Discovery of orally bioavailable NK1 receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(02)00956-3

Benzyloxyphenethylpiperazines are a new class of high affinity NK₁ receptor antagonists. Oral bioavailability and selectivity can be fine tuned by the nature of the substituents on the basic nitrogen atom. Addition of substituents with a carboxylic acid group led to very selective and orally active NK₁ antagonists free of interaction with L-type calcium channels.

Full text of DOI:10.1016/S0960-894X(02)00956-3

Bioorganic & Medicinal Chemistry Letters 13 (2003) 437–442
Discovery of Orally Bioavailable NK1 Receptor Antagonists
C. Genicot,a,* B. Christophe,b P. Collart,d M. Gillard,b L. Goossens,e J.-P. Henichart,e
M.-A. Lassoie,a F. Moureau,a M. Neuwels,a J.-M. Nicolas,d P. Pasau,a
L. Quere,a T. Ryckmans,a F. Stiernet,c T. Tavernee
and B. J. Van Keulena
aChemistry Department, UCB Pharma, Chemin du Foriest, B-1420, Braine-l’Alleud, Belgium
bIn vitro Pharmacology, UCB Pharma, Chemin du Foriest, B-1420, Braine-l’Alleud, Belgium
cA/R Pharmacology, UCB Pharma, Chemin du Foriest, B-1420, Braine-l’Alleud, Belgium
dIn vitro DMPK, R&D UCB Pharma, Chemin du Foriest, B-1420, Braine-l’Alleud, Belgium
e
´
Institut de Chimie Pharmaceutique A. Lespagnol, Universite de Lille II,
3 rue du Professeur Laguesse, F-59006, Lille, France
Received 23 July 2002; revised 30 September 2002; accepted 29 October 2002
Abstract—Benzyloxyphenethylpiperazines are a newclass of high affinity NK receptor antagonists. Oral bioavailability and
1
selectivity can be fine tuned by the nature of the substituents on the basic nitrogen atom. Addition of substituents with a carboxylic
acid group led to very selective and orally active NK1 antagonists free of interaction with l-type calcium channels.
# 2003 Elsevier Science Ltd. All rights reserved.
The endogenous undecapeptide Substance P (SP) elicits
a wide range of physiological responses both centrally
and peripherally. Through binding to NK1 receptors,
SP has been implicated in the induction of neurogenic
inflammation, in the transmission of nociceptive stimu-
lus, in the control of vomiting reflex and in affective
disorders.1 Thus, NK1 receptor antagonists are poten-
tially useful for the treatment of a wide range of clinical
diseases such as asthma, chronic pain, emesis, anxiety
and depression. The disclosure by Pfizer of the first two
non peptide NK1 antagonists, CP-96,345 and CP-99,994
(1)2 (Fig. 1), has spurred intensive research in this field.
To date, only two NK1 antagonists have reached phase
III clinical trials. A prodrug of MK-869 46 is currently
under development for the treatment of emesis whereas
CP-122,721 has shown robust antidepressant effect in
humans and is nowundergoing further evaluation.
Herein, we would like to report our own efforts directed
towards the discovery of orally bioavailable NK1
antagonists free of interaction with calcium channels.
Our starting point 6 suffered from several drawbacks
During the last fewyears, several other NK receptor
1
antagonists have been reported including benzyloxy-
aminopiperidines such as L-733,060 23 (Fig. 1). How-
ever, clinical development of the early NK1 antagonists
has been hampered by poor bioavailability and/or car-
diovascular side effects resulting from interaction with
L-type calcium channels. Subsequent efforts have been
devoted to the conception of NK1 antagonists with
improved in vivo activity and reduced interaction with
calcium channels such as MK-869 34 and CP-122,721 5.5
*Corresponding author: Tel.: +32-2-386-2797; fax: +32-2-386-2704;
e-mail: christophe.genicot@ucb-group.com
Figure 1. Selected NK1 antagonists.
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00956-3

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C. Genicot et al. / Bioorg. Med. Chem. Lett. 13 (2003) 437–442
(Fig. 2). In addition to its modest affinity for the NK1
receptor, absorption issues were anticipated due to the
poor aqueous solubility (s: 0.6 mg/mL, pH: 7.4) and high
lipophilicity (log D >5.0) of the compound. Sequential
removal of the benzyl and trimethoxybenzyl groups on
the piperazine ring led to 7, a molecule with a much
better affinity for the NK1 receptor and lower lipophili-
city (log D: 2.40). These results bear out the currently
established pharmacophoric model consisting of two
aromatic rings, a hydrogen bond acceptor and a nitro-
gen atom.7 However, the presence of the second nitro-
gen of the piperazine greatly improved solubility (s:
0.2 mg/mL for 7) and gave us the opportunity to further
modulate the physicochemical and ADME properties of
the molecule (vide infra).
(Scheme 3). Homogeneous hydrogenation with Wilk-
inson’s catalyst and deprotection of the nitrogen affor-
ded the expected products as a mixture of two
diastereoisomers that were separated by chromato-
graphy. Absolute configuration of the second chiral
center was determined by X-ray diffraction analysis on
the hydrochloride salt of 59a (Fig. 3).
Biology
As can be seen in Table 1, introduction of substituents
other than CF3 on the distal aromatic ring led to a loss
of affinity for the NK1 receptor. The drop was moderate
when chlorine or bromine atoms were used instead of
CF3 and was much more severe when fluorine or alkyl
groups (Me, tBu) were introduced instead of CF3.
Attempts to modify the position of the substituents was
unconvincing, for example when the CF3 groups were
shifted to the 2,5-position, a 100-fold loss of affinity was
observed (21). Hence, CF3 substituents in 3,5-position
appear to be optimal for high affinity.
Chemistry
Preparation of the benzyloxyphenethylpiperazines (9–
23) started from 2-phenyl-2-piperazinylethanol (8)
which was easily obtained by reaction of the N-pro-
tected piperazine with bromophenylacetate followed by
reduction of the ester (Scheme 1). Enantiomerically pure
8 was obtained by gently heating the N-protected
piperazine with the chiral styrene epoxide in ethanol.
The nucleophilic attack on the epoxide gave a mixture
of two regioisomers that were readily separated by
chromatography. Synthesis of substituted-phenyl or
heteroarylpiperazinylether (24–39) relies on the recently
reported arylboronic acid Mannich reaction.8 This ele-
gant three-component coupling reaction provided a
straightforward access to alcohol intermediates by sim-
ply stirring at room temperature in methylene chloride
the commercially available arylboronic acid, the N-Boc
piperazine and the hydroxyacetaldehyde. Alkylation of
the alcohols with benzyl bromides under standard con-
ditions followed by cleavage of the carbamate group
gave the target compounds 24.
Introduction of substituents in the para position of the
proximal phenyl was detrimental to the interaction with
the NK1 receptor (Table 2). Surprisingly, even a fluorine
Substituents on the basic nitrogen were introduced
through alkylation or acylation with the appropriate
reagents. Compounds bearing either a carboxylic acid
or a tetrazole were prepared from the corresponding
amide or nitrile following experimental procedures
described in the literature (Scheme 2). The Williamson
reaction could not be used for the preparation of the
a-methylsubstitutedbenzylethers (58–61) since rapid
decomposition of the benzylbromides took place.
Therefore condensation of the (R) or (S) alcohol 8 with
benzoic acid and methylenation of the resulting ester
with dimethyltitanocene9 yielded the enol ethers
Scheme 1. (i) K2CO3, DMF, rt, 84%; (ii) LAH, THF, ꢀ10ꢁC, 93%;
(iii) EtOH, reflux, 92%; (iv) NaH, NaI, THF, rt; (v) KOH, EtOH, 80 ꢁC;
(vi) CH2Cl2, 50ꢁC; (vii) NaH, NaI, THF, rt; (viii) KOH, EtOH, 80ꢁC.
Figure 2.

C. Genicot et al. / Bioorg. Med. Chem. Lett. 13 (2003) 437–442
439
Scheme 2. (i) Cl(CH2)2OCH2CONH2, K2CO3, DMF; (ii) HCl 1 N, reflux; (iii) BrCH2CONH2, Na2CO3, MeOH, reflux; (iv) BrCH2CO2Me, K2CO3,
DMF; (v) HCl 1 N, reflux; (vi) Me3SiNCO, THF; (vii) ClCO(CH2)3CO2Me, CH2Cl2/NaOH aq; (viii) KOH, MeOH, rt; (ix) HCHO, NaBH3CN,
AcOH, MeOH; (x) Br(CH2)3CH3, KI, NaHCO3, EtOH; (xi) Cl(CH2)2OH, Na2CO3, MeOH, reflux; (xii) Cl(CH2)2morpholine, NaI, NaHCO3,
EtOH; (xiii) 2-pyridinecarboxaldehyde, NaBH3CN, AcOH, MeOH; (xiv) ClCH2CN, H2NNHCO2CH3, MeOH then K2CO3, DMF, 140 ꢁC;
(xv) Br(CH2)4CN, K2CO3, DMF; (xvi) Me3SiN3, Bu2SnO, toluene, reflux.
observe subtle variations when the alkyl side chain con-
tained polar functions such as alcohols and amides.
Hence, compound 44 with an acetamide side chain dis-
plays a lower affinity than the parent compound
whereas compounds with an ethanolamine side chain
(46) or a longer amide (53) showsimilar affinity to 7.
The difference between 44 and 53 could lie in the pKa of
the basic nitrogen. pKa values for compounds 44 and 53
(6.2 and 7.4, respectively) indicate that the nitrogen
atom of 53 is half protonated at pH 7.4 whereas 44 is
not. A protonated nitrogen seems to be favorable but
not necessary for the interaction with the NK1 receptor.
This assumption is supported by the observation that
acylation of the nitrogen further reduces the affinity.
However, since compound 43 (urea) retains a reason-
able affinity, the protonated nitrogen is not involved in
an ionic interaction with the receptor but would rather
act as a hydrogen bond donor. The interaction with the
receptor is not stereoselective since we did not notice
any difference between the two enantiomers 44a and
44b. Compounds with acidic functions also have a
diminished binding affinity in comparison to simple
alkyl derivatives.
Figure 3. X-ray diffraction of 59a.
atom in this position gave a compound with much
weaker affinity than the unsubstituted analogue. Sub-
stitution in the meta or ortho position was well toler-
ated. Compound 36 with an a-naphthyl has similar
affinity to 7 whereas the one with a b-naphthyl 37 has
lower affinity. Replacement of phenyl with a heteroaryl
nucleus revealed that thiophene but not furan could be
used as a surrogate for phenyl. We then turned our
attention to the introduction of substituents on the basic
nitrogen of the piperazine nucleus (Table 3).
The incorporation of a methyl group in the benzylic
position next to the oxygen atom led to the creation of
a second chiral center. The four were prepared and
tested separately. The absolute configuration of the
carbon atom next to the piperazine nucleus has only a
Introduction of alkyl groups (CH3, (CH2)3CH3) did not
affect the affinity for the NK1 receptor. However, we did

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C. Genicot et al. / Bioorg. Med. Chem. Lett. 13 (2003) 437–442
Table 2. Affinities of compounds 24–39 for the NK1 receptora
Compd
R1
R2
R3
pKi NK1
7
H
F
Cl
CH3
OCH3
H
H
H
H
H
H
H
H
H
H
F
Cl
CH3
OCH3
H
H
H
H
H
H
H
H
H
8.8
8.3
8.8
9.0
8.2
8.7
8.8
8.5
9.0
7.1
7.3
8.5
8.2
8.8
7.5
8.1
6.8
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
H
F
OCH3
H
Cl
H
CH=CH
—
H
F
H
F
Cl
H
CH=CH
Scheme 3. (i) (EtO)3P(O)CH2COOEt, NaH, THF; (ii) Cp2TiMe2, tol-
uene/THF, rt; (iii) H2, RhCl(PPh3)3, EtOH; (iv) HCl aq 60 ꢁC;
(v) BrCH2CONH2, Na2CO3, MeOH, reflux then HCl 1 N reflux;
(vi) Cl(CH2)2OCH2CONH2, K2CO3, DMF, then HCl 1 N, reflux.
CH=CH
CH=CH
—
H
—
—
—
—
aDisplacement of [3H]-substance
expressed in CHO cells (n=3).
P from human NK1 receptors
Table 1. Affinitiesa of compounds 9–23 for the NK1 receptor
In parallel to the binding assay, physicochemical and
ADME properties of a selected set of compounds were
investigated in order to detect possible flaws that might
impair oral activity such as poor aqueous solubility, low
membrane permeation and high first pass effect (Table 5).
Solubility appears to be strongly influenced by the nature
of the substituents on the basic nitrogen. Introduction of
a butyl group on 7 (see 41) markedly reduced solubility.
Oddly enough, solubility at physiological pH was still low
when an acetamide group was anchored to 7 (see 44). This
is likely to be due to the reduced pKa value of the basic
nitrogen. Compounds with acidic functions (47, 54, 57,
60b) showed improved solubility with respect to 7. The
ability of the compounds to cross the intestinal mem-
brane was evaluated with Caco-2 cells. Papps values
(>10ꢀ6 cm/s) indicate that permeability is not an issue for
the compounds tested. Metabolic stability, assessed using
guinea pig microsomes, varied widely. Addition of an
acidic function considerably increases the metabolic sta-
bility of these compounds. On the other hand, it has
already been shown that the introduction of a methyl
group in the benzyloxy position is favorable.11 As far as
potential cardiovascular effects were concerned, we found
that compounds with acidic side chains did not interact
significantly with L-type calcium channels. Based on in
vitro screening, compound 60b was the most interesting.
Compd
R1
R2
R3
R4
pKi NK1
7
9
H
H
OCH3
OCF3
Cl
H
H
H
H
H
H
H
H
CF3
H
OCH3
CF3
H
H
H
H
Cl
Cl
Cl
Br
F
H
H
H
H
H
H
Cl
H
H
H
H
H
H
CF3
OCH3
H
CF3
H
H
H
H
H
H
Cl
Br
F
CF3
CH3
C(CH3)3
H
OCH3
Br
8.8
+
+
10
11
12
13
14
15
16
17
18
19
20
21
22
23
++
++
6.4
7.0
8.5
8.3
6.7
7.9
7.6
7.0
7.8
6.9
8.1
F
CH3
C(CH3)3
H
OCH3
Br
aDisplacement of [3H]-substance
P from human NK1 receptors
expressed in CHO cells (n=3); +: <20% inhibition at 10ꢀ5 M; ++:
20–50% inhibition at 10ꢀ5 M.
marginal influence on the affinity (Table 4). However,
it appears that the absolute configuration of the carbon
atom bearing the methyl group has a marked effect on
the affinity10 with the (S) enantiomers being much
more potent than the non-methylated derivatives while
the (R) enantiomers are essentially inactive. The gain in
affinity may be ascribed to a conformational effect and/
or to an additional hydrophobic interaction with the
NK1 receptor.
In vivo NK1 antagonist activity of the selected com-
pounds after oral administration was evaluated in the
guinea-pig (Table 6). In agreement with the in vitro
results, 60b was the most potent compound to inhibit
SP induced pulmonary edema. This compound dis-
played also a much longer duration of action being still
active after 4 h at 1 mg/kg oral dose.

C. Genicot et al. / Bioorg. Med. Chem. Lett. 13 (2003) 437–442
441
Table 3. Affinties of compounds 40–57 for the NK1 receptora
Table 5. Physicochemical/in vitro ADME properties and calcium
channel blocking effect of selected compounds
Cpds
Log Da
Sb
Pappsc
Clmd
pD02e
6
7
5.20
2.41
4.09
3.26
2.35
2.17
2.71
2.64
0.0006
0.2
0.001
0.002
1.3
>2.0
0.2
>2.0
—
—
54
—
5.38
4.60
4.27
<4.0
<4.0
<4.0
<4.0
2.1
5.2
7.9
9.6
4.2
2.6
8.7
41
44
47
54
57
60b
170
50
<10
<10
<10
<10
aLog Doct/w, buffer PBS, pH: 7.4.
bS: solubility (mg/mL, pH: 7.4).
Compd
R
pKi NK1
cPapps, caco-2 permeability (ꢂ10ꢀ6 cm/s).
dClint, intrinsec clearance (mL/min/mg protein).
7
40
41
42
H
CH3
(CH2)3CH3
CO2CH2CH3
CONH2
CH2CONH2
CH2CONH2
CH2CO2CH3
CH2CH2OH
CH2CO2H
CH2CN
8.8
8.8
8.6
7.2
7.7
8.0
8.1
7.4
8.7
7.8
8.1
7.2
7.8
7.8
8.7
8.5
7.8
8.0
+
eDetermined on isolated rat aorta [inhibition of contraction induced
by KCl (100 mM)].
43
44a(R)
44b(S)
45
46
47
48
49
50
51
52
53
54
55
56
57
Table 6. Effect of orally administered compounds on SP-induced
pulmonary oedema in guinea-pig12
Cpds
Dosea
30 minb
60 minb
120 minb
240 minb
41
43
47
54
57
60b
10
10
10
10
10
1
62c
15c
84c
53c
65c
77c
—
—
26c
68c
54c
65c
—
58c
—
53c
29c
20c
70c
47c
76c
61c
99c
75c
CH2G1
CH2G2
(CH2)2G3
CH2G4
(CH2)2OCH2CONH2
(CH2)2OCH2CO2H
(CH2)4CO2H
CO(CH2)3CO2H
(CH2)4G1
amg/kg.
bTime of compound preadministration (before SP-induced extravas-
ation).
8.2
c% of inhibition.
aDisplacement of [3H]-substance
expressed in CHO cells (n=3).
P
from human NK1 receptors
pounds. Indeed, 60b has been screened at 10 mM against
a variety of G-protein coupled receptor and ion chan-
nels and no significant interactions have been identified.
Table 4. Affinties of compounds 59–61 for the NK1 receptora
Acknowledgements
The authors wish to thank A. Debeuckelaere, N. Price,
F. Coloretti, J.-C. Hayez, V. Verbois, N. Pirlot and C.
Delaunoy for their skillfull synthetic assistance and L.
Ellens for physicochemical property measurements.
Compd
R
R0
pKi NK1
47
H
CH2CO2H
CH2CO2H
CH2CO2H
CH2CO2H
CH2CO2H
7.8
+
8.9
8.5
+
7.8
+
8.9
8.6
+
58a(R,R)
58b(S,S)
59a(R,S)
59b(S,R)
54
60a(R,R)
60b(S,S)
61a(R,S)
61b(S,R)
CH3
CH3
CH3
CH3
H
CH3
CH3
CH3
CH3
References and Notes
(CH2)2OCH2CO2H
(CH2)2OCH2CO2H
(CH2)2OCH2CO2H
(CH2)2OCH2CO2H
(CH2)2OCH2CO2H
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