Synthesis and structure-activity relationship of 5-pyridazin-3-one phenoxypiperidines as potent, selective histamine H3 receptor inverse agonists

Article abstract of DOI:10.1016/j.bmcl.2011.11.118

Optimization of the R² and R⁶ positions of (5-{4-[3-(R)-2-methylpyrrolin-1-yl-propoxy]phenyl}-2H-pyridazin-3-one) 2a with constrained phenoxypiperidines led to the identification of 5-[4-(cyclobutyl- piperidin-4-yloxy)-phenyl]-6-meth

Full text of DOI:10.1016/j.bmcl.2011.11.118

Bioorganic & Medicinal Chemistry Letters 22 (2012) 1073–1077
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and structure–activity relationship of 5-pyridazin-3-one
phenoxypiperidines as potent, selective histamine H3 receptor inverse agonists
⇑
Ming Tao , Lisa D. Aimone, John A. Gruner, Joanne R. Mathiasen, Zeqi Huang, Jacquelyn Lyons,
Rita Raddatz, Robert L. Hudkins
Discovery Research, Cephalon, Inc., 145 Brandywine Parkway, West Chester, PA 19380, USA
a r t i c l e i n f o
a b s t r a c t
Optimization of the R2 and R6 positions of (5-{4-[3-(R)-2-methylpyrrolin-1-yl-propoxy]phenyl}-2H-pyri-
dazin-3-one) 2a with constrained phenoxypiperidines led to the identification of 5-[4-(cyclobutyl-piper-
idin-4-yloxy)-phenyl]-6-methyl-2H-pyridazin-3-one 8b as a potent, selective histamine H3 receptor
antagonist with favorable pharmacokinetic properties. Compound 8b had an excellent safety genotoxo-
city profile for a CNS-active compound in the Ames and micronucleus tests, also displayed potent H3R
antagonist activity in the brain in the rat dipsogenia model and robust wake activity in the rat EEG/
EMG model.
Article history:
Received 21 October 2011
Revised 22 November 2011
Accepted 28 November 2011
Available online 4 December 2011
Keywords:
Phenoxypiperidine
5-pyridazin-3-one
Histamine H3 receptor
CEP-26401 irdabisant
Sleep–wake
Ó 2011 Elsevier Ltd. All rights reserved.
Histamine plays an important role in different physiological
processes by four G-protein coupled receptors (H1R–H4R) and ex-
erts a variety of functions in the central nervous system (CNS).1
The histamine H3 receptor (H3R) is predominantly expressed in
the central nervous system (CNS), where it functions both as an
autoreceptor to modulate histamine release and as an inhibitory
heteroreceptor regulating the release of multiple neurotransmit-
ters including acetylcholine, histamine, norepinephrine, and dopa-
mine.2 H3R antagonists are currently being evaluated as potential
therapeutic agents for the treatment of cognitive deficits associ-
ated with a variety of CNS disease states including Alzheimer’s dis-
ease (AD), attention deficit hyperactivity disorder (ADHD) and
schizophrenia.3
nists/inverse agonists with excellent drug properties, safety and
in vivo profiles and advanced CEP-26401 1f as a clinical candidate,
which recently completed Phase I.5
As part of our H3R discovery project studying the structure–
activity relationships (SAR) around 1f, we synthesized and reported
the profile of the 5-regiomer 2a (5-{4-[3-(R)-2-methylpyrrolin-1-yl-
propoxy]-phenyl}-2H-pyridazin-3-one) (Fig. 2). Compound 2a had
Me
N
NC
N
Me
O
N
N
O
O
There have been a number of H3R antagonists advance to pre-
clinical and clinical development studies (Fig. 1). For example,
ABT-239 (1a) was nominated for clinical development for cogni-
tion promotion, but was halted because of QTc prolongation ob-
served in monkeys.1,3g,f MK-0249 (1b) from Merck has completed
Phase II trails for ADHD, AD and cognitive impairment associated
with schizophrenia.1,3g,f JNJ-31001074 (bavisant, 1c) is reported
in Phase II for ADHD.1a Pfizer has reported PF-03654746 (1d) in
Phase II trail for ADHD and was discontinued.3f GSK-189254 (1e)
enhanced cognitive performance pre-clinically and advanced to
Phase II for narcolepsy and was in early clinical trails for AD.3f
We recently reported a novel class of pyridazin-3-one H3R antago-
1b MK-0249
1a ABT-239
O
O
O
N
Et
N
H
N
N
1d PF03654746
1c JNJ-31001074
O
H
Me
O
N
N
N
N
N
Me
O
O
N
H
1e GSK-189254
1f CEP-26401
⇑
Corresponding author.
E-mail address: mtao@cephalon.com (M. Tao).
Figure 1. Structure of pre-clinical and clinical H3R antagonists.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.11.118

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M. Tao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1073–1077
H
N
The synthesis of 5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phe-
O
O
R2
N
nyl]-pyridazinone analog 8a is shown in Scheme 1. Alkylation of
4-hydroxy-phenyl ethanol 3 with 4-methanesulfonyloxy-1-Boc-
piperidine 4 in the presence of cesium carbonate in DMF afforded
the ether 5. Dess–Martin oxidation of 5, followed by condensation
with glyoxylic acid, and subsequent cyclization with hydrazine
hydrate yielded the pyridazinone 7.8 Deprotection of 7 with HCl
in dioxane, followed by reductive amination with cyclobutanone
in the presence of NaCNBH3 gave 8a. The 6-methyl substituted
pyridazinones 8b and 8d were synthesized as shown in Scheme
2. Mitsunobu coupling of 4-hydroxyphenyl acetone 9 and 4-hydro-
xy-1-Boc-piperidine 10 in the presence of 40% of diethylazodicarb-
oxylate in toluene and triphenylphosphine gave ether 11.
Condensation of 11 with glyoxylic acid, followed by cyclization
with hydrazine afforded the 6-methyl pyridazinone 12. Final
targets 8b and 8d were synthesized following the same steps as
described in Scheme 1. The synthesis of 5-[4-(1-cyclobutyl-piperi-
din-4-yloxy)-phenyl]-5-methyl-4,5-dihydro-2H-pyridazin-3-one 8c
is shown in Scheme 3. Alkylation of 11 with ethyl bromoacetate
in the presence of KHMDS, and cyclization with hydrazine yielded
the Boc-piperidinyl-6-methyl dihydropyridazinone 14. Deprotec-
tion and reductive amination as described in Scheme 1 gave 8c.
The 2-aryl substituted pyridazinone analogs 8e–h were synthe-
sized as shown in Scheme 4. The 5-(4-methoxy-phenyl)-6-
methyl-2H-pyridazin-3-one 16 was coupled with aryl bromide in
the presence of copper(I) iodide in DMF to give 2-aryl substituted
17.9 Deprotection of 17 with BBr3, followed by alkylation with 1-
Boc-4-methanesulfonyloxy-piperidine 4 afforded 12. Deprotection
and reductive amination as shown in Scheme 1 gave the 2-aryl
pyridazinones 8e–h. The synthesis of 24 is shown in Scheme 5. Su-
zuki coupling 2-chloropyridyl-4-boronic acid 20 with 2-hydroxy-
methyl-5-iodo-2H-pyridazin3-one, followed by substitution with
4-hydroxy-1-Boc-piperidine afforded the Boc-protected pyridyl
pyridazinone 22. Deprotection of 22 with HCl in dioxane, and
reductive amination gave compound 24.
N
N
O
N
N
O
2a R2 = H
1f
2b R2 = 2-Py
O
R2
N
N
N
R6
O
8
Figure 2. Pyridazinone H3R antagonists.
high affinity for both the human (hH3R Ki = 2.8 nM) and rat H3Rs
(rH3R Ki = 8.5 nM), but displayed low oral bioavailability (%F = 24)
in the rat.5 To explore the structure–activity relationships (SAR) of
5-pyridazin-3-one core R2 and R6 positions around 2a with a focus
on improving the pharmacokinetic (PK) properties, we identified
5-{4-[3-(R)-2-methyl-pyrrolidin-1-yl)-propoxy]-phenyl}-2-pyri-
din-2-yl-2H-pyridazin-3-one 2b (hH3R Ki = 1.7 nM and rH3R
Ki = 3.7 nM) with favorable oral bioavailability (%F = 78 in rat).6
However, 2b showed positive results in the Ames test, and was dis-
continued for development.
Most non-imidazole H3R antagonists reported share common
structural features: a basic tertiary amine attached to an aromatic
ring through a linker-space group; for example, the propyloxy, and
its constrained piperidinyloxy moiety.7,1a There have been many
reports describing a variety of histamine H3R receptor antagonist
pharmacophores in the past few years.4,1a One design strategy
we used in our search for 5-aryl analogs with a clean genotoxicity
profile and to improved PK was to rigidify the core to reduce in the
number of rotatable bonds.7d Based on this, we synthesized a series
of 5-regiomers 8 (5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-
2H-pyridazin-3-one) where the pyrrolidinylpropyloxy side chain
was constrained as the N-cyclobutylpiperidinyloxy moiety; a
maneuver explored by the Merck and Johnson and Johnson groups
on their respective cores.7e In this Letter, we report the synthesis,
SAR, selectivity, pharmacokinetics (PK) and the in vivo activities
The substituted pyridazinone analogs were tested using in vitro
binding assays by displacement of [3H]N-
a-methylhistamine
([3H]NAMH) in membranes isolated from CHO cells transfected
with cloned human H3 or rat H3 receptors.10 Rat and human H3R
binding data for the analogs is shown in Table 1 in comparison
with 2a.5 The N-cyclobutyl was established to be optimum and
was fixed for comparison while exploring SAR.11 As previously
reported, the 5-pyridazin-3-one regiomer of 2a had high affinity
(hH3R Ki = 2.8 nM, rH3R Ki = 8.5 nM), but displayed low oral
of
5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-6-methyl-2H-
pyridazin-3-one 8b in the rat dipsogenia model and in the rat
EEG/EMG model of wakefulness.
O
O
S
O
O
OH
a
b
HO
+
N
O
O
HO
N
O
O
O
O
5
3
4
O
HN
N
H
c
d
N
O
N
O
O
O
O
6
7
O
HN
N
N
O
8a
Scheme 1. Reagents and conditions: (a) Cs2CO3, DMF, 100 °C, 59%; (b) Dess–Martin [O], CH2Cl2, 0 °C to rt, 35%; (c) (i) HCOCOOHÁH2O, 135 °C; (ii) NH2NH2, EtOH, 90 °C, 34% in
2 steps; (d) (i) 4 N HCl in dioxane, 50 °C; (ii) cyclobutanone, NaCNBH3, DMF/MeOH/acetic acid, 60 °C, 1 h, 56%.

M. Tao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1073–1077
1075
O
Me
Me
a
O
N
OH
+
O
N
O
O
HO
O
O
10
9
11
O
O
R2
R2
c
N
O
b
N
N
H
Cl
N
N
O
NH
Me
Me
O
O
12
13
O
R2
d
N
N
N
Me
O
R2 = H 8b
= Me 8d
Scheme 2. Reagents and conditions: (a) DEAD, PPh3, THF, 0 °C?rt, 63%; (b) (i) HCOCOOHÁH2O, 135 °C; (ii) R2NHNH2, EtOH, 90 °C, 23% in 2 steps; (c) 4 N HCl in dioxane,
dioxane, 50 °C; (d) cyclobutanone, NaCNBH3, DMF/MeOH/acetic acid, 60 °C, 2 h, 80%.
O
(IC50 >30 lM, 5 CYP isoforms). Compound 8b had an improved
Me
O
PK profile in rat (%F = 68, Table 2). The dihydropyridazinone analog
8c showed ꢀ2–5-fold weaker affinity compared to 2a (hH3R
Ki = 15 nM, rH3R Ki = 13 nM). Compound 8c had acceptable
in vitro metabolic stability in liver microsomes from rat, dog and
a
HN
N
O
N
O
O
O
N
O
Me
O
14
11
human (t >40 min) and weak CYP inhibition (IC50 >30 lM, 5 CYP
½
O
isoforms). However, plasma levels of 8c were low following oral
administration (%F = 6, Cmax = 73 ng/mL, AUC = 370 ngÃh/mL), and
8c was extensively aromatized to the metabolite 8b (ꢀ40% in plas-
ma, Cmax = 34 ng/mL, AUC = 152 ngÃh/mL) in a rat PK experiment.
The N-2-methyl analog 8d, with high affinity, acceptable in vitro
metabolic stability in liver microsomes from rat, dog and human
O
b
HN
N
c
TFA
NH
HN
N
N
Me
O
Me
O
15
8c
(t >40 min) and weak CYP inhibition (IC50 >30 lM, 5 CYP iso-
½
Scheme 3. Reagents and conditions: (a) (i) KHMDS, BrCH2COOEt, THF, À78 °C; (ii)
NH2NH2, EtOH, 90 °C, 56% in 2 steps; (b) TFA, CH2Cl2, 0 °C to rt; (c) cyclobutanone,
NaCNBH3, DMF/MeOH/acetic acid, 60 °C, 2 h, 30%.
forms), was profiled in a rat PK experiment. Following oral admin-
istration of 8d (%F = 19, Cmax = 107 ng/mL, AUC = 337 ngÃh/mL) in
the rat, high levels of the demethylated active metabolite 8b
(ꢀ95% in plasma, Cmax = 68 ng/mL, AUC = 320 ngÃh/mL) was
formed in plasma, therefore the compound was not further ad-
vanced. The N-2-pyridyl analog 8e was synthesized with the goal
to find metabolically stable R2 analogs while maintaining a low
logP value (<3).6 Compound 8e had high affinity in both human
and rat H3R and displayed acceptable in vitro metabolic stability
bioavailability in the rat (%F = 24).5 Replacement of (R)-2-meth-
ylpyrrolidinylpropoxy of 5-pyridazin-3-one with the constrained
cyclobutylpiperidinyloxy (8a) showed high affinity in both human
and rat H3R (hH3R Ki = 2 nM, rH3R Ki = 2 nM), but did not show
improvement on rat oral bioavailability (%F = 21) compared to 2a.
Further profiling showed 8a had moderate selectivity for the hERG
in liver microsomes from rat, dog and human (t >40 min), and
½
weak CYP inhibition (IC50 >30
filing showed 8e displayed an acceptable PK profile (Table 2), weak
hERG activity in a patch clamp assay (IC50 >30 M), and showed
high selectivity against hH1, hH2, and hH4 receptor subtypes
(<30% inhibition at 10 M). However, it had moderate affinity for
lM for 5 CYP isoforms). Further pro-
channel in a patch clamp assay (IC50 = 2 lM). Compound 8b, with a
methyl substitution at the 6-position, designed to increase the tor-
sional angle between the two rings, retained high affinity in both
human and rat H3R (hH3R Ki = 5 nM, rH3R Ki = 6 nM), and displayed
acceptable in vitro metabolic stability in liver microsomes from rat,
l
l
the muscarinic M2 subtype (IC50 = 0.34 lM) when screened against
dog and human (t >40 min), and showed weak CYP inhibition
½
O
O
O
R2
R2
N
N
N
N
HN
N
a
b
Me
18
Me
Me
Me
Me
O
OH
N
O
O
17
16
O
R2
d
c
R2
O
N
N
N
N
N
O
Me
Me
O
O
R2 = 2-py
= 6-Me-2-py
= 3-Me-2-py
8e
12
8f
8g
= 3-thiophenyl 8h
Scheme 4. Reagents and conditions: (a) K2CO3, ArBr, CuI, DMF, 150 °C, 36–87%; (b) BBr3, CH2Cl2, 0 °C to rt, ꢀ80%; (c) 4-methanesulfonyloxy-piperidine-1-carboxylic acid tert-
butyl ester, Cs2CO3, DMF, 100 °C, 40–70%; (d) (i) 4 N HCl in dioxane, dioxane, 50 °C, (ii) cyclobutanone, NaCNBH3, DMF/MeOH/acetic acid, 60 °C, 1 h, ꢀ70%.

1076
M. Tao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1073–1077
O
HO
O
OH
N
O
B
NH
N
O
HN
N
b
a
N
N
OH
+
N
O
I
Cl
N
N
O
Cl
21
22
19
20
O
O
d
c
HN
N
HN
N
Cl
H
NH
N
N
O
N
O
23
24
Scheme 5. Reagents and conditions: (a) Pd(PPh3)4, K2CO3, DME/water, 85 °C, 36%; (b) 4-hydroxy-piperidine-1-carboxylic acid tert-butyl ester, K2CO3, DMSO, 110 °C, 54%; (c)
4 N HCl in dioxane, dioxane, 50 °C; (d) cyclobutanone, NaCNBH3, DMF/MeOH/acetic acid, 60 °C, 1 h, 56%.
Table 1
and potentially improve hERG activity. However, this maneuver
showed much weaker H3R affinity compared to 8a.
5-Pyridazin-3-one in vitro binding data
O
Compound 8b was further tested for H3R functional activity
R2
using a [35S]GTP
cS hH3R binding assay and displayed full inverse
N
N
agonist activity with an EC50 = 1.1 nM.10 Compound 8b showed
X
N
R6
high selectivity against hH1, hH2, and hH4 receptor subtypes
O
(<20% inhibition at 10
nels and enzymes inhibited the
greater than 50% at 10 M. It showed acceptable hERG selectivity
with an IC50 value of 7 M and displayed acceptable drug-like
lM) and in a panel of 101 GPCRs, ion chan-
a-adrenergic receptor subtype
Compd Bond
X
R2
R6
hH3 (Ki, nM) rH3 (Ki, nM)
l
8a
8b
8c
8d
8e
8f
8g
8h
24
Double CH
Double CH
H
H
H
H
Me
2
5
2
6
l
properties with high water solubility (pH 2 and pH 7.4 >0.14 mg/
mL), low lipophilicity (clogP = 1.5), and high permeability in the
Caco-2 assay (Papp = 14.9 Â 10À6 cm/s). Compound 8b had low
binding to plasma proteins of rat (54%), dog (43%), and human
(58%) in vitro. In genotoxicity profiling, 8b did not induce mutation
in the Ames assay (Salmonella typhimurium strains TA98, TA100,
TA102, TA1535 and TA1537) both in the presence and absence of
rat liver S9 metabolic activation and did not induce micronuclei
in vitro in a micronucleus test in mouse lymphocytes with and
without metabolic activation.
Single
CH
Me 15
13
8
9
Double CH Me
Double CH 2-Py
Double CH 6-Me-2-py
Double CH 3-Me-2-py
Double CH 3-Thiophenyl Me
Double
Me
Me
Me
Me
4
3
4
6
3
10
17
8
N
H
H
14
20
Ki values are an average of two or more determinations. The assay-to-assay varia-
tion was typically within 2.5-fold.
The rat dipsogenia model was used as an in vivo surrogate mea-
sure of H3R functional inhibition in the brain following peripheral
administration, and the activity in this model may be predictive of
efficacy in cognitive models. Histamine and the H3R-selective ago-
71 GPCRs, ion channels and enzymes (MDS Pharma Services, Lead
Profiler). The methyl substituted N2-2-pyridine analogs 8f, and 8g
also showed high affinity for both human and rat H3R, good
in vitro metabolic stability, weak CYP inhibition (IC50 >30
lM, 5
nist, R-a-methylhistamine (RAMH), induce water drinking in the
CYP isoforms), and acceptable hERG selectivity (IC50 >30 M) in a
l
rat when administered either peripherally or centrally, an effect
that is blocked by H3R antagonists.12 Compound 8b inhibited
RAMH-induced water drinking, demonstrating potent inhibition
of central H3R following ip administration of compound with an
ED50 value of 0.02 mg/kg, consistent with the high brain exposure
patch clamp assay. However, it showed no improvement in rat
PK compared to 2a (Table 2). Further R2 modification, the N-2-
(3)-thiophene analog 8h also showed high affinity for both human
and rat H3R but had significantly lower solubility. Changing the
central phenyl ring to pyridyl 24 was designed to lower the clogP
Table 2
Pharmacokinetic properties in rat
2aa
8ba
8eb
8fb
8gb
ivc
t
(h)
1.0 0.1
0.8 0.1
9.6 1.0
1.5 0.5
3.4 1.5
25
1.3 0.0
1.4 0.1
13 1.3
1.1 0.1
1.7 0.6
16
1.1 0.1
1.1 0.2
11 2.4
½
Vd (L/kg)
CL (mL/min/kg)
4
4
poc
AUC (ng h/mL)
Cmax (ng/mL)
Tmax (h)
F (%)
4209 258
446 65
4869 524
673 44
1502 103
287 31
3.2 0.7
1273 300
332 129
2.0 1.0
1860 347
505 137
2.3 0.9
2
24
0
2
1
68
0
7
22
2
20
5
23
4
B/Pd
1.8 0.2
2.3 0.1
1.6 0.2
1.6 0.1
1.0 0.0
a
b
c
Administration at 1 mg/kg iv and 10 mg/kg po.
Administration at 1 mg/kg iv and 5 mg/kg po.
iv formulation (3% DMSO, 30% solutol, 67% phosphate buffered saline) oral formulation (50% Tween 80, 40% propylene carbonate and 10% propylene glycol).
d
B/P = brain to plasma ratio measured 6 h post 10 mg/kg ip dose.

M. Tao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1073–1077
1077
240
180
120
60
References and notes
*
1. For reviews see (a) Berlin, M.; Boyce, C. W.; de Lera Ruiz, M. J. Med. Chem. 2011,
54, 26; (b) Brown, R.; Stevens, D. R.; Haas, H. L. Prog. Neurobiol. 2001, 63, 637;
(c) Leurs, R.; Bakker, R. A.; Timmerman, H.; de Esch, J. Nat. Rev. Drug Disc. 2005,
4, 107; (d) Wijtmans, M.; Leurs, R.; de Esch, I. Expert Opin. Investig. Drugs 2007,
16, 967; (e) Esbenshade, T. A.; Fox, G. B.; Cowart, M. D. Mol. Interv. 2006, 6, 77;
(f) Esbenshade, T. A.; Browman, K. E.; Bitner, R. S., et al Br. J. Pharmacol. 2008,
154, 1166; (g) Hudkins, R. L.; Raddatz, R. Annu. Rep. Med. Chem. 2007, 42, 49; (h)
Raddatz, R.; Tao, M.; Hudkins, R. L. Curr. Top. Med. Chem. 2010, 10, 153; (i)
Brioni, J. D.; Esbenshade, T. A.; Garrison, T. R.; Bitner, S. R.; Cowart, M. D. J.
Pharmacol. Exp. Ther. 2011, 336, 38.
*
*
*
0
2. (a) Oda, T.; Morikawa, N.; Saito, Y.; Masuho, Y.; Matsumoto, S. J. Biol. Chem.
2000, 275, 36781; (b) Liu, C.; Ma, X.; Jiang, X.; Wilson, S. J.; Hofstra, C. L.; Blevitt,
J.; Pyati, J.; Li, X.; Chai, W.; Carruthers, N.; Lovenberg, T. W. Mol. Pharmacol.
2001, 59, 420; (c) Cowart, M. D.; Altenbach, R. J.; Liu, H.; Hsieh, G. C.; Drizin, I.;
Milicic, I.; Miller, T. R.; Witte, D. G.; Wishart, N.; Fix-Stenzel, S. R.; McPherson,
M. J.; Adair, R. M.; Wetter, J. M.; Bettencourt, B. M.; Marsh, K. C.; Sullivan, J. P.;
Honore, P.; Esbenshade, T. A.; Brioni, J. D. J. Med. Chem. 2008, 51, 6547; (d)
Cowart, M. D.; Faghih, R.; Curtis, M. P.; Gfesser, G. A.; Bennani, Y. L.; Black, L. A.;
Pan, L.; Marsh, K. C.; Sullivan, J. P.; Esbenshade, T. A.; Fox, G. B.; Hancock, A. J.
Med. Chem. 2005, 48, 38; (e) Nagase, T.; Mizutani, T.; Ishikawa, S.; Sekino, E.;
Sasaki, T.; Fujimura, T.; Ito, S.; Mitobe, Y.; Miyamoto, Y.; Yoshimoto, R.; Tanaka,
T.; Ishihara, A.; Takenaga, N.; Tokita, S.; Fukami, T.; Sato, N. J. Med. Chem. 2008,
51, 4780.
Veh
1
3
10
30
Compound 8b (mg/kg, i.p.)
Figure 3. Compound 8b-induced wake promotion. Cumulative wake time (min) for
4 h post dosing following administration of vehicle (Veh), Compound 8b in rats
chronically implanted with electrodes for recording EEG and EMG activity.
Mean + SEM; n = 6–7/group. ⁄p <0.05, Dunnett’s test versus vehicle.
3. (a) Arrang, J. M.; Garbarg, M.; Schwartz, J. C. Nature 1983, 302, 832; (b)
Lovenberg, T. W.; Roland, B. L.; Wilson, S. J.; Jiang, X.; Pyati, J.; Huvar, A.;
Jackson, M. R.; Erlander, M. G. Mol. Pharmacol. 1999, 55, 1101; (c) Bongers, G.;
Bakker, R. A.; Leurs, R. Biochem. Pharmacol. 2007, 73, 1195; (d) Wulff, B. S.;
Hastrup, S.; Rimvall, K. Eur. J. Pharmacol. 2002, 453, 33; (g) Lin, J.; Sergeeva, O.
A.; Haas, H. L. J. Pharmacol. Exp. Ther. 2011, 336, 17; (f) http://clinicaltrails.gov
(assessed July 2011).
4. Raddatz, R.; Tao, M.; Hudkins, R. L. Curr. Top. Med. Chem. 2010, 10, 153.
5. Hudkins, R. L.; Raddatz, R.; Tao, M.; Mathiasen, J. R.; Aimone, L. D.; Becknell, N.
C.; Prouty, C. P.; Knutsen, L.; Yazdanian, M.; Moachon, G.; Ator, M. A.; Mallamo,
J. P.; Marino, M. J.; Bacon, E. R.; Williams, M. J. Med. Chem. 2011, 54, 4781.
6. Tao, M.; Aimone, L. D.; Huang, Z.; Mathiason, J.; Raddatz, R.; Lyons, J.; Hudkins,
R. L. J. Med. Chem., in press. Nov. 22. [Epub ahead of print].
7. (a) Apodaca, R.; Dvorak, C. A.; Xiao, W.; Barbie, A. J.; Boggs, J. D.; Wilson, S. J.;
Lovenberg, T. W.; Carruthers, N. I. J. Med. Chem. 2003, 46, 3938; (b) Stark, H.
Expert Opin. Ther. Pat. 2003, 13, 851; (c) Celanire, S.; Wijtmans, M.; Talaga, P.;
Leurs, R.; de Esch, I. J. P. Drug Discovery Today 2005, 10, 1613; (d) Dvorak, C. A.;
Apodaca, R.; Barbier, A. J.; Berridge, C. W.; Wilson, S. J.; Boggs, J. D.; Xiao, W.;
Lovenberg, T. W.; Carruthers, N. I. J. Med. Chem. 2005, 48, 2229; (e) Nagase, T.;
Mizutani, T.; Sekino, E.; Ishikawa, S.; Ito, S.; Mitobe, Y.; Miyamoyo, Y.;
Yoshimoto, R.; Tanaka, T.; Ishihara, A.; Takenaga, N.; Tokita, S.; Sato, N. J.
Med. Chem. 2008, 51, 6889.
and potent rat H3R affinity. Histamine-producing neurons are an
important part of the monoaminergic arousal system and H3R
antagonists have been documented to increase wakefulness in
a number of species. However, potency of H3R antagonist in
the rat dipsogenia model has been reported to correlate with
lower levels of receptor occupancy compared with wake promo-
tion, which required much higher doses, and 80–100% receptor
occupy for robust wake activity.13,14 Compound 8b was evalu-
ated in the rat EEG/EMG sleep–wake model. Wake-promoting
activity in the rat was measured as previously described using
male Sprague Dawley rats surgically implanted for chronic
recording of EEG (electroencephalographic) and EMG (electro-
myographic) signals.15 Cumulative wake time for 4 h post dosing
was evaluated during the normal quiet period of the rat. While
the wake activity at 1 and 3 mg/kg was significantly greater than
vehicle, the corresponding slow-wave sleep onset values
(34.9 5.1, 35.3 7.0, and 45.0 7.5 for vehicle, 1, and 3 mg/kg
groups, respectively) were not different. Compound 8b signifi-
cantly increased waking at 10 and 30 mg/kg ip (167 11 min,
and 226 2.9 min) by 4 h AUC values (Fig. 3). Maximal cumula-
tive wake surplus at 30 mg/kg was 163 min at 6 h post dosing,
and dropped by only 20 min over the next 11 h (data not
shown). At 30 mg/kg ip, 8b demonstrated robust wake promo-
tion, with the treated animals being awake 94% of the time up
to 4 h post dose and the increase in wake time over vehi-
cle = 145 min which is 178% of the vehicle wake time (Fig. 3).
No hypersomnolence was observed in any group (data not
shown).
8. (a) Wermuth, C. G.; Schlewer, G.; Bourguignon, J. J.; Maghioros, G.; Bouchet, M.
J.; Moire, C.; Kan, J. P.; Worms, P.; Biziere, K. J. Med. Chem. 1989, 32, 528; (b)
Coates, W. J.; McKillop, A. Synthesis 1993, 334.
9. Sugahara, M.; Ukita, T. Chem. Pharm. Bull. 1997, 45, 719.
10. Bacon, E. R.; Bailey, T. R.; Becknell, N. C.; Chatterjee, S.; Dunn, D.; Hostetler, G.
A.; Hudkins, R. L.; Josef, K. A.; Knutsen, L.; Tao, M.; Zulli, A. L. US2010273779,
2010.
11. Hudkins, R. L.; Zulli, A. L.; Dandu, R.; Tao, M.; Josef, K. A.; Aimone, L. D.;
Haltwanger, C.; Huang, Z.; Lyons, J. A.; Raddatz, R.; Mathiasen, J. R.; Grunner, J.
A. Bioorg. Med. Chem. Lett., submitted for publication.
12. Clapham, J.; Kilpatrick, G. J. Eur. J. Pharmacol. 1993, 232, 99; (b) Medhurst, A. D.;
Atkins, A. R.; Beresford, I. J.; Brackenborough, K.; Briggs, M. A.; Calver, A. R.;
Cilia, J.; Cluderay, J. E.; Crook, B.; Davis, J. B.; Davis, R. K.; Davis, R. P.; Dawson, L.
A.; Foley, A. G.; Gartlon, J.; Gonzalez, M. I.; Heslop, T.; Hirst, W. D.; Jennings, C.;
Jones, D. N.; Lacroix, L. P.; Martyn, A.; Ociepka, S.; Ray, A.; Regan, C. M.; Roberts,
J. C.; Schogger, J.; Southam, E.; Stean, T. O.; Trail, B. K.; Upton, N.; Wadsworth,
G.; Wald, J. A.; White, T.; Witherington, J.; Woolley, M. L.; Worby, A.; Wilson, D.
M. J. Pharmacol. Exp. Ther. 2007, 321, 1032; (c) Le, S.; Gruner, J. A.; Mathiasen, J.
R.; Marino, M. J.; Schaffhauser, H. J. Pharmacol. Exp. Ther. 2008, 325, 902.
13. (a) Fox, G. B.; Esbenshade, T. A.; Pan, J. B.; Radek, R. J.; Krueger, K. M.; Yao, B. B.;
Browman, K. E.; Buckley, M. J.; Ballard, M. E.; Komater, V. A.; Miner, H.; Zhang,
M.; Faghih, R.; Rueter, L. E.; Bitner, R. S.; Drescher, K. U.; Wetter, J.; Marsh, K.;
Lemaire, M.; Porsolt, R. D.; Bennani, Y. L.; Sullivan, J. P.; Cowart, M. D.; Decker,
M. W.; Hancock, A. A. J. Pharmacol. Exp. Ther. 2005, 313, 176; (b) Medhurst, A.
D.; Atkins, A. R.; Beresford, I. J.; Brackenborough, K.; Briggs, M. A.; Calver, A. R.;
Cilia, J.; Cluderay, J. E.; Crook, B.; Davis, J. B.; Davis, R. K.; Davis, R. P.; Dawson, L.
A.; Foley, A. G.; Garlton, J.; Gonzalez, M. I.; Heslop, T.; Hirst, W. D.; Jennings, C.;
Jones, D. N.; Lacroix, L. P.; Martyn, A.; Ociepka, S.; Ray, A.; Regan, C. M.; Roberts,
J. C.; Schogger, J.; Southam, E.; Stean, T. O.; Trail, B. K.; Upton, N.; Wadsworth,
G.; Wald, J. A.; White, T.; Witherington, J.; Woolley, M. L.; Worby, A.; Wilson, D.
M. J. Pharmacol. Exp. Ther. 2007, 321, 1032; (c) Raddatz, R.; Hudkins, R. L.;
Mathiasen, J. R.; Gruner, J. A.; Flood, D. G.; Aimone, L. D.; Le, S.; Schaffhauser,
H.; Gasior, M.; Bozyczko-Coyne, D.; Marino, M. J.; Ator, M. A.; Bacon, E. R.;
Mallamo, J. P.; Williams, M. J. Pharmacol. Exp. Ther. 2011, 340, in press.
doi:10.1124/jpet.111.186585.
In summary, optimization of the 5-pyridazin-3-one R2 and R6
positions of 8 with constrained phenoxypiperidine amine led to
the identification of 5-[4-(cyclobutyl-piperidin-4-yloxy)-phenyl]-
6-methyl-2H-pyridazin-3-one 8b as a potent, selective histamine
H3 receptor antagonist. Compound 8b showed favorable pharma-
cokinetic properties with improved oral bioavailability (%F = 61
in rat), and displayed an excellent safety genotoxicity profile
for a CNS-active compound in the Ames and mouse lymphocyte
micronucleus in vitro tests. Compound 8b displayed potent H3R
antagonist activity in the brain in the rat dipsogenia model and
robust wake activity in the rat EEG/EMG model.
Acknowledgments
The authors acknowledge the support and contributions from
Edward R. Bacon, Mark A. Ator, Michael J. Marino, Mehran Yazdanizn,
Amy Decamillo, Bob Bendesky, Nathalie Bourrit, and Debra Galinis.
14. Le, S.; Grunner, J. A.; Mathiasen, J. R.; Marino, M. J.; Schaffhauser, H. J.
Pharmacol. Exp. Ther. 2008, 325, 902.
15. Edgar, D. M.; Seidel, W. F. J. Pharmacol. Exp. Ther. 1997, 283, 757.