Synthesis and biological assays of new H3-antagonists with imidazole and imidazoline polar groups

Article abstract of DOI:10.1016/S0014-827X(99)00115-9

New histamine H₃-receptor antagonists were synthesised and tested on rat brain membranes and on electrically stimulated guinea-pig ileum. The new compounds have a central polar group represented by a 2-alkylimidazole or a 2-thioimidazoline nucleus. The effect of the polar group basicity on the optimal length of the alkyl chain, connecting this group to a 4(5)-imidazolyl ring, was investigated. The best affinity values, obtained by displacement of [³H]-RAMHA from rat brain, were obtained for the 2-alkylimidazole derivatives (2a-f) with tetramethylene chain (pK(i) 8.03-8.97), having an intermediate basicity between that of the previously reported 2- thioimidazoles (1a-i) and that of 2-alkylthioimidazolines (3a-h). In contrast, a general lowering of affinity (pK(i) 5.90-7.63) was observed for compounds of the last series (3a-h), with a complex dependence on the terminal lipophilic group and chain length. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0014-827X(99)00115-9

www.elsevier.nl/locate/farmac
Il Farmaco 55 (2000) 27–34
Synthesis and biological assays of new H3-antagonists with
imidazole and imidazoline polar groups
Marco Mor a, Fabrizio Bordi a, Claudia Silva a, Silvia Rivara a, Valentina Zuliani a,
Federica Vacondio a, Giovanni Morini a, Elisabetta Barocelli b, Vigilio Ballabeni b,
Mariannina Impicciatore b, Pier Vincenzo Plazzi a,*
a Dipartimento Farmaceutico, Uni6ersita` degli Studi di Parma, 6iale delle Scienze, I-43100 Parma, Italy
b Istituto di Farmacologia e Farmacognosia, Uni6ersita` degli Studi di Parma, 6iale delle Scienze, I-43100 Parma, Italy
Received 6 September 1999; accepted 29 October 1999
Abstract
New histamine H3-receptor antagonists were synthesised and tested on rat brain membranes and on electrically stimulated
guinea-pig ileum. The new compounds have a central polar group represented by a 2-alkylimidazole or a 2-thioimidazoline
nucleus. The effect of the polar group basicity on the optimal length of the alkyl chain, connecting this group to a 4(5)-imidazolyl
ring, was investigated. The best affinity values, obtained by displacement of [3H]-RAMHA from rat brain, were obtained for the
2-alkylimidazole derivatives (2a–f) with tetramethylene chain (pKi 8.03–8.97), having an intermediate basicity between that of the
previously reported 2-thioimidazoles (1a–i) and that of 2-alkylthioimidazolines (3a–h). In contrast, a general lowering of affinity
(pKi 5.90–7.63) was observed for compounds of the last series (3a–h), with a complex dependence on the terminal lipophilic group
and chain length. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Histamine; H3-receptor antagonists; 2-Alkylimidazoles; 2-Thioimidazolines
1. Introduction
tion-deficit hyperactive disorder, epilepsy and obesity
[13,14]. These possibilities prompted the search for new
potent, selective and brain-penetrating H3-receptor
antagonists.
CNS access can be reduced for various reasons, such
as a high binding to plasma proteins (e.g. for thio-
peramide [15,16]) or drug protonation (e.g. for cloben-
propit); these phenomena lead to high peripheral con-
centrations compared to brain levels, which should be
considered as being a drawback, given the possible
actions of H3-antagonists on vessels, lung and the gas-
trointestinal tract [17,18].
The basicity of the so-called ‘polar group’, connected
to an imidazole headgroup by an alkyl spacer in the
classical H3-antagonist formula [19], is therefore a cru-
cial feature for the pharmacological optimisation of
these compounds. Indeed, a classification of H3-antago-
nists based on the nature of the polar group can be
proposed from a structural viewpoint, grouping them
into basic (e.g. clobenpropit) and neutral (e.g. thiop-
eramide) ones [20].
Histamine, besides its well-known actions on blood
vessels and gastric secretion, is known to act as a
neurotransmitter in the central nervous system (CNS)
[1], exerting its pharmacological actions through the
activation of distinct receptor subtypes, named H1, H2
and H3. The histamine H3-receptor [2] is located presy-
naptically on various kinds of central neurons, regulat-
ing the synthesis and the release of histamine [3,4] and
of other neurotransmitters [5], such as noradrenaline
[6], acetylcholine [7], dopamine [8] and serotonin [9,10].
Moreover, H3-receptors can also play a modulating role
in peripheral neurotransmission [11,12].
The central effects of the H3-receptor antagonists
suggest a potential therapeutic role for the treatment of
some neurological disorders, such as narcolepsy, atten-
* Corresponding author. Tel.: +39-0521-905062; fax: +39-0521-
905006.
E-mail address: chifar@ipruniv.cce.unipr.it (P.V. Plazzi)
0014-827X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(99)00115-9

28
M. Mor et al. / Il Farmaco 55 (2000) 27–34
Histamine H3-receptor antagonist potency was evalu-
Recently, a new series of H3-antagonists has been
ated on electrically stimulated guinea-pig ileum, by in-
hibition of RAMHA-induced responses [27].
described, having no traditional polar group, but a
double or a triple bond between the alkyl chain and a
lipophilic ending group [21]. While this casts doubts on
the classical definition cited above, the defects of which
are further underlined by the report of potent antago-
nists lacking the imidazole headgroup [22], the study of
the possible interactions occurring at the polar group is
still important for the design of new compounds, as it
can provide information on a group that could be
tightly bound to the receptor. Such a group could
therefore be considered as a good starting point for
further structural modifications, aimed at the optimisa-
tion of pharmacokinetic and toxicological properties.
In this field, we have tested different heterocyclic
groups [23], directing our attention towards the impor-
tance of basicity in polar groups and its relation with
the alkyl chain length. In fact, it is our opinion that
SAR profiles for the two classes of H3-antagonists cited
are different and that, while strongly basic compounds
should be avoided to ensure an easy crossing of the
blood–brain barrier, polar interactions of basic groups
(such as the isothiourea one in clobenpropit, or the
guanidine one in thioperamide derivatives [24]) at the
H3-receptor could be exploited for the design of potent
ligands. The rationale of the present study was to test
the possibility of obtaining good affinity with hetero-
cyclic polar groups having intermediate basicity, and
thus of being able to take polar interaction at the
receptor, while conserving a significant fraction of neu-
tral species in solution at physiological pH, to allow
passive penetration of lipophilic membranes.
3. Chemistry
4(5)Phenyl-2-[v-[4(5)imidazolyl]alkyl]imidazoles 2a,
2c, 2e were prepared starting from the corresponding
4(5)-(v-cyanoalkyl)imidazoles, as described in Scheme
1. These were obtained by substitution with sodium
cyanide from alkyl halides, prepared following different
synthetic routes, depending on the length of the chain.
4(5)-(b-Chloroethyl)imidazole was prepared from his-
tamine by diazotisation and substitution with SOCl2, as
described in a previous paper [28]; the homologue
chloropropyl derivative was obtained in the same way
from 4(5)-(g-hydroxypropyl)imidazole, which was syn-
thesised from 3,4-dihydro-2H-pyran by a series of reac-
tions described in the literature [29,30].
4(5)-(d-Cyanobutyl)imidazole was prepared from 2-
(t - butyldimethylsilyl) - 5 - (4 - chlorobutyl) - 1 - (dimethyl-
sulfamoyl)imidazole, described by Vollinga et al. [31],
by treatment with sodium cyanide of the intermediate
partially deprotected in position 2, and following de-
protection of position 1 by aqueous HCl.
All the cyano-compounds were then treated with
absolute EtOH and gaseous HCl to give the iminoether
derivatives which, by cyclisation with v-bromoace-
tophenone, gave the final products.
4(5)Cyclohexyl-2-[v-[4(5)imidazolyl]alkyl]imidazoles
2b, 2d, 2f were prepared by the hydrogenation of
compounds 2a, 2c, 2e with rhodium catalyst.
Imidazoline derivatives 3a–3h were obtained by con-
densation of the appropriate 4(5)-(v-chloroalkyl)-
imidazole with substituted 2-thioimidazolines [32–34],
obtained by cyclisation of the corresponding diamine
with CS2, as described in Scheme 2.
Starting from a series of known H3-antagonists with
2-alkylthioimidazole polar group (1) [20], which are not
protonated at physiological pH (pKaꢀ4, [25]), we syn-
thesised and tested two new series of compounds (2-
alkylimidazoles (2) and 2-alkylthioimidazolines (3))
which have a polar group with a higher basicity than
the previous series 1.
We also investigated the effect of the polar group
basicity on the optimal length of the alkyl chain,
preparing compounds with different spacer lengths. In
fact, in the neutral 2-alkylthioimidazole derivatives (1),
the best H3-receptor antagonist activity and affinity
values were obtained with the shortest chains (two-
methylene spacer) [20], and this is in contrast with what
has been reported for basic H3-antagonists [26].
4. Experimental
4.1. Chemistry
Melting points were not corrected and were deter-
mined using a Bu¨chi instrument (Tottoli). The newly
synthesised substances were analysed for C, H and N.
The percentages we found were within 90.4 of the
1
theoretical values. The H NMR spectra were recorded
on a Bruker 300 spectrometer (300 MHz) with te-
tramethylsilane (TMS) as an internal standard. Mass
spectra were recorded using a Finnigan MAT SSQ 710
instrument, and IR spectra were recorded using a Jasco
FTIR 300E instrument. Reactions were followed by
TLC, on a Kieselgel 60 F 254 (DC-Alufolien, Merck).
2. Pharmacology
Newly synthesised compounds were tested for their
H3-receptor affinity on rat brain membranes, by dis-
placement
of
[3H]-(R)-a-methylhistamine
([3H]-
RAMHA) binding to cerebral cortex synaptosomes.

M. Mor et al. / Il Farmaco 55 (2000) 27–34
29
Scheme 1.
Scheme 2.
Final compounds and intermediates were purified by
chromatography on a preparative Gilson HPLC, using
an SiO2 column (LiChroprep, Si 60, 15–25 mm, Merck).
When indicated, the methanol phase was saturated with
gaseous NH3.
of 4.5 mmol of the appropriate 4(5)-(v-chloroalkyl)-
imidazole (prepared according to Refs. [28–30]) in the
minimum amount of DMF. The reaction mixture was
stirred at 70°C for 20 h, under dry nitrogen. The
solvent was then evaporated under reduced pressure
and the residual oil was purified by column chromatog-
raphy (SiO2, CH2Cl2–CH3OH(NH3)=20:1), giving
solid products.
Yields and characteristic data of the final products
are described in Table 1.
4.1.1. 4(5)-(i-cyanoethyl)imidazole and
4(5)-(k-cyanopropyl)imidazole
Sodium cyanide (6.7 mmol) was added to a solution
M.p.
4(5)-(b-cyanoethyl)imidazole
96–99°C
[35]; m.p. 4(5)-(g-cyanopropyl)imidazole 132–134°C
[35].

30
M. Mor et al. / Il Farmaco 55 (2000) 27–34
Table 1
Yields and characteristic data of final products
Comp.
Yield (%)
Crystallisation solvent
M.p. (°C) a
Analysis
2a
2b
2c
2d
2e
2f
3a
3b
3c
3d
3e
3f
49
80
51
82
47
79
46
58
46
42
38
18
30
20
i-PrOH/Et2O
abs EtOH/Et2O
i-PrOH/Et2O
abs EtOH/Et2O
abs EtOH/Et2O
abs EtOH
234–236
175–176
253–254
156–159
205–206
105–107
213–215
182–184
205–207
190–193
188–189
142–144
221–223
135–136
C14H14N4·2HCl
C14H20N4·2C4H4O4
C15H16N4·2HCl
C15H22N4·2C6H3N3O7
C16H18N4·2HCl
C16H24N4·2C2H2O4
C8H12N4S·2HCl·0.5H2O
C9H14N4S·1.5C2H2O4
C14H16N4S·HCl·HI
C15H18N4S·2HCl
abs EtOH
MeOH/acetone
abs EtOH/Et2O
abs EtOH/acetone
abs EtOH
abs EtOH
abs EtOH
C9H14N4S·2HCl
C10H16N4S·2C2H2O4
C15H18N4S·2C10H8N4O5
C16H20N4S·2C2H2O4
3g
3h
abs EtOH/acetone
a The melting points refer to the analysed salts.
4.1.2. 4(5)-(4-cyanobutyl)imidazole
1H NMR (DMSO-d6) 2a (dihydrochloride): l 3.31 (t,
J=6.9 Hz, 2H, CH2), 3.43 (t, J=6.9 Hz, 2H, CH2),
7.41 (t, 1H, Ph-4-H), 7.49 (t, 2H, Ph-3-H), 7.52 (s, 1H,
Im-5-H), 7.90 (d, 2H, Ph-2-H), 8.03 (s, 1H, Im-5-H),
9.02 (s, 1H, Im-2-H).
2-(t-Butyldimethylsilyl)-1-(dimethylsulfamoyl)-5-(4-
chlorobutyl)imidazole (8 mmol), prepared according to
Ref. [31], was deprotected in position 2 by 1 N HCl and
dissolved in the minimum amount of DMF with 24
mmol of sodium cyanide. The reaction mixture was
stirred at 130°C for 4 h, under dry nitrogen. The
solvent was then evaporated under reduced pressure
and the semisolid residue was dissolved in water. Ex-
traction with ether and evaporation of the dried organic
phase gave 1-(dimethylsulfamoyl)-5-(4-cyanobutyl)-
imidazole (yield 53%). The N-deprotection was
achieved with 2 N HCl. The aqueous acid solution was
then treated with K2CO3 and the product was extracted
with CH2Cl2. The organic layer was dried with Na2SO4
and evaporated under reduced pressure to give 0.49 g of
4(5)-(4-cyanobutyl)imidazole (yield 71%).
1H NMR (DMSO-d6) 2c (free base): l 1.91–2.00 (m,
2H, CH2ꢀCH2ꢀCH2), 2.53 (t, 2H, CH2), 2.66 (t, 2H,
CH2), 6.75 (s, 1H, Im-5-H), 7.15 (t, 1H, Ph-4-H), 7.31
(t, 2H, Ph-3-H), 7.40 (s, 1H, Im-5-H), 7.51 (s, 1H,
Im-2-H), 7.70 (d, 2H, Ph-2-H).
1H NMR (DMSO-d6) 2e (free base): l 1.58–1.72 (m,
4H, CH2ꢀCH2ꢀCH2ꢀCH2), 2.52 (t, 2H, CH2), 2.65 (t,
2H, CH2), 6.71 (s, 1H, Im-5-H), 7.14 (t, 1H, Ph-4-H),
7.31 (t, 2H, Ph-3-H), 7.38 (s, 1H, Im-5-H), 7.49 (s, 1H,
Im-2-H), 7.68 (d, 2H, Ph-2-H).
4.1.4. General method of preparation of
4(5)cyclohexyl-2-[ꢀ-[4(5)imidazolyl)alkyl]imidazoles
(2b, 2d, 2f)
1H NMR (DMSO-d6) (free base): l 1.61 (m, 4H,
CH2ꢀCH2ꢀCH2ꢀCH2), 2.49 (t, 2H, CH2), 2.52 (t, 2H,
CH2), 6.77 (s, 1H, Im-5-H), 7.56 (s, 1H, Im-2-H).
A solution of compounds 2a, 2c or 2e (1.1 mmol) in
35 ml of 3 N HCl was hydrogenated using a Parr
apparatus with 0.5 g of Rh/C (5%) at room tempera-
ture, for 65 h. The products obtained after filtration of
the catalyst were purified by column chromatography
(SiO2, 2b: CH2Cl2–CH3OH(NH3)=7:2; 2d: CH2Cl2–
CH3OH(NH3)=12:1; 2f: CH2Cl2–CH3OH(NH3)=
9:1). Yields and characteristic data of these compounds
are reported in Table 1.
4.1.3. General method of preparation of 4(5)-
phenyl-2-[ꢀ-[4(5)imidazolyl)alkyl]imidazoles (2a, 2c, 2e)
Gaseous HCl (9 mmol) was added to a solution of 3
mmol of the appropriate 4(5)-(v-cyanoalkyl)imidazole
in 3.1 mmol of absolute EtOH, maintaining the temper-
ature below 5°C. The resulting mixture was kept at 4°C
for 5 days. v-Bromoacetophenone (3.1 mmol) and
gaseous NH3 (2.6 g, 0.15 mol) were added to a solution
of the crude product (dihydrochloride) in monoethylene
glycol, and the mixture was cooled to −20°C. The
solution was then heated under pressure at 110°C for
about 30 h.
1H NMR (DMSO-d6) 2b (difumarate): l 1.20–1.38 (m,
6H, C6H11), 1.63–1.72 (m, 5H, C6H11), 2.92 (t, J=6.9
Hz, 2H, CH2), 3.03 (t, J=6.9 Hz, 2H, CH2), 6.58 (s, 1H,
Im-5-H), 6.79 (s, 1H, Im-5-H), 7.66 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 2d (free base): l 1.23–1.29 (m,
6H, C6H11), 1.62–1.89 (m, 7H, CH2ꢀCH2ꢀCH2ꢀCH2ꢀ
CH2ꢀCH2; C6H11), 1.86 (t, 2H, CH2), 2.51 (t, 2H, CH2),
6.47 (s, 1H, Im-5-H), 6.72 (s, 1H, Im-5-H), 7.48 (s, 1H,
Im-2-H).
The compounds were purified by column chromatog-
raphy (SiO2, 2a: CH2Cl2–CH3OH(NH3)=12:1; 2c and
2e: CH2Cl2–CH3OH(NH3)=10:1). Yields and charact-
eristic data of these compounds are reported in Table 1.

M. Mor et al. / Il Farmaco 55 (2000) 27–34
31
1H NMR (DMSO-d6) 2f (free base): l 1.17–1.28 (m,
6H, C6H11), 1.56–1.69 (m, 9H, CH2ꢀCH2ꢀCH2ꢀCH2;
C6H11), 2.49 (t, 2H, CH2), 2.57 (t, 2H, CH2), 6.52 (s, 1H,
Im-5-H), 6.69 (s, 1H, Im-5-H), 7.50 (s, 1H, Im-2-H).
imidazoline), 7.10 (s, 1H, Im-5-H), 8.06 (s, 1H, Im-2-
H).
1H NMR (DMSO-d6) 3g (free base): l 1.93 (m, 2H,
CH2ꢀCH2ꢀCH2), 2.60 (t, 2H, CH2ꢀIm), 3.05 (t, 2H,
CH2ꢀS), 3.27 (dd, J=11.1, J=8.2 Hz, 1H, imidazo-
line), 3.96 (app. t, J=10.7 Hz, 1H, CHꢀimidazoline),
4.90 (app. t, J=9.1 Hz, 1H, imidazoline), 6.76 (s, 1H,
Im-5-H), 7.30 (m, 5H, Ph), 7.49 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3h (dioxalate): l 1.64 (d, 2H,
CH2ꢀBz), 1.94 (m, 2H, CH2ꢀCH2ꢀCH2), 2.70 (t, J=7.1
Hz, 2H, CH2ꢀIm), 2.85–2.98 (m, 1H, imidazoline), 3.19
(t, J=7.1 Hz, CH2ꢀS), 3.60 (dd, J=11.0, J=6.6 Hz,
1H, imidazoline), 3.87 (app. t, J=10.8 Hz, 1H, imida-
zoline), 7.26 (s, 5H, Ph), 7.60 (s, 1H, Im-5-H), 8.43 (s,
1H, Im-2-H).
4.1.5. General method of cyclisation of the diamines
with CS2
A solution of 0.125 mol of the appropriate diamine
in 30 ml of EtOH was added to a solution of 32 ml of
CS2 and 32 ml of EtOH. The mixture was stirred at
40°C for 15 min to give a product that was washed with
EtOH (yield 75%).
M.p. 4(5)methyl-2-mercaptoimidazoline decomposed
at 129–160°C [32], 4(5)phenyl-2-mercaptoimidazoline
at 191–193.5°C [33], and 4(5)benzyl-2-mercaptoimi-
dazoline at 152–154°C [34].
4.2. Pharmacology
4.1.6. General method of preparation of
4(5)-R-2-[ꢀ-[4(5)imidazolyl]alkyl]thio]imidazolines
(3a–3h)
4.2.1. Binding assays
Rat (Wistar) brain membranes were incubated for 30
min with [3H]-RAMHA, in Tris–HCl 50 mM, pH 7.4,
NaCl 50 mM, EDTA 0.5 mM and rapidly filtered
under vacuum. Specific binding was defined as the
binding inhibited by thioperamide 10 mM, and the pKi
values were calculated from the inhibition curves of the
compounds tested versus 0.5 nM [3H]-RAMHA accord-
ing to Cheng and Prusoff’s equation [36].
The appropriate 2-mercaptoimidazoline (5 mmol)
was added to a solution of 5.8 mmol of 4(5)-(v-
chloroalkyl)imidazole and 5.8 mmol of KI in the mini-
mum amount of DMF. The mixture was stirred at 60°C
for 20 h. The solvent was then evaporated under re-
duced pressure and the residue was purified by column
chromatography (SiO2, 3a–3d: CH2Cl2–CH3OH=
18:1; 3e–3h: CH2Cl2–CH3OH=20:1). Yields and char-
acteristic data of these compounds are reported in
Table 1.
4.2.2. Functional assays
Portions of guinea-pig ileum were mounted on a
coaxial platinum electrode assembly in a 10 ml water-
jacketed organ-bath containing Krebs–Henseleit solu-
tion aerated with 95% O2:5% CO2 and maintained at
37°C. The preparation was then equilibrated for 60 min
under 1 g of resting tension, with replacement of fresh
solution every 15 min. Single electrical pulses were
delivered to the tissue at 0.1 Hz frequency and 1 ms
duration from a stimulator (LACE Elettronica model
ES-3, Ospedaletto PI, Italy) with submaximal voltage
(1.5–3.0 V). Cumulative concentration–response curves
for the inhibition of electrically stimulated contractions
were determined for the H3 selective agonist RAMHA
(1 nM–1 mM). The tissues were allowed to equilibrate
with the compounds under study (1 nM–10 mM) for 30
min before the generation of concentration–response
curves to the agonist. The antagonist potencies of the
drugs tested were estimated using pA2 determination as
described by Arunlakshana and Schild [37]; otherwise,
when only two high concentrations were tested, pKB
values (‘apparent pA2’) were determined according to
Furchgott’s equation [38]:
1H NMR (DMSO-d6) 3a (dihydrochloride): l 3.01 (t,
2H, CH2ꢀIm), 3.68 (t, 2H, CH2ꢀS), 3.85 (s, 4H, imida-
zoline), 7.57 (s, 1H, Im-5-H), 9.07 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3b (oxalate): l 1.24 (d, 3H,
CH3), 2.93 (t, 2H, CH2ꢀIm), 3.37–3.48 (m, 3H, CH2ꢀS;
imidazoline), 3.95 (app. t, J=10.1 Hz, 1H, imidazo-
line), 4.23–4.38 (m, 1H, imidazoline), 7.06 (s, 1H, Im-5-
H), 7.95 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3c (hydrochloride): l 3.15 (t,
2H, CH2ꢀIm), 3.69 (m, 3H, CH2ꢀS; imidazoline), 4.37
(app. t, J=11.1 Hz, 1H, imidazoline), 5.43 (dd, J=
11.1, J=8.3 Hz, 1H, imidazoline), 7.42 (m, 5H, Ph),
7.59 (s, 1H, Im-5-H), 9.08 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3d (dihydrochloride): l 2.94 (d,
2H, CH2ꢀPh), 3.10 (t, 2H, CH2ꢀIm), 3.57–3.67 (m, 3H,
CH2ꢀS; imidazoline), 3.88 (app. t, 1H, imidazoline),
4.54–4.64 (m, 1H, imidazoline), 7.31 (m, 5H, Ph), 7.54
(s, 1H, Im-5-H), 9.05 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3e (dihydrochloride): l 2.05
(m, 2H, CH2ꢀCH2ꢀCH2), 2.76 (t, 2H, CH2ꢀIm), 3.30 (t,
2H, CH2ꢀS), 3.84 (s, 4H, CH2ꢀimidazoline), 7.45 (s,
1H, Im-5-H), 8.99 (s, 1H, Im-2-H).
1H NMR (DMSO-d6) 3f (oxalate): l 1.19 (d, 3H,
CH3), 1.87 (m, 2H, CH2ꢀCH2ꢀCH2), 2.94 (t, 2H,
CH2ꢀIm), 3.35–3.45 (m, 3H, CH2ꢀS; imidazoline),
3.90–4.00 (m, 1H, imidazoline), 4.24–4.35 (m, 1H,
pA2 app=log([E%]/[E]−1)−log[B]
where [E%] and [E] are the concentrations of the agonist
producing the half-maximum effect in the presence and

32
M. Mor et al. / Il Farmaco 55 (2000) 27–34
Table 4
absence of the antagonist, respectively; [B] is the con-
H3-receptor affinity (pKi) and antagonist activity (pA2) of the 2-
alkylthioimidazole derivatives
centration of the antagonist.
5. Results and discussion
Biological data on H3-receptor affinity and antago-
nist potency for newly synthesised 2-alkylimidazole
derivatives, with alkyl spacers ranging from
dimethylene to tetramethylene chains and phenyl or
cyclohexyl lipophilic ending groups, are reported in
Table 2, while those for ethylthio- and propylthio-imi-
dazoline derivatives are reported in Table 3. Table 4
reports the biological data for thioimidazole derivatives
[20,23,25,39] to be used for comparison.
Comp.
n
R
pKi
pA2
7.14 [39]
7.91 [20]
8.19 [25]
9.07 [20]
1a
1b
1c
1d
2
2
2
2
H
7.24 [23]
7.42 [20]
7.85 [25]
pKi,1=7.99 [20]
pKi,2=9.89 [20]
7.03 [20]
6.52 [20]
6.66 [20]
CH3
C6H5
cC6H11
1e
1f
1g
1h
1i
2
3
3
3
3
CH2C6H5
H
CH3
C6H5
cC6H11
8.17 [20]
B6 [20]
6.88 [20]
B6 [20]
7.27 [20]
Table 2
H3-receptor affinity (pKi) and antagonist activity (pA2) of the 2-
alkylimidazole derivatives
7.68 [20]
pKi,1=6.76 [20]
pKi,2=7.97 [20]
The compounds tested behaved as H3-antagonists on
guinea-pig ileum, and displaced [3H]-RAMHA from rat
brain membranes with Ki values spanning three orders
of magnitude, from millimolar to nanomolar values.
Potency data on guinea-pig ileum (pA2) generally con-
firmed the affinity values measured on rat brain mem-
branes (pKi), even if individual differences arose,
probably due to the different biological procedures.
2-Alkylimidazole derivatives (2a–f), having the cen-
tral heterocycle endowed with an intermediate basicity
(its pKa is expected to be close to 7), showed the best
affinity values (pKi 7.00–8.97), particularly with the
longest tetramethylene chain. In contrast, the basic
imidazoline derivatives (3a–h) gave poorer affinity val-
ues (pKi 5.90–7.63), with a complex SAR pattern.
For the latter compounds (3a–h), the interpretation
of the biological data is not straightforward, nor can a
clear preference between the ethylthio and the propyl-
thio chains be stated, although the positive influence of
the 4(5)-lipophilic group, which had been observed for
2-thioimidazole derivatives 1a–i, can still be recognised.
Starting from very similar pKi values for the parent
compounds, 3a and 3e, the phenyl group gave a slight
improvement in affinity, although this was not suffi-
cient to reach good pKi values. As this group steps out
of the imidazoline ring plane, we tried to reduce the
out-of-plane hindrance through the synthesis of a 4(5)-
methyl and of a flexible 4(5)-benzyl derivative. Slight,
but significant, improvement in affinity was obtained
on the whole, although the best pKi values, achieved for
the benzyl derivatives, hardly reached those obtained
for 2-alkylthioimidazole derivatives (1a–i).
Comp.
n
R
pKi
pA2
2a
2b
2c
2d
2e
2f
2
2
3
3
4
4
C6H5
cC6H11
C6H5
cC6H11
C6H5
cC6H11
7.7490.04
7.0090.07
7.0790.20
7.9090.04
8.0390.07
8.9790.06
6.8190.11
8.0990.14
8.4890.35
8.1290.30
7.3190.09
8.4590.31
a
a pKB value obtained according to Furchgott’s method [38].
Table 3
H3-receptor affinity (pKi) and antagonist activity (pA2) of the 2-
alkylthioimidazoline derivatives
Comp.
n
R
pKi
pA2
3a
3b
3c
3d
3e c
3f
2
2
2
2
3
3
3
3
H
CH3
C6H5
CH2C6H5
H
CH3
C6H5
CH2C6H5
6.2590.10
5.9090.08
6.9090.11
7.6390.03
6.5390.06
7.3290.05
6.9890.07
7.5390.05
6.7890.27
6.6890.36
a
b
7.4490.17
B6
7.3890.35
7.5490.35
7.2090.30
3g
3h
a Non-competitive antagonism was observed at 10−7 M.
b pKB value obtained according to Furchgott’s method [38].
c a2-Agonist, pD2=5.5 (versus yohimbine).
Significantly higher affinities were obtained for 2-
alkylimidazoles (2a–f). In this series the cyclohexyl

M. Mor et al. / Il Farmaco 55 (2000) 27–34
33
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presence of the three- and four-methylene chains. In
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Financial support from Italian MURST is gratefully
acknowledged.
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