Renin inhibitors for the treatment of hypertension: Design and optimization of a novel series of pyridone-substituted piperidines

Article abstract of DOI:10.1016/j.bmcl.2011.05.013

An SAR campaign aimed at decreasing the overall lipophilicity of renin inhibitors such as 1 is described herein. It was found that replacement of the northern appendage in 1 with an N-methyl pyridone and subsequent re-optimization of the benzyl amide hand

Full text of DOI:10.1016/j.bmcl.2011.05.013

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3970–3975
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Renin inhibitors for the treatment of hypertension: Design and optimization
of a novel series of pyridone-substituted piperidines
Austin Chen a, , Louis-Charles Campeau a, Elizabeth Cauchon a, Amandine Chefson a, Yves Ducharme a,
⇑
Daniel Dubé a, Jean-Pierre Falgueyret a, Pierre-André Fournier a, Sébastien Gagné a, Erich Grimm a,
Yongxin Han a, Robert Houle a, Jing-Qi Huang a, Patrick Lacombe a, Sébastien Laliberté a,
Jean-François Lévesque a, Susana Liu a, Dwight MacDonald a, Bruce Mackay a, Dan McKay a,
M. David Percival a, Chris Regan b, Hillary Regan b, René St-Jacques a, Sylvie Toulmond a
a Merck Frosst Centre for Therapeutic Research, 16711 Trans Canada Highway, Kirkland, Québec, Canada H9H 3L1
b Merck Research Laboratories, Sumneytown Pike, PO Box 4, West Point, Pennsylvania 19486, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 14 May 2011
An SAR campaign aimed at decreasing the overall lipophilicity of renin inhibitors such as 1 is described
herein. It was found that replacement of the northern appendage in 1 with an N-methyl pyridone and
subsequent re-optimization of the benzyl amide handle afforded compounds with in vitro and in vivo
profiles suitable for further profiling. An unexpected CV toxicity in dogs observed with compound 20
led to the employment of a time and resource sparing rodent model for in vivo screening of key com-
pounds. This culminated in the identification of compound 31 as an optimized renin inhibitor.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Renin
Inhibitor
Anti-hypertensive
Essential hypertension is known to affect over a billion people
worldwide. If left untreated, hypertension can lead to multiple or-
gan failure. Compounds capable of perturbing the renin-angioten-
sin-aldosterone system (RAAS, Fig. 1), a tightly regulated biological
cascade known to play a pivotal role in the control of blood pres-
sure,1 have the potential to address this medical need.2 Effective
anti-hypertensive therapies involving either the inhibition of
angiotensin I converting enzyme (ACE)3 or the antagonism of
angiotensin II type I receptor4 have already been developed. How-
ever, it is believed that the inhibition of renin, an enzyme that is
involved in the first and rate-limiting step of RAAS,5 would offer
the best potential for blood pressure control and end organ protec-
tion. Furthermore unlike ACE inhibition,6 renin blockade should
not be plagued by mechanism-based adverse events as the only
known substrate for the renin enzyme is angiotensinogen, which
is a protein with no known biological function.7
tion, high affinity for the hERG channel, poor physicochemical
properties and potential to induce phospholipidosis. We attempted
to address these four issues through the judicious incorporation of
polar functional groups onto these privileged scaffolds.9 Although
the resulting compounds (e.g., 2, Fig. 2) did show the desired
improvements across these key parameters while keeping their
intrinsic potency against renin, they were, however, not orally bio-
available in either rodents or dogs.
Having demonstrated with 2 that most of the issues associated
with 1 could be addressed by decreasing its overall ‘greasiness’, an
alternate approach to achieving this goal could involve truncating
one of the compound’s two lipophilic appendages. Since we were
also cognizant of the highly mobile nature of the renin flap under
which the 2-(2,6-dichloro-4-methylphenoxy)-ethoxy residue sits,
we decided to initiate a program targeting the synthesis of trun-
cated renin inhibitors.
It has been previously reported that compounds bearing a
piperidine warhead flanked at the 3,4-position by two optimized
but lipophilic appendages (e.g., 1, Fig. 2) can serve as highly potent
and orally-bioavailable inhibitors of renin.8,9 Unfortunately the
same structural features responsible for both renin potency and
selectivity against other aspartic proteases also rendered the
development of these compounds challenging. Specifically, the
majority of these lipophilic amines suffered from CYP 3A4 inhibi-
Although a complete replacement of the 2-(2,6-dichloro-4-
methylphenoxy)ethoxy group in compound 1 by hydrogen (i.e.,
3) led to a precipitous drop in renin potency, we were however
encouraged by the attendant improvement made on both the hERG
and CYP 3A4 profiles (Table 1). When compound 3 was docked into
the renin active site, computer modeling suggested that this trun-
cation would now force the renin flap to adopt a half-closed
conformation thereby leading to a small clash between Trp45
and the phenyl stub. This unfavorable interaction could however
be alleviated by replacing the offending benzene with a pyridine.
Indeed, on going from compound 3 to 4, we were able to improve
⇑
Corresponding author. Tel.: +1 450 218 4017.
E-mail address: austin.chihyu.chen@gmail.com (A. Chen).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.05.013

A. Chen et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3970–3975
3971
group in b-ketoester 11 was cleaved and the resulting free amine
re-protected in situ as the corresponding BOC carbamate 12. High
conversion was achieved with Pd(OH)2 as the catalyst, ethanol as
the solvent and triethylamine as the base under pressurized H2
atmosphere (50 psi). Subsequent reaction with sodium hydride
and triflic anhydride resulted in the clean formation of triflate
13, which could in turn be converted to the corresponding pinacol
boronate 14 using standard Miyaura conditions.11 Suzuki coupling
of boronate 14 with 4-bromo-2-methoxypyridine was best accom-
plished with Pd(dppf)Cl2 and 2 N aq Na2CO3 in n-propanol at
80 °C.12 Mg-mediated single electron reduction of the tetra-substi-
tuted olefin in ester 15 in methanol afforded the reduced ester 16
as a mixture of diastereomers, with the cis-isomer constituting the
bulk of the mass balance. Although the two diastereomers could be
readily separated, it was more convenient to heat an ethanol solu-
tion of the crude product mixture from the reduction step in the
presence of excess sodium ethoxide. After 18 h, only the desired
trans-isomer 17 remained. It was discovered that prolonged heat-
ing of methoxypyridine 17 with an excess of iodomethane and so-
dium iodide in acetonitrile would trigger a formal N-to-O methyl
migration to furnish pyridone 18 in nearly quantitative yield. Race-
mic 18 could then be resolved on a chiral AD column with
72:14:14 (v/v/v) hexanes/ethanol/i-propanol as the eluent. Finally,
saponification of the slower eluting isomer with lithium hydroxide
in a 2:1 (v/v) mixture of THF and methanol afforded the desired
pyridone acid 10.
Figure 1. The renin-angiotensin-aldosterone system.
All the amides shown in Table 2 could be rapidly synthesized
from 10 via a two-step sequence: HATU-mediated amide coupling
with an appropriate amine in DMF, followed by removal of the BOC
protecting group. Although the latter step could often be per-
formed in the presence of a large excess of 4 M HCl in dioxane,
BOC-deprotection from more acid-sensitive substrates was best
accomplished with zinc(II) bromide in CH2Cl2.13
It became apparent that the best amine identified for our earlier
generations of non-truncated renin inhibitors was no longer opti-
mal.14 We hypothesized that the downward movement of the re-
nin flap observed with truncated inhibitors such as 5 would also
lead to some re-organization of the renin s3 pocket. Consequently
when the benzyl amine used for the synthesis of 5 was switched
for N-(2,3-dichlorobenzyl)cyclopropanamine (i.e., 19), an equipo-
tent but orally bioavailable compound was obtained. Re-introduc-
tion of a 3-methoxypropyl tail designed to reach into the renin s3
sub-pocket afforded compound 20, an inhibitor with a renin plas-
ma IC50 value of 1.3 nM. Both chlorines in 20 proved to be impor-
tant for renin potency as the removal of either led to less potent
the renin potency by 4-fold without sacrificing the gains made in
the off-target profile. Furthermore, compound 4 was also found
to be orally bioavailable in rodents. The proximity of Trp45 to
the pyridine nitrogen of compound 4 offered us another opportu-
nity to improve the renin potency further. While the compound
4 pyridine did not possess the necessary reach to engage in an
effective H-bond with the indole NH of Trp45, its replacement by
a pyridone delivered a compound (i.e., 5) with another 4-fold in-
crease in renin potency (Fig. 3). Unfortunately, compound 5 suf-
fered from poor rodent PK. Our attempts to address this and
improve the renin potency via further fine-tuning of the pyridone
scaffold afforded only limited success. Indeed, while compound 9
did demonstrate an improvement in both renin potency and rodent
PK over 5, its associated off-target profile was not optimal.
Keeping the simple N-methyl pyridone plate in 5, an optimiza-
tion campaign of the benzyl amide handle led to our next break-
through. The requisite pyridone acid 10 could be accessed using
the chemistry shown in Scheme 1. Briefly, the N-benzyl protecting
Figure 2. Piperidine-based renin inhibitors.

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A. Chen et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3970–3975
Figure 3. Modeling of compound 5 in the renin active site.
compounds (i.e., 21, 22). Further alterations of the p3 sub-pocket
tail of compound 20; specifically, shortening the propyl chain by
one methylene unit (i.e., 23), replacing the methoxy capping group
with either cyanide (i.e., 24), N-acetamide (i.e., 26) or methyl car-
bamate (i.e., 27), and changing the linking atom from carbon to
nitrogen (i.e., 28), all led to a drop in renin potency, oral bioavail-
ability or both. On the other hand, the two chlorines in 20 could
be replaced with methyl groups (i.e., 29) without sacrificing renin
potency. Other preferred p3 aromatic plates include naphthalene
(33), quinoline (34, 35) and indole (36 to 45). Although some of
these derivatives were found to be sub-nanomolar renin inhibitors
in our plasma assay (i.e., 37–39, 41–43, and 45), compound 20 still
stood out by virtue of its renin potency, clean off-target profile (i.e.,
hERG, CYP 3A4 inhibition) and good pharmacokinetics in rodents.
When compound 20 was infused intravenously at a dose of
10 mpk over 1.5 h into anesthetized, vagotomized and ventilated
mongrel dogs, we observed significant prolongation of QTc, PR
and QRS intervals (Table 3). The same CV signals were also ob-
served in vivo with 36, a compound that was found to be inactive
(IC50 >30 lM) in the IKr, Ca L-type, hCaV1.2, hNaV2, and hNaV1.5
in vitro channel assays. Faced with the scenario of having to screen
all the key compounds in an in vivo setting, a more time and re-
source sparing rodent alternative to the CV dog model was estab-
lished.15 Although the cardiac electrophysiology (EP) rat model
proved to be unsuitable for assessing a given compound’s potential
to induce QTc prolongation in either dog or human, there was how-
ever good agreement in the magnitude of both QRS and PR prolon-
gations between what was observed in the EP rat and the CV dog
model for compounds 20 and 36. With this tool in hand, all key
compounds were screened leading to the identification of com-
pound 31 as a renin inhibitor suitable for further development.
The key characteristics of compound 31 are summarized in
Table 4. It is a potent renin inhibitor with moderate oral bioavailabil-
ity in rats, dogs, and cyno monkeys. The corresponding HCl salt was
found to be crystalline, non-hygroscopic and highly soluble in water.
Compound 31 was also found to be efficacious in the double trans-
genic rat model (dTGR),16 imparting a robust blood pressure de-
crease when given orally at 10 mpk. Compound 31 inhibited CYP
3A4 reversibly with an IC50 of 6.5 lM and demonstrated little poten-
tial to inactivate CYP 3A4 in a time-dependent manner. This com-
pound also exhibited minimal potential to covalently label
proteins in the presence of liver microsomes. Most importantly,
compound 31 did not cause significant prolongation of QTc, PR nor
QRS intervals in vivo in both rats and dogs.

A. Chen et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3970–3975
3973
Scheme 1. Synthesis of 10: Reagents and conditions: (a) Pd(OH)2, BOC2O, TEA, ethanol, 50 psi H2, 4 h, 82%; (b) NaH (60% dispersion in oil), Tf2O, THF, 0 °C to rt, 2 h, 79%; (c)
Pd(dppf)Cl2, KOAc, B2Pin2, dioxane, 80 °C, 14 h; (d) Pd(dppf)Cl2, 2 N aq Na2CO3, 4-bromo-2-methoxypyridine, n-PrOH, 80 °C, 14 h, 84% over two steps; (e) Mg powder, MeOH,
sonication, 2 h; (f) NaOEt, EtOH, 80 °C, 16 h, 70% over two steps; (g) iodomethane, NaI, CH3CN, sealed tube, 45 °C, 72 h, 97%; (h) chiral resolution, AD column, 72:14:14
hexanes/EtOH/iPrOH; (i) 1 N aq LiOH, MeOH, THF, 16 h, 48% over two steps.
Table 2
SAR of select renin inhibitors: Amide modifications
Compound R1
R2
R3
R4
Renin Potencya
hERG Ki (nM)
CYP 3A4 inhibition
Reversible Time dependent
(% activity)b (% loss)c
F in SD rats at 20 mpk po (%)
Buffer (nM) Plasma (nM)
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Cl
Cl
Cl
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
20
0.3
8
42
1.3
25
6.5
2.2
10
13,000
4,800
28,000
7,600
11,000
8,400
17,000
>29,000
15,000
18,000
>29,000
5,000
61
53
75
23
68
55
52
97
45
71
74
15
22
23
54
95
85
59
41
48
63
74
19
37
16
29
1
23
20
15
0
56
0
15
45
0
23
24
14
(CH2)3OMe
(CH2)3OMe
(CH2)3OMe
(CH2)2OMe
(CH2)3CN
(CH2)2CN
(CH2)2NHAc
3
28 at 30 mpk
18
7
8
0
1
1
7 at 30 mpk
15
30
8
4
0.6
4.9
8.5
4.5
17
9
(CH2)2NH(C@O)OMe 61
210
26
1.0
3.5
2.4
81
1.4
3.0
2.2
5.2
0.87
0.64
0.57
8.4
0.62
NH(CH2)2OMe
(CH2)3OMe
(CH2)3OMe
7.1
0.6
1.7
1.0
18
0.4
1.4
0.8
2.3
0.32
0.12
0.07
12
Me Me
H
H
H
—
—
—
H
F
Br
Br
Ph
—
—
—
—
—
—
—
—
—
Me (CH2)3OMe
18,000
1,700
H
—
—
—
—
—
—
—
—
—
(CH2)3OMe
—
—
0
17,000
>29,000
27,000
>29,000
>29,000
23,000
11,000
>29,000
950
34
13
16
45
0
19
48
0
1
—
2 at 30 mpk
8 at 30 mpk
8 at 30 mpk
22 at 30 mpk
3 at 3 mpk
0 at 3 mpk
16 at 3 mpk
(CH2)3OMe
(CH2)3OMe
(CH2)3OMe
(CH2)3OMe
H
Cl
Br
F
F
CH2Ph
0.02
43
(continued on next page)

3974
A. Chen et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3970–3975
Table 2 (continued)
Compound R1
R2
R3
R4
Renin Potencya
hERG Ki (nM)
CYP 3A4 inhibition
Reversible Time dependent
(% activity)b (% loss)c
F in SD rats at 20 mpk po (%)
Buffer (nM) Plasma (nM)
42
43
44
45
F
F
F
F
—
—
—
—
—
—
—
—
CH2(3-FPh)
CH2(4-FPh)
CH2(3-Pyr)
CH2(4-Pyr)
0.06
0.05
0.88
0.24
0.63
0.48
2.0
1,200
750
12,000
15,000
18
16
61
44
39
70
44
29
30 at 30 mpk
17 at 30 mpk
0 at 3 mpk
0.53
0 at 3 mpk
a
Average of at least two replicates. See Ref. 10 for assay protocols.
b
Calculated as a percentage of the difference in the rate of CYP3A4-mediated conversion of testosterone (250
M in DMSO) versus blank DMSO. 50% activity corresponds to a reversible CYP 3A4 inhibition IC50 of 10 M. 10% activity corresponds to a reversible CYP 3A4 inhibition
M.
Calculated as a percentage of the difference in the rate of CYP3A4-mediated conversion of testosterone (250
lM) to 6-b-hydroxytestosterone in the presence of compound
(10
l
l
IC50 of 2
l
c
lM) to 6-b-hydroxytestosterone before and after 30 min
incubation period with the compound (10
lM in DMSO). A 0% loss corresponds to no measurable time-dependent CYP 3A4 inhibition.
Table 3
CV profiles of representative compounds
20
36
31
CV Dog (10 mpk/1.5 h)
QTc (%)a
PR (%)a
10
50
33
15
35
54
7.4
6
0
5
4
9.3
16
14
7.9
14
12
25
23
30
12.7
QRS (%)a
[Plasma] (
PR (%)a
l
l
M)
M)
EP Rat (10 mpk/0.5 h)
QRS (%)a
[Plasma] (
a
Prolongation expressed as a percentage change from baseline.
Table 4
Key profile of compound 31
a,b
Renin IC50 (nM)
Buffer
Plasma
F (%)
1.0
2.4
30
SD rat (20 mpk po, 5 mpk iv)
po AUC (
Cl (mL/min/kg)
T1/2 (h)
Vdss (L/kg)
F (%)
l
M h)
4.2
67
5.2
17
Beagle dog (3 mpk po, 1 mpk iv)
Cyno monkey (3 mpk po, 1 mpk iv)
36
po AUC (
Cl (mL/min/kg)
T1/2 (h)
Vdss (L/kg)
F (%)
l
M h)
2.1
16
13.7
4.5
14
po AUC (
Cl (mL/min/kg)
l
M h)
0.9
14
T1/2 (h)
Vdss (L/kg)
With NADPH pmole eq/(mg at 1 h)
Without NADPH pmole eq/(mg at 1 h)
Max BP ; (mm Hg)
7.8
4.8
34.5
8.3
35
COBILMc, human
Efficacy in dTGR (10 mpk po)
CYP 3A4 IC50 (nM)
Duration (h)
24
6,500
16,000/20,000
Reversible
30 min Pre-Inc/0 min Pre-Incd
a
See Ref. 10 for assay protocols.
Average of at least two replicates.
b
c
COBILM assay (covalent binding in liver microsomes): incubations (n = 3) were done with human liver microsomes at 10 lM, with 1 mg protein/mL, for 60 min with or
without NADPH (1 mM) as additive. For complete protocol, see Ref. 17.
d
TDI assay (time dependent inhibition): the CYP 3A4 IC50 observed with or without a 30 min pre-incubation period with the compound in question.
6. Husain, A. J. Hypertens. 1993, 11, 1155.
7. Cooper, M. E. Am. J. Hypertens. 2004, 17, 16S.
In summary, we have described a novel series of pyridone-
substituted piperidines with a profile suitable for their further clin-
ical development as renin inhibitors.
ˇ
8. Corminboeuf, O.; Bezençon, O.; Remen, L.; Grisostomi, C.; Richard-Bildstein, S.;
Bur, D.; Prade, L.; Strickner, P.; Hess, P.; Fischli, W.; Steiner, B.; Treiber, A.
Bioorg. Med. Chem. Lett. 2010, 20, 6291.
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Lévesque, J. F.; MacDonald, D.; McKay, D.; Percival, M. D.; Roy, P.; Soisson, S.;
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