3H, CH3)
7.21–7.14 (m, 2H, phenyl-H), 6.89–6.81 (m,
2H, phenyl-H), 3.93 (t, J = 6.4 Hz, 2H, OCH2),
3.58 (s, 2H, CH2CO), 1.83–1.69 (m, 2H, CH2),
1.47-1.28 (m, 12H, 66CH2), 0.89 (t, J = 6.4 Hz,
3H, CH3)
recryst. from lp
colourless crystals
mp 80–828C
4-OC9H19
C17H26O3 (278.39)
60%
1701
279
recryst. from lp
colourless crystals
mp 84–868C
lp = light petroleum; dipe = diisopropyl ether.
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Arch. Pharm. Chem. Life Sci. 2006, 339, 559–563
Novel Aldose Reductase Inhibitors
563
[15, 16], (4-pentyloxyphenyl)acetic acid 4bc [15, 17], (4-hexyloxy-
phenyl)acetic acid 4bd [15], (4-octyloxyphenyl)acetic acid 4be
[15, 16], (4-iso-butoxyphenyl)acetic acid 4bb [17], (4-nonyloxyphe-
nyl)acetic acid 4bf [16]. However, with the exception of 4bd [18]
no spectroscopic data are given.
the oxidation of NADPH catalyzed by ALR 2. The assay was per-
formed at 378C in a reaction mixture containing 0.25 M potas-
sium phosphate buffer pH 6.8, 0.38 M ammonium sulfate,
0.11 mM NADPH, and 4.7 mM D,L-glyceraldehyde as substrate in
a final volume of 1.5 mL. All inhibitors were dissolved in DMSO.
The final concentration of DMSO in the reaction mixture was
1%. To correct for the nonenzymatic oxidation of NADPH, the
rate of NADPH oxidation in the presence of all the components
except the substrate was subtracted from each experimental
rate. Each dose-effect curve was generated using at least three
concentrations of inhibitor causing an inhibition between 20
and 80%. Each concentration was tested in duplicate and IC50
values as well as the 95% confidence limits (95% CL) were
obtained by using CalcuSyn software [20] for dose effect analysis.
General procedure for O-alkylation of 1a and 1b
Powdered potassium carbonate (4 equiv.) was added to a solution
of the appropriate hydroxy compound 1a or 1b (2 equiv.,
5.6 mmol) in 10 mL of dry N,N-dimethylformamide under atmo-
sphere of nitrogen. After stirring for 30 min at room tempera-
ture, one equivalent of the appropriate alkyliodide was added
and the mixture was stirred until TLC indicated no further con-
version (room temperature to 508C). Then, the mixture was
poured into cold water, acidified with 2N HCl and the product
was extracted exhaustively with diethyl ether or dichloro-
methane, respectively. The organic layer was washed succes-
sively with 2N NaOH, water and brine, dried over anhydrous
sodium sulfate, and evaporated to dryness. The residue thus
obtained was purified to give 3aa, 3ad, 3ba, and 3bd (Table 2) or
directly used for further conversion (3ab and 3bb).
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alkyloxyphenylacetates which were directly used for further
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NADPH, D,L-glyceraldehyde and dithiothreitol (DTT) were pur-
chased from Sigma Chemical Co. DEAE-cellulose (DE-52) was
obtained from Whatman. Sorbinil was a gift from Prof. Dr. Luca
Costantino, University of Modena (Italy) and was used as stan-
dard [IC50 = 0.9 (l0.3) lM]. All other chemicals were commercial
samples of good grade. Calf lenses for the isolation of ALR 2 were
obtained locally from freshly slaughtered animals. The enzyme
was purified by a chromatographic procedure as previously
described [19]. Briefly, ALR 2 was released by carving the capsule
and the frozen lenses were suspended in potassium phosphate
buffer pH 7 containing 5 mM DTT and stirred in an ice-cold bath
for 2 h. The suspension was centrifuged at 4000 rpm at 48C for
30 min and the supernatant was subjected to ion exchange chro-
matography on DE-52. Enzyme activity was assayed spectropho-
tometrically on a Cecil Super Aurius CE 3041 spectrophotometer
(Cecil Instruments, Cambridge, England) by measuring the
decrease in absorption of NADPH at 340 nm which accompanies
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