Synthesis and evaluation of pyridone-phenoxypropyl-R-2-methylpyrrolidine analogues as histamine H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2011.09.091

6-{4-[3-(R)-2-Methylpyrrolidin-1-yl)propoxy]-phenyl}-2H-pyridazin-3-one 6 (Irdabisant; CEP-26401) was recently reported as a potent H₃R antagonist with excellent drug-like properties and in vivo activity that advanced into clinical evaluation. A series of pyridone analogs of 6 was synthesized and evaluated as H₃R antagonists. Structure-activity relationships revealed that the 5-pyridone regiomer was optimal for H ₃R affinity. N-Methyl 9b showed excellent H₃R affinity, acceptable pharmacokinetics and pharmaceutical properties. In vivo evaluation of 9b showed potent activity in the rat dipsogenia model and robust wake-promoting activity in the rat EEG model.

Full text of DOI:10.1016/j.bmcl.2011.09.091

Bioorganic & Medicinal Chemistry Letters 21 (2011) 7076–7080
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and evaluation of pyridone-phenoxypropyl-R-2-methylpyrrolidine
analogues as histamine H3 receptor antagonists
⇑
Nadine C. Becknell , Jacquelyn A. Lyons, Lisa D. Aimone, John A. Gruner, Joanne R. Mathiasen,
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
Article history:
6-{4-[3-(R)-2-Methylpyrrolidin-1-yl)propoxy]-phenyl}-2H-pyridazin-3-one
6 (Irdabisant; CEP-26401)
Received 25 August 2011
Revised 19 September 2011
Accepted 21 September 2011
Available online 29 September 2011
was recently reported as a potent H3R antagonist with excellent drug-like properties and in vivo activity
that advanced into clinical evaluation. A series of pyridone analogs of 6 was synthesized and evaluated as
H3R antagonists. Structure–activity relationships revealed that the 5-pyridone regiomer was optimal for
H3R affinity. N-Methyl 9b showed excellent H3R affinity, acceptable pharmacokinetics and pharmaceuti-
cal properties. In vivo evaluation of 9b showed potent activity in the rat dipsogenia model and robust
wake-promoting activity in the rat EEG model.
Keywords:
Histamine H3
H3 Antagonists
CEP-26401
Pyridone
Ó 2011 Elsevier Ltd. All rights reserved.
Sleep-wake
Histamine elicits physiological responses mediated by four
G-protein coupled receptors (H1R–H4R) and exerts a variety of func-
tions in the central nervous system (CNS).1,2 H3Rs are highly ex-
pressed in the CNS and function as presynaptic autoreceptors
regulating histamine release and as presynaptic heteroreceptors
regulating release of multiple neurotransmitters including
acetylcholine, dopamine, norepinephrine and serotonin.3,4 H3R
antagonists therefore have potential use for treatment of a variety
of CNS diseases including sleep disorders, cognitive disorders,
attention-deficit hyperactivity disorder (ADHD) and Alzheimer’s
disease (AD).3,4
A number of H3R antagonists have advanced to pre-clinical and
clinical development stages (Fig. 1). For example, ABT-239 (1) was
nominated for clinical development as a cognition enhancing agent
but was ultimately terminated due to cardiovascular liabilities.5,11
GSK-189254 (2), a potent and selective H3R ligand advanced into
clinical trials for narcolepsy and AD.6,11 The Merck compound
MK-0249 (4) completed phase II trials for ADHD, AD and cognitive
impairments in schizophrenia.8,11 The Pfizer compound PF-
3654746 (5) failed a Phase II ADHD trial and was discontinued
while JNJ-31001074 (Bavisant, 3) is reportedly still in Phase II for
ADHD.7,9,11 We identified a novel class of pyridazin-3-one H3R
antagonists/inverse agonists with exceptional drug-like properties
and in vivo profiles.10 6-{4-[3-(R)-2-Methylpyrrolidin-1-yl)pro-
poxy]-phenyl}-2H-pyridazin-3-one 6 (Irdabisant; CEP-26401) was
selected as a clinical candidate and recently completed phase I.10
As part of our H3 discovery project studying the structure–activity
relationships (SAR) around 6, we actively pursued a variety of struc-
tural core modifications. One strategy, which is the focus of this
Letter, was to synthesize and evaluate the pyridone replacements
for the pyridazinone and evaluate the H3R binding SAR, pharmaco-
kinetics (PK), selectivity and drug-like properties for a series of pyri-
done-phenoxypropyl-R-2-methylpyrrolidine analogues.
A general synthesis of pyridones 9 and 10 is shown in Scheme 1.
Alkylation of 4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane-2-yl)-
phenol (7) with 1-bromo-3-chloropropane and the subsequent
reaction of the terminal chloride with (R)-2-methylpyrrolidine gave
compound 8.10 Suzuki coupling of 8 with various bromopyridones
provided analogs 9 and 10.12 N-Methyl-bromopyridone fragments
leading to compounds 9b-e were synthesized by simple N-methyl-
ation of their commercially available bromopyridones with MeI in
the present of K2CO3 in DMSO at room temperature. Syntheses of
the required bromopyridone precursors to compounds 10a–e are
shown in Scheme 2. N-Arylation of 11 with arylboronic acids pro-
vided compound 12a–b.13 N-Benzylation of 5-bromopyridone 11
with BnBr gave compound 13. N-Methylation of pyridone 14 with
MeI provided compound 15. Bromopyridone 17 was synthesized
in a three steps sequence: N-methylation of 3-bromopyridone 16
with MeI, then Suzuki coupling of the bromide group with
PhB(OH)2, followed by region selective bromination at the 5-posi-
tion with NBS.14a
⇑
Corresponding author. Tel.: +1 610 738 6240.
E-mail address: nbecknell@cephalon.com (N.C. Becknell).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.09.091

N. C. Becknell et al. / Bioorg. Med. Chem. Lett. 21 (2011) 7076–7080
7077
Me
NC
O
Me
N
H
N
N
N
O
O
2 (GSK-189254)
1 (ABT-239)
N
O
Me
O
N
N
O
N
N
CF3
O
N
3 (JNJ-31001074)
4 (MK-0249)
H
F
O
N
N
O
Me
Et
N
H
N
F
N
O
5 (PF03654746)
6 (CEP-26401)
Figure 1. Structures of pre-clinical and clinical H3R antagonists.
O
B
a, b
O
B
c
O
Me
O
N
O
OH
8
7
R1
N
R
N
O
O
R2
Me
N
O
Me
N
O
9
10
R = H, Me
a: R1 = Ph, R2 = H
b: R1 = 4-F-Ph, R2 = H
c: R1 = Bn, R2 = H
d: R1 = Me, R2 = CN
e: R1 = Me, R2 = Ph
Scheme 1. Reagents and conditions: (a) BrCH2CH2CH2Cl, K2CO3, CH3CN, 80 °C, 18 h; (b) (R)-2-methylpyrrolidine–HCl, NaI, K2CO3, CH3CN, 80 °C, 24 h, 88% two steps; (c)
bromopyridones, Pd(PPh3)4, Na2CO3, LiCl, PhCH3–EtOH–H2O (1:1:1.5), 100 °C, 18 h, 26–58% (except 9a = 8%).
Syntheses of compounds 20 and 22 are presented in Scheme 3.
N-Methylation of isoquinolinone 18 followed by region selective
bromination at the 4-position gave compound 19.14b Suzuki cou-
pling of 19 with 8 afforded compound 20. Suzuki coupling of 21
with 5-bromo-1-methyl-pyridin-2-one, and the subsequence
alkylation of the phenol with 1-bromo-3-chloropropane followed
by reaction of the terminal chloride with (R)-2-methylpyrrolidine
provided compound 22.
In general, the pyridone analogues bound the hH3R with single
digit nanomolar affinity comparable with the pyridazinone class.
N-methyl pyridone 9b (hH3R Ki = 0.7 nM, rH3R Ki = 8.5 nM) had
excellent H3R binding affinity for both human and rat receptors
(Table 1) whereas the N-H pyridone 9a (hH3R Ki = 7.7 nM, rH3R
Ki = 9.0 nM) had 11-fold weaker hH3R affinity but similar rH3R
affinity. The pyridone regiochemistry appeared to play an
important role in the H3R binding affinity. The 4-pyridone 9c
(hH3R Ki = 3.2 nM, rH3R Ki = 12 nM) and 6-pyridone regiomers 9d
(hH3R Ki = 4.9 nM, rH3R Ki = 21 nM) with the meta-carbonyl orien-
tation had comparable H3R binding affinities in both species. How-
ever, the 5-pyridone regiomer 9b had 5–7-fold higher hH3R affinity
compared to 9c and 9d. The 6-regiomer 9e (hH3R Ki = 18 nM, rH3R
Ki = 144 nM) was over 16-fold weaker for both human and rat H3R
than 9b. Thus, the optimum pyridone regiomer had the carbonyl
oriented at the para-position.

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N. C. Becknell et al. / Bioorg. Med. Chem. Lett. 21 (2011) 7076–7080
Ph
R
N
H
O
O
N
O
N
b
a
Br
Br
Br
12
11
13
a: R = Ph
b: R = 4-F-Ph
Me
N
Me
N
H
H
O
N
O
N
c, d, e
O
O
c
NC
Br
Br
NC
Br
Ph
Br
16
14
15
17
Scheme 2. Reagents and conditions: (a) ArB(OH)2, Cu(OAc)2, pyridine, molecular sieves, CH2Cl2, 80–88%; (b) BnBr, K2CO3, DMSO, rt, 98%; (c) MeI, K2CO3, DMSO, rt, 97%; (d)
Ph(OH)2, Pd(PPh3)4, Na2CO3, LiCl, PhCH3–EtOH–H2O (1:1:1.5), 100 °C, 17 h; (e) NBS, CHCl3, rt, 15 h, 78% (2 steps).
Me
Me
N
O
N
H
N
O
O
c
a, b
Me
Br
O
N
20
19
18
Me
N
Me
O
OH
d,e,f
B
OH
N
O
HO
21
22
Scheme 3. Reagents and conditions: (a) MeI, K2CO3, DMSO, rt, 2 h; (b) NBS, CHCl3, rt, 1 h, 42% (2 steps); (c) 8, Pd(PPh3)4, Na2CO3, LiCl, PhCH3–EtOH–H2O (1:1:1.5), 100 °C,
21 h, 37%; (d) 5-bromo-1-methyl-pyridin-2-one, Pd(PPh3)4, Na2CO3, LiCl, PhCH3–EtOH–H2O (1:1:1.5), 100 °C, 5 h, 66%; (e) ClCH2CH2CH2Br, K2CO3, CH3CN, 80 °C, 24 h, 98%; (f)
(R)-2-methylpyrrolidine–HCl, NaI, K2CO3, CH3CN, 80 °C, 21 h, 33%.
The effect of substitution on the pyridone ring was also investi-
gated (Table 2). Replacement of N-methyl with N-phenyl and
N-benzyl groups (10a–c) or substitution at the 3-position of the
pyridone ring with nitrile and phenyl (10d and 10e) gave no
improvement in hH3R or rH3R affinities with the exception of com-
pound 10d (rH3R Ki = 3.0 nM). Interestingly, the 3,4-fused phenyl
20 (hH3R Ki = 0.9 nM, rH3R Ki = 2.4 nM) displayed about a 3-fold
improvement in affinity compared to 3-phenyl 10e. Moving the
3-((R)-2-methylpyrrolidin-1-yl)-propan-1-ol fragment from the
4- to the 3-position as in 22 resulted in significant loss of H3R bind-
ing affinity, demonstrating that attachment of the pyridone at the
4-position of the central ring was clearly preferred.
hH3R selectivity over hH1R, hH2R, and hH4R subtypes
(Ki >10 M), acceptable drug-like properties with aqueous solubil-
ity at pH 7.4 (>0.3 mg/mL), stability in liver microsomes (human,
l
mouse, rat, and monkey t >40 min) and low to no inhibition of
½
cytochrome P450 enzymes (CYP1A2, 2C9, 2C19, 2D6, and 3A4
IC50 >30
The pyridone analogues were potent antagonists and inverse
agonists of H3R activity in vitro, in the guanosine 50-(
thio)
trisphosphate ([35S]GTP
S) binding assay, as demonstrated with
the pyridazinone series.15 Compound 9b inhibited R-
-methylhis-
tamine (RAMH)-induced [35S]GTP
S binding in membranes pre-
lM).
c
c
a
c
pared from CHO cells recombinantly expressing hH3R with a Kb,
Based on the target affinities, analogues 9a, 9b, 10a, 10c and 20
were screened for PK properties in the rat. Compared to the
pyridazinone series,10 the pyridone series showed poor rat PK pro-
files. Compounds 9a, 10a, 10c and 20 all suffered poor PK proper-
ties with high clearance (9a = 96 ml/min/kg, 10a = 138 ml/min/kg,
10c = 171 ml/min/kg, and 20 = 42 ml/min/kg), and low to no oral
exposure (9a %F = 0, 10a %F = 17, 10c %F = 0, 20 %F = 8). N-Methyl
9b demonstrated a slight improvement in the overall rat PK profile.
The oral exposure was low (%F = 11), however the iv intrinsic prop-
value of 0.2 nM and inhibited basal activity in this assay with
an EC50 value of 0.7 nM.
app
The rat dipsogenia model was initially used in the project as an
in vivo surrogate measure of H3R functional inhibition in the brain
following peripheral administration of compounds. Histamine and
the H3-selective agonist, RAMH induce water drinking in the rat
when administered either peripherally or centrally, an effect that
is blocked by H3R antagonists.16 9b potently and dose-dependently
inhibited RAMH-induced dipsogenia with an ED50 value of
0.03 mg/kg ip. Following the demonstration of potent in vivo H3R
functional activity in the brain, 9b was further evaluated for
wake-promoting activity in the rat.4,19,20
erties were acceptable (t = 1.4 h, Vd = 1.4 L/kg, CL = 11 mL/min/kg)
½
(Table 3). Following oral administration, the brain exposure was
acceptable (brain to plasma ratio B/P = 1.7) and following adminis-
tration of a 10 mg/kg ip dose 1 h brain levels of 2.9
achieved (B/P = 1.9), sufficient to allow for proof-of-concept
in vivo evaluation. Further, the N-methyl analog 9b had excellent
l
M were
A number of H3R antagonists promote wake activity in preclin-
ical species, and this effect has recently been reported in clinical
trials with the H3R antagonists pitolisant and MK-0249.17,18

N. C. Becknell et al. / Bioorg. Med. Chem. Lett. 21 (2011) 7076–7080
7079
Table 1
Table 2 (continued)
Human and rat H3Rs binding data for pyridone analogues15
Compounds
R
Regiomer
4-
hH3RKi (nM)a
rH3RKi (nM)a
R
Me
Me
N
O
O
N
10e
3.5
7.9
Ph
O
Compounds
R
hH3RKi (nM)a
rH3RKi (nM)a
Me
N
H
N
O
20
22
4-
3-
0.9
2.4
9a
7.7
0.7
9.0
Me
N
Me
N
O
O
9b
9c
8.5
12
O
112
>500
Me
N
a
Ki values are an average of two or more determination. The assay-to-assay
variation was typically within 2.5-fold.
3.2
Me
N
Table 3
Rat PK data for compound 9b
O
9d
9e
4.9
18
21
iv (1 mg/kg)
t1/2 (h)
Vd (L/kg)
1.4
1.4
11
Me
N
O
CL (mL/min/kg)
144
po (5 mg/kg)
AUC0–t (ng h/mL)
Cmax (ng/mL)
%F
867
190
11
a
Ki values are an average of two or more determination. The assay-to-assay
variation was typically within 2.5-fold.
B/P
1.7
Compound 9b was tested in the rat EEG/EMG model of wake pro-
motion as previously described (Fig. 2).4,19,20 Compound 9b in-
creased wake activity dose dependently at 10 and 30 mg/kg ip
based on cumulative wake 4 h post dosing (4 h AUC). AUC values
were 160 16 min at 10 mg/kg and 227 5 min at 30 mg/kg ip
compared to vehicle (114 9 min). At 30 mg/kg the treated ani-
mals were awake 95% of the time for 4 h post dosing, a two-fold in-
crease in wake time compared to the vehicle group. The maximal
cumulative wake surplus (time awake compared to the vehicle
group) at 30 mg/kg ip was 146 8 min reached at 6 h post dosing.
From 6 to 22 h post dosing, this group recovered sleep at a constant
rate of 6.3 min per hour, and at 22 h post dosing, retained 35% of
the surplus wake time (51 27 min, p <0.05 vs 6 h, paired t-test).
At 10 mg/kg ip, the maximum cumulative wake surplus was
Table 2
The substitution effect on pyridone ring15
3
R
Me
N
O
4
Compounds
R
Regiomer
4-
hH3RKi (nM)a
rH3RKi (nM)a
Ph
N
O
10a
1.8
4.7
19
F
10b
4-
9.6
O
O
N
Ph
N
10c
10d
4-
4-
2.0
3.4
10
Me
N
O
3.0
Figure 2. EEG wake activity of compound 9b. Compound 9b-induced wake
promotion; cumulative wake 4 h AUC values shown for each dose (mean + SEM,
n = 8, 5, and 10 for vehicle, 10, and 30 mg/kg groups). Compound administered i.p.
to rats with chronically implanted electrodes for recording EEG and EMG activity.
⁄p < 0.05 Dunnett’s post hoc vs. vehicle.
NC

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Regan, C. M.; Roberts, J. C.; Schogger, J.; Southam, E.; Stean, T. O.; Trail, B. K.;
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53 10 min at 3 h and was not different at 22 h (33 12 min). No
adverse EEG activity was observed in either treatment group.
In conclusion, a series of pyridone phenoxypropyl-R-2-methyl-
pyrrolidine analogues were synthesized and evaluated as H3R li-
gands. They were found to have excellent binding affinities for
both hH3R and rH3R. The pyridone series in general displayed
unacceptable rat PK properties compared to the pyridazinone ser-
ies. However, results from this study demonstrated that N-methyl
9b showed acceptable target affinity, PK exposure and pharmaceu-
tical properties for in vivo proof of concept studies.
11. http://clinicaltrials.gov (accessed July 2011).
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6. Medhurst, A. D.; Atkins, A. R.; Beresford, I. J.; Brackenborough, K.; Briggs, M. A.;
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P.; Dawson, L. A.; Foley, A. G.; Gartlon, J.; Gonzales, M. I.; Heslop, T.; Hirst, W.
D.; Jennings, C.; Jones, D. N. C.; Lacroix, L. P.; Martyn, A.; Ociepka, S.; Ray, A.;