Identification of acidic heterocycle-substituted 1H-pyrazolo[3,4-b] pyridines as soluble guanylate cyclase stimulators

Article abstract of DOI:10.1016/j.bmcl.2013.01.028

Novel guanylate cyclase stimulators are disclosed. Design, synthesis, SAR, and pharmacological profile of the compounds are discussed.

Full text of DOI:10.1016/j.bmcl.2013.01.028

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1197–1200
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Identification of acidic heterocycle-substituted 1H-pyrazolo[3,4-b]
pyridines as soluble guanylate cyclase stimulators
a
a
a
a
Nils Griebenow a, , Hartmut Schirok , Joachim Mittendorf , Alexander Straub , Markus Follmann ,
⇑
Johannes-Peter Stasch b, Andreas Knorr b, Karl-Heinz Schlemmer c, Gorden Redlich c
a Bayer Pharma AG, Global Drug Discovery, Medicinal Chemistry Wuppertal, D-42096 Wuppertal, Germany
b Bayer Pharma AG, Global Drug Discovery, Cardiology Research, D-42096 Wuppertal, Germany
c Bayer Pharma AG, Global Drug Discovery, Research Pharmacokinetics, D-42096 Wuppertal, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel guanylate cyclase stimulators are disclosed. Design, synthesis, SAR, and pharmacological profile of
the compounds are discussed.
Received 10 December 2012
Revised 4 January 2013
Accepted 7 January 2013
Available online 16 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Guanylate cyclase
Stimulator
Acidic heterocycles
Carboxylic acid isosteres
SAR
Soluble guanylate cyclase (sGC) is the only proven receptor for
the ubiquitous biological messenger nitric oxide (NO). Stimulation
of the enzyme by NO facilitates the conversion of guanosine-50-tri-
phosphate (GTP) to the intracellular second messenger cyclic
guanosine-30,50-monophosphate (cGMP), which regulates various
cGMP-specific effector systems such as PDEs, ion channels, and
protein kinases.1 Thus, the NO/cGMP pathway is important in
many physiological processes including vasodilatation, neuro-
transmission, and platelet aggregation.2 Since the emergence of
sGC as a therapeutic target for cardiovascular and pulmonary
disease, two classes of molecules have been developed: NO-inde-
pendent but heme-dependent sGC stimulators and NO- and
heme-independent sGC activators.1,2 The first reported direct
stimulator of sGC is YC-1,3 followed by the discovery of more
potent compounds such as BAY 41-8543 and the advanced clinical
candidate riociguat (BAY 63-2521) by Bayer4 (Figure 1).5 Riociguat
is currently in phase III clinical trials for the treatment of chronic
thromboembolic pulmonary hypertension (CTEPH) and pulmonary
arterial hypertension (PAH).6 These compounds have a dual mode
of action: they sensitize sGC to the body’s own NO and can also in-
crease sGC activity in the absence of NO, causing vasorelaxation,
anti-proliferation and anti-fibrotic effects. It is postulated that they
bind to an allosteric binding site within the catalytic domain and
stabilize the nitrosyl-haem complex to keep sGC in its active
conformation.7
To further improve physicochemical and pharmacokinetic prop-
erties, we envisaged to synthesize weakly acidic compounds instead
of weakly basic aminopyrimidine congeners. A similar approach was
published recently by Roberts et al. (Pfizer, example 25), yielding
acidic triazoles.8 As a starting point we reinvestigated the acidic
tetrazole congener 1 (Table 1), which has been reported earlier.4
Tetrazole 1 was found to have an excellent physicochemical and
DMPK profile (Table 2), but its potency was insufficient. Thus, we
synthesized a series of other heterocyclic carboxylic acid isosteres,9
with the aim to improve potency (Table 1), while maintaining the
favorable physico chemical and pharmacokinetic profile.
Compounds 2, 3, and 4 were synthesized from the intermediate
13, which was built up in four linear steps (Scheme 1) in analogy to
our work on 7-azaindoles.11 2-Fluoropyridine (10) was converted
to the pyrazolopyridine 11 in two steps.12 Upon lithiation at the
3-position it reacted with ethyl trifluoroacetate to give the corre-
sponding trifluoroketone. Subsequent reaction with hydrazine hy-
drate provided 11. The trifluoromethyl group in pyrazolopyridine
11 was anionically activated and underwent aminolysis via a
methine intermediate and thus furnished nitrile 12.13 Regioselec-
tive benzylation in the presence of cesium carbonate gave interme-
diate 13. The nitrile 13 was elaborated further to hydroxyamidine
14. Treating 14 with 1,10-thiocarbonyldiimidazole (TCDI) under
either basic (DBU)14 or acidic (boron trifluoride etherate)15
⇑
Corresponding author. Tel.: +49 202 36 5866.
E-mail address: nils.griebenow@bayer.com (N. Griebenow).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.01.028

1198
N. Griebenow et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1197–1200
F
F
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NH
N
N
NH2
O
N
NH2
O
H2N
OH
H2N
F3C
O
O
Pfizer
example 25
YC-1
BAY 41-8543
BAY 63-2521
Figure 1. Chemical structure of some sGC stimulators.
Table 1
SAR of several heterocyclic substituents
F
N
N
N
Het
Solubilityb (mg/L)
Fmaxc (%)
cLogDd
a
Compd
Het
Rabbit aorta IC50
(l
M)
*
N
N
nde
76
1.78
H2N
NH2
BAY 41-8543
0.10
N
O
*
1
2
8.60
1.10
150
280
100
69
0.96
0.90
HN
N
N
N
*
*
*
HN
S
N
O
HN
O
N
S
3
4
5
6
7
8
0.52
1.26
0.61
3.10
0.87
0.73
11
320
10
16
4
90
98
59f
59
74
69
0.95
0.94
0.97
1.77
1.96
2.21
HN
O
N
O
*
O
N
N
H
O
*
*
*
HN
O
N
N
H
HN
O
N
N
HN
O
nde
N
N

N. Griebenow et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1197–1200
1199
Table 1 (continued)
a
Compd
Het
Rabbit aorta IC50
0.61
(
lM)
Solubilityb (mg/L)
Fmaxc (%)
91
cLogDd
*
HN
O
N
N
9
nde
2.56
F
F
F
a
Values are means of three experiments. Relaxing effect on pre-contracted rabbit aortic rings.10
Pseudo-thermodynamic solubility assay (at pH 6.5).
Bioavailability determined from incubation in rat liver hepatocytes.
Calculated logD (at pH 7.5).
Solubility was not measurable.
Bioavailability determined from incubation in rat liver microsomes.
b
c
d
e
f
Table 2
F
F
Pharmacokinetics in Wistar ratsa
AUCnorm (kg h/L) CL (L/h/kg) T1/2 (h) MRTe (h) Ff (%)
b
Compound
N
N
4.5d
0.28c
0.7d
1.2
2.4
1.4
—
—
1.9
25
94
75
N
N
BAY 41-8543 0.32
a
b
N
O
N
1
4
3.63
2.79
5
H
N
a
Mean values derived by intravenous (bolus) and oral (gavage) administration of
0.3 mg/kg in EtOH/PEG400/H2O.
16
O
O
15
c
NH2
b
Calculated from concentration/time–curve after intravenous administration.
Total plasma clearance.
Total blood clearance.
Mean residence time.
c
F
d
F
e
f
Oral bioavailability.
N
N
N
N
d
e
N
N
F
H
N
H
N
N
N
H
N
N
F
N
N
a,b
c
N
O
O
N
N
DMB
N
H
NH
17
CF3
N
DMB
H
18
10
11
12
O
F
F
F
d
e
N
N
N
N
N
N
N
N
f
N
N
N
6
7
8
R = -H
R = -Me
R = -Et
N
13
14
N
N
N
H2N
HN
N
OH
DMB
N
N
N
R
R
R = -CH2CF3
9
19
O
O
Scheme 2. Reagents and conditions: (a) Hydrazine hydrate, MeOH, THF, 65 °C, 4 h;
(b) 1,10-carbonyldiimidazole, THF, rf, 1.5 h (49% over two steps); (c) 2,4-dime-
thoxybenzyl isocyanate (DMB–NCO), DCM, rt, 16 h (84%); (d) 2% aq NaOH, rf, 16 h
(36%); (e) MeI or EtI or 2,2,2-trifluoroethyl trichloromethanesulfonate, Cs2CO3,
DMF, rt or 60 °C, 16 h or 3 h, in case of R@H the step was not performed; (f) TsOH,
toluene, rf, 16 h.
f
g,h
i,,j
2
3
4
Scheme 1. Synthesis of 2,
trifluoroacetate, THF, ꢀ75 °C, 4 h; (b) hydrazine hydrate, 70 °C, 6 h (55% over two
steps); (c) aq NH3, 140 °C W, 0.2 h (90%); (d) 1-(bromomethyl)-2-fluorobenzene,
Cs2CO3, DMF, 16 h (81%); (e) hydroxylamine hydrochloride, NEt3, DMSO, 75 °C, 16 h
(quant.); (f) 1,10-thiocarbonyldiimidazole, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), MeCN, rt, 24 h (64%); (g) 1,10-thiocarbonyldiimidazole, THF, rt, 2 h; (h)
boron trifluoride etherate, THF, rt, 16 h (44% over two steps); (i) 2-ethylhexyl
carbonochloridate, pyridine, DMF, 0 °C, 0.5 h; (j) xylenes, rf, 32 h (48% over two
steps).
3 and 4. Reagents and conditions: (a) LDA, ethyl
l
The ester 15 was converted to hydrazide 16 upon treatment
with hydrazine hydrate. Cyclization with 1,10-carbonyldiimidazole
provided 1,3,4-oxadiazol-2-one 5. Further elaboration of hydrazide
16 to the DMB-protected 1,2,4-triazole-3-one 18 was performed
via reaction with 2,4-dimethoxybenzyl isocyanate16 followed by
alkaline cyclization. N-Alkylation and acidic deprotection yielded
the target 1,2,4-triazole-3-ones 7, 8, and 9.17 In case of compound
6 the alkylation step was omitted.
conditions yielded 1,2,4-oxadiazol-5-thione 2 and 1,2,4-thiadiazol-
5-one 3, respectively. Upon treatment with 2-ethylhexyl carbo-
nochloridate followed by cyclization in refluxing xylenes, 14 was
converted into the 1,2,4-oxadiazol-5-one 4.12
Compared with the tetrazole derivative 1, the potency of ana-
logs 2–9 was 3- to 17-fold improved. Of the range of heterocycles
prepared, 1,2,4-thiadiazol-5(4H)-one 3 and 1,3,4-oxadiazol-2(3H)-
Starting from key intermediate 15, which has been described
earlier,4,5 compounds 5–9 were synthesized as outlined in
Scheme 2.
one 5 showed the best potencies of 0.52 lM and 0.61 lM, respec-
tively. Introduction of a second hydrogen bond donor, such as in
heterocycle 6 caused a significant loss of potency. Substitution of

1200
N. Griebenow et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1197–1200
In conclusion, we have synthesized a novel series of acidic het-
erocycle-substituted 1H-pyrazolo[3,4-b]pyridines. Lead optimiza-
tion resulted in the identification of oxadiazol-one 4, which
showed the expected profile of an sGC stimulator combined with
a favorable pharmacokinetic and physicochemical profile and im-
proved solubility. Further in vivo characterization of 4 will be re-
ported in due course.
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