Rodent selectivity of piperidine-4-yl-1H-indoles, a series of CC chemokine receptor-3 (CCR3) antagonists: Insights from a receptor model

Article abstract of DOI:10.1016/j.bmcl.2014.11.063

Rodent selectivity data of piperidine-4-yl-1H-indoles, a series of CC chemokine receptor-3 (CCR3) antagonists, are presented and discussed as part of an overall optimization effort within this lead compound class. Although attachment of an acidic moiety t

Full text of DOI:10.1016/j.bmcl.2014.11.063

Bioorganic & Medicinal Chemistry Letters 25 (2015) 229–235
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Rodent selectivity of piperidine-4-yl-1H-indoles, a series of CC
chemokine receptor-3 (CCR3) antagonists: Insights from a receptor
model
b
b
b
b
c
Jan M. Kriegl a, , Domnic Martyres , Marc A. Grundl , Ralf Anderskewitz , Horst Dollinger , Georg Rast ,
⇑
Bernhard Schmid c, Peter Seither d, Christofer S. Tautermann a
a Lead Identification and Optimization Support, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Str. 65, D-88397 Biberach a.d. Riss, Germany
b Medicinal Chemistry, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Str. 65, D-88397 Biberach a.d. Riss, Germany
c Drug Discovery Support, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Str. 65, D-88397 Biberach a.d. Riss, Germany
d Respiratory Diseases Research, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Str. 65, D-88397 Biberach a.d. Riss, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Rodent selectivity data of piperidine-4-yl-1H-indoles, a series of CC chemokine receptor-3 (CCR3)
antagonists, are presented and discussed as part of an overall optimization effort within this lead
compound class. Although attachment of an acidic moiety to the 1-position of the indole led to an overall
balanced in vitro profile, in particular reducing inhibition of the hERG channel, potency on the rat and
mouse receptor worsened. These findings could be rationalized in the context of a CCR3 homology model.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 29 September 2014
Revised 19 November 2014
Accepted 22 November 2014
Available online 27 November 2014
Keywords:
CCR3
Chemokine
Piperidine-4-yl-1H-indoles
Rodent selectivity
hERG
Receptor model
The CC chemokine receptor-3 (CCR3), a G-protein coupled
receptor (GPCR), plays an important role in the migration and
activation of eosinophils. Numerous in vivo studies suggest that
diseases such as asthma, allergic rhinitis or atopic dermatitis are
closely linked to high eosinophil levels.1,2 Therefore, inhibition of
CCR3 with small molecule antagonists has been proposed as a
valuable approach to treating these diseases. So far, many CCR3
antagonists spanning a variety of structural classes have been
published.2–6 However, it has been repeatedly reported that small
molecule chemokine receptor antagonists potent on the human
receptor display a lower potency on rodent receptor orthologues.1,7
In the absence of genetically modified animals expressing the
human receptor orthologue, species selectivity becomes a key
property to optimize before compound efficacy can be character-
ized in preclinical in vivo models such as models of allergic airway
inflammation.
An initial HTS screen using a human CCR3 binding assay8,9
yielded substituted 1-alkyl-4-aryl-piperidines 1 (see Supporting
material for synthesis details), from which piperidine-4-yl-1H-
benzimidazoles were developed (see Fig. 1). An initial potency
optimization resulted in the identification of compound 2.10 It
binds to human and mouse CCR3 with Ki values of 3.6 nM and
42 nM, respectively. Furthermore, compound 2 inhibits eotaxin-
induced Ca2+ influx into human eosinophils11 with an IC50 of
11 nM. Despite its short half-life in liver microsomes this com-
pound was chosen for in vivo proof of concept studies and showed
efficacy in a mouse model of chronic airway inflammation.12 To
further optimize the series, we sought to establish a further point
of substitution. This was achieved by substituting the benzimid-
azole ring of 2 with a C-linked indole.13–15 The indole analogue 3
displays comparable human CCR3 binding (cf. Table 1) and compa-
rable inhibition of Ca2+ influx into human eosinophils (IC50 34 nM).
The discussion of SAR around all four substitution positions of the
piperidine-4-yl-1H-indole core structure as indicated in Figure 2
follows. It should be mentioned that all molecules shown are
CCR3 antagonists, as confirmed by efficacy in the human eosino-
phil Ca2+ influx assay. The syntheses of compounds 2–34 are
described elsewhere.10,13–15
In the following letter we describe the optimization of piperi-
dine-4-yl-1H-indoles as CCR3 antagonists, with a special focus on
rodent selectivity and hERG inhibition of this compound class.
⇑
Corresponding author. Tel.: +49 7351 54 7504; fax: +49 7351 83 7504.
E-mail address: jan.kriegl@boehringer-ingelheim.com (J.M. Kriegl).
http://dx.doi.org/10.1016/j.bmcl.2014.11.063
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

230
J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
R3
F
1
F
R4
N
O
N
O
N
S
N
R2
F
F
R1
Figure 2. General formula of the class of piperidine-4-yl-1H-indoles.
2
N
compounds possessing substituents R2 = Et and R3 = F (compound
3, compounds 21–34) were selected for further investigation.
Like the majority of small molecule CCR3 antagonists published
to date,2–6 the piperidine-4-yl-1H-indoles also possess a sp3
nitrogen in the center of the molecule. This basic center has a cal-
culated16 pKa of around 9.8 (compound 3) and is situated between
two aromatic ring systems. This combination of features made the
series susceptible to significant interaction with the hERG channel.
Compound 3, which was further profiled in vitro as the hitherto
most advanced compound with respect to potency in binding
and cellular assays, inhibited the hERG channel in a manual patch
clamp assay17 with an IC50 of 30 nM. To reduce hERG inhibition we
pursued different strategies: first, we attenuated the pKa value of
the basic nitrogen by introducing electron withdrawing groups at
the 3 position of the central piperidine ring.18 Although the pKa
could be reduced significantly by a fluoro (experimental pKa 6.5)
or hydroxyl substituent (experimental pKa 7.6), the hERG inhibi-
S
N
F
Figure 1. Representative of the class of substituted 1-alkyl-4-aryl-piperidines (1)
and the result of a first potency optimization campaign (2).
Table 1 summarizes SAR around the 4 position of the thiophenyl
ring (R1), with all other substituents fixed (R2 = Et, R3 = F, R4 = H).
The unsubstituted analogue 4 shows a 10-fold reduced binding Ki
on human CCR3 compared to the fluoro derivative 3. Other small
substituents such as methyl (compound 5) or methoxy (compound
7) also lead to lower potency. This reduction in potency is most
prominent, however, for polar substituents such as hydroxyl 6 or
amide 8, suggesting that the phenyl moiety resides in a largely
lipophilic environment within the receptor with almost no space
for further attachments at the 4 position. Binding to the rat recep-
tor is shifted by at least a factor of 8 towards higher Ki values for all
examples shown here, whereas binding to the mouse orthologue
stays within a factor of 1–3.
tion remained above 80% at a compound concentration of 1 lM.
As a second strategy we introduced acidic substituents in the
periphery of the molecule. Zwitterionic CCR3 antagonists had been
reported from other groups,19,20 suggesting that it was possible to
reduce hERG inhibition by the introduction of an acidic moiety
without compromising potency on human CCR3. From previous
SAR on human CCR3 binding (cf. Tables 1 and 2) it was clear that
polar residues are not tolerated at positions R1, R2, and R3. This left
position R4 for further variations. The exact positioning of the
acidic moiety is crucial for its impact on hERG inhibition, as
observed for instance for a series of sertindole analogues.21 There-
fore, we included also small as well as lipophilic substituents to
fully explore the range of variations allowed at R4. The results for
representative examples are summarized in Table 3. Lipophilic
substituents such as methyl (compound 21) and phenyl (com-
pound 22) were tolerated, but they resulted in a 5–10 fold loss in
potency, as did an acetic acid substituent (compound 23). Remark-
ably, the zwitterionic compound 23 still inhibited hERG with an
IC50 value below 100 nM. In close analogy to the sertindole
The influence of variations at positions R2 and R3 is shown in
Table 2. A fluorine at position R3 leads to higher potency on the
rat and mouse receptor (compounds 3, 9–11), compared with their
H-substituted analogues (compounds 13–15) although changes in
human receptor potency are less pronounced. The introduction of
small groups such as CH3 or OMe (compounds 16 and 18) or more
polar groups (compounds 17–20) results in reduced human recep-
tor potency, which is pronounced with carboxylate 20. Noteworthy
is the introduction of the somewhat larger methoxy-propane moi-
ety at R2 leading to molecules having the highest potency on
human CCR3 in both the H- and F-substituted sub-series which
are summarized in Table 2 (compounds 11 and 15, both of which
are racemic). Compared to the ethyl analogue, potency on mouse
CCR3 is reduced. Due to the beneficial impact on rodent activity,
Table 1
SAR around R1
Compound
R1
hCCR3a binding Ki (nM)
mCCR3b binding Ki (nM)
rCCR3c binding Ki (nM)
Ki ratio mouse/human
Ki ratio rat/human
3
4
5
6
7
8
–F
–H
–CH3
–OH
–OMe
–CONH2
5.7
60
32
78
96
10
65
62
210
130
1300
260
880
630
2300
950
4000
1.8
1.1
2
2.8
1.3
2.9
44
15
20
30
9.9
8.8
450
R
2 = Et, R3 = F, R4 = H.
For standard errors of mean pKi values and a detailed analysis of the error propagation we refer to Table S3 of the Supporting material.
a
hCCR3 = human CCR3.
mCCR3 = mouse CCR3.
rCCR3 = rat CCR3.
b
c

J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
231
Table 2
SAR around R2 and R3
Compound
R2
R3
hCCR3 binding Ki (nM)
mCCR3 binding Ki (nM)
rCCR3 binding Ki (nM)
Ki ratio mouse/human
Ki ratio rat/human
3
9
10
–Et
–Me
–CH2OH
–F
–F
–F
5.7
33
99
10
37
18
260
730
1400
1.8
1.1*
0.2
44
22
14
*
11
12
–F
4.3
52
100
nt
12
24
O
O
*
–F
480
nt
nt
nt
O
13
14
–Et
–CH2OH
–H
–H
31
66
400
92
4600
1100
13
1.4
150
17
*
15
–H
4.8
24
420
5*
87
O
16
17
18
19
20
–Et
–Et
–Et
–Et
–Et
–CH3
–OH
–OMe
–CONH2
–COOH
46
86
190
310
4000
48
370
nt
380
nt
560
3700
nt
1700
nt
1
12
43
nt
5.6
nt
4.2
nt
1.2
nt
R
1 = F, R4 = H.
For standard errors of mean pKi values and a detailed analysis of the error propagation we refer to Table S3 of the Supporting material.
*
Species selectivity ratios which display an uncertainty greater than 5 fold.
Table 3
SAR around R4
Compound R4
hCCR3 binding Ki
(nM)
mCCR3 binding Ki
(nM)
rCCR3 binding Ki
(nM)
Ki ratio mouse/
human
Ki ratio rat/
human
hERG
(lM)
3
21
–H
–CH3
5.7
38
10
85
260
560
1.8
2.2
44
15
0.03
nt
22
23
67
650
5500
9.7
83
nt
*
O
O
61
74
1300
1.2
21
0.04
*
*
O
O
24
25
26
27
4.9
4.7
120
72
2600
1100
nt
26
15
nt
530
230
nt
2.7
N
N
N
46% @ 1
>3
*
*
N
O
O
6900
16
nt
O
N
630
4700
40
300
>10
*
O
R1 = F, R2 = Et, R3 = F.
For standard errors of mean pKi values and a detailed analysis of the error propagation we refer to Table S3 of the Supporting material.
example we further explored benzoic acids. The para benzoic acid
derivative 24 is very potent on the human CCR3 receptor, with a Ki
value of 4.9 nM. Compared to the acetic acid derivative 23, a signif-
icant reduction in hERG inhibition was observed. Unfortunately, rat
receptor potency was significantly reduced. Compound 24 was
more than 500 times less potent on the rat receptor than on the
human orthologue, and mouse receptor potency was reduced by
a factor of more than 20. These shifts are much higher than for
any other analogue described so far. A similar profile is displayed
by compound 25, in which the carboxylic acid is replaced by the
bioisosteric tetrazole. Activity on human CCR3 was almost com-
pletely lost when the benzene ring was attached to the indole via
a methyl spacer (compound 26).
Like the benzoic acid analogues 24 and 25, the nicotinic acid
analogue 27 displayed a reduced hERG interaction. The shift
between hERG inhibition and human CCR3 binding is >600. This
value is much higher than the factor obtained for the starting point
of our optimization efforts (compound 3, factor 5). Frustratingly,
potency on rodent receptor orthologues was again reduced.
The steep structure-activity and structure-species selectivity
relationship around aryl substituents at position R4 can be ratio-
nalized in the context of a CCR3 receptor model. In such a model
the amino acid differences between human, rat, and mouse CCR3
can be mapped onto the putative transmembrane binding pocket.
To build a CCR3 receptor model we employed the recently pub-
lished structure of human CC chemokine receptor-5 (CCR5) in

232
J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
complex with maraviroc.22 Human CCR3 and CCR5 share
a
sequence identity of more than 50%, thus making CCR5 a well sui-
ted template for the generation of a human CCR3 homology model.
The homology modeling procedure is based on the sequence align-
ment shown in Figure S1 (cf. Supporting material). The compara-
tive modeling step was done by Modeller23 following standard
homology modeling procedures. The resulting homology model
was protonated with the Protonate3D procedure as implemented
in MOE.24 In a next step an energy optimization of the receptor
with high tethers on the heavy atoms was conducted, with a sub-
sequent reduction of the tethers. In a final step the protein back-
bone was kept fixed, and the side chains were energy minimized.
All minimizations were done with the MMFF94x force field as
implemented in MOE. Placement of the ligand (compound 27) into
the receptor was done manually by generating an ensemble of low
energy conformations of the ligand, followed by manual docking of
the various conformations into the receptor. Thereby the ionic
interaction between the positively charged center of the ligand
and Glu287 on transmembrane helix (TM) 7 was enforced. Confor-
mations which caused major clashes were discarded. This ionic
interaction is reported to be formed for most positively charged
chemokine receptor ligands and the corresponding glutamate in
TM 7.25 As a first plausibility check, the shape overlay with marav-
iroc in hCCR5 was used. As a second check we requested the forma-
tion of a salt bridge between the acidic part of the compound and
the positively charged His97 on TM 2, which we considered as the
most likely interaction partner for a negatively charged moiety in
this part of the TM binding region. Finally, the most plausible bind-
ing pose underwent stepwise geometry optimization (as previ-
TM4
TM1
Glu 287
TM7
A
His 97
ously described) and was followed by
a 60 ns molecular
dynamics simulation of the resulting complex in a DMPC lipid
bilayer employing the TIP3P water model with the Amber99sb
force field using GROMACS 4.5.26 The ligand did not undergo major
rearrangements, suggesting stability of the modeled binding mode.
Figure 3 shows the modeled binding mode of compound 27 in
the homology model of human CCR3, overlaid with the crystal
structure of human CCR5 in complex with maraviroc22 (Fig. 3 A).
The modeled binding mode displays the second salt bridge which
is formed between the carboxylate of compound 27 and the pro-
tonated His97. A sequence alignment (cf. Supplementary material)
indicates that rat and mouse CCR3 carry an asparagine at this posi-
tion, which is a much less favorable interaction partner for the neg-
atively charged carboxylate of 27 than the histidine in the human
receptor orthologue. This is reflected in the SAR around the aryl
group at position R4. The unsubstituted benzene ring (compound
22) resulted in a loss in potency on all three species orthologues.
Introduction of an acidic group (compounds 24, 25, and 27)
regained potency on human CCR3, but not on mouse or rat CCR3.
The presence of a salt bridge with His97 in the human orthologue,
which cannot be formed in rat and mouse CCR3, can explain this
observation. Substitution of the carboxylate by a neutral group
such as the corresponding ethyl ester should in general reduce
the potency shift between human and rodent receptors. This was
indeed observed in a consistent manner for several matched pairs
of carboxylates and ethyl esters, as summarized in Table 4. The
only exception was found when the carboxylate or the correspond-
ing ethyl ester is linked via a methyl spacer to the indole ring (com-
pounds 23 and 28). However, the reduced species selectivity for
the aryl ethyl ester analogues is accompanied by a reduced overall
potency on human CCR3. In the context of the CCR3 homology
model this can be attributed to the missing ionic interaction with
histidine His97.
B
Figure 3. (A) Superimposition of CCR5 in complex with maraviroc (grey; PDB entry
4MBS) and the homology model of CCR3 with compound 27 modeled into the TM
binding pocket (magenta). (B) Surface representation of the CCR3 binding pocket.
Key residues are shown as sticks.
it appears very unlikely that polar and/or large substituents at this
position would lead to very potent compounds, as also seen in the
SAR data (Table 2).
On the opposite end of the molecule, that is, at the R1 position,
the prediction of the correct binding mode is challenging. Various
placements of the flexible fluorophenylsulfanyl-propyl tail in the
receptor model are possible, and it is difficult to generate clear
evidence for the biologically relevant binding pose. However, some
observations provide support for our hypothesis. With the piperi-
dine-4-yl-1H-indole part in our proposed binding pose being fixed
between TMs 1, 2, 3 and 7, the fluoro-phenylsulfanyl-propyl tail
protrudes deeply into the predominantly hydrophobic transmem-
brane region of TMs 3, 6 and 7 and overlaps with the benzene ring
of maraviroc when overlaid with the hCCR5—maraviroc complex
(see Fig. 3A). The model suggests that there is a hydrophobic sub
pocket which offers only little space for moieties larger than fluo-
rophenyl and where the introduction of polar substituents might
be less favorable. This is indeed reflected in the SAR trends
observed around R1, in particular for mouse and rat CCR3 (cf.
Table 1). An increase in size and polarity resulted in compounds
that are less active than the reference molecule 3. The picture is
less consistent for human CCR3, where this trend is more obvious
for nonpolar substituents. Elimination of the fluorophenylsulfanyl-
propyl tail resulted in inactive compounds, suggesting that the
hydrophobic interactions formed by this part of the molecule con-
tribute significantly to the overall free energy of binding. However,
the model as such is not accurate enough to explain the beneficial
The homology model can be further used to interpret SAR at
positions R1 and R3 of the piperidine-4-yl-1H-indole series. It sug-
gests that the R3 position points towards a small, lipophilic sub
pocket between TM 7 and TM 1. From the surrounding residues

J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
233
Table 4
Matched pairs at R4
Compound R4
hCCR3 binding Ki
(nM)
mCCR3 binding Ki
(nM)
rCCR3 binding Ki
(nM)
Ki ratio mouse/
human
Ki ratio rat/
human
hERG
(lM)
O
O
O
23
61
74
1300
1.2
21
0.04
*
O
28
82
230
120
2600
630
740
1300
2600
11,000
4700
5200
2.8
26
23
40
14
16
*
O
24
4.9
120
16
530
94
2.7
0.3
>10
*
O
O
*
29
27
30
O
O
N
300
96
*
*
O
O
N
54
O
O
O
O
O
O
O
31
32
5
110
410
1700
6400
21
330
110
N
N
*
*
60
6.9
33
34
5
79
390
990
16
78
10
11.4
*
*
N
N
O
O
98
110
1.1
R
1 = F, R2 = Et, R3 = F.
For standard errors of mean pKi values and a detailed analysis of the error propagation we refer to Table S3 of the Supporting material.
impact of the fluoro substituent on the affinities which were
F
observed across all species (comparison of compounds 3 and 4).
F
N
The mouse/rat CCR3 structure-selectivity relationships around R1
O
N
N
N
cannot be directly explained by the amino acids (F83, Y113,
L117, W252, Y255) which define the sub pocket that is supposed
to be occupied by the substituted benzene ring and which are
conserved across species. We speculate that second shell effects
of amino acids not directly interacting with the ligand as well as
solvent effects may contribute to this distinct mouse/rat selectivity
behavior. The floor of the sub pocket is formed by the W252 from
the conserved WxP GPCR motif on TM 6, which is involved in the
conserved receptor-spanning water network.27,28 Therefore we
cannot rule out the involvement of water contacts in the binding
of the fluorophenyl substructure.
N
According to our superimposition model with maraviroc (cf.
Figs. 3 and 4), polar substituents on the distal alkyl chain of 3
should be tolerated in CCR3 binding. This would offer additional
opportunities for addressing hERG activity, as structural
modifications in this central part of the molecule had significant
effects on the hERG inhibition of maraviroc analogues.29 Ureas or
Figure 4. Superimposition of maraviroc (grey, 3D conformation taken from PDB
entry 4MBS), compound 27 (blue) and 36 (orange). The inset displays the 2D
structure of maraviroc (CAS entry 376348-65-1).
amides attached in c-position relative to the piperidine nitrogen

234
J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
Table 5
Branched piperidine-4-yl-1H-indoles
Compound Structure
hCCR3 binding Ki
(nM)
mCCR3 binding Ki
(nM)
rCCR3 binding Ki
(nM)
Ki ratio mouse/
human
Ki ratio rat/
human
hERG
(lM)
F
N
S
N
3
5.7
10
64
260
320
1.8
5.8
44
29
0.03
0.01
F
N
Cl
N
N
O
35
11
N
Cl
N
F
O
O
Cl
Cl
N
N
O
36
37
N
4
270
660
3500
3300
68
19
880
97
>3
N
F
O
N
O
Cl
Cl
N
N
O
N
34
0.1
N
F
For standard errors of mean pKi values and a detailed analysis of the error propagation we refer to Table S3 of the Supporting material.
Table 6
Profile of compound 27
analogy to maraviroc inhibited hERG with an IC50 below 2
Hence, this sub-series was not further pursued.
lM.
Property
Despite their low potency on the rodent receptors, the zwitter-
ionic unbranched aryl acids were further profiled in cellular and
in vitro pharmacokinetic assays. Table 6 summarizes the key
in vitro parameters of 27, which was found to be the most potent
compound among the zwitterionic piperidine-4-yl-1H-indoles in a
human eosinophil shape change assay30 whilst still showing
hCCR3 receptor binding Ki (nM)
hEos Ca2+ influx IC50 (nM)
hEos shape change IC50 (nM)
16
40
531.8
Stability in human/rat/mouse liver microsomes t1/2 (min) >90/>90/>90
Caco2 AB permeability (10À6 cm/s)
Caco2 efflux ratio
19
2.2
0.039
Solubility at pH 7.4 (mg/ml)
potency on the mouse CCR3 receptor below 1 lM. This particular
Off-target
hERG inhibition IC50
<50% inhibition @ 3
compound is stable in liver microsomes, and has a Caco2 perme-
(
l
M)
>10
ability of 19 Â 10À6 cm/s (apical to basolateral).
lM
77/79 off-targets
In summary, optimization of a series of potent CCR3 antagonists
ultimately led to compounds with markedly reduced hERG
inhibition. These compounds were however found to be weak
antagonists of the rodent receptor orthologues, thus hampering
their further progression into in vivo efficacy and tolerability stud-
ies. A binding model that potentially helps to explain the observed
potency differences against the rodent orthologues was generated.
It could provide useful information in guiding subsequent lead
optimization campaigns.
were tolerated as branched linker modifications, and replacement
of the 4-fluorophenyl residue by a 3,4-dichlorophenyl was found to
be advantageous for CCR3 potency. Overall, we were able to
establish a sub-series with robust CCR3 antagonistic efficacy.
Without further attachments to the indole nitrogen compounds
of this series displayed a similar species selectivity profile like their
unbranched counterparts such as 3 (cf. Table 5). Moreover, the
nicotinic acid derivatives 36 and 37 (Table 5) reveal a very similar
species selectivity profile to their direct analogues from the
thiophenyl-series (compounds 27 and 30). Hence, replacing the
acidic functionality by a methyl ester going from 36 to 37 signifi-
cantly reduces affinity to human CCR3, whilst binding to the
mouse and rat CCR3 orthologues is less affected. This helps to sup-
port our hypothesis that the branched piperidine-4-yl-1H-indoles
in fact bind to the CCR3 receptor in a very similar fashion as the
unbranched thio-propyl sub-series or maraviroc to CCR5 (Fig. 4).
Interestingly, in contrast to the series of maraviroc analogues29
we did not observe any impact of structural modifications made to
the urea or amide moiety on hERG inhibition. Compound 35 was as
potent as compound 3 in the hERG manual patch clamp assay, and
the 4,4-difluoro-cyclohexyl-amide derivative (not shown) in direct
Acknowledgments
We thank Dirk Gester, Melanie Trojahn, Frank Schmiedt, Bernd
Schulz, Silke Zeh, Petra Breh and Silvia Bender for excellent techni-
cal assistance, and Daniel Seeliger for setting up the molecular
dynamics simulations.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.11.
063.

J. M. Kriegl et al. / Bioorg. Med. Chem. Lett. 25 (2015) 229–235
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