Piperidinyl-nicotinamides as potent and selective somatostatin receptor subtype 5 antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.06.026

Nicotinamides of benzyl-substituted 4-aminopiperidines and their seven-membered analogs of generic structure 2 and 2′ have been discovered as potent and selective SST5 antagonists. The activity (K_i) ranges from 2.4 to 436 nM. Most compounds exh

Full text of DOI:10.1016/j.bmcl.2010.06.026

Bioorganic & Medicinal Chemistry Letters 20 (2010) 4521–4525
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Piperidinyl-nicotinamides as potent and selective somatostatin
receptor subtype 5 antagonists
André Alker a, Alfred Binggeli b, Andreas D. Christ c, Luke Green b, Hans Peter Maerki b,
Rainer E. Martin b, Peter Mohr b,
*
a F. Hoffmann-La Roche Ltd, Pharmaceuticals Division, Molecular Structure Research, CH-4070 Basel, Switzerland
b F. Hoffmann-La Roche Ltd, Pharmaceuticals Division, Medicinal Chemistry, CH-4070 Basel, Switzerland
c F. Hoffmann-La Roche Ltd, Pharmaceuticals Division, Discovery Metabolic & Vascular Diseases, CH-4070 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Nicotinamides of benzyl-substituted 4-aminopiperidines and their seven-membered analogs of generic
structure 2 and 20 have been discovered as potent and selective SST5 antagonists. The activity (Ki) ranges
from 2.4 to 436 nM. Most compounds exhibit decent physicochemical properties and follow a clear SAR pat-
tern. Interestingly enough, the receptor is strongly enantiodiscriminating and binds in the amino-azepane-
series only the (R)-enantiomer.
Received 10 May 2010
Revised 2 June 2010
Accepted 4 June 2010
Available online 8 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
GPCR
Somatostatin
Pancreas
Antagonists
Somatostatin (SST) or somatotropin release-inhibiting factor
(SRIF), discovered more than 30 years ago,1 is a mammalian pep-
tide hormone comprising 14 or 28 amino acids (SST-14 and SST-
28, respectively) that is widely distributed throughout the body
where it exhibits a plethora of biological functions, including anti-
proliferative, hormonal, and neuron-transmitter activity.2 Periphe-
ral SST-28 is predominantly expressed in the mucosa of ileum and
colon and in b-cells of the endocrine pancreas, whereas SST-14 can
be found in the foregut and enteric nervous system.3 As a hormone
SST is mostly inhibitory and, for instance, impedes the release of
growth hormone (GH), pancreatic insulin, glucagon, and gastrin.4
SST acts via five distinct G-protein coupled receptors (GPCR)
O
()n
O
N
N
N
H
N
H
N
O
N
S
O
H2N
R1
O
O
R2
R4
R3
O
NH2
2 n=1
2' n=2
O
1
N
N
N
O
O
F
13
* Corresponding author. Tel.: +41 61 688 64 59; fax: +41 61 688 87 14.
E-mail address: peter.mohr@roche.com (P. Mohr).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.06.026

4522
A. Alker et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4521–4525
SST1–5, which all have been cloned and characterized.4,5 Particu-
larly, SST acting via SST5 receptors has been found to activate
and up-regulate NMDA receptor function3,6,7 and to control hor-
monal secretions (e.g., insulin and growth hormone).3,7 For in-
stance in the pancreas, the SST5 receptor is prominently
expressed in up to 100% of all b-cells.8–11 Due to the numerous,
in detail not yet very well understood physiological functions of
SST, selective receptor specific ligands would be highly valuable
tool compounds to study and explore its rich pharmacology in
more detail.12 Whether they deserve also attention as putative
drugs remains to be established. We have recently reported on
the identification of receptor subtype 5 selective SST5 antagonists
containing a benzoxazole headgroup connected to a 4-amino-
piperidine unit,13 represented by the prototypic compound 1. It
soon became clear that 4-amino-piperidine is a key element of
the pharmacophore. In this Letter we disclose a further extension
of our work aiming at optimizing potency and selectivity as well
as fine tuning of physicochemical properties, and describe synthe-
sis and in vitro properties of piperidin-4-yl-nicotinamides 2 and
their homologous azepane-derivatives, respectively. The lead
structure was found by screening combinatorially prepared acyl-
ated 4-aminopiperidines to identify products with more favorable
pharmacokinetic behavior. This series complements the recently
published SAR study around the benzoxazoles14 and describes
the identification of lead compound 2Ai profiled in in vivo
studies.15
All compounds were synthesized in a straightforward manner
from the appropriate nicotinic acids 3 which were either commer-
cially available or prepared according to standard methods. Amide
coupling to BOC-protected aminopiperidine 4 or its homolgue 40
yielded after ensuing deprotection intermediates 5 or 50 which
were then transformed by reductive amination17,18 with the corre-
sponding m-ethoxy-benzaldehydes 6 into the target molecules 2
and 20, respectively (see Scheme 1). Intermediates 6, if not com-
mercially available or prepared by trivial transformations, were ob-
tained either by an SNAr reaction of the fluoride 1119 or by a
modified Suzuki-reaction (see Scheme 3). (4RS)-Aminoazepan-1-
tert-butylcarbamate 40, indispensable starting material to produce
the seven-membered analogs, was prepared from 4-piperidone hy-
drate as described in the literature.20,21 Resolution was achieved by
performing a reductive amination of ketone 7 with (S)-1-phenyl-
ethylamine 8 to give a roughly 1:1 epimeric mixture of 9 which
O
O
()n
()n
O
O
a)
N
OH
b)
NH
N
N
H
H2N
N
+
R1
R1
R2
3
R2
5 n=1
5' n=2
4 n=1
4' n=2
H
O
c)
6
R4
O
R3
O
()n
N
N
N
H
R1
O
R2
R4
R3
2 n=1
2' n=2
Scheme 1. Reagents and conditions: (a) benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate, N-ethyldiisopropylamine, THF, rt, 14 h, 60–95%; (b)
4 M HCl in dioxane, 2 h, rt, ꢀquant.; or 20% TFA in CH2Cl2, 2 h, rt, ꢀquant.; (c) 3 equiv Ti(OiPr)4, 2 equiv NaCNBH3, iPrOH, 14 h, rt, 40–70%.
H
H
N
N
O
a)
N
N
O
N
O
O
NH2
+
+
O
O
O
7
8
(S,S)-9
(R,S)-9
c)
H
b)
H
N+
O
O
N
O
H2N
N
N
O
O
H2N
+
O
O
O
(R)-4'
(S)-4'
Br
(S,S)-10
Scheme 2. Reagents and conditions: (a) 3 equiv Ti(OiPr)4, 2 equiv NaCNBH3, iPrOH, 14 h, rt, 80%, followed by preparative HPLC; (b) Pd/C (10%), H2 (1 atm), AcOH, 14 h, rt,
ꢀquant.; (c) crystallize S,S-9 (which leads to inactive isomers) in presence of 1 equiv of 4-bromobenzoic acid in CH2Cl2.

A. Alker et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4521–4525
4523
H
O
H
O
H
O
H
O
a)
b)
R4
R4
O
R4
O
R4
O
O
N
F
I
N
R3'
12
11
6 R3 = Imidazolyl
6 R3 = Aryl
Scheme 3. Reagents and conditions: (a) 2 equiv imidazole, K2CO3, DMSO, 110°, 2 h, 65–85%; (b) 2 equiv arylboronic acid, 20% tricyclohexylphosphine, 10% Pd(OAc)2, K3PO4,
toluene/water, 14 h, 110°, 80–90%.
could readily be separated by preparative HPLC into the (S,S)- and
(S,R)-isomer. Hydrogenolytic cleavage under acidic conditions led
then to homochiral (S)-4 and (R)-4, respectively. The absolute con-
figuration was determined by X-ray analysis of compound 10, the
p-bromobenzoate salt of (S,S)-9; this intermediate led to inactive
SST5 ligands (Scheme 2).22
Table 1
SST5-activity and physicochemical properties of nicotinamides 2 (aminopiperidines) and 20 (aminoazepanes)
Compound
R1
R2
R3
R4
hSST5a
(nM)
Selectivity
hSST1
H1 %inhib
Lysab
g/mL)
Pampa Pec
log Dd
pKa (B)e (piperidine
nitrogen)
at 3
lM
(
l
(10ꢁ6 cm/s)
2Aa
2Ba
H
NH2
Me
H
Me
Me
H
H
21
19
>500
>500
81
85
>433
375
2.94
2.40
3.03
2.24
8.08
8.27
2Ab
H
H
Me
H
4.9
11
84
89
48
101
1.32
0.03
4.10
7.87
7.96
F
2Ac
Me
H
F
156
<1
>4.0
F
F
F
(RS)-20Bd
2Ae
NH2
H
H
F
436
11
nd
nd
33
nd
nd
nd
nd
O
N
Me
213
473
3.17
2.61
7.34
N
O
O
O
2Af
H
H
Me
Me
Me
4.0
4.4
nd
nd
nd
nd
106
216
2.36
3.82
3.61
3.43
7.92
7.46
2Ag
Cl
2Ah
2Bi
H
Me
2.4
7.1
743
407
12
<1.0
123
ꢀ0
>4.0
3.15
nd
F
F
O
F
O
NH2
H
4.7
1.39
7.86
F
N
O
O
2Cj
H
H
F
6.3
6.9
141
439
35
13
>520
155
2.41
2.07
3.39
3.13
7.84
7.90
N
2Dk
O
2Ei
Cl
H
7.9
54
7.1
76
nd
3.55
7.80
F
(continued on next page)

4524
A. Alker et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4521–4525
Table 1 (continued)
Compound
R1
R2
R3
R4
hSST5a
(nM)
Selectivity
hSST1
H1 %inhib
Lysab
g/mL)
Pampa Pec
log Dd
4.01
pKa (B)e (piperidine
nitrogen)
at 3
lM
(l
(10ꢁ6 cm/s)
O
2Ai
2Ak
2Di
H
Me
Me
H
4.4
5.4
2.5
59
5.9
94
0.46
0.64
1.23
7.69
7.74
7.86
F
O
O
H
224
211
ꢁ1.2
152
153
3.73
3.83
N
11
F
F
O
CH2
2Bl
2Dj
NH2
H
H
6.7
18
660
ꢁ1.2
5.0
0.14
2.98
3.94
2.66
7.66
7.83
N
O
F
>500
55
330
(R)-20Am
(S)-20Am
2Am
H
H
H
Me
Me
Me
Cl
Cl
Cl
H
H
H
18
637
15
175
nd
335
98
nd
nd
nd
nd
172
nd
nd
1.93
nd
nd
3.05
nd
nd
7.63
O
O
(R)-20Aj
H
Me
F
22
253
58
nd
nd
nd
nd
2Aj
H
H
Me
Me
F
25
22
180
256
25
97
443
nd
3.48
nd
2.90
nd
7.70
nd
(R)-20Aa
Me
H
O
(RS)-20Ai
H
Me
9.3
86
nd
54
nd
81
1.56
1.93
3.31
2.44
8.34
7.19
F
O
13 (4-ring)
(H)
(Me)
(F)
>1000
>499
a
Radioligand binding assay.16
b
c
Lyophilisation solubility assay. Solubility was measured from lyophilized DMSO stock solutions spectrophotometrically at pH 6.5 in a 50 mM phosphate buffer.
Pampa (parallel artificial membrane permeation assay).
log D values were measured spectrophotometrically at pH 7.4 in a 1-octanol/50 mM TAPSO buffer system containing 5% (v/v) DMSO.
pKa values were determined spectrophotometrically on a ProfilerSGA instrument in a SGA buffer system containing 10% (v/v) methanol at an ionic strength of 150 mM.
d
e
Table 1 summarizes the hSST5 binding affinity, selectivity
against the hSST1 receptor, %inhibition at the human histamine 1
receptor at 3
M,23 and the physicochemical key parameters solu-
much weaker ligands at the SST5-receptor or do not bind at all,25 the
seven-membered analogs exhibit almost identical activity to the
achiral piperidine derivatives (cf. 2Aa with 20Aa, 2Ai with 20Ai,
2Am with 20Am, and 2Aj with 20Aj). Interestingly enough, the recep-
tor is highly enantiodiscriminating, almost all activity resides in the
(R)-series (cf. (R)-20Am being 35 times more potent than the (S)-iso-
mer). Solubility is found to be typically in a favorable range, with the
exception of the very lipophilic analogs containing a trifluorometh-
ylphenyl- or a trifluoromethoxyphenyl-residue. And the PAMPA
values parallel this trend; again, all but very few compounds are
blessed with decent to excellent permeability. SAR around the nic-
otinamide moiety is less straightforward; obviously, and in full
agreement with our earlier observations, the receptor is quite toler-
ant with respect to structural changes in this region. Even signifi-
cantly modulating polarity, shape, or steric encumbrance exerts
only a marginal effect on the binding affinity, cf., for example, 2Aa
and 2Ba, or 2Ai, 2Bi, and 2Ei. This is in perfect agreement with our
earlier observation that benzoxazoles, if coupled to appropriate
aminopiperidines, deliver excellent SST5 antagonists as well.14,26
l
bility, permeability, log D value, and pKa of the piperidine nitrogen,
respectively, of selected analogs. We knew from our former experi-
ence13,14 that two substituents, among them a small meta-alkoxy-
residue, is indispensable to get decent potency. Therefore, we fo-
cused on 3-ethoxy-derivatives and include in this Letter only such
analogs. Close inspection of the data reveal that three exit vectors
in the benzylamine part are not only necessary for achieving single
digit nanomolar affinity, this additional substituent in the other
meta-position also helps to get rid of the undesired hH1-activity
as previously described (cf. 2Aa with 2Ag, 2Ah, or 2Ai). And again,
the ethoxy side-chain outperformed all the few other residues
tested. Fluorine is clearly inferior ((RS)-20Bd)) whereas benzyloxy
(2Bl) does not boost the affinity any further but worsens signifi-
cantly the physicochemical properties. Selectivity towards SST1,
on the other hand, is hardly ever an issue and always 50-fold or bet-
ter.24 In contrast to the four- and five-membered analogs which are

A. Alker et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4521–4525
7. Osapay, G.; Osapay, K. Expert Opin. Ther. Patent 1998, 8, 855.
4525
X-Ray
8. Kumar, U.; Sasi, R.; Suresh, S.; Patel, A.; Thangaraju, M.; Metrakos, P.; Patel, S.
C.; Patel, Y. C. Diabetes 1999, 48, 77.
9. Mitra, S. W.; Mezey, E.; Hunyady, B.; Chamberlain, L.; Hayes, E.; Foor, F.; Wang,
Y.; Schonbrunn, A.; Schaeffer, J. M. Endocrinology 1999, 140, 3790.
10. Portela-Gomes, G. M.; Stridsberg, M.; Grimelius, L.; Oberg, K.; Janson, E. T. Appl.
Immunohistochem. Mol. Morphol. 2000, 8, 126.
11. Strowski, M. Z.; Blake, A. D. Mol. Cell. Endocrinol. 2008, 286, 169.
12. Wolkenberg, S. E.; Thut, C. J. Curr. Opin. Drug Disc. Dev. 2008, 11, 446.
13. Martin, R. E.; Green, L. G.; Guba, W.; Kratochwil, N.; Christ, A. J. Med. Chem.
2007, 50, 6291.
14. Martin, R. E.; Mohr, P.; Maerki, H. P.; Guba, W.; Kuratli, C.; Gavelle, O.; Binggeli,
A.; Bendels, S.; lvarez-Sanchez, R.; Alker, A.; Polonchuk, L.; Christ, A. D. Bioorg.
Med. Chem. Lett. 2009, 19, 6106.
15. Sprecher, U.; Mohr, P.; Martin, R. E.; Maerki, H. P.; Sanchez, R. A.; Binggeli, A.;
Kuennecke, B.; Christ, A. D. Regul. Pept. 2010, 159, 19.
16. A CHO cells stably expressing human SSTR5 (GenBank Accession No. D16827,
Euroscreen, Brussels, Belgium) was used for binding and functional assays;
cells expressing human SSTR1,
2 and 3, and rat and mouse SSTR5 were
established in-house. SST-14 was purchased from Bachem (Bubendorf,
Switzerland). Membranes from cells expressing SST receptors were prepared
by sonication and incubated with radiolabeled tracer (11 Tyr SST-14; Perkin-
Elmer, Schwerzenbach, Switzerland, or Amersham, Dübendorf, Switzerland)
and either test compound in varying concentration or, for the quantification of
non-specific binding, non-labeled SST-14. The incubation was stopped by
filtration through glass-fiber filters and the bound radioactivity measured to
estimate the concentration of test compound required for half maximal
inhibition of binding (IC50
) and the binding affinity (Ki). For functional
experiments, transfected cells were incubated with forskolin and test
compound in varying concentration. Subsequently, cellular cAMP
concentration was measured using a FRET (fluorescence resonance energy
transfer) based assay as previously published Roth, D.; Matile, H.; Josel, H.-P.;
Enderle, T. Fast-TRF: Novel time-resolved assays for drug discovery. In Society
for Biomolecular Screening, 11th Annual Conference and Exhibition, Geneva,
2005; p 265. The concentration of the test compound necessary to induce a half
maximal effect (EC50) and the efficacy compared to 0.15 nM SST-14 were
determined from concentration-versus-cAMP graphs. For the determination of
potential antagonism, 0.15 nM SST-14 was applied together with the test
compound, and the concentration of the test compound to half maximally
reverse the effect of SST-14 (i.e., IC50) was deduced from concentration-versus-
cAMP graphs.
X-ray structure of (S)-4-((S)-1-phenyl-ethylamino)-azepane-1-
carboxylic acid tert-butyl ester as p-Br-benzoate ((S,S)-10, only cat-
ion shown).
In conclusion, we have identified a novel series of highly potent,
selective SST5-antagonists, many of them showing single digit
nanomolar binding Ki-values. Compound 2Ai had been chosen,
thanks to its balanced overall profile, rather straightforward syn-
thetic access, and its favorable pharmacokinetic properties for an
exploratory in vivo study.15 Irrespective of its mixed outcome we
think that these molecules should become valuable tools for fur-
ther exploring and exploiting the SST receptors in general and
SST5 in particular as pharmacological target.
17. Mattson, R. J.; Pham, K. M.; Leuck, D. J.; Cowen, K. A. J. Org. Chem. 1990, 55,
2552.
18. It proved to be advantageous to work in iPrOH instead of MeOH or EtOH in
order to suppress the formation of the benzyl–methyl- or benzyl–ethyl ether,
generated via hemiacetal formation of the corresponding benzaldehyde and
ensuing reduction.
19. Meciarova, M.; Podlesna, J.; Toma, S. Monatsh. Chem. 2004, 135, 419.
20. Roglans, A.; Marquet, J.; Moreno-Manas, M. Synth. Commun. 1992, 22, 1249.
21. Surivet, J.-P.; Zumbrunn, C.; Hubschwerlen, C.; Perez Frutos Hoener, A.
Application: WO 2004035569, 2004, 97 pp.
Acknowledgments
22. Data were collected on
a STOE Imaging Plate Diffraction System (STOE,
Darmstadt) with Mo-radiation (0.71 Å) and data processed with STOE
IPDSsoftware. The crystal structure was solved and refined with the program
SHELXTL (Bruker AXS, Karlsruhe). The coordinates of the structure of
compound (S,S)-10 have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication numbers CCDC
775436. These data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: +44 (0)1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
The authors gratefully acknowledge Stefan Bürli and Thomas
Burger for chemical syntheses, Angele Costanzo and Astride Schno-
ebelen for in vitro testing, Dr. Christoph Ulmer for histamine
screening, and Virginie Micallef, Isabelle Parrilla, Frank Senner,
and Björn Wagner for physicochemical measurements.
23. Since our first seeding hit turned out to be Astemizole, a potent H1-antagonist,
we routinely checked all our SST5-antagonists for this putative off-target
effect.
References and notes
24. We spot-checked selected antagonists against SST2, SST3 as well as—to some
minor extent—against SST4 and never found any significant binding.
1. Brazeau, P.; Vale, W.; Burgus, R.; Ling, N.; Butcher, M.; Rivier, J.; Guillemin, R.
Science 1973, 179, 77.
2. Gillies, G. Trends Pharmacol. Sci. 1997, 18, 87.
3. Chisholm, C.; Greenberg, G. R. Am. J. Physiol. 2002, 283, E311.
4. Reisine, T.; Bell, G. I. Endocr. Rev. 1995, 16, 427.
5. Hoyer, D.; Bell, G. I.; Berelowitz, M.; Epelbaum, J.; Feniuk, W.; Humphrey, P. P.
A.; O’Carroll, A. M.; Patel, Y. C.; Schonbrunn, A., et al Trends Pharmacol. Sci. 1995,
16, 86.
25. Knowing from our earlier series that 3-aminopyrrolidine is
a much less
favorable pharmacophore than 4-aminopiperidine we did not prepare and
screen the exact analogs of 2 or 20. To complement the picture, however, we
synthesized a couple of aminoazetidines which were found to be devoid of any
activity (see compound 13).
26. Most compounds, specifically 2Ba, 2Ab, 2Bd, 2Ae, 2Ag, 2Bh, 2Ah, 2Bi, 2Cj, 2Dk,
2Ei, 2Ai, 2Ak, 2Di, 2Dj, 2Aj, (RS)-20Ai were tested in a functional assay as
6. Pittaluga, A.; Feligioni, M.; Longordo, F.; Arvigo, M.; Raiteri, M. J. Pharmacol. Exp.
Ther. 2005, 313, 242.
described in Ref. 16 and were found to be devoid of any activity (EC50 >1 lM).