Fluorinated non-imidazole histamine H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2009.02.110

Fluorine substituents have become a widespread and important component in drug design and development. Here, the synthesis of fluorine containing compounds and some corresponding precursor molecules are presented for potential isotope labelling as well as their data obtained with in vitro and in vivo screenings. The compounds vary in the basic centres (piperidine or pyrrolidine) and are fluoro substituted in different positions of the basic alicyclic moiety. Pharmacological evaluation resulted in ligands with high affinities at hH₃ receptor in the nanomolar and subnanomolar concentration range and some with high antagonist in vivo potencies.

Full text of DOI:10.1016/j.bmcl.2009.02.110

Bioorganic & Medicinal Chemistry Letters 19 (2009) 2172–2175
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Fluorinated non-imidazole histamine H3 receptor antagonists
K. Isensee a, M. Amon a, A. Galaparti a,b, X. Ligneau c, J.-C. Camelin c, M. Capet c, J.-C. Schwartz c, H. Stark a,
*
a Johann Wolfgang Goethe-Universität, Institut für Pharmazeutische Chemie, Biozentrum, ZAFES/LiFF/CMP, Max-von-Laue-Str. 9, 60438 Frankfurt, Germany
b Department of Medicinal Chemistry, University College of Pharmaceutical Sciences, Kakatiya University, Warangal 506 009, AP, India
c Bioprojet-Biotech, 4 rue du Chesnay Beauregard, BP 96205, 35762 Saint Grégoire Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorine substituents have become a widespread and important component in drug design and develop-
ment. Here, the synthesis of fluorine containing compounds and some corresponding precursor mole-
cules are presented for potential isotope labelling as well as their data obtained with in vitro and
in vivo screenings. The compounds vary in the basic centres (piperidine or pyrrolidine) and are fluoro
substituted in different positions of the basic alicyclic moiety. Pharmacological evaluation resulted in
ligands with high affinities at hH3 receptor in the nanomolar and subnanomolar concentration range
and some with high antagonist in vivo potencies.
Received 5 February 2009
Revised 25 February 2009
Accepted 26 February 2009
Available online 3 March 2009
Keywords:
Histamine
H3
Ó 2009 Elsevier Ltd. All rights reserved.
Receptor
GPCR
Medicinal chemistry
PET
CNS
Neurotransmitter
The histamine H3 receptor (H3R) is one of four different G pro-
spacer has also been successfully used in other H3R antagonists like
UCL 2138.9 The diamine approach in our compounds is based on
previously described related benzylic derivatives like FUB 88010
and JNJ-520785211 (Fig. 1). Different lead developments with such
a diamine element was described, in some cases with some
potential therapeutic long-term treatment drawbacks like
phospholipidosis.12,13
1
tein-coupled histamine receptors known as H1–4
. The H3R was
first discovered in 1983 in rats2 and cloned in 1999.3 As presynap-
tic auto- and heteroreceptor the H3R is acting mainly in the central
nervous system (CNS) controlling the synthesis and release of his-
tamine, but also modulating the liberation of several other neuro-
transmitters, for example, dopamine, serotonin,
c-aminobutyric
acid, noradrenaline, or acetylcholine. The abundance and localiza-
tion of H3Rs in the CNS suggests an integrative role in neuronal
functions and behaviour, for example, arousal, cognition, memory
or food intake.4
We combined the phenoxy moiety with heterogeneous ali-
phatic mono- and difluorinated substitutions. These compounds
were screened for hH3R affinity displaying influence of fluoro
Numerous H3R antagonists demonstrate positive effects in ani-
mal models of various CNS-related dysfunctions5 and might be
useful for the treatment of these disorders such as daytime sleep-
iness, attention deficit and hyperactivity, Alzheimer’s disease,
schizophrenia or obesity.6
One clinical candidate is the inverse H3R agonist Tiprolisant, the
former BF2.649 (Fig. 1), which shows high H3R affinity.7 The com-
pound is now in phase II of clinical testing and targeting on narco-
lepsy, and epilepsy.8
We have exploited of the common ‘amino-propyloxy’ H3R phar-
macophore of Tiprolisant and others and linked it to benzyl moie-
ties as central element to obtain our lead structures. This aromatic
* Corresponding author. Fax: +49 69 798 29258.
E-mail address: h.stark@pharmchem.uni-frankfurt.de (H. Stark).
Figure 1. Non-imidazole histamine H3 receptor antagonists.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.02.110

K. Isensee et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2172–2175
2173
substitution in comparison to other halogenated derivatives or an-
other leaving group. The high affinity hH3R ligands were screened
to determine their in vivo potencies after p.o. application. Novel
compounds might be useful as pharmacological tools for drug
development with changed basicity or for radioactive labelling eli-
gible tool to get rapid information on receptor occupation in
vivo.14,15
Briefly, the compounds have been pharmacologically screened
for H3R affinities by [125I]iodoproxyfan displacement assay on
HEK-293 cells stably expressing the hH3R.16 The non-specific bind-
ing was determined using imetit. Central in vivo H3R potency has
been determined after p.o. administration of the compounds to
s
Swiss mice measuring the increase of N -methylhistamine level
in the cerebral cortex.17
The compounds have been synthesized in two different sequen-
tial approaches, described in Figure 2 as Method A or Method B.
The synthesis route in Method A has started with alkylation of a
5- or 6-membered aliphatic heterocycle (piperidine and pyrroli-
dine) with 3-chloropropan-1-ol under basic conditions resulting
in synthon 1a and 1b, respectively. The alcohol group has been
activated by reaction with thionyl chloride. These alkyl chlorides
have been directly coupled to different phenols in a Williamson
reaction. Based on this linear synthesis strategy, compounds 2, 3,
4a, and 4b have been obtained in good yields. The benzaldehydes
(4a, 4b) have been used as starting material for reductive amina-
tion with corresponding secondary amines to 5a–h.
The mesylate compound 7 has not been synthesized directly by
reductive amination, but it has been formed from the 4-hydroxypi-
peridine derivate 6 by transformation with mesyl chloride.
In Method B the central part of the structure has been built up
firstly by Williamson reaction of 3-chloropropan-1-ol with 4-cya-
nophenol for compound 8 or 4-hydroxybenzaldehyde for com-
pound 9, respectively, under iodide catalysis. Afterwards, the
different fluorinated aminergic rings have been linked on the cor-
responding functional group on either side. 4,4-Difluoropiperidine
has been coupled with 4-(3-hydroxypropoxy)benzonitrile (8),
which was previously activated by reaction with thionyl chloride,
to obtain compound 10.
Introduction of fluorine changes properties of neighbouring
functional groups, especially pKA values.18 Thereby, it modifies
pharmacodynamic and pharmacokinetic properties.19 Fluoro sub-
stitution can be used to solve problems unique to the CNS, for
example, blood brain barrier (BBB) permeation.20 Additionally it
may exert a substantial effect on the conformation of a molecule
and it is used to enhance the binding affinity to the target
protein.21
Our initial starting point was the difluorination of the piperidine
of a modern monoamine non-imidazole H3R antagonist (com-
pound 10) (Table 1). This derivatisation disappointingly caused a
complete loss in affinity. Consequently, our studies mainly focused
on modification and fluorination of the lipophilic pharmacophore
on the right-hand part of the molecule. The monofluorination to
fluorobenzyl derivate 2 resulted in moderate affinity, which even
decreased with elongation of the spacer to fluoroethylphenyl
derivative 3.
The development of dibasic structures in combination of mono-
fluorination on the right-hand basic centre (5a–e) involved high
affinities in low subnanomolar concentration ranges. Compound
5a fluorinated on the second piperidine moiety showed the highest
affinity in this short series.
After changing piperidine to pyrrolidine fluorination in 3-posi-
tion introduced a stereocenter. Stereoselective differentiation of
eutomer and distomer has not been observed with the enantio-
meric compounds 5b and 5c. Change of the left-hand basic ring
system from piperidine to pyrrolidine (5a ? 5d, 5c ? 5e) did not
result in any improvement of affinity.
For compound 12 the aldehyde functionality of 9 has been re-
acted with pyrrolidine in a reductive amination to give the alcohol
11, which has been finally activated by thionyl chloride and cou-
pled with 4-fluoropiperidine.
Figure 2. Synthesis of compounds: (i) (1) SOCl2, toluene, 0 °C ? 60 °C, 3 h; (2) different phenols, KI, K2CO3, acetone, reflux, 48 h; (ii) sec-amine base, NaBH(AcO)3, dichloroethane,
rt, 1–12 h; (iii) (1) SOCl2, toluene, 0 °C ? 60 °C, 3 h; (2) sec-amine base, KI, K2CO3, acetone, reflux, 48 h; (iv) for compound 7 only: CH3SO2Cl, DCM, 0 °C ? rt.

2174
K. Isensee et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2172–2175
Table 1
mice. All tested compounds showed central antagonistic efficacies
with good oral potencies. Here, the best compound is the difluori-
nated derivative 5f (ED50 value of 0.23 mg/kg). Metabolic lability,
distribution differences or other potential reasons for the related
monofluorinated compound 5a in comparison to difluoro com-
pound 5f might taken into account, but need additional
investigations.
hH3R affinities and potencies of compounds 2, 3, 5a–h, 7, 10 and 12
b
No.
Structure
hH3R Kia (nM)
ED50 (mg/kg)
10
>1000
In addition to fluorination other halogenations (5g, 5h) or intro-
duction of other leaving groups (7) have been introduced has also
led to bioisosteric replacements. These compounds have not been
tested in vivo due to their chemical reactivity.
2
17.3 3.4
In summary, all diamine compounds with fluorination on the
right-hand side showed subnanomolar affinities at hH3R. The
high affinity of 5a recommends this compound as a potential
candidate for [18F]-labelling to receive a novel pharmacological
tool for drug discovery in CNS by means of positron emission
tomography (PET)22 or ligand-based [19F]NMR binding screen-
ing.23 With compounds 5g, 5h, and 7 suitable precursors are
available for a convenient and last-step labelling. Further studies
in this direction are in progress. In addition, all high affine fluo-
rinated antagonists exhibit antagonist potency in vivo after p.o.
administration indicating that this structural modification is a
new lead element in compound optimization (cf. 5f). This offers
the possibility of a potent, orally available hH3R antagonist. Fur-
ther non-target studies and pharmacokinetic investigations are
awaited for compound evaluation.
3
43
5
5a
5b
5c
5d
5e
12
5f
5g
5h
7
0.094 0.011
0.28 0.07
0.28 0.16
0.26 0.02
0.31 0.08
0.99 0.16
0.24 0.05
0.20 0.03
0.068 0.004
0.84 0.08
7.4 3.9
3.9 0.4
12
1
Acknowledgement
This study was supported by the LOEWE Lipid Signaling Fors-
chungszentrum Frankfurt (LiFF) and the Deutscher Akademischer
Austauschdienst (D/06/25529), Germany.
Supplementary data
Supplemental material with analytical data of target com-
pounds will be freely available. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.bmcl.2009.02.110.
0.40 0.01
References and notes
1. Hill, S. J.; Ganellin, C. R.; Timmerman, H.; Schwartz, J. C.; Shankley, N. P.; Young,
J. M.; Schunack, W.; Levi, R.; Haas, H. L. Pharmacol. Rev. 1997, 49, 253.
2. Arrang, J. M.; Garbarg, M.; Schwartz, J. C. Nature 1983, 302, 832.
3. Lovenberg, T. W.; Roland, B. L.; Wilson, S. J.; Jiang, X.; Pyati, J.; Huvar, A.;
Jackson, M. R.; Erlander, M. G. Mol. Pharmacol. 1999, 55, 1101.
4. Sander, K.; Kottke, T.; Stark, H. Biol. Pharm. Bull. 2008, 31, 2163.
5. Isensee, K.; Petroianu, G.; Stark, H. J. Appl. Biosci. 2007, 5, 57.
6. Celanire, S.; Lebon, F.; Stark, H. In The Third Histamine Receptor: Selective Ligands
as Potential Therapeutic Agents in CNS Disorders; Vohora, D. S., Ed.; Taylor &
Francis CRC Press: Boca Raton, Fl, 2009; p 103.
7. Ligneau, X.; Perrin, D.; Landais, L.; Camelin, J. C.; Calmels, T. P.; Berrebi-
Bertrand, I.; Lecomte, J. M.; Parmentier, R.; Anaclet, C.; Lin, J. S.; Bertaina-
Anglade, V.; la Rochelle, C. D.; d’Aniello, F.; Rouleau, A.; Gbahou, F.; Arrang, J.
M.; Ganellin, C. R.; Stark, H.; Schunack, W.; Schwartz, J. C. J. Pharmacol. Exp.
Ther. 2007, 320, 365.
a
HEK-293 cells expressing hH3R.16
p.o. Administration to Swiss mice.17
b
8. Raga, M. M.; Sallares, J.; Guerrero, M.; Guglietta, A.; Arrang, J. M.; Schwartz, J.
C.; Lecomte, J. M.; Ligneau, X.; Schunack, W.; Ganellin, C. R.; Stark, H.; Patent
WO2006/084833 A1, 2006.
9. (a) Schwartz, J. C.; Arrang, J. M.; Garbarg, M.; Lecomte, J. M.; Ligneau, X.;
Schunack, W.; Stark, H.; Ganellin, C. R.; Leurquin, F.; Sigurd, E.; Patent
EP0978512 A1, 2000.; (b) Ganellin, C. R.; Leurquin, F.; Piripitsi, A.; Arrang, J.
M.; Garbarg, M.; Ligneau, X.; Stark, H.; Schunack, W.; Schwartz, J. C. In
Histamine Research in the New Millennium;; Watanabe, T., Timmerman, H.,
Yanai, K., Eds.; Elsevier Science: Amsterdam, 2001; pp 25–31.
Fluorination on the left-hand side as in compound 10 is ac-
cepted by maintaining affinity (12) although the Ki value is about
1 nM, which constitutes the worst Ki in the diamine series. In con-
trast to the derivatisation of 10 the difluorination of piperidine on
the right-hand moiety of compound 5f involved an affinity in the
subnanomolar concentration range.
Interestingly, the chlorinated precursor molecule 5h for poten-
tial [18F]-labelling showed the highest affinity pointing out the dis-
tinct steric demands for hH3R binding pocket interaction.
Selected compounds with high affinity have been tested for
their central antagonist in vivo potencies by p.o. application to
10. Mikó, T.; Ligneau, X.; Pertz, H. H.; Ganellin, C. R.; Arrang, J. M.; Schwartz, J. C.;
Schunack, W.; Stark, H. J. Med. Chem. 2003, 46, 1523.
11. (a) Barbier, A. J.; Berridge, C.; Dugovic, C.; Laposky, A. D.; Wilson, S. J.; Boggs, J.;
Aluisio, L.; Lord, B.; Mazur, C.; Pudiak, C. M.; Langlois, X.; Xiao, W.; Apodaca, R.;
Carruthers, N. I.; Lovenberg, T. W. Br. J. Pharmacol. 2004, 143, 649; (b) Apodaca,
R.; Carruthers, N. I.; Dvorak, C. A.; Rudolph, D. A.; Shah, C. R.; Xiao W. Patent
WO02/12214 A2, 2001.

K. Isensee et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2172–2175
2175
12. Bonaventure, P.; Letavic, M.; Dugovic, C.; Wilson, S.; Aluisio, L.; Pudiak, C.; Lord,
B.; Mazur, C.; Kamme, F.; Nishino, S.; Carruthers, N.; Lovenberg, T. Biochem.
Pharmacol. 2007, 73, 1084.
13. Cowart, M.; Gfesser, G. A.; Browman, K. E.; Faghih, R.; Miller, T. R.; Milicic, I.;
Baranowski, J. L.; Krueger, K. M.; Witte, D. G.; Molesky, A. L.; Komater, V. A.;
Buckley, M. J.; Diaz, G. J.; Gagne, G. D.; Zhou, D.; Deng, X.; Pan, L.; Roberts, E. M.;
Diehl, M. S.; Wetter, J. M.; Marsh, K. C.; Fox, G. B.; Brioni, J. D.; Esbenshade, T. A.;
Hancock, A. A. Biochem. Pharmacol. 2007, 73, 1243.
16. Ligneau, X.; Morisset, S.; Tardivel-Lacombe, J.; Gbahou, F.; Ganellin, C. R.; Stark,
H.; Schunack, W.; Schwartz, J. C.; Arrang, J. M. Br. J. Pharmacol. 2000, 131, 1247.
17. Garbarg, M.; Arrang, J. M.; Rouleau, A.; Ligneau, X.; Tuong, M. D.; Schwartz, J.
C.; Ganellin, C. R. J. Pharmacol. Exp. Ther. 1992, 263, 304.
18. Hagmann, W. K. J. Med. Chem. 2008, 51, 4359.
19. Kirk, K. L. Curr. Top. Med. Chem. 2006, 6, 1447.
20. Sun, S.; Adejare, A. Curr. Top. Med. Chem. 2006, 6, 1457.
21. Böhm, H. J.; Banner, D.; Bendels, S.; Kansy, M.; Kuhn, B.; Müller, K.; Obst-
Sander, U.; Stahl, M. ChemBioChem 2004, 5, 637.
22. Cai, L.; Lu, S.; Pike, V. W. Eur. J. Org. Chem. 2008, 2853.
23. Dalvit, C. Prog. Nucl. Mag. Reson. Spectrosc. 2007, 51, 243.
14. Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37,
320.
15. Shah, P.; Westwell, A. D. J. Enzyme Inhib. Med. Chem. 2007, 22, 527.