The discovery of fused oxadiazepines as gamma secretase modulators for treatment of Alzheimer's disease

Article abstract of DOI:10.1016/j.bmcl.2012.11.055

In an attempt to further improve overall profiles of the oxadiazine series of GSMs, in particular the hERG activity, conformational modifications of the core structure resulted in the identification of fused oxadiazepines such as 7i which had an improved

Full text of DOI:10.1016/j.bmcl.2012.11.055

Bioorganic & Medicinal Chemistry Letters 23 (2013) 466–471
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
The discovery of fused oxadiazepines as gamma secretase modulators
for treatment of Alzheimer’s disease
Hongmei Li a, Jun Qin a, Pawan Dhondi a, Wei Zhou a, Monica Vicarel a, Thomas Bara a, David Cole a,
Hubert Josien a, Dmitri Pissarnitski a, Zhaoning Zhu a, Anandan Palani a, Robert Aslanian a, John Clader a,
Michael Czarniecki a, William Greenlee a, Mary Cohen-Williams b, Lynn Hyde b, Lixin Song b, Lili Zhang b,
Inhou Chu c, Xianhai Huang a,
⇑
a Department of Medicinal Chemistry, Merck Research Laboratory, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
b Department of Neuroscience, Merck Research Laboratory, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
c Drug Metabolism, Merck Research Laboratory, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In an attempt to further improve overall profiles of the oxadiazine series of GSMs, in particular the hERG
activity, conformational modifications of the core structure resulted in the identification of fused oxa-
diazepines such as 7i which had an improved hERG inhibition profile and was a highly efficacious
GSM in vitro and in vivo in rats. These SAR explorations offer opportunities to identify potential drugs
to treat Alzheimer’s disease.
Received 30 September 2012
Revised 10 November 2012
Accepted 14 November 2012
Available online 29 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Alzheimer’s disease
Oxadiazepine
Amyloid precursor protein
Gamma secretase modulator
hERG
Alzheimer’s disease (AD) is an age-related neurodegenerative
disorder that affects millions of older people in the United States.
AD is characterized by neuronal loss, neurofibrillary tangle (NFT)
formation, and extracellular deposition of amyloid-b (Ab) peptide
plaques. Patients normally show symptoms of cognitive impair-
ment, as well as disturbance in language, memory, movement,
attention, and orientation. It is estimated that more than 35 million
people currently suffer from AD worldwide, with an annual cost of
over $600 billion and a potential population increase to more than
115 million patients by 2050.1 Despite enormous efforts AD, there
is no solution to this medical problem.2 Due to this urgent unmet
medical need, academic and industrial laboratories are working
very aggressively to develop therapies to halt or even reverse AD.
It is believed that the accumulation of amyloid-beta (Ab) peptide
and hyperphosphorylated protein tau contribute to AD progres-
sion.3 Ab peptide is formed from a larger amyloid precursor protein
holds promise for the treatment of AD,4 recent preclinical experi-
ments demonstrated that inhibition of -secretase resulted in
mechanism-based GI toxicity such as thymus atrophy and intesti-
c
nal goblet cell hyperplasia. This toxicity is due to disruption of the
processing of other c-secretase substrates such as Notch, which is
important for cellular gene transcription,5 and ERB4. Recent clini-
cal results of the GSI semagacestat showed that it not only failed
to slow disease progression but also increased the incidence of skin
cancer of patients in the treatment group compared to the placebo
group.6 Although the molecular mechanism of action remains lar-
gely unknown,
at an allosteric site to shift the predominant site of
cleavage toward shorter, non-amyloidogenic peptides (e.g.,
Ab38), without blocking overall
-secretase function.7 This poten-
c
-secretase modulators (GSMs) are believed to act
c
-secretase
c
tially offers a better selectivity window over GSIs versus for exam-
ple notch processing. From an in vitro point of view, Abtotal/Ab42
ratio can be used to distinguish GSMs from GSIs with a ratio of
>10 for GSMs and <3 for GSIs. There are two major classes of GSMs
in clinical trials: the non-steroidal anti-inflammatory acids (NSA-
IDs),8 and the non-NSAID class such as the lactam E-2012 from Ei-
sai.9 Among our efforts in the AD area,10 we have recently
identified cyclic hydroxyamidines such as oxadiazolines (1) and
oxadiazines (2) as highly efficacious GSMs in both in vitro studies
and in vivo animal models. They were found to not only be
(APP) via sequential proteolytic cleavage by b- and
c-secretases.
c-Secretase cleaves the APP C-terminal fragment at multiple sites
leading to Ab peptides of 37–42 amino acids. Of these peptides
Ab42, the more hydrophobic form, is the most amyloidogenic and
neurotoxic. While development of
c-secretase inhibitors (GSIs)
⇑
Corresponding author. Tel.: +1 732 594 4734.
E-mail address: xianhai.huang@merck.com (X. Huang).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.11.055

H. Li et al. / Bioorg. Med. Chem. Lett. 23 (2013) 466–471
467
2
O
O
N
3
1
O
N
N
8
OH
4
5
F
MeO
N
MeO
N
MeO
N
N
N
N
Ar
6
9
F
7
F
N
N
N
F
1 (oxadiazoline)
3 (oxadiazepine)
2 (oxadiazine)
Aβ42 IC50 = 33 nM
Aβtotal/Aβ42 = 607
Figure 1. Design rationale of oxadiazepine GSMs.
O
O
F
F
N
N
MeO
MeO
N
N
F
N
N
N
F
N
4
5
Aβ42 IC50 (nM) = 580
Aβtotal/Aβ42 = 35
Aβ42 IC50 (nM ) = 88
Aβtotal/Aβ42 = 32
Figure 2. Activity of fused azepane oxadiazine analogs.
Table 1
SAR studies of C3 mono-substituted oxadiazepinesa
Ar
O
N
MeO
N
N
N
Ab42 IC50 (nm)
41
6
Ar
Abtotal/Ab42 ratio
hERG inhibitionb (%)
489
93
6a
F
43
465
93
6b
F
F
41
64
371
262
nd
61
6c
F
F
Cl
F
6d
F
53
364
89
6e
a
Each IC50 value is an average of at least two determinations.
At 10 lM.
b

468
H. Li et al. / Bioorg. Med. Chem. Lett. 23 (2013) 466–471
Table 2
SAR studies of C5 mono-substituted oxadiazepinesa
O
N
N
MeO
Ar
N
N
7
Ar
Ab42 IC50 (nm)
Abtotal/Ab42ratio
CSF Ab42 reductionb (%)
hERG inhibitionc (%)
81
49
409
ꢀ40
F
F
7a
35
37
24
572
392
466
nd
86
49
56
7b
F
F
ꢀ39
ꢀ48
F
7c
F
7d
F
52
354
ꢀ23
nd
F
7e
F
16
40
95
480
340
210
ꢀ59
ꢀ38
ꢀ57
53
39
nd
Cl
7f
Cl
F
7g
7h
F
Cl
F
39
510
ꢀ61
33
F
F
7i
a
b
c
Each IC50 value is an average of at least two determinations.
10 mpk Oral acute dosing at 3 h time point on average 2–3 tests.
At 10 lM.
chemically stable but also possess highly desirable pharmacoki-
netic and toxicological profiles. To further improve the overall pro-
file, especially the hERG activity of compound 2 (76% inhibition of
hERG at 10 lM), we continued our search for next generation
GSMs. Herein we report the identification of a novel class of fused
interaction with the hERG protein. Thus we prepared the 7,6-mem-
ber ring fused azepane oxadiazines 4 and 5 (Fig. 2), but found that
4 lost much in vitro activity. Compound 5 had good Ab42 inhibition
activity but selectivity decreased by almost 20-fold. This modifica-
tion suggested that fused piperidinyl ring in 2 was important for
both activity and selectivity. Based on this information, we turned
our attention to the preparation of fused piperidinyloxadiazepines
(3) and focused on C3, C4 and C5 modifications.
oxadiazepine GSMs (3, Fig. 1).
Structural properties such as the conformation of the molecule,
basicity and p–p interactions impact the hERG activity of a mole-
cule.11 We reasoned that it might be possible to improve the hERG
profile observed with compound 2 by modifying the oxadiazine
core structure (first-degree SAR studies12), thereby impacting its
We first prepared C3 mono-substituted oxadiazepines and fo-
cused on introducing aryl substitutions since these were important
for potency in the oxadiazine series.10a As summarized in Table 1,

H. Li et al. / Bioorg. Med. Chem. Lett. 23 (2013) 466–471
469
Table 3
SAR studies of C4 mono-, C4–C5 and C3–C5 di-substituted oxadiazepinesa
O
N
MeO
N
N
N
8
Ar
Ab42 IC50 (nm)
Abtotal/Ab42 ratio
hERG inhibitionb (%)
140
140
nd
8a
8b
F
130
240
150
83
90
55
OH
F
8c
F
O
1260
16
42
F
F
8d
F
41
480
nd
8e
F
F
a
Each IC50 value is an average of at least two determinations.
At 10 lM.
b
all these modifications retained good in vitro activity and selectiv-
ity (6a–6e) compared to the oxadiazine series. However, little
reduction of hERG activity was observed. While this was disap-
pointing, we nonetheless turned our attention to C5 modifications
to complete the SAR (Table 2). Gratifyingly, C5-aryl substituted
oxadiazepines again showed very good in vitro Ab42 activity and
selectivity. With respect to hERG inhibition, mono fluorinated
compounds 7a and 7b were not improved, however dihalogenation
(7c–7h) moderately reduced hERG activity. The trifluorinated com-
dose dependently reduced Ab42 levels in CSF at the 3 h time point
(ꢀ78% at 30 mpk, ꢀ61% at 10 mpk, and ꢀ43% at 3 mpk). The oral
10 mpk dose showed very good exposure with a plasma AUC of
15,800 nM h, a high brain concentration (1803 ng/g) and a brain/
plasma ratio of 3.2 at 6 h post-dose. The compound was clean in
CYP P450 inhibition (CYP3A4, 2D6, 2C9 IC50 >30 lM) and PXR en-
zyme induction studies (relative signal strength to control,
CYP1A1: 2%, 2B1/2: 0%, 3A1: 0%), when tested at concentrations
up to 30 lM.
pound 7i had a hERG inhibition of 33% at 10
l
M. In order to differ-
We also prepared C4 mono-, C4–C5 and C3–C5 di-substituted
oxadiazepines to determine whether further improvement of
hERG activity could be achieved. As shown in Table 3, C4 mono-
substitution didn’t help to improve either activity (8a and 8b) or
the hERG profile (8b). The introduction of a polar hydroxy group
at C4 of the C4–C5 di-substituted compound 8c led to a small de-
crease in activity and resulted in comparable hERG activity to 7c
(without OH at C4). Alkylation on C4 hydroxy (8d) resulted in fur-
ther loss of activity and selectivity. On the other hand, C3–C5 di-
substitution (8e, mixture of isomers) showed good Ab42 inhibition
and selectivity which provided opportunity for further SAR
studies.
entiate these compounds for further follow up, they were tested in
in vivo rat CSF Ab42 reduction studies (10 mpk oral acute dosing at
3 h time point). Monofluorinated compound 7a showed 40% Ab42
reduction in vivo, while the 3,5- (7c) and 3,4-di-fluorinated (7d)
compounds showed similar in vivo activity whereas 2,4-di-fluori-
nation resulted in loss of activity (7e). On the other hand, fluoro-
chlorination and trifluorination improved in vivo activity with
compounds 7f, 7h and 7i having similar Ab42 CSF reduction. With
an improved hERG profile and good in vivo activity, compound 7i
was further characterized in rising dose in vivo studies, PK and
ancillary profile evaluations. Acute oral administration of 7i to rats

470
H. Li et al. / Bioorg. Med. Chem. Lett. 23 (2013) 466–471
O
O
P
O
MeO
N
O
P
O
EtO
EtO
OH
Cl
a
b, c
EtO
EtO
OtBu
Cl
OtBu
N
9
11
13
OH
O
N
O
MeO
MeO
N
F
F
d, e
f, g, h
N
N
F
N
N
F
N
F
F
15
7i
OH
O
MeO
N
H
F
O
O
H2N
N
OH
I
Cl
N
F
F
10
12
14
16
Scheme 1. The Synthesis of fused oxadiazepine GSMs. Reagents and conditions: (a) 10, NaH, THF, 95%; (b) 12, LiOH, THF; (c) TFA, 90% over two steps; (d) 14, EDCI/HOBt, DMF;
(e) NaOMe, MeOH, 75% over two steps; (f) 16, 1,10-(azodicarbonyl)dipiperidine, PBu3, THF, 80 °C, 65%; (g) NH2NH2ꢁxH2O, CH3CN, 75%; (h) P2O5, EtOH, 80 °C, 35%.
Med. Chem. Lett. 2012, 3, 265; (e) Zhang, H.; Ma, Q.; Zhang, Y.-W.; Xu, H. J.
The synthesis of the fused oxadiazepine analogs was similar to
Neurochem. 2012, 120, 9.
that of the fused oxadiazines10a (Scheme 1). Commercially avail-
4. (a) Josien, H. Curr. Opin. Drug Discovery Dev. 2002, 5, 513; (b) Harrison, T.;
able ester 9 was alkylated with iodide 10 under basic conditions
to give chloride 11. A Wittig reaction between 11 and aldehyde
12 proceeded smoothly under mild basic conditions to give an E-al-
kene, which was deprotected with TFA to provide acid 13. Com-
pound 13 was then coupled with the appropriate aminoalcohol
14 using EDCI/HOBt, and subsequently cyclized in the presence
of NaOMe to give primary alcohol 15. Compound 15 was reacted
with N-hydroxyphthalimide 16 under Mitsunobu reaction condi-
tions, followed by deprotection of the nitrogen with hydrazine hy-
drate to give an alkoxyamine intermediate which was cyclized to
the desired final product 7i under dehydrative conditions.
In summary, in an attempt to improve the hERG profile of lead
oxadiazine GSM 2, conformational modification of the core struc-
ture resulted in the identification of fused oxadiazepines as potent
GSMs with compound 7i showing comparable in vitro and in vivo
activity to 2 and an improved hERG profile. This compound was
highly efficacious in vitro and in vivo in rats with highly desirable
physical and pharmacological properties. Further exploration of
these compounds as potential drugs to treat Alzheimer’s disease
is underway and will be the subject of future communications.
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Acknowledgments
We thank Drs. Andrew Stamford, Christopher Boyce and Chad
Bennett for comments on the preparation of the manuscript. We
thank Dr. Eric Parker for his strong support of the program. We
thank Drs. Alexei Buevich and Li-Kang Zhang for technical support.
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