Development of CXCR3 antagonists. Part 3: Tropenyl and homotropenyl-piperidine urea derivatives

Article abstract of DOI:10.1016/j.bmcl.2007.10.109

The optimization of a series of 1-aryl-3-piperidinyl urea derivatives is described in which incorporation of tropenyl and homotropenyl moieties has led to significant improvements in activity and drug-like properties. Replacement of the central piperidine

Full text of DOI:10.1016/j.bmcl.2007.10.109

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 147–151
Development of CXCR3 antagonists. Part 3: Tropenyl
and homotropenyl-piperidine urea derivatives
Robert J. Watson,* Daniel R. Allen, Helen L. Birch, Gayle A. Chapman,
Frances C. Galvin, Louise A. Jopling, Roland L. Knight, Dorica Meier,
Kathryn Oliver, Johannes W. G. Meissner, David A. Owen,
Elizabeth J. Thomas, Neil Tremayne and Sophie C. Williams
UCB Inflammation Discovery, Granta Park, Great Abington, Cambridge CB21 6GS, United Kingdom
Received 10 October 2007; revised 29 October 2007; accepted 30 October 2007
Available online 4 November 2007
Abstract—The optimization of a series of 1-aryl-3-piperidinyl urea derivatives is described in which incorporation of tropenyl and
homotropenyl moieties has led to significant improvements in activity and drug-like properties. Replacement of the central piper-
idine with an exo-tropanyl unit led to the identification of compound 15 which provides a combination of excellent potency against
human and murine receptors, drug-like properties and pharmacokinetics, thus providing a valuable tool for the evaluation of
CXCR3 antagonists in models of human disease.
Ó 2007 Elsevier Ltd. All rights reserved.
The chemokine receptor CXCR3 binds and is activated by
the interferon c-inducible chemokines, CXCL9, CXCL10
and CXCL11, leading to cellular activation and chemo-
taxis.1,2 The receptor and its ligands have been implicated
in human diseases including multiple sclerosis,3 arthritis,4
IBD,5 asthma,6 COPD7 and transplant rejection.8
that there was an opportunity to further improve our
series by modifying this part of the template.
Our goal was to provide a series of potent and drug-like
molecules suitable for use in vivo to elucidate the poten-
tial of CXCR3 antagonists in inflammatory diseases. In
this paper, we describe the identification of replacements
for the cycloalkene which result in dramatically im-
proved physicochemical and DMPK characteristics
and potency (Fig. 1).
A number of groups have reported the development of
small molecule CXCR3 antagonists,9 although only
Amgen’s AMG487 has so far progressed into clinical tri-
als.10 This compound has been reported to have efficacy
in a murine bleomycin-induced cell recruitment assay.9a
In order to examine the cycloaliphatic area of the mole-
cule, we selected a set of 100 heteroatom-containing car-
boxylic acids and aldehydes based on similarity in shape
and volume to our commonly used myrtenyl right hand
side according to Verloop parameters.
In our previous publications,11 we have described the
development of the screening hit 1a into potent urea
1b and arylazole 1c derivatives. In the development of
these molecules we found that the bulky cycloalkenyl
moiety was important for potency; simpler cycloali-
phatic rings and aryl groups in this position gave rise
to much reduced potency. The cycloalkene brought with
it a contribution to high logD, often resulting in low sol-
ubility and limited metabolic stability. We therefore felt
Preparation of the substituted ureas 3 was performed as
shown in Scheme 1. The bis-trifluoromethyl urea 2 was
coupled with selected carboxylic acids, followed by
reduction with lithium aluminium hydride; or reacted
with aldehydes in the presence of sodium triacetoxy-
borohydride. In general, the products were found to
be inactive, however the cyclic sulfone 3a and the piper-
idines 3b and 3c were found to have interesting activity.
Keywords: CXCR3 antagonists; Piperidine urea; Tropanyl urea.
*
We were particularly intrigued that the modestly active
sulfonyl derivative 3a showed a dramatic improvement
Corresponding author. Tel.: +44 12 238 96515; fax: +44 12 238
96400; e-mail: bob.watson@ucb-group.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.10.109

148
R. J. Watson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 147–151
O
N
N
N
H
H
1a
CF
CF
3
3
O
N
N
N
CF
N
N
H
CF
3
3
N
H
H
N
S
1c
1b
Figure 1. Hit molecules.
F3C
F3C
F3C
O
O
a or b
N
N
NH
N
H
N
N
H
H
H
R
2
F3C
3
O
R =
S
O
N
N
3c 60 nM
O
O
3a
3b
180 nM
1100 nM
Scheme 1. Reagents and condition: (a) i—RCO2H, HOBT, EDC,
DMF, 50–85%; ii—LAH, THF, reflux; (b) RCHO, NaBH(OAc)3,
DCE, 4A molecular sieves.
in physicochemical properties and metabolic stability
compared to the myrtenyl analogue 1b (Table 1). The
properties translated into good pharmacokinetics fol-
lowing oral dosing to mice, with 3a showing 52% bio-
availability and a plasma half-life of 5.5 h. The
acetylpiperidine 3b was more active and again showed
significantly improved properties over 1b and potency
was further improved in the pivaloyl derivative 3c,
though at the expense of both physicochemical proper-
ties and metabolic stability. These observations gave
us confidence that, by suitable replacement of the right
hand cycloalkene, we could significantly improve the
physicochemical properties in this series while retaining
potency.
Figure 2. Overlay of compound 3c (gold) and myrtenyl analogue
(green) carried out using Sybyl (Tripos).
We found that methylation at the 2, 3 and 4-positions
around the piperidine ring resulted in reduced potency
(not shown), and instead targeted bridged piperidine
derivatives, which appeared to fill a similar volume to
the myrtenyl group. We therefore selected tropanyl
and homotropanyl derivatives for synthesis.
Construction of the required aldehydes was carried out
from commercially available N-methyl tropanone 4a
and N-methyl homotropanone 4b (Scheme 2). These
were N-demethylated and converted to carbamates, then
into their enol triflates 6 under standard conditions. The
aldehydes 7 were prepared in one step by carbonylation
in the presence of triethyl silane.12 These were coupled
with the urea building block 8 and the intermediate alke-
nyl products 9 deprotected and acylated in good overall
yields. Hydrogenation of the tropene derivative 9b gave
10 as a one-to-one mixture of endo and exo isomers.
While the overall profiles of the acylpiperidine deriva-
tives were encouraging, they were generally less potent
than equivalent myrtenyl derivatives, and modelling
showed that acyl piperidines and the myrtenyl ring
system occupied quite different volumes, the latter
appearing more globular (Fig. 2). We reasoned that
modification of the piperidine ring could restore potency.
The tropene derivatives 9b–9e gave good potency
against the human receptor as shown in Table 2.
Increasing the size of the terminal acyl group improved
activity at the expense of increased logD and impaired
metabolic stability. The N-acetyl homotropene 9f gave
excellent activity against both human and murine recep-
tors. In this case, the pivaloyl compound 9g offered no
advantage over 9f, giving significantly increased logD
and microsomal instability with no compensating in-
crease in potency.
Table 1. Comparison of potency and properties of compounds 1a and
3a–c
Compound hCXCR3 LogD Solb
CLINT
PPB
c
Kia (lM)
(lg/ml) (lL/min/mg)
1b
3a
3b
3c
0.026
1.1
5.1
2.8
3.1
4.2
<1
1360
850
45
82
3.5
4.5
80
99.9
97
0.18
0.06
95
99
a GTPcS35 assay. Results are an average of two values.11
b At pH 6.5.
c Human microsomal clearance at 0.5 lM. LogD was measured by
octanol–water partition at pH 7.4.
We had previously found that favourable changes to
physicochemical properties could be achieved by incor-

R. J. Watson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 147–151
149
O
OTf
O
)
CHO
(
)
(
)
(
n
n
(
)
n
n
d
b, c
a
N
H
N
N
N
Me
O
OEt
6a n = 1
6b n = 2
O
OEt
7a n = 1
7b n = 2
4a n = 1
4b n = 2
5a n = 1
5b n = 2
R'
O
e, f, g
NH
N
H
N
F
H
8
O
R'
N
h
O
F3C
N
N
H
N
N
H
(
)
n
N
N
N
H
H
O
R''
10
9
Scheme 2. Reagents and conditions: (a) 1-chloroethylchloroformate, DCE, 80%; (b) EtOCOCl, Et3N, DCM, 100%; (c) LiHMDS, THF, Tf2NPh,
90%; (d) CO, Et3SiH, DMF, Pd(Oac)2, dppp, i-Pr2NEt 75%; (e) NaBH(OAc)3, DCM, 4A molecular sieves, 65–80%; (f) TMSI, CH3CN, reflux, 96%;
(g) RCOCl, Et3N, DCM; (h) H2, Pd/C, MeOH, 86%.
Table 2. Tropene and homotropene derivatives
Compound
R0
R00
n
LogD
CLINT (lL/min/mg)
Ki (lM) human
Ki (lM) mouse
9a
9b
9c
9d
9e
9f
3-F, 5-CF3
3-F, 5-CF3
3-F, 5-CF3
3-F, 5-CF3
3-F, 5-CF3
3-F, 5-CF3
3-F, 5-CF3
3-COMe
CO2Et
1
1
1
1
1
2
2
2
2
2
1
2
4.6
3.0
3.9
3.4
4.2
3.4
4.5
2.8
2.5
2.7
2.8
3.1
48
11
58
9
0.19
0.027
0.95
0.12
COMe
CO-i-Pr
CO-c-Pr
CO-t-Bu
COMe
CO-t-Bu
COMe
COMe
COMe
COMe
COMe
0.028
0.020
0.18
0.08
60
9
0.018
0.009 3.2 (n = 5)
0.10
0.038 15.6 (n = 4)
9g
9h
9i
74
10
22
50
0
0.010
0.28
0.057
ND
3-O-i-Pr
3-F, 5-O-i-Pr
0.011
0.003
0.033
0.006
9j
10
15
0.12
0.007 2.9 (n = 5)
0.65
0.006 1.3 (n = 5)
7
n refers to the bridge position (Scheme 2).
poration of polar aromatic substituents11 and found
that such modifications were tolerated in the piperi-
dine–homotropene template. For example, the 3-acetyl
derivative 9h gave moderate activity while the isopropyl
ether 9i provided similar potency to 9f against both hu-
man and murine receptor, along with a reduction in
logD. A further improvement in potency was achieved
with the 3-fluoro-5-isopropoxy derivative 9j. On the ba-
sis of these results, we concluded that the homotropene
moiety provided a binding motif, which could tolerate a
range of aromatic substitutions and contributed to a
drug-like profile.
give the aminotropane 12. This was coupled with 3-flu-
oro, 5-trifluoromethylphenyl isocyanate and the product
deprotected to give 13 in excellent overall yield.
The N-acetyl homotropene aldehyde 14 was constructed
in good overall yield using the chemistry described in
Scheme 2. Reductive amination was efficiently achieved
using a titanium tetraisopropoxide mediated imine
formation followed by reduction with sodium triacetoxy-
borohydride,13 providing a convergent synthesis suitable
for the preparation of multigram quantities of 15.
Compound 15 provided similar potency against the hu-
man receptor compared with 9f, with an improvement in
activity against murine CXCR3. Both logD and micro-
somal stability were in the desired range and compounds
9f and 15 were compared to determine their suitability
as tools for in vivo pharmacology.
In metabolite identification studies on 9f we observed
several sites of oxidation around the central piperidine
ring. We rationalized that bridging or substituting this
moiety could protect from metabolism and further im-
prove the overall profile of the series. Modelling sug-
gested that an exo-tropanyl unit could be
accommodated and this was synthesized as shown in
Scheme 3.
Compounds 9f and 15 were orally dosed to Balb/c mice
at 30 mg/kg, and both compounds demonstrated excel-
lent bioavailability with 15 giving sustained plasma con-
centrations greater than 3 times its plasma pA2 value14
beyond the 12 h time point (Fig. 3). In order to translate
exposure to a pharmacological effect a pharmacody-
namic readout was also obtained using the murine
Commercially available tropine 11 was reacted with
phthalimide under Mitsunobu conditions. The product
was demethylated and protected as its N-Boc derivative
and the phthalimide group cleaved by hydrazinolysis to

150
R. J. Watson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 147–151
F
e, f
NMe
a, b, c, d
NBoc
H2N
NH.HCl
N
N
H
O
OH
H
O
F3C
12
13
11
CHO
g, h, i
j
N
N
H
O
5b
14
F
N
N
N
H
O
N
O
F3C
15
Scheme 3. Reactions and conditions: (a) Phthalimide, diisopropylazodicarboxylate, PPh3, DCM, 75%; (b) 1-chloroethyl chloroformate, DCE, 87%;
(c) Boc2O, DCM, Et3N, 95%; (d) hydrazine, ethanol; (e) 3-fluoro, 5-trifluoromethyl phenyl isocyanate, DCM, Et3N, 90%; (f) HCl, ether, MeOH,
95%; (g) AcCl, Et3N, DCM, 93%; (h) LiHMDS, Tf2NPh, THF, 90%; (i) CO, Et3SiH, Pd(OAc)2, dppp, DMF, 78%; (j) Ti(OiPr)4, THF,
NaBH(OAc)3, 75%.
compound 15 (30mg/kg p.o.)
compound 9f (30mg/kg p.o.)
compound 15 (30mg/kg p.o.)
compound 9f (30mg/kg p.o.)
10000
1000
100
10
80
60
40
20
0
1
0
2
4
6
8
10 12 14 16 18
0.0 0.3 0.5 1.0 2.0 4.0 7.0 10.0 16.0
time post p.o. dose (hours)
time post p.o. dose (hours)
Figure 3. Pharmacokinetics of compounds 9f and 15 following oral
dosing to Balb/c mice at 30 mg/kg.
Figure 4. Ex vivo inhibition of murine CXCR3 receptor internalisa-
tion, response to 3 nM CXCL11, following administration of com-
pounds 9f and 15 to mice (30 mg/lg po).
CXCR3 receptor internalisation assay (Fig. 4).15,16
Compound 9f demonstrated antagonism of the CXCR3
receptor internalisation response up to 4 h post dose
however this declined by 7 h post dose, while the supe-
rior pharmacokinetics of compound 15 translated into
antagonism of CXCR3 receptor internalisation at 10 h
post dose (Fig. 4). These data demonstrated a good cor-
relation between exposure and effect and discriminated
between the compounds in vivo, allowing us to select
15 for further evaluation.
Table 3. Selectivity profile of compound 15
Target
Ki (lM)
Fold difference
CXCR3
M1
M2
0.007
0.23
0.28
1.2
NA
33
40
M3
170
40
5HT1A
Na Channel
5HT5A
0.28
0.57
3.0
81
430
We found that 15 gave good aqueous solubility, human
plasma protein binding of 93% and only weak inhibition
of CYPs 3A4 (32 lM) and 2D6 (15 lM). Selectivity
against a panel of 50 receptors was determined and Ki
values obtained for those targets where the compound
gave greater than 50% inhibition at 10 lM.17 As shown
in Table 3, the compound was selective with less than
50% inhibition of 44 out of the 50 tested targets and a
minimum 33-fold separation from its activity against
the muscarinic M1 receptor.
When compared to the profile of our original screening
hit 1a and the latter derivative 1b it was apparent that
improvements in potency, physicochemical properties
and DMPK characteristics had been achieved as shown
in Table 4.
In conclusion, we have developed a series of highly po-
tent piperidinyl and tropanyl urea CXCR3 antagonists,
leading to the identification of compound 15, which pro-

R. J. Watson et al. / Bioorg. Med. Chem. Lett. 18 (2008) 147–151
151
Table 4. Comparison of compounds 1a, 1b and 15 showing improvements in potency and properties
Compound
hCXCR3 (lM) mCXCR3 (lM) Sol (lg/ml) LogD
CLINT (lL/min/mg) CYP 2D6 (lM) PPB (%) % F
1a
1b
15
0.110
0.07
0.007
0.40
0.40
0.006
0.1
0.7
4.95
5.1
72
105
7
0.6
5
>99.9
99.7
93
1
ND
70
40
400
3.1
1.85 log units
15
25
Fold change 1a–15 16
67
10
70
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