Discovery of 3-hydroxy-4-carboxyalkylamidino-5-arylamino-isothiazoles as potent MEK1 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2006.05.019

3-Hydroxy-4-carboxyalkylamidino-5-arylamino-isothiazoles were discovered as potent in vitro MEK1 inhibitors.

Full text of DOI:10.1016/j.bmcl.2006.05.019

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3975–3980
Discovery of 3-hydroxy-4-carboxyalkylamidino-5-arylamino-
isothiazoles as potent MEK1 inhibitors
Chamakura V. N. S. Varaprasad,* Dinesh Barawkar, Hassan El Abdellaoui,
Subrata Chakravarty, Matthew Allan, Huanming Chen, Weijian Zhang,
Jim Z. Wu, Robert Tam, Robert Hamatake, Stanley Lang and Zhi Hong
Drug Discovery, Valeant Pharmaceutical Research and Development, 3300 Hyland Avenue, Costa Mesa, CA 92626, USA
Received 8 April 2006; revised 4 May 2006; accepted 4 May 2006
Available online 24 May 2006
Abstract—3-Hydroxy-4-carboxyalkylamidino-5-arylamino-isothiazoles were discovered as potent in vitro MEK1 inhibitors.
Ó 2006 Elsevier Ltd. All rights reserved.
Chemotherapy of cancer by traditional cytotoxic agents
has limited efficacy and lacks specificity. This has led to
the development of therapies aimed at aberrant molecu-
lar pathways of tumor growth in the hope of greater effi-
cacy and fewer side effects. Recent studies validated
protein kinases as key regulators of all pathways of can-
cer and many kinase inhibitors are now being developed.
Imatinib,1 a tyrosine kinase inhibitor, is the first molec-
ular targeted agent to be approved in US and other
countries for the treatment of chronic myelogenous leu-
kemia (CML). Other drugs, recently approved, for the
treatment of cancer based on target approach include,
Gefitinib,2a Erlotinib,2b Trastuzumab,2c Cetuximab,2d
and Bevacizumab.2e While drugs such as Sorafenib, Su-
tent, Zactima, and AG-013736, in clinical development
for different kinds of cancers, rely on intervention of
multiple target pathways.3 The aforementioned drugs
have proven that therapies targeting specific enzymes
in signal transduction pathways are successful.
Further it was found overexpression and activation of
these enzymes were associated with various human can-
cers including kidney, breast, colon, and oral carcino-
mas, leukemia, and glial neoplasmas.7 MEK1/2 are the
only enzymes known to activate ERK1/2 and it is
believed their inhibition might be useful in controlling
the growth of cancer cells. At present, several pharma-
ceutical companies are pursuing intervention of this
pathway in the treatment of various cancers. PD-
0325901 and ARRY-142886, the lead compounds in
clinical development, represent such examples wherein
MEK8 is inhibited to suppress the tumor growth.
During a high-throughput screening of Valeant library
of compounds several 4-cyano-3-hydroxy-5-arylamino-
isothiazoles, such as 1 (IC50: 7 lM), were found as lead
in vitro inhibitors of MEK.1 Isothiazole9 scaffold is pres-
ent in a wide range of compounds possessing a wide
range of activities. Fused isothiazole moiety has been
present in drugs such as antipsychotics Geodon10 and
Perispirone11 (Sumitomo Chemical, Japan). Further,
many isothiazole derivatives show antiviral,12 histone
acetyltransferase inhibitor,13 cholinergic channel activa-
Mitogen activated protein kinase (MEK4), a dual speci-
ficity kinase, has central role in extracellular signal reg-
ulated kinase (ERK) pathway in regulating cell growth
and survival, differentiation, and angiogenesis. MEK
phosphorylates threonine and tyrosine (T183 and
Y185) residues of its substrates ERK 1 and 2 (MAP
kinase) resulting in their activation.5 MEK and ERK
are frequently dysregulated in human cancers.6
H
N
S
N
NC
OH
Keywords: Isothiazoles; 3-Hydroxy-isothiazoles; Cancer; MEK1;
MAPK; Protein kinases.
1, IC50: 7 µM
*
Figure 1.
Corresponding author. E-mail: vchamakura@valeant.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.05.019

3976
C. V. N. S. Varaprasad et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3975–3980
tor,14 and herbicide and fungicide15 activities. In light of
few studies involving isothiazoles as kinase inhibitors,16
we initiated SAR to find novel and potent MEK inhib-
itors. In this communication, we present preliminary re-
sults of this study.
The SAR studies of lead isothiazole 1 (Fig. 1) to im-
prove the potency involved modifications at four posi-
tions, viz. 3-hydroxy group, linker between aryl group
and isothiazole moiety, 5-arylamino, and 4-cyano
groups. The required 3-hydroxy-4-nitrile-5-arylamino-
isothiazoles17 (D) have been synthesized from the
corresponding isothiocyanates (B) which were either
commercially available or readily prepared from the
respective amines (A) (Scheme 1). Isothiocyanates (B)
were treated with cyanoacetamide in presence of
powdered KOH in DMF to give substituted cyanoac-
etamides (C) which were cyclised by Br2 to give the
target 5-arylamino-isothiazole derivatives (D). Howev-
er, some 5-alkyl or cycloakylamino-isothiazoles,
prepared in the course of SAR studies, were obtained
by the reaction of corresponding amines with sulfone
4 (Scheme 2) which was synthesized from malononitri-
le following the literature procedure.18 Thus, smalono-
nitrile was treated with CS2 in presence of NaOH to
obtain di-sodium salt 2 which was subjected to oxida-
tive cyclization followed by its methylation leading to
sulfide 3. The sulfide 3 was then oxidized to desired
sulfone 4.
CN
(b)
(a)
H
R NCS
R NH2
N
NH2
R
A
B
S
O
C
(c)
NHR'
OH
NC
OH
HN
(d)
N
N
R N
H
R N
H
S
S
D
E
R = Alkyl or Aryl; R' = Alkyl or hydroxy-alkyl
Scheme 1. Reagents and conditions: (a) CSCl2, aq K2CO3, CHCl3, rt,
2 h; (b) cyanoacetamide, KOH, DMF, rt, 16 h; (c) Br2, EtOAc, rt,
1–2 h; (d) R0NH2, EtOH, 90 °C, 16 h.
O
H3C
O
H3C
S
S
CN
OH
CN
OH
CN
CN
(a)
NC CN
(c)
(b)
S
S
N
N
NaS SNa
2
With regard to SAR of 1 we first examined the
relevance of 3-hydroxy group. Accordingly, 3-meth-
oxy-4-cyano-5-phenylamino isothiazole 5 and 4-cya-
4
3
Scheme 2. Reagents: (a) CS2, NaOH, EtOH; (b) i—H2O2, MeOH; ii—
MeI, MeOH; (c) H2O2, Ac2O, AcOH.
no-5-phenylamino-N-methyl-3-isothiazolone
6
were
synthesized from 5-phenylamino-4-cyano-3-hydroxy-iso-
thiazole 1 (Scheme 3). Both modifications (5 and 6)
resulted in reducing the activity.19 Further replacement
of hydroxyl with an halogen or a proton also resulted
in the loss of activity (data not shown). These results
indicate that a free hydroxyl at position 3 is essential
for activity. The preferred tautomer of hydroxyl group
is expected to be determined by X-ray crystallographic
studies of MEK1 enzyme in presence of inhibitor.
(a)
NH
S
CN
O
NH
S
NH
S
CN
OH
CN
O
+
N
N
N
CH3
CH3
6
1
5
IC50 = > 5.00 μM
IC50 = > 5.00 μM
Scheme 3. Reagents and conditions: (a) NaH, MeI, DMF, rt, 4 h.
O
O
H3C
O
S
CN
(b)
(a)
O
CN
OH
S
CN
OH
S
OH
S
N
S
N
N
8
4
7
IC50 = > 5.00 μM
IC50 = > 5.00 μM
(c)
N
CN
OH
S
N
9
IC50 = > 5.00 μM
Scheme 4. Reagents and conditions: (a) 30% aq NaOH, PhOPhOH, 80 °C, 16 h; (b) 30% aq NaOH, p-iPr–PhSH, 80 °C, 16 h; (c) piperidine, EtOH,
80 °C, 16 h.

C. V. N. S. Varaprasad et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3975–3980
3977
Table 1. SAR of 5-alkyl/arylamino-4-cyano-3-hydroxy-isothiazole
These results would be discussed in future
communications.
H
N
S
R
N
Next, we focused our attention on the linker of iso-
thiazole core to aryl group at position 5. To this extent,
we prepared the corresponding phenoxy 7 and phenyl
sulfide 8 derivatives as shown in Scheme 4 from sulfone
4. These modifications resulted in further loss of
activity.
NC
OH
Compound
R
IC50 (lM)
10
>5.00
>5.00
>5.00
>5.00
H3C
The intermediates sulfide 3 and sulfone 4 used in syn-
theses of the above modifications were also inactive.
Additionally, 5-cycloalkylamino-isothiazole 9, repre-
senting N,N-substituted amino group at 5-position,
also showed loss of activity. The loss of activity from
all of the above modifications indicated that a hydro-
gen bond donor, such as an amino group (NH), is also
essential to retain the activity apart from 3-hydroxyl
group (vide supra).
H3C
H3C
11
12
13
O
CH3
N
O
14
>5.00
In order to explore the suitable substitution on amino
group at 5 position of isothiazole 1, a variety of alkyl
and arylamino groups (Table 1) were examined. In
general, alkyl groups (10–13) resulted in increased
IC50 values indicating loss of activity. Notably, ali-
phatic rings (14) or aromatic groups (15–18) attached
to 5-amino-isothiazole core by a spacer (1–2 carbons)
also lost activity. This result suggests that inclusion of
a spacer (1–2 carbons) between 5-alkyl or aromatic-
amino group and isothiazole core is not tenable. Het-
erocycles on 5-amino group (19) did not improve the
potency either.
15
16
17
>5.00
>5.00
>5.00
S
HO
CH3
18
>5.00
19
20
21
22
23
>5.00
>5.00
>5.00
>5.00
>5.00
After establishing the essential requirements, viz. pres-
ence of 3-hydroxy group, a free NH group at position
5 and exclusion of a spacer between aryl and amino
groups at this position; to retain the activity, we pre-
pared several substituted 5-arylamino derivatives
(Table 1). A variety of groups were examined to
improve potency further involving substitutions such
as methyl (20, o, p, or m –Me), electron donating
(21: o, p, or m –OCH3) and withdrawing groups (22:
o, p, or m –NO2, 23–25) on the aryl group. None of
the single modifications had improved activity except
for halogen (26–27) derivatives. However, introduction
of two halogen substitutions (28–31) on phenyl ring
has substantially improved (28-fold over lead 1) activ-
ity as in the case of 31.
N
H3C
MeO
O2N
CF3
24
>5.00
NC
Meanwhile 4-nitrile group on isothiazole was not affect-
ed in any of the above modifications thus far. Hence, we
wanted to explore whether its modifications would fur-
ther improve the activity. We prepared the correspond-
ing amide, hydrazide, hydroxyamidine, tetrazole, and
amidine (32–36, Table 2) derivatives following standard
literature procedures involving nitriles. Amongst these
modifications only N-alkylcarboxyamidines (E, Scheme
1; 36, Table 2) had retained the potency gained previous-
ly (26–31, Table 1).
25
26
27
>5.00
0.50
CO2CH3
Br
0.41
Cl
Cl
28
0.80
This prompted us to find whether the activity of carbo-
xyalkylamidines can be improved further by modifying
5-phenylamino group. Hence, a variety of cyclohexylm-
Cl
(continued on next page)

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C. V. N. S. Varaprasad et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3975–3980
Table 3. SAR of 5-arylamino-4-carboxyalkylamidino-3-hydroxy-
isothiazole
Table 1 (continued)
Compound
R
IC50 (lM)
N
OH
NH
S
29
30
31
0.65
Cl
Cl
R
NH
CF3
HN
0.62
0.25
Cl
Compound
R
IC50 (lM)
37
1.20
Cl
Cl
H3C
38
39
1.44
0.42
OMe
Table 2. SAR of 5-arylamino-4-cyano-3-hydroxy-isothiazole
Compound
IC50 (lM)
Cl
O
O
N
N
O
N
NH
S
NH
32
>2.00
>2.00
>2.00
>2.00
0.74
40
41
42
0.50
0.43
0.50
OH
N
Cl
Br
H
N
NH
S
33
34
35
36
OH
N
Br
N
HN
NH
H
N OH
43
44
1.23
0.35
S
OH
N
MeO2C
HN
NH
NH N
OH
S
N
45
46
47
48
1.45
1.39
0.67
0.69
F
HN
NH
NH
OH
Cl
NO2
S
N
O2N
OMe
CH3
ethylene amidines20 were prepared with various substi-
tutions on 5-arylamino group. However, as in the case
of 4-nitrile SAR studies (26–31, Table 1), only mono-
and di-halogen derivatives (39–50) had displayed low
micromolar inhibition concentrations except for p-ester
derivative (44) unlike its related nitrile (25, Table 1
and Table 3).
H3C
Cl
49
50
0.79
0.41
Cl
Cl
Cl
Among other substitutions studied on amidines small al-
kyl groups and alkyl groups with hydrophilic groups
(OH) are favored and had improved the activity sub-
stantially. Further, it was also found that ortho substitu-
tion on phenyl ring has improved the potency (51 vs 53;
52 vs 54, Table 4) by 8- to 20-fold. ortho Substitution on
5-phenylamino group coupled with hydroxyalkyl-carb-
oxyamidino substitution has further enhanced potency
by 6-fold (59 vs 60, Table 4).
no-5-arylamino-isothiazoles are potent in vitro MEK1
inhibitors. The present systematic structure–activity
relationship studies of 5-phenylamino-4-cyano-3-hy-
droxy-isothiazole led to more than 180-fold (1 vs 60)
improvement in vitro inhibition of MEK1. Future
reports will examine other modifications of 5-arylami-
no-4-carboxyalkylamidino-3-hydroxy-isothiazole scaf-
fold and its cellular assays and bioavailability studies.
In summary, we designed and synthesized and have
shown that a variety of 3-hydroxy-4-carboxyalkylamidi-

C. V. N. S. Varaprasad et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3975–3980
3979
Table 4. SAR of 5-arylamino-4-carboxyalkylamidino-3-hydroxy-iso-
thiazole
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S
H
N
N
R
H
HN
R'
IC50 Compound R0 = OH
Compound R0 = H
R
IC50
(lM)
(lM)
R
51
0.51 52
1.44
I
I
CH3
CH3
Cl
53
0.062 54
0.042 56
0.068 58
0.073
0.044
0.086
I
I
I
Cl
55
I
Cl
Cl
57
59
F3C
Cl
F3C
Cl
Cl
Cl
0.22 60
0.038
Acknowledgment
11. (a) Shiwa, T.; Amano, T.; Matsubayashi, H.; Seki, T.;
Sasa, M.; Sakai, N. Jpn. J. Pharmacol. Sci. 2003, 93, 114;
(b) Matsushita, M.; Egashira, N.; Harada, S.; Okuno, R.;
Mishima, K.; Iwasaki, K.; Nishimura, R.; Fujiwara, M.
Jpn. J. Pharmacol. Sci. 2005, 99, 154.
The authors thank colleagues from HTS and analytical
groups.
´
`
12. (a) Cutrı, C. C. C.; Garozzo, A.; Siracusa, M. A.; Sarva,
M. C.; Castro, A.; Geremia, E.; Pinizzotto, M. R.;
Guerrera, F. Bioorg. Med. Chem. Lett. 1999, 7, 225; (b)
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2006.05.019.
´
`
Cutrı, C. C. C.; Garozzo, A.; Siracusa, M. A.; Sarva, M.
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19. For IC50 assay procedure and conditions, please see the
supporting data.
20. 1H and ES-MS data of select compounds are given in the
supporting data.