Analogues and derivatives of ciproxifan, a novel prototype for generating potent histamine H3-receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(00)00473-X

Novel derivatives of the highly potent and selective histamine H₃-receptor antagonist ciproxifan (3) with different chain lengths as well as with structural variants of the cyclopropyl ketone moiety have been prepared and screened for their antagonist H₃-receptor potencies in vitro and in vivo. Some derivatives (2, 6-8, 12) containing other functionalities were effective in vitro in the same (sub)nanomolar concentration range and in vivo in a remarkably low oral dose. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00473-X

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2379±2382
Analogues and Derivatives of Ciproxifan, a Novel Prototype for
Generating Potent Histamine H3-Receptor Antagonists
Holger Stark,a,* Xavier Ligneau,b Bassem Sadek,a C. Robin Ganellin,c
Jean-Michel Arrang,d Jean-Charles Schwartzd and Walter Schunacka
aInstitut fur Pharmazie, Freie Universitat Berlin, Konigin-Luise-Strasse 2+4, 14195 Berlin, Germany
È
È
È
bLaboratoire Bioprojet, 30 rue des Francs-Bourgeois, 75003 Paris, France
cChristopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ, UK
  Â
Unite de Neurobiologie et Pharmacologie Moleculaire de l'INSERM (U.109), 2ter rue d'Alesia, 75014 Paris, France
d
Received 12 May 2000; revised 14 August 2000; accepted 17 August 2000
AbstractÐNovel derivatives of the highly potent and selective histamine H3-receptor antagonist ciproxifan (3) with di€erent chain
lengths as well as with structural variants of the cyclopropyl ketone moiety have been prepared and screened for their antagonist
H3-receptor potencies in vitro and in vivo. Some derivatives (2, 6±8, 12) containing other functionalities were e€ective in vitro in the
same (sub)nanomolar concentration range and in vivo in a remarkably low oral dose. # 2000 Elsevier Science Ltd. All rights
reserved.
It is widely recognized that histamine acts as neuro-
transmitter in the mammalian central nervous system by
interacting with three di€erent receptors.1 Whereas
antagonists for H1 and H2 receptors were introduced to
therapy a long time ago, ligands for the histamine H3
receptor are still missing in the repertoire for treatment
of diseases. For H3-receptor antagonists a number of
di€erent therapeutic indications have been proposed,
e.g., `cognitive enhancers' in Alzheimer's disease or
attention de®cit hyperactivity disorder (ADHD), as well
as in schizophrenia, and epilepsy.2 4 Various structural
developments of H3-receptor antagonists have been
reported (thioperamide, clobenpropit, GT-2331 (Per-
ceptinTM) etc.)1,3 6 in which the `proxifan' class with a
3-(1H-imidazol-4-yl)propoxy structure ([125I]iodoproxy-
fan,7 FUB 470,8 UCL 13909 etc.) seemed most promis-
ing for us having ciproxifan as a novel prototypic agent
of high potency (Fig. 1).10
Structural variants of chain length between the imida-
zole and phenoxy moieties as well as of the cyclopropyl
ketone moiety have been prepared and investigated for
their in¯uence on antagonist H3-receptor potency in
vitro and in vivo.
Figure 1. Histamine H3-receptor antagonists.
*Corresponding author. Fax: +49-30-838 53854; e-mail: stark@schunet.pharmazie.fu-berlin.de
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00473-X

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H. Stark et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2379±2382
Chemistry
as an intermediate for 9, the ketal butanol derivative
10, on the 4-hydroxybutyrophenone 11 via acidic
hydrolysis of 10. Compound 12 was obtained from 6
by C C cleavage, elimination and simultaneous
substitution (Scheme 2).
Alkyl aryl ethers were prepared from known sodium o-
(1H-1-(triphenylmethyl)imidazol-4-yl)alkanolates7,9 and
cyclopropyl 4-¯uorophenyl ketone by an SNAr reaction,
under milder conditions by the Mitsunobu reaction of
the corresponding alcohols (Scheme 1)11 or by William-
son synthesis from the corresponding alkyl chloride.12
Acidic deprotection of the intermediates resulted in
compounds 1±4 with di€erent alkyl chain lengths.13
All compounds were puri®ed chromatographically,
characterized in the form of their hydrogen maleates (2,
3, 5±7, 10, 11) or hydrogen oxalates (1, 8, 9, 12), and
gave satisfactory analytical results. Compound 4 was
free base and analysed for having retained some solvent
(EtOH and H2O).
The trityl-protected intermediates for ciproxifan (3) were
reduced under di€erent conditions with complex hydrides
and then deprotected to give 5 and 6, respectively (Scheme
2). Ciproxifan was transformed into the corresponding
oxime derivative 7 or catalytically reduced to the n-butyl
derivative 8. Treatment under strong acidic conditions
at high temperature for 3±48 h led to ring opening (9,
10, 12). According to the conditions and reagents the
4-chlorobutyrophenone derivative (not shown) served
Pharmacology
Histamine H3-receptor in vitro assay on synaptosomes of
rat cerebral cortex
The compounds were tested for their H3-receptor
antagonist potencies in an assay with K+-evoked
depolarization-induced release of [3H]histamine from
Scheme 1. Synthesis of ciproxifan (3) and analogues thereof. (a) For n=1±3: i: NaH, toluene; ii: 4-¯uorophenyl cyclopropyl ketone, toluene, re¯ux;
(b) 2 N HCl/THF, re¯ux; (c) for n=3: Ph3P, diethyl azodicarboxylate (DEAD), 4-hydroxyphenyl cyclopropyl ketone, THF; (d) for n=4: NaH,
4-hydroxyphenyl cyclopropyl ketone, DMF, (n-C4H9)4NI (cat.).
Scheme 2. Synthesis of ciproxifan and derivatives thereof. (a) LiAlH4/AlCl3, THF, re¯ux; (b) 2 N HCl/THF, re¯ux; (c) LiAlH4, THF/Et2O, re¯ux;
.
(d) H2NOH HCl, K2CO3, EtOH, re¯ux; (e) H2 (10 bar), Pd/C, MeOH; (f) i: MeOH, concd HCl, re¯ux; ii: pyrrolidine, NaI (cat.), re¯ux; (g) p-
H3CC6H4SO3H, HOC2H4OH, re¯ux; (h) F3CCOOH, re¯ux; (k) SOCl2, i-PrOH/HCl, re¯ux.

H. Stark et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2379±2382
2381
synaptosomes of rat cerebral cortex according to Gar-
barg et al.14 (Table 1).
this result of 5 with the analogous n-butyl derivative 8
shows that distinct steric parameters in vitro or phar-
macokinetic e€ects in vivo could be the basis for these
dramatic di€erences.
Histamine H3-receptor antagonist in vivo potency in mice
Increase in Nt-methylhistamine levels in Swiss mice
brain 90 min after p.o. administration of compounds
was selected to screen histamine H3-receptor antagonist
in vivo potency14 (Table 1). ED50 values were calculated
Transformation of a related acetyl derivative (FUB
372)11,15 into an oxime functionality recently led to
imoproxifan showing a strong increase in antagonist
potency and being one of the most potent histamine H3-
receptor antagonists known so far.13,16 The equivalent
transformation of 3 into the oxime derivative 7 main-
tained potency, but did not give the improvement
observed with imoproxifan.
1
.
as mg free base kg
.
Results and Discussion
Depending on alkyl chain length, compounds with two
(2) or three methylene (3) groups between the imidazole
ring and phenoxy moiety were clearly more e€ective
than those with shorter (1) or longer chains (4) in vitro,
®nding an optimum in the propyl derivative, but sur-
prisingly not di€ering statistically signi®cantly in in vivo
potency. Since it had activity in both test systems and a
good selectivity pro®le,10,15 3 was selected as a lead for
further optimization. Reduction of the ketone function-
ality in 3 to a hydroxy group (6) maintained potencies in
vitro as well as in vivo, whereas reduction to an alkane
(compound 5) showed no in vivo activity. Comparing
Despite diverse or alike electronic properties, com-
pounds were found which in some cases had almost
equie€ective (3!6; 8!10!11) or also in other cases
divergent receptor anities (3!7; 3!8; 3!10; 3!11)
giving a hint for the importance of steric e€ects.
Introduction of additional binding sites with hydrogen
bonding capabilities (9±11) or basic groups (9) on the
phenyl side chain led to compounds with low if any in
vivo potencies. The in vitro anity in a nanomolar
concentration range for the aliphatic primary alcohols
10 and 11 indicates that in these cases pharmacokinetic
Table 1. Antagonist histamine H3-receptor in vitro and in vivo potenciesa
Compound,
formula,
mp [ꢀC]
In vitro
Ki [nM]
In vivo p.o.
ED50 [mg kg
Compound,
formula,
mp [ꢀC]
In vitro
Ki [nM]
In vivo p.o.
ED50 [mg kg
1
1
]
]
Á
Á
Á
Á
ꢀ
ꢀ
ꢀ
ꢀ
x Æ sXꢀ
x Æ sXꢀ
x Æ sXꢀ
x Æ sXꢀ
1,
>500
2.1Æ0.7
0.49Æ0.09c
113Æ23
n.d.
2.8Æ0.7
0.21Æ0.05
0.14Æ0.03c
>10
7,
C16H19N3O2
4.4Æ1.4
60Æ23
n.d.
0.21Æ0.05
0.96Æ0.30
>10
.
.
C14H14N2O2
C2H2O4 H2O
177
1
4
1
.
.
C4H4O4 H2O
4
139±140
2,
8,
.
.
.
.
C15H16N2O2
C16H21N2O
34C2H2O4
190
C4H4O4
132±133
3b,c
,
9,
C20H27N3O2 2
.
C16H18N2O2
C4H4O4
118±120
1
.
C2H2O4 12H2O
134±136
4,
10,
40Æ8
30Æ8
n.d.
>10
.
C
17H20N2O2
1
C
C
18H24N2O4
C4H4O4
104±105
.
1.7H2O 4C2H5OH
98±100
5,
>10
11,
>10
.
.
C16H20N2O
16H20N2O3
C4H4O4
107±108
C4H4O4
113±114
6b,
0.58Æ0.17
0.18Æ0.02
12,
C16H19ClN2O
0.72Æ0.22
.
.
C16H20N2O2
C4H4O4
84±85
1
.
C2H2O4 H2O
2
159±161
Thioperamide
Clobenpropit
4Æ1c
1.0Æ0.5c
ꢁ 25c
FUB 470
GT-2331
2.3Æ0.8d
0.12Æ0.07d
0.6Æ0.2c
(0.12Æ0.04)e
(0.12)f
aData in parentheses are obtained from di€erent test systems. n.d., not determined.
bRef. 15.
cRef. 10.
dRef. 8.
eRef. 6.
fRef. 17.

2382
H. Stark et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2379±2382
diculties such as oral absorption, penetration of the
blood±brain barrier or rapid metabolization seem likely
explanations for the lack of in vivo activity.
3. Stark, H.; Schlicker, E.; Schunack, W. Drugs Future 1996,
21, 507.
4. Schwartz, J.-C.; Arrang, J.-M.; Garbarg, M.; Trai€ort, E.
In Psychopharmacology: The Fourth Generation of Progress;
Bloom, F. E., Kupfer, D. J., Eds; Raven: New York, 1995;
pp 397±406.
5. Leurs, R.; Vollinga, R. C.; Timmerman, H. Prog. Drug
Res. 1995, 45, 107.
6. Ali, S. M.; Tedford, C. E.; Gregory, R.; Handley, M. K.;
Yates, S. L.; Hirth, W. W.; Phillips, J. G. J. Med. Chem. 1999,
42, 903.
7. Stark, H.; Purand, K.; Huls, A.; Ligneau, X.; Garbarg, M.;
Schwartz, J.-C.; Schunack, W. J. Med. Chem. 1996, 39, 1220.
8. Krause, M.; Ligneau, X.; Stark, H.; Garbarg, M.;
Schwartz, J.-C.; Schunack, W. J. Med. Chem. 1998, 41, 4171.
9. Ganellin, C. R.; Hosseini, S. K.; Khalaf, Y. S.; Athmani,
S.; Tertiuk, W.; Garbarg, M.; Ligneau, X.; Schwartz, J.-C. J.
Med. Chem. 1996, 39, 3806.
10. Ligneau, X.; Lin, J.-S.; Vanni-Mercier, G.; Jouvet, M.;
Muir, J. L.; Ganellin, C. R.; Stark, H.; Elz, S.; Schunack, W.;
Schwartz, J.-C. J. Pharmacol. Exp. Ther. 1998, 287, 658 and
references therein.
11. Stark, H.; Sadek, B.; Krause, M.; Huls, A.; Ligneau, X.;
Ganellin, C. R.; Arrang, J.-M.; Schwartz, J.-C.; Schunack, W.
J. Med. Chem. 2000, 43, in press.
12. Vollinga, R. C.; Menge, W. M. P. B.; Timmerman, H.
Rec. Trav. Chim. Pays-Bas 1993, 112, 123.
The 4-chloro-1-butenyl derivative 12 was active in the
same range as the butyl derivative 8 showing that steric
e€ects are more important for receptor/ligand interac-
tion than electronic properties of the substituents on the
phenyl moiety (cf. 5) or that pharmacokinetic reasons
have been taken into account. The same is true for dif-
ferent cyclopropyl derivatives comparing compound 3
to compounds 6 or 7. In this series the lack of in vivo
activity of 5 was unexpected.
It can be concluded that structural variants of the
ciproxifan lead result in some highly potent histamine
H3-receptor antagonists, regardless of di€erent electro-
nic e€ects of substituents in the phenyl ring. The ethyl
derivative 2, the cyclopropyl methanol derivative 6, and
the cyclopropyl ketone oxime derivative 7 show that
various structural modi®cations can be performed with
maintenance of potency, especially the high p.o. potency
in vivo. Therefore, these compounds represent new
leads for further improvements. Additional studies are
currently in progress.
13. Schwartz, J.-C.; Arrang, J.-M.; Garbarg, M.; Quemener,
A.; Lecomte, J.-M.; Ligneau, X.; Schunack, W.; Stark, H.;
Purand, K.; Huls, A.; Reidemeister, S.; Athmani, S.; Ganellin,
C. R.; Pelloux-Leon, N.; Tertiuk, W.; Krause, M.; Sadek, B.
PCT WO 96/29 315 (03.21.1995).
Acknowledgements
We gratefully thank A. Piripitsi for the synthesis of
compound 4. This work was supported by the Biome-
dical & Health Research Programme BIOMED of the
European Union and by the Fonds der Chemischen
Industrie, Verband der Chemischen Industrie (Frank-
furt/Main, Germany).
14. Garbarg, M.; Arrang, J.-M.; Rouleau, A.; Ligneau, X.;
Dam Trung Tuong, M.; Schwartz, J.-C.; Ganellin, C. R. J.
Pharmacol. Exp. Ther. 1992, 263, 304.
15. Kathmann, M.; Schlicker, E.; Marr, I.; Werthwein, S.;
Stark, H.; Schunack, W. Naunyn-Schmiedeberg's Arch. Phar-
macol. 1998, 358, 623.
16. Stark, H.; Sadek, B.; Elz, S.; Ligneau, X.; Arrang, J.-M.;
Ganellin, C. R.; Schwartz, J.-C.; Schunack, W. 18th Annual
Meeting of the European Histamine Research Society
(EHRS), Lyon, France (May 12±15, 1999), Abstract Book,
poster No. 400.
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 and references therein.
2. Leurs, R.; Blandina, P.; Tedford, C.; Timmerman, H.
Trends Pharmacol. Sci. 1998, 19, 177.
17. Tedford, C. E.; Pawlowski, G. P.; Handley, M. K.; Phil-
lips, J. G.; Ali, S. M.; Hirth, W. W.; Yates, S. L. 28th Annual
Meeting of the Society for Neuroscience, Los Angeles, CA,
USA (Nov. 7±12, 1998); Soc. Neurosci. Abstr. 1998, 24 (Part
2), 1838 Ð Abstr. 730.11.