Design, synthesis, and structure-activity relationship of novel orally efficacious pyrazole/sulfonamide based dihydroquinoline γ-secretase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2009.07.092

In this Letter, we report our strategy to design potent and metabolically stable γ-secretase inhibitors that are efficacious in reducing the cortical Aβx-40 levels in FVB mice via a single PO dose.

Full text of DOI:10.1016/j.bmcl.2009.07.092

Bioorganic & Medicinal Chemistry Letters 19 (2009) 4920–4923
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis, and structure–activity relationship of novel orally
efficacious pyrazole/sulfonamide based dihydroquinoline
c-secretase inhibitors
a
a
Anh P. Truong a, Danielle L. Aubele a, Gary D. Probst a, , Martin L. Neitzel , Chris M. Semko ,
Simeon Bowers a, Darren Dressen a, Roy K. Hom a, Andrei W. Konradi a, Hing L. Sham a,b, Albert W. Garofalo a,
Pamela S. Keim b, Jing Wu b, Michael S. Dappen b, Karina Wong c, Erich Goldbach c, Kevin P. Quinn c,
John-Michael Sauer c, Elizabeth F. Brigham d, William Wallace e, Lan Nguyen e, Susanna S. Hemphill e,
Michael P. Bova e, Guriqbal Basi e
*
a Department of Medicinal Chemistry, Elan Pharmaceuticals, 800 Gateway Boulevard, South San Francisco, CA 94080, United States
b Department of Process and Analytical Chemistry, Elan Pharmaceuticals, 1000 Gateway Boulevard, South San Francisco, CA 94080, United States
c Department of Lead Finding, Drug Disposition, and Safety Evaluation, Elan Pharmaceuticals, 800 Gateway Boulevard, South San Francisco, CA 94080, United States
d Department of In Vivo Pharmacology, Elan Pharmaceuticals, 800 Gateway Boulevard, South San Francisco, CA 94080, United States
e Department of Biology, Elan Pharmaceuticals, 1000 Gateway Boulevard, South San Francisco, CA 94080, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
In this Letter, we report our strategy to design potent and metabolically stable c-secretase inhibitors that
are efficacious in reducing the cortical Abx-40 levels in FVB mice via a single PO dose.
Received 22 June 2009
Accepted 20 July 2009
Available online 23 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
c
-Secretase inhibitors
Alzheimer’s disease
Alzheimer’s disease (AD) is a form of senile dementia, charac-
terized by a progressive loss of memory and cognitive ability,
affecting 24 million elderly people worldwide.1 The pathology of
this neurodegenerative disorder uniquely manifests itself with
the presence of extraneuronal aggregation of plaques composed
of b-amyloid peptides (Fb).2 Ab-peptides are derived from the
sequential proteolytic cleavage of the b-amyloid precursor protein
chemical activity and oxidative metabolic stability. Finally,
in vivo reduction of cortical Ab in wild type mice will be discussed.
The initial synthetic route outlined in Scheme 1 installed the
aryl ring and potency piece in the first step using a reductive ami-
nation which was followed by sulfonylation. Saponification of the
Cl
O
Sulfonamide
Aryl
(b-APP) by two aspartic acid proteases, referred to as b- and
c-
O S
secretase, respectively.3 Inhibition of either protease has been
demonstrated to result in reduction of brain Ab-peptide in preclin-
ical studies.4 Inhibitors of either protease offer attractive candi-
dates as disease-modifying treatments for people afflicted with
this debilitating malady.5
N
Potency
Pyrazole
1
N
NH
Figure 1. Representative c-secretase inhibitor.
Tetrahydroquinoline sulfonamides have been previously re-
ported as
properties of our pyrazole/sulfonamide based dihydroquinoline
series of
-secretase inhibitors7 that can be divided into four
components: the aryl, sulfonamide, pyrazole, and potency regions
(Fig. 1). First, we will discuss the synthetic preparation of the inhib-
itors. This will be followed by a disclosure of their in vitro bio-
c
-secretase inhibitors.6 Herein, we will describe the
Cl
c
O
O S
N
NH2
O
O
a-f
+
O
2
3
4
N
NH
Scheme 1. Reagents and conditions: (a) NaBH(OAc)3, AcOH, 1,2-DCE; (b) 4-
chlorobenzenesulfonyl chloride, pyridine; (c) aq NaOH, MeOH; (d) TFAA, TFA,
reflux; (e) DMA, DMF, 110 °C; (f) hydrazine, AcOH, EtOH.
* Corresponding author.
E-mail address: gary.probst@elan.com (G.D. Probst).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.07.092

A. P. Truong et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4920–4923
4921
order to build in a bi-directional manner which would allow for
the introduction of variation into the other three regions late in
the synthesis (Scheme 2). The protected pyrazole 6 bearing differ-
entiated synthetic handles, an iodide and aldehyde, was prepared
in five steps.8 A Suzuki–Miyaura coupling with 6 and the appropri-
ate 2-bromoarylboronic acid effectively installed the aryl portion
of the quinoline.9 Condensation of the aldehyde with the requisite
CO2Et
I
CHO
H2N
CHO
a-e
f
Br
N
N
N
N
N
H
5
N
SEM
SEM
6
7
Cl
O
O
O
S
Cl
O S
N
g-h
NH
i-j
Table 3
Br
Sulfonamide SARa
N
N
R1
O
O S
SEM
8
4
N
NH
R2
R3
R4
N
18
Scheme 2. Reagents and conditions: (a) isoamylnitrite CH2I2, 0 °C to 100 °C, 66%;
(b) NaH, SEM-Cl, THF, 95%; (c) aq NaOH, MeOH; (d) BH3-DMS, THF, 50 °C; (e) Dess–
Martin periodinane, NaHCO3, CH2Cl2, 80% over three steps; (f) 2-bromophenylbo-
ronic acid, Pd(PPh3)4, K2CO3, DME, H2O, 80 °C, 75%; (g) 4-chlorobenzenesulfona-
mide, Ti(O-iPr)4, THF; (h) MeMgBr, THF, ꢀ78 °C, 74% over two steps; (i) CuI, CsOAc,
DMSO, 120 °C, 67%; (j) 4 N HCl in dioxane, MeOH, 60 °C, 100%.
N
NH
R4
b
#
R1
R2
R3
IC50 (nM)
OxMet., (m, h)c
na
19d
cPr
H
H
H
4.2
ester, cyclization to the quinolone, and finally pyrazole formation
completed the synthesis. Since we wished to maintain the pyrazole
as a constant and vary the other three regions, this synthetic route
was highly inefficient. Furthermore, this sequence was plagued by
low yields and/or incompatible chemistry (e.g., the cyclization step
with electron-deficient aryl rings).
20
cPr
H
0.42
11, 8
F
21
22
cPr
cPr
H
H
H
H
3.2
2, 3
OMe
CN
0.74
11, 9
A second synthetic sequence was required where the pyrazole
would be introduced early with two distinct synthetic handles in
23
24
cPr
Me
H
H
H
H
0.31
140
49, 45
3, 5
CF3
Table 1
N
SAR of potency region substituentsa
Cl
N
N
25
26
27
28
29
30
31
32
33d
Me
cPr
cPr
cPr
Me
Me
cPr
cPr
cPr
H
H
H
H
H
H
F
H
F
F
F
F
F
F
F
F
290
1.3
46, 73
49, 62
56, 68
55, 60
45, 76
65, 76
5, 1
O
O S
R
Potency
Region
N
Cl
9
N
NH
OxMet., (m, h)c
b
#
R
IC50 (nM)
0.76
0.37
280
5500
2.9
OCHF2
1
H
Me
Et
iPr
cPr
CF3
27
1.2
0.36
0.28
0.17
0.21
2, 0
16, 3
7, 2
2, 1
15, 4
10, 13
4
10
11
12
13
OCF3
N
N
a
Compounds are racemic.
b
c
Values are means of three experiments. See Ref. 11.
See Ref. 12.
N
N
S
S
Table 2
SAR of fluoride on the aryl regiona
Cl
F
0.52
0.46
9, 4
Cl
O
O S
R1
R2
N
N
14
F
4, 23
Cl
S
R3
R3
N
NH
34
35
cPr
cPr
F
F
F
F
85
40
35, 53
na
b
NH2
NO2
#
R1
R2
IC50 (nM)
OxMet., (m, h)c
15
16
17
F
H
H
H
F
H
H
H
F
2.1
1.6
1.4
23, 6
23, 13
11, 3
a
b
c
Compounds are racemic unless stated otherwise.
Values are means of three experiments. See Ref. 11.
See Ref. 12.
a
Compounds are racemic.
b
c
Values are means of three experiments. See Ref. 11.
See Ref. 12.
d
Pure enantiomer.

4922
A. P. Truong et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4920–4923
sulfonamide, and subsequent addition of the desired Grignard re-
agent to the sulfonylimine afforded the cyclization precursor 8. A
copper-mediated cyclization10 followed by removal of the protect-
ing group produced the desired compounds.
O
S
O
NH
I
CHO
Cl
I
a-b
N
N
N
N
SEM
With synthetic chemistry in-hand, we endeavored to focus in-
tensely on increasing the oxidative metabolic stability of 1 along
with the potency in hopes of creating efficacious inhibitors. Our
initial efforts were to explore the potency region. Placement of al-
kyl groups in this zone dramatically increased the potency as
shown in Table 1. The larger alkyl groups generated increased po-
tency with cyclopropyl 12 having the greatest effect with an in-
crease in potency of over 150-fold. A trifluoromethyl group 13
afforded a similar potency with a slight increase in metabolic sta-
bility across both species.
SEM
6
44
Cl
O
O S
c-d
39
N
N
N
N
NH
Scheme 3. Reagents and conditions: (a) 4-chlorobenzenesulfonamide, Ti(O-iPr)4,
THF; (b) MeMgBr, THF, ꢀ78 °C, 54% over two steps; (c) 2-bromoimidazole, CuI,
trans-N,N0-dimethylcyclohexane-1,2-diamine, Cs2CO3, dioxane, reflux, 70%; (d) 4 N
HCl in dioxane, MeOH, 60 °C, 100%.
The effect of fluorine on the aryl portion of the quinoline was
explored (Table 2). Little effect was observed except for a modest
increase in oxidative metabolic stability in both species with a
fluorine at the 8-position of 16.
on the aryl region. Unfortunately, all of the heteroaryl groups,
38–41 and 43, resulted in a significant loss in potency except for
the thiophene 37 and pyridyl 42, but both suffered from metabolic
inadequacies (Table 4). Of synthetic interest from this series is the
preparation of the bridgehead nitrogen compound 39 which was
prepared via a tandem N-arylation with 44 and 2-bromoimidazole
(Scheme 3).
Finally, the structure–activity relationship on the aryl region re-
vealed that the 7-position was highly tolerant of an eclectic mix of
functionalities as shown in Table 5. This allowed for the alteration
of the physiochemical properties, thus the modulation of the phar-
Exploration of the sulfonamide region revealed wide fluctua-
tions in potency and metabolic stability as shown in Table 3. The
placement of lipophilic groups at the para-position provided
approximately a 10-fold increase in potency (e.g., 12 vs 19). Polar
groups at that position had a detrimental effect on potency (21,
34, and 35), except for the nitrile 22. Potent and metabolically sta-
ble compounds were realized with lipophilic electron-withdrawing
groups13a at the para-position of the sulfonamide in conjunction
with a cyclopropyl moiety13b as the potency piece (23, 27, and
28). Alternatively, metabolic stability could be achieved by lower-
ing the c log P via the incorporation of nitrogen into the aryl ring of
the sulfonamide 26. Interestingly, the thiophene 32 and thiazole 33
were potent, but each proved to be metabolically unstable.
Since lowering the c log P proved to be a fruitful strategy to im-
prove metabolic stability, further investigations were warranted
Table 5
SAR at the 7-region of the aryl regiona
R2
O
O S
N
R3
R1
Table 4
SAR of heteroarylsa
45
R1
O
N
NH
O S
R2
N
Het
#
R1
R2
R3
IC50 (nM)
OxMet., (m, h)c
b
36
46
47
48
49d
50
51
52
OMe
OBn
OH
I
Cl
Cl
Cl
Cl
CF3
CF3
CF3
CF3
Me
Me
Me
cPr
cPr
cPr
cPr
2.0
1.5
1, 1
1, 0
N
R2
NH
b
#
Het
R1
IC50 (nM)
OxMet., (m, h)c
18, 3
0.65
0.26
0.44
0.17
0.26
27, 50
78, 80
85, 87
77, 80
na
S
37
CF3
Me
cPr
Me
cPr
cPr
Me
cPr
4.5
24
Cl
CN
CO2Me
S
O
N
38
39
40
41
42
43
CF3
Cl
8, 31
Me
N
53
54
55
56
57
58
Cl
Me
cPr
cPr
cPr
cPr
cPr
0.93
0.26
0.24
0.41
0.58
0.20
0, 0
N
O
O
27
56, 74
53, 59
25, 44
3, 6
CF3
CF3
CF3
CF3
CF3
13, 32
55, 59
32, 60
17, 29
3, 27
O
N
N
N
N
N
CF3
CF3
Cl
1.1
2.1
2.6
2.4
S
N
N
O
N
N
N
N
O
N
CF3
88, 87
a
b
c
Compounds are racemic unless stated otherwise.
Values are means of three experiments. See Ref. 11.
See Ref. 12.
a
Compounds are racemic.
Values are means of three experiments. See Ref. 11.
See Ref. 12.
b
c
d
Pure enantiomer.

A. P. Truong et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4920–4923
4923
Table 6
In vitro properties of the
3 mpk (Fig. 2). Our strategy of improving the oxidative metabolic
stability was a major determinant that led to more efficacious
compounds, although there certainly could be additional factors
contributing to the increased efficacy.
c
-secretase inhibitors
Cl
CF3
N
O
O
O S
O S
In conclusion, we have discovered potent and metabolically sta-
N
N
ble
c-secretase inhibitors with potential utility as a therapy for
F
F
Alzheimer’s disease. These inhibitors dramatically reduced cortical
N
N
NH
Abx-40 in FVB mice at pharmaceutically relevant oral doses.19
NH
59
60
Sol.f
M)
a
d
#
IC50
OxMet.,
(m, h)b
GlucMet.,
(m, h)c
Papp
(nm/s)
P-gp
(nM)
Effluxe
(
l
Acknowledgments
59
60
0.62
0.67
7, 1
83, 95
91, 77
89, 97
198
249
0.81
1.47
>100
95
We would like to thank Jennifer Marugg, Wes Zmolek, Jackie
Kwong, David Nakamura, Rose Lawler, Juri Fukada, Lany Ruslim,
Michael Lee, Kang Hu, Kevin Tanaka, Terence Hui, Tracy Cole, Cris-
tian Cabrera, Pamela Santiago, Ferdie Soriano, Brain Peterson, Ron
Morgan, Jenifer Smith, John Maynard, David Quincy, and Lee Lati-
mer for their contributions to this work.
a
Values are means of three experiments. See Ref. 11.
b
c
d
e
f
See Ref. 12.
See Ref. 14.
See Ref. 15.
See Ref. 16.
See Ref. 17.
References and notes
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4. (a) Sankaranarayanan, S.; Holahan, M. A.; Colussi, D., et al J. Pharmacol. Exp.
Ther. 2009, 328, 131; (b) Hussain, I.; Hawkins, J.; Harrison, D., et al J. Neurochem.
2007, 100, 802; (c) Nishitomi, K.; Sakaguchi, G.; Horikoshi, Y., et al J. Neurochem.
2006, 96, 1; (d) Dovey, H. F.; John, V.; Anderson, J. P., et al J. Neurochem. 2001,
76, 173.
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80
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40
20
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5. Schmidt, B.; Baumann, S.; Braun, H. A.; Larbig, G. Curr. Top. Med. Chem. 2006, 6,
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W.; Greenlee, W. J.; Guzik, H. S.; Josien, H. B.; Wei, L.; Parker, E. M.; Pissarnitski,
D. A.; Song, L.; Zhang, L.; Zhao, Z. Bioorg. Med. Chem. Lett. 2007, 17, 205; (c)
Asberom, T.; Guzik, H. S.; Josien, H. B.; Pissarnitski, D. A., 2003.
WO2003014075.
7. Neitzel, M. L.; Aubele, D. L.; Hom, R.; Konradi, A. W.; Probst, G.; Semko, C. M.;
Truong, A. P.; Garofalo, A. W., 2007. WO2007143523.
8. (a) Gavina, B. D.; Forte, B.; Mantegani, S.; Varasi, M.; Vianello, P., 2005.
WO2005005414.; (b) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155.
9. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
10. Yamada, K.; Kubo, T.; Tokuyama, H.; Fukuyama, T. Synlett 2002, 231.
11. Compound, initially diluted with DMSO, was incubated with gamma secretase
prepared from IMR-32 cell membranes. The reaction, at 37 °C, was initiated by
the addition of MBPC-125 Swedish substrate for 2 h, and then quenched by the
addition of SDS. Quantification of cleaved substrate was determined by an
Ab40 specific ELISA assay.
3 mg/kg
30 mg/kg
3 mg/kg
#60
#59
12. Percentage of compound (1
lM) remaining after 30 min incubation in liver
Figure 2. Reduction of cortical Abx-4018 after 3 h in FVB mice after a single oral
dose as compared to the vehicle-dosed animals. Each circle or square represents
one animal.
microsomes (0.5 mg/ml protein) supplemented with 1 mM NADPH at 37 °C in
phosphate buffer. (m = mouse, h = human).
13. (a) Smart, B. E. J. Fluorine Chem. 2001, 109, 3; (b) Manoury, P. M.; Binet, J. L.;
Rousseau, J.; Lefevre-Borg, F. M.; Cavero, I. G. J. Med. Chem. 1987, 30, 1003.
14. Percentage of compound (2
lM) remaining after 30 min incubation in liver
microsomes (0.5 mg protein) supplemented with 1 mM UDPGA, 100 mM
macokinetic parameters of these compounds. Compounds 49–51
displayed excellent potency and metabolic stability.
MgCl2 and 25
h = human).
lg/mL alamethacin at 37 °C in phosphate buffer. (m = mouse,
15. Compound (5
l
M) in mHBSS (pH 7.4) were incubated with MDCK II cell
To validate the strategy of focusing on alleviating the metabolic
liability, it was necessary to test two compounds in vivo with sim-
ilar in vitro characteristics. Enantiomerically pure samples of com-
pounds 59 and 60 were obtained by chiral HPLC. Their in vitro
properties are nearly identical except for their oxidative stability
which are 7% and 83% for 59 and 60, respectively, in mouse liver
microsomes (Table 6). Both of these compounds were tested
in vivo by administration of a single PO dose to FVB mice. Com-
pound 59 reduced Abx-40 by 7% and 37% at 3 and 30 mpk, respec-
tively, in the cortex, whereas compound 60 proved to be
significantly more efficacious by reducing Abx-40 by 43% at
monolayers for 120 min at 37 °C. Samples were taken from apical and
basolateral chambers, and analyzed using LC/MS/MS.
16. Compound (5 lM) in mHBSS (pH 7.4) were incubated with MDR1-MDCK cell
monolayers for 120 min at 37 °C with and without a P-gp inhibitor. Samples
were taken from apical (A) and basolateral (B) chambers, and analyzed using
LC/MS/MS. The efflux ratio was determined by dividing the rate of the A to B
direction with and without a P-gp inhibitor.
17. Compound (20 mM) were serial diluted in DMSO and aliquoted to buffer (HBSS
pH 7.4), 1:200 dilution. After 1 h incubation, plates were read on a solubility
scanner.
18. Zhou, B.; Yu, M.; Neitzel, M.; Marugg, J.; Jagodzinska, J.; Lee, M.; Hu, K.; Schenk,
D.; Yednock, T.; Basi, G. J. Biol. Chem. 2008, 283, 2927.
19. Details of in vivo and in vitro experiments will be published separately.