Benzazepinone Nav1.7 blockers: Potential treatments for neuropathic pain

Article abstract of DOI:10.1016/j.bmcl.2007.09.032

A series of benzazepinones were synthesized and evaluated as hNa_v1.7 sodium channel blockers. Several compounds from this series displayed good oral bioavailability and exposure and were efficacious in a rat model of neuropathic pain.

Full text of DOI:10.1016/j.bmcl.2007.09.032

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 17 (2007) 6172–6177
Benzazepinone Nav1.7 blockers: Potential treatments
for neuropathic pain
Scott B. Hoyt,a,* Clare London,a Hyun Ok,a Edward Gonzalez,a Joseph L. Duffy,a
Catherine Abbadie,b Brian Dean,c John P. Felix,d Maria L. Garcia,d Nina Jochnowitz,b
Bindhu V. Karanam,c Xiaohua Li,a Kathryn A. Lyons,a Erin McGowan,b
D. Euan MacIntyre,b William J. Martin,b Birgit T. Priest,d McHardy M. Smith,d
Richard Tschirret-Guth,c Vivien A. Warren,d Brande S. Williams,d
Gregory J. Kaczorowskid and William H. Parsonsa
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Pharmacology, Merck Research Laboratories, Rahway, NJ 07065, USA
cDepartment of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA
dDepartment of Ion Channels, Merck Research Laboratories, Rahway, NJ 07065, USA
Received 6 August 2007; revised 4 September 2007; accepted 5 September 2007
Available online 11 September 2007
Abstract—A series of benzazepinones were synthesized and evaluated as hNav1.7 sodium channel blockers. Several compounds
from this series displayed good oral bioavailability and exposure and were efficacious in a rat model of neuropathic pain.
Ó 2007 Elsevier Ltd. All rights reserved.
Neuropathic pain is a chronic, debilitating pain state that
results from injury to the peripheral or central nervous
system. It is estimated to affect 4 million people in the
US, and can be triggered by a variety of events or condi-
tions, including diabetes, shingles and chemotherapy.1
Because few effective therapies exist, patients suffering
neuropathic pain are often prescribed anticonvulsants
or topical anesthetics as treatment. Optimized for other
indications, these agents typically offer only modest pain
relief, and frequently elicit dose-limiting CNS-based side
effects.
shown preclinical and/or clinical efficacy in the treat-
ment of neuropathic pain, thereby providing validation
for this approach.5–8
Recent data from human genetic studies have implicated
hNav1.7, a subtype located primarily in the PNS, as a
key constituent in pain signaling. Individuals with gain
of function mutations in SCN9A, the gene that encodes
hNav1.7, experience bouts of intense pain that are either
evoked by mild stimuli or spontaneous in nature.9,10 As
such, their symptoms resemble those presented by neu-
ropathic pain patients. In contrast, individuals with loss
of function mutations in SCN9A—human Nav1.7
knockouts—are viable, healthy, and normal in seem-
ingly every regard, save one: they have a complete
inability to sense pain.11,12 Collectively, these studies
provide compelling genetic validation for hNav1.7 as
an important pain target.
Neuropathic pain signaling begins with the aberrant fir-
ing of action potential bursts in damaged axons. The ini-
tiation and propagation of these action potentials
typically require the opening of voltage-gated sodium
channels (Nav1.x). Because they can inhibit action po-
tential firing, Nav1 blockers have been investigated as
treatments for neuropathic pain.2–4 Weak blockers such
as lidocaine, carbamazepine and ralfinamide have
Our goal is to develop hNav1.7 blockers as treatments
for neuropathic pain. Toward that end, we recently re-
ported the discovery of a structurally novel class of ben-
zazepinone hNav1.7 blockers.13 An exemplar of this
class, compound 1, displayed potent, state-dependent
block of hNav1.7 in vitro, blocked spontaneous
Keywords: Sodium channel; Nav1.7; Neuropathic pain; Benzazepinone.
*
Corresponding author. Tel.: +1 732 594 3753; fax: +1 732 594
5350; e-mail: scott_hoyt@merck.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.09.032

S. B. Hoyt et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6172–6177
6173
hydride and 2-iodopropane effected lactam alkylation
to give N-isopropyl derivative 4. Acid-catalyzed detrity-
lation of 4 then furnished an amine that was coupled with
N-Boc-D-2-OCF3-phenylalanine to yield 1.15 Finally,
exposure of 1 to standard conditions for N-Boc deprotec-
tion (TFA, CH2Cl2) gave an amine salt that could be
coupled with 4-fluoro-2-(trifluoromethyl)benzoic acid
to afford 5, or with other commercially available carbox-
ylic acids to provide products 6–19 (Table 1).
O
O
OCF3
N
N
H
*
*
NH
O
*
O
1
*
= putative sites of metabolic oxidation
Figure 1. Benzazepinone Nav1.7 sodium channel blocker 1.
Once synthesized, compounds were then assayed for
their ability to block hNav1.7. The extent of channel
block was determined in a functional, membrane poten-
tial-based assay that measures the fluorescence reso-
nance energy transfer (FRET) between two
membrane-associated dyes. Specific details of the exper-
imental protocols employed have recently been de-
scribed.16 Target compounds were also screened
against other ion channels that are known to impact car-
diac function. Because block of hERG K+ channels has
been associated with potentially lethal ventricular
arrhythmias, compounds were tested in a binding assay
that measures displacement of 35S-labelled MK-0499, a
known hERG blocker.17
neuronal firing in vivo, and was orally efficacious in a rat
model of neuropathic pain (Fig. 1). Because 1 exhibited
modestpharmacokinetics(PK)inratanddog, subsequent
work in this series has focused specifically on improving
PK. These efforts, described herein, have led to the dis-
covery of compounds that display improved oral bio-
availability and exposure as well as increased efficacy
in a rat model of neuropathic pain.
The oral exposure of 1 was modest in rat and dog (rat: PO
AUCN = 0.31 lM h/mpk; dog: PO AUCN = 0.27 lM h/
mpk) and was limited by relatively high rates of oxidative
metabolism and clearance (rat: Clp = 24 mL/min/kg; dog:
Clp = 15 mL/min/kg). We thus sought to increase expo-
sure by improving metabolic stability and reducing clear-
ance. To determine its primary sites of metabolic
oxidation, we incubated 1 in the presence of rat liver
microsomes. Mass spectral analysis of the major metabo-
lites revealed oxidation at the N-Boc tert-butyl group, at
one or more sites on the benzazepinone phenyl ring
(Fig. 1, C6–C9) and/or at the benzazepinone benzylic po-
sition (C5). Blocking or deactivating those sites became
the main focus of our chemistry efforts, described below.
Prior work had shown that hNav1.7 block was opti-
mized when the side chain incorporated a secondary
amide or carbamate. Knowing that, we synthesized a
series of analogs wherein the N-Boc group of 1 was re-
placed by a variety of secondary amides. In the alkyla-
mide series, analogs with sterically smaller R1 groups
(Table 1, compounds 6–7) displayed weak hNav1.7
block, while those with bulkier R1 groups (compounds
8–10) proved more potent. In the arylamide series, the
simple benzamide derivative 11 exhibited best-in-class
potency, but suffered from high activity in the MK-
0499 counterscreen. Several substituted benzamides
(compounds 12–15), albeit less potent, were consider-
Analogs of 1 wherein the N-Boc group had been replaced
were synthesized as shown in Scheme 1. The requisite
starting material, (R)-3-amino-2,3,4,5-tetrahydro-1H-
[1]-benzazepin-2-one 2, was prepared according to the
procedure of Armstrong and coworkers, then tritylated
to yield compound 3.14 Treatment of 3 with sodium
Table 1. Effect of the R1 group on hNav1.7 potency
O
O
OCF3
N
N
H
NH
R1
H
N
O
O
O
N
4
b
c
NHR
NHTr
Compound
R1
hNav1.7
(IC50, nM)
MK-0499
(% inh at 10 lM)
2 (R = H)
3 (R = Tr)
a
6
CH3
CF3
>1000
868
131
203
172
22
9
40
18
10
39
83
26
43
0
7
O
O
8
C(CH3)3
O
OCF3
O
OCF3
N
N
5
d
9
C(CF3)2CH3
(c-Pr)CH3
Ph
N
H
N
H
10
11
12
13
14
15
16
17
18
19
NH
Boc
NH
O
4-F-Ph
94
1
4-CF3-Ph
2-CF3-Ph
2-CF3-4-F-Ph
2-Pyridyl
175
98
F3C
F
128
177
453
>1000
>1000
31
61
44
15
Scheme 1. Reagents and conditions: (a) TrCl, Et3N, DMF (60%); (b)
NaH, DMF, 2-iodopropane, 0–60 °C (72%); (c) HCl, MeOH; N-Boc-
D-2-OCF3Phe, EDC, HOBt, i-Pr2NEt, THF; (d) TFA, CH2Cl2; 4-F-2-
CF3-benzoic acid, EDC, HOBt, i-Pr2NEt, THF.
4-Pyridyl
2-Pyrimidinyl
5-Pyrimidinyl

6174
S. B. Hoyt et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6172–6177
ably cleaner in that counterscreen. Finally, a wide vari-
ety of 5- and 6-membered heteroarylamides were pre-
pared. Of the selected examples listed in Table 1, only
2-pyridyl analog 16 exhibited potent hNav1.7 block.
H
N
O
O
O
N
N
N
N
N
a
b
c, d
I
20
N
21
22
Several of the more potent secondary amides displayed
improved rat PK profiles relative to N-Boc derivative
1 (Table 2).18 Alkylamides 9 and 10, for instance,
showed increases in oral bioavailability and exposure
(AUCN). In the arylamide series, while the simple benz-
amide 11 exhibited a modest profile, substituted benz-
amide 15 and 2-pyridyl derivative 16 both improved
upon the oral bioavailability (F%) and exposure of 1.
Along with 9 and 10, these amides were deemed useful
N-Boc replacements, and were incorporated into subse-
quent designs.
O
O
O
OCF3
N
N
N
e
NH2
N
H
NH
Boc
23
24 (fast-eluting diastereomer)
25 (slow-eluting diastereomer)
Scheme 2. Reagents and conditions: (a) NaH, DMF, 2-iodopropane,
0–60 °C (75%); (b) TMSI, TMEDA, I2, THF, À15 °C (66%); (c) NaN3,
DMF, rt (87%); (d) H2 (1 atm), 10% Pd/C, EtOH; (e) N-Boc-D-2-
OCF3Phe, BOP, i-Pr2NEt, CH2Cl2.
With a set of N-Boc replacements in hand, we turned
our attention toward blocking metabolic oxidation of
the benzazepinone core. As noted above, this oxidation
was thought to occur at the benzazepinone benzylic po-
sition, and/or at various unspecified sites on the lefthand
phenyl ring. Analogs wherein those positions were
blocked or otherwise deactivated were synthesized as
shown in Schemes 2 and 3.
H
N
O
NHBoc
O
26
H
N
H
N
O
O
a
9-Aza-benzazepinones were prepared from known inter-
mediate 20 (Scheme 2).19 Upon treatment with sodium
hydride and 2-iodopropane, 20 underwent alkylation to
yield N-isopropyl derivative 21. Exposure of that species
to tetramethylethylenediamine (TMEDA), iodotrimeth-
ylsilane (TMSI) and iodine then effected iodination alpha
to the lactam carbonyl to give 22. Nucleophilic displace-
ment of iodide with azide and subsequent hydrogenation
furnished racemic amine 23. That amine was then cou-
pled with N-Boc-D-2-OCF3-Phe under standard condi-
tions (BOP, i-Pr2NEt) to give two diastereomeric
products, 24 and 25, that were separable via flash chro-
matography on silica gel. For these compounds, the
absolute stereochemistry of the stereocenter contained
within the lactam (C7) was not determined.
NHBoc
NHBoc
Cl
27
28
Scheme 3. Reagents and conditions: (a) NCS, DMF, 0 °C to rt (96%).
Benzazepinones substituted at the C5 or C7 positions
were prepared from intermediates shown in Scheme 3.
((R)-8-Oxo-6,7,8,9-tetrahydro-5-oxa-9-aza-benzocyclo-
hepten-7-yl)- carbamic acid tert-butyl ester 26 was syn-
thesized via the procedure of DeVita and coworkers.20
Once in hand, it was then processed via the reaction se-
quence shown in Scheme 1 for the conversion of 3–1 to
yield 5-oxo analog 29 (Table 3). 7-Chlorobenzazepinon-
es were prepared from known ((R)-2-oxo-2,3,4,5-tetra-
hydro-1H-benzo[b]azepin-3-yl)- carbamic acid tert-butyl
ester 27.21 Treatment of 27 with N-chlorosuccinimide
(NCS) in dimethylformamide (DMF) resulted in regio-
selective chlorination to yield 28. That intermediate
was then subjected to the reaction sequence shown in
Scheme 1 for the conversion of 3–5 to yield 7-chloroben-
zazepinone products 30–37 (Tables 3 and 5).
Table 2. Rat pharmacokinetic data for secondary amides
O
O
OCF3
N
N
H
NH
R1
O
Compound R1
F (%) AUCN
a
b
Cmax Clp t1/2
c
d
Using the methods outlined in Schemes 2 and 3, we syn-
thesized a set of compounds wherein the C5–C9 posi-
tions were blocked or deactivated. As shown in Table
3, replacement of the C5 benzylic methylene unit with
oxygen afforded a compound (29) that was equipotent
to 1. Conversion from the benzazepinone core of 1 to
the 9-aza-benzazepinone of 24 and 25 was also well tol-
erated, with each diastereomer exhibiting potent
hNav1.7 block. Finally, chlorination at C7 was permit-
ted as evidenced by compound 30.
1
OC(CH3)3
C(CH3)3
24
16
42
35
18
50
0.31
0.16
0.50
0.65
0.13
0.23
0.45
0.42
0.30
0.14
0.33
0.67
0.15
0.24
0.43
0.55
24
31
23
20
43
66
51
43
2.3
1.8
2.5
1.4
1.5
2.1
2.6
1.2
8
9
C(CF3)2CH3
(c-Pr)CH3
Ph
10
11
12
15
16
4-F-Ph
2-CF3-4-F-Ph 86
2-Pyridyl 60
a (po, lM h/mpk).
b (lM).
c (mL/min/kg).
d (h).
Compounds 24, 25, 29, and 30 were submitted for rat
PK determination. Though the C5 benzylic methylene

S. B. Hoyt et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6172–6177
6175
Table 3. Effect of benzazepinone substitution on hNav1.7 potency
Table 5. Optimization of the benzazepinone R3 substituent
O
O
O
OCF3
O
N
N
X
N
H
N
H
R3
*
R2
Cl
Y
NH
O
NH
O
O
F3C
F
Compound
X
Y
R2 hNav1.7
MK-0499
(IC50, nM) (% inh at
*
Compound
R3
hNav1.7
(IC50, nM)
MK-0499
(% inh at 10 lM)
10 lM)
31
32
33
34
35
36
37
2-OCF3-Ph
2-F-Ph
169
30
7
32
3
1
C
C
N
N
C
CH2
O
R
R
H
H
H
H
35
44
80
79
17
17
7
29
24
25
30
CH2 Fasta
2,3-Di-F-Ph
2,4-Di-F-Ph
2,5-Di-F-Ph
2,6-Di-F-Ph
2,6-Di-Cl-Ph
146
>1000
232
290
107
CH2 Slowb
18
32
10
7
CH2
R
Cl 108
a Fast-eluting diastereomer.
b Slow-eluting diastereomer.
20
focused re-optimization of that domain (Table 5). Prior
work had shown that a lipophilic aromatic group, pref-
erably one bearing a 2-substituent, was required at R3
for optimal potency and PK. Accordingly, the 2-
OCF3-Ph and 2-F-Ph derivatives 31 and 32 were synthe-
sized and found to be potent hNav1.7 blockers. A series
of di-F-Ph and di-Cl-Ph analogs were also prepared.
Though the 2,4-di-F-Ph derivative 34 proved weak, the
others all displayed comparably potent hNav1.7 block.
Table 4. Rat pharmacokinetic data for selected compounds
O
O
OCF3
N
X
N
H
R2
Y
NH
O
O
Compound
X
Y
R2 F (%) AUCN
Clp
t1/2
a
b
c
Compounds from this optimized series displayed im-
proved rat PK profiles relative to benchmark 1. The 2-
OCF3-Ph and 2-F-Ph derivatives 31 and 32, for instance,
both exhibited increased oral bioavailability and expo-
sure (Table 6). While the 2,5-di-F-Ph compound 35 suf-
fered a bit in comparison, the analogous 2,6-di-F-Ph
and 2,6-di-Cl-Ph derivatives 36 and 37 provided addi-
tional gains in oral exposure and clearance.
1
C
C
N
N
C
CH2
O
H
H
H
H
24
25
4
0.31
0.19
0.03
0.35
0.71
24
40
46
24
13
2.3
2.0
1.9
1.4
3.0
29
24
25
30
CH2
CH2
27
31
CH2 Cl
a (po, lM h/mpk).
b (mL/min/kg).
c (h).
Compounds 32 and 35 proved to be highly efficacious in
a rat spinal nerve ligation model of neuropathic pain.22
In this assay, rats undergo surgical ligation and transec-
of 1 was considered a potential site of metabolic oxida-
tion, block of that site as in 29 actually yielded a com-
pound with higher clearance and lower oral exposure
(Table 4). Metabolic oxidation was also thought to oc-
cur at one or more sites on the benzazepinone phenyl
ring. To mute that metabolism, the phenyl ring was re-
placed with a more electron-poor pyridine as in 24 and
25. In accordance with prior results in this series, the
two diastereomers exhibited markedly different pharma-
cokinetic profiles. Neither one, though, showed any
improvement relative to 1. In the end, it was block of
a specific position—C7—that led to significant PK
improvement. When compared with 1, the 7-chloro
derivative 30 displayed reduced clearance, increased oral
exposure and a longer half-life, thus implicating C7 as a
likely site of oxidative metabolism.
Table 6. Rat pharmacokinetic data for selected compounds
O
O
N
N
H
R3
Cl
NH
O
F3C
F
a
b
F (%) AUCN Cmax Clp t1/2
c
d
Compound R3
31
32
35
36
37
2-OCF3-Ph
2-F-Ph
65
90
0.57
0.58
0.35
0.97
1.51
0.52
0.33
0.26
0.74
1.47
29
44
32
19
12
2.6
2.8
2.2
2.0
2.9
2,5-Di-F-Ph 42
2,6-Di-F-Ph 68
2,6-Di-Cl-Ph 70
At this point, we had identified several PK-enhancing
changes to the structure of 1 and were ready to combine
them. We thus synthesized a set of 7-chlorobenzazepi-
nones that incorporated the 2-CF3-4-F-benzamide
N-Boc replacement. Because substitution at the R3 posi-
tion was known to affect hNav1.7 block, we ran a
a (po, lM h/mpk).
b (lM).
c (mL/min/kg).
d (h).

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S. B. Hoyt et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6172–6177
Table 7. Pharmacodynamic efficacy data for selected compounds
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O
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32
35
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tion of the L5 spinal nerve as a means of initiating neu-
ropathy. Tactile allodynia is assessed both before and
one week after surgery using calibrated Von Frey
filaments. In rats that present significant allodynia, test
compound is administered orally, and reversal of
allodynia is then determined. When dosed orally at
10 mg/kg, compound 32 elicited significant reversal of
allodynia at 2 h post dose (Table 7; % reversal @ 2 h,
4 h = 56%, 28%, n = 8). Compound 35 was equally effi-
cacious at 2 h, and even more active at 4 hours post dose
(% reversal @ 2 h, 4 h = 55%, 61%, n = 8). These results
compare favorably with those we obtained using a
30 mg/kg dose of the clinical standard mexiletine (mex-
iletine: % reversal @ 2 h, 4 h = 40%, 24%).
Interestingly, several closely related analogs (36 and 37)
that exhibited comparable in vitro potencies and oral
exposures proved to be less efficacious in this model.
The various factors underlying this discrepancy are not
clear at this time.
In summary, we have reported studies aimed at improv-
ing pharmacokinetics in a previously disclosed series of
benzazepinone hNav1.7 blockers. These efforts have cul-
minated in the discovery of compounds that display sig-
nificantly improved oral bioavailability and exposure
when compared with a prior benchmark. Two com-
pounds from this series also exhibit good efficacy in a
rat model of neuropathic pain. Additional in vitro and
pharmacological characterization of these compounds
has been completed, and will be disclosed in due course.
18. Rat PK experiments were conducted as follows: test
compounds were typically formulated as 1.5 mg/mL solu-
tions in mixtures of PEG300/water or DMSO/PEG300/
water. Fasted male Sprague–Dawley rats were given either
a 1.0 mg/kg iv dose of test compound solution via a cannula
implanted in the femoral vein (n = 3) or a 3.0 mg/kg
po dose by gavage (n = 3). Serial blood samples were
collected at 5 (iv only), 15, and 30 min, and at 1, 2, 4, 6,
and 8 h post dose. Plasma was collected by centrifugation,
and plasma concentrations of test compound were deter-
mined by LC–MS/MS following protein precipitation with
acetonitrile.
Acknowledgment
We thank Ramona Gray and Joe Leone for their out-
standing technical contributions to this work.
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