2-Aryloxymethylmorpholine histamine H3 antagonists

Article abstract of DOI:10.1016/j.bmcl.2008.09.077

The synthesis and biological activity of a new series of 2-aryloxymethylmorpholine histamine H₃ antagonists is described. The new compounds are high affinity histamine H₃ ligands that penetrate the CNS and occupy the histamine H₃ receptor in rat brain.

Full text of DOI:10.1016/j.bmcl.2008.09.077

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5796–5799
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
2-Aryloxymethylmorpholine histamine H3 antagonists
*
Michael A. Letavic , John M. Keith, Kiev S. Ly, Pascal Bonaventure, Mark A. Feinstein, Brian Lord,
Kirsten L. Miller, S. Timothy Motley, Diane Nepomuceno, Steven W. Sutton, Nicholas I. Carruthers
Johnson & Johnson Pharmaceutical Research & Development L.L.C., 3210 Merryfield Row, San Diego, CA 92121-1126, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and biological activity of a new series of 2-aryloxymethylmorpholine histamine H3 antag-
onists is described. The new compounds are high affinity histamine H3 ligands that penetrate the CNS and
occupy the histamine H3 receptor in rat brain.
Received 8 August 2008
Revised 12 September 2008
Accepted 15 September 2008
Available online 24 September 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Histamine
Histamine H3 receptor
Histamine plays an important role in a variety of physiological
processes, including inflammatory responses, which are regulated
by the histamine H1 receptor,1 and gastric acid secretion, which
is controlled by histamine H2 receptors.2 The cloning of the human
histamine H3 receptor in the late 1990s,3 and reports that the his-
tamine H3 receptor is widely expressed in the CNS, led to numer-
ous reports on the therapeutic potential for histamine H3
agonists and antagonists. After much research, it is now clear that
histamine H3 receptors play a role in regulation of the sleep-wake
cycle and that histamine H3 antagonists may have therapeutic util-
ity for the treatment of a variety of conditions, including excessive
daytime sleepiness, ADHD, Narcolepsy and Alzheimer’s disease,
among other conditions.4
Following the cloning of the human histamine H3 receptor
numerous pharmaceutical companies initiated drug discovery ef-
forts designed to identify histamine H3 antagonists that were suit-
able for clinical trials.5 Much progress has been made in this area
and many chemotypes have been disclosed that are potent and
selective histamine H3 antagonists. Several recent reviews have de-
scribed medicinal chemistry efforts in this area.5–7 One class of his-
tamine H3 antagonists contains a tertiary basic amine linked to an
aromatic core by a three to five atom spacer. A few of the many
possible representative examples are shown below. These com-
pounds include 18 and 2,9 which incorporate a propyloxypiperidine
moiety present in many of the reported H3 antagonists, and com-
pounds 3,10 4,11 and 5,12 which have slightly different linkers to
the terminal basic amine. As part of our efforts to discover novel
histamine H3 antagonists we now report a series of high affinity
morpholine-based ligands that penetrate the CNS.
Using 6 as a conceptual starting point,13 we devised a variety of
strategies to prepare conformationally restricted linkers that might
generate useful H3 ligands. One of these strategies was to tether
the tertiary basic amine to the linker as shown in compound 7. This
led generally to compounds 8 and includes the morpholine-based
structures described below. We chose to prepare these morpholine
compounds based on the hypothesis that they may have improved
physical and pharmacokinetic properties as a result of the reduced
basicity of the morpholine ring nitrogen as compared to the piper-
idine ring nitrogen in compounds such as JNJ-5207852 (6).
The desired morpholines can be prepared by several routes,
including those shown in Schemes 1 and 2. Initially, we prepared
compounds 12 using a Mitsunobu reaction of racemic 2-hydroxy-
methyl-morpholine-4-carboxylic acid tert-butyl ester 10 with phe-
nols such as 9. While we could obtain compounds 12 using this
procedure, the Mitsunobu reaction was plagued by poor and vari-
able yields. Two alternative routes are also shown in Schemes 1
and 2. In these examples, the morpholine methanol 10 was either
condensed with 4-flourobenzaldehyde to give 14 or was activated
as the tosylate 15 and reacted with the appropriate phenol to ob-
tain the required intermediates. These intermediates were then
converted to the final products by simple reductive amination
reactions as shown in the schemes. Compounds 19 were prepared
by the route shown in Scheme 1 using the appropriate phenol.
Compounds 21 and 22 were prepared using the procedures shown
in Scheme 2 using (R)- or (S)-2-hydroxymethyl-morpholine-4-car-
boxylic acid tert-butyl ester, which was made by literature
methods.14
Rat and human in vitro binding and functional data for the
compounds prepared are detailed in Table 1.15,16 As can be seen
in the table, compounds 19, lacking the benzyl amine moiety,
have only weak affinity for the histamine H3 receptor. In addi-
* Corresponding author.
E-mail address: mletavic@its.jnj.com (M.A. Letavic).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.09.077

M. A. Letavic et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5796–5799
5797
Cl
O
N
N
O
1
N
N
N
O
O
N
2
3
NC
O
H
N
N
N
O
O
N
N
5
4
R
N
linker
N
N
R
X
O
O
O
N
N
N
R
JNJ-5207852
6, hH3 pKi = 9.24, Ki = 0.97nM
7
8
tion to this apparent requirement for the benzyl amine function-
ality for high affinity, branched alkyl groups at R2 appear to have
important interactions with the H3 receptor. It is interesting to
note that the meta substituted compounds 18a–c consistently
have lower affinity than the corresponding para substituted ana-
logs 12c–e.
O
O
N
N
N
a
c, d
b
H
O
O
O
N
O
O
O
HO
N
HO
R2
O
a
O
9
11
12
O
N
H
O
O
O
N
n
O
O
a, c, d
H
e
10
+
N
R2
F
O
O
20a, n = 1
20b, n = 2
13
14
Scheme 1. Synthesis of compounds 12. Reagents and conditions: (a) amine, NaBH(OAc)3, MeOH, 23 °C, 7 h, 62–93%; (b) 2-hydroxymethyl-morpholine-4-carboxylic acid tert-
butyl ester (10), PPh3, DEAD, CH2Cl2, 23 °C, 24 h, 0–34%; (c) TFA, CH2Cl2; (d) ketone or aldehyde, NaBH(OAc)3, MeOH, 23 °C, 29–97% (two steps); (e) NaH, DMF, 90 °C, 24 h,
64%.
O
O
N
N
O
N
O
N
OTs
OH
c, d
b
a
O
O
N
O
O
O
O
O
O
N
R2
O
O
17
18
10
15
Scheme 2. Synthesis of compounds 18. Reagents and conditions: (a) TsCl, CH2Cl2, DIPEA, pyridine, 18 h, 23 °C, 96%; (b) 3-morpholin-4-ylmethyl-phenol (16), Cs2CO3, DMSO,
50 °C, 4 h, 96%; (c) TFA, CH2Cl2, quant.; (d) ketone or aldehyde, NaBH(OAc)3, MeOH, 23 °C, 27–48%.

5798
M. A. Letavic et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5796–5799
Table 1
Binding data for the rat and human H3 receptors for compounds 12 and 18–22
R1
N
N
O
O
O
N
O
O
N
O
O
O
O
O
R1
N
R2
N
R2
19
20
21
22
R1
R2
Rat H3 Ki (nM)a
Rat H3 pA2
Human H3 Ki (nM)a
Human H3 pA2
a
a
Compound
12a
12b
12c
12d
12e
12f
18a
18b
18c
19a
19b
19c
19d
19e
19f
—
—
—
—
—
—
—
—
—
H
H
OEt
OEt
OEt
OEt
Me
Et
—
—
—
—
—
—
—
2100 1500
165 46
106 16
—
—
—
—
c-Pr
i-Pr
c-Bu
c-Pent
c-Pr
i-Pr
c-Bu
H
i-Pr
i-Pr
c-Bu
c-Pr
c-Pent
92 32
75
20
49
18
9
6
7
3
7.72 0.00
8.32 0.07
—
6.60 (n = 1)
7.10 (n = 2)
—
—
—
—
—
—
8.51 0.05
9.02 0.08
—
7.35 0.23
7.85 0.05
—
—
—
—
—
—
3.6 0.9
566 193
102 24
37 12
>10,000
4800 2100
—
562 99
277 59
—–
—
—
—
—
—
9000
0
7100 2300
>10,000
5300 2900
N
N
20a
c-Pent
9.8 2.9
8.80 0.08
2.2 0.1
2.5 0.3
9.48 (n = 2)
20b
c-Pent
10
3
8
8.81 0.14
9.83 (n = 2)
21
22
—
—
—
—
121
7.36 0.07
8.67 0.06
31
2
8.10 0.02
9.42 0.04
9.7 2.1
1.9 0.1
a
Except where indicated values are the means of at least three experiments in triplicate. Ki SEM is reported.
In the case of compound 12f both enantiomers were prepared
(21 and 22), and the (S)-isomer (22) was clearly better accommo-
dated by the receptor. As such, 22 was chosen for in vivo evalua-
tion in rats.
Figures 1 and 2 show dose response data for ex vivo receptor
occupancy and compound levels in the brain and plasma at one
hour following oral administration of compound 22 to rats. These
data indicate good plasma and brain exposures (Fig. 1) and robust
receptor occupancy after oral administration (Fig. 2). Compound 22
gave an 80% receptor occupancy in the ex vivo occupancy assay at
0.3 mg/kg (po). Considering that compound 22 has higher func-
tional activity at the human histamine H3 than at the rat receptor
110
100
90
80
70
60
50
40
30
20
10
0
0.001
0.01
0.1
1
10
100
Dose (mg/kg)
100
Figure 2. Ex vivo histamine H3 receptor occupancy data in rat striatum for
compound 22: oral dose dependency at 60 min. Results are represented as
average SEM of n = 3.
10
(human pA2 = 9.42 vs rat pA2 = 8.67) it is possible that the expo-
sure required for occupancy in human may be even lower than that
required in rat.
Figure 3 shows the brain and plasma concentrations of com-
pound 22 as a function of time in rats following oral administra-
tion. The data support our hypothesis that these morpholines
have improved pharmacokinetics as compared to more basic piper-
idines such as JNJ-5207852 (6), which has been reported to have
sustained drug levels in the brain (out to 48 h post dose) upon oral
administration to rats.17
Plasma
1
Brain
0.1
0.1
1
10
100
Dose (mg/kg)
In conclusion, we have demonstrated that this new series of 2-
aryloxymethylmorpholines are high affinity histamine H3 receptor
Figure 1. Plasma and brain concentrations for compound 22 in the rat after oral
administration at 60 min. Results are represented as average SEM of n = 3.

M. A. Letavic et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5796–5799
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1
0.1
0.01
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0
4
8
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Time (hr)
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expressed in SK-N-MC cells was determined by competitive radioligand
Figure 3. Plasma and brain concentrations for compound 22 as a function of time in
the rat after oral administration. Results are represented as average SEM of n = 3.
binding using [
125I]-iodoproxyfan as the radioligand. Cells expressing the
human H3 receptor were grown to confluence in tissue culture plates, washed
with phosphate-buffered saline, and scraped into 50 mL tubes. After
centrifugation, the supernatant was aspirated, and the pellets frozen and
stored at À80 °C. Thawed pellets were homogenized with a polytron tissue
grinder for 15 s in 50 mM Tris–HCl, 5 mM EDTA at pH = 7.5. The homogenate
was centrifuged at 1000g for 5 min. The supernatant was recovered and
centrifuged at 25,000g for 25 min. The resulting pellet was resuspended in
binding buffer (50 mM Tris–HCl, 5 mM EDTA, pH = 7.5). Membranes were
incubated with [125I]-iodoproxyfan (1 nM) in the presence or absence of test
compound for 1 h at room temperature. Reactions were stopped by filtration
through GF/B filter plates pre-soaked in 0.3% polyethylenimine and
subsequently washed with Tris 50 mM, 5 mM EDTA buffer at pH = 7.5. Plates
were dried for 1 h in a 55 °C oven. Scintillation fluid was added and the
radioactivity was counted in a Packard TopCount. Non-specific binding to the
H3 receptor was defined by radioactivity that was bound in the presence of
antagonists and that one member of the series shows a high level
of receptor occupancy after oral administration. In addition, these
morpholine-based histamine H3 antagonists have improved phar-
macokinetic properties over some of the previously described
propyloxypiperidines.
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