4-Benzyl-1H-imidazoles with oxazoline termini as histamine H3 receptor agonists

Article abstract of DOI:10.1021/jm7014149

Research on the therapeutic applications of the histamine H₃ receptor (H₃R) has traditionally focused on antagonists/inverse agonists. In contrast, H₃R agonists have received less attention despite their potential use in s

Full text of DOI:10.1021/jm7014149

2944
J. Med. Chem. 2008, 51, 2944–2953
4-Benzyl-1H-imidazoles with Oxazoline Termini as Histamine H3 Receptor Agonists
Maikel Wijtmans,‡ Sylvain Celanire,† Erwin Snip,‡ Michel R. Gillard,† Edith Gelens,‡ Philippe P. Collart,† Bastiaan J. Venhuis,‡
Bernard Christophe,† Saskia Hulscher,‡ Henk van der Goot,‡ Florence Lebon,† Henk Timmerman,‡ Remko A. Bakker,‡
Be´ne´dicte I. L. F. Lallemand,† Rob Leurs,‡ Patrice E. Talaga,† and Iwan J. P. de Esch*,‡
Leiden/Amsterdam Center for Drug Research, DiVision of Medicinal Chemistry, Faculty of Exact Sciences, VU UniVersity Amsterdam, De
Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, UCB, Chemin du Foriest, B-1420 Braine-l’Alleud, Belgium
ReceiVed NoVember 9, 2007
Research on the therapeutic applications of the histamine H3 receptor (H3R) has traditionally focused on
antagonists/inverse agonists. In contrast, H3R agonists have received less attention despite their potential
use in several disease areas. The lower availability of H3R agonists not only hampers their full therapeutic
exploration, it also prevents an unequivocal understanding of the structural requirements for H3R activation.
In the light of these important issues, we present our findings on 4-benzyl-1H-imidazole-based H3R agonists.
Starting from two high throughput screen hits (10 and 11), the benzyl side chain was altered with lipophilic
groups using combinatorial and classical chemical approaches (compounds 12-31). Alkyne- or oxazolino-
substituents gave excellent affinities and agonist activities up to the single digit nM range. Our findings
further substantiate the growing notion that basic ligand sidechains are not necessary for H3R activation and
reveal the oxazolino group as a hitherto unexplored functional group in H3R research.
Introduction
combating obesity, narcolepsy, allergic rhinitis, cognitive dis-
orders, and others.10,13–15
The neurotransmitter histamine exerts its physiological actions
through four distinct GPCRsa known as the histamine H1, H2,
H3, and H4 receptors.1 The former two have already proven to
be excellent drug targets, whereas the latter one was discovered
seven years ago2 and is currently the subject of explorative
chemical and pharmacological endeavors.3,4 The histamine H3
receptor (H3R) was discovered by classical pharmacological
means in the early 1980s.5 Evidence gathered over the years
has revealed a prominent role of neuronal H3Rs in modulation
of histamine biosynthesis and release as well as in regulation
of the release of other neurotransmitters such as dopamine,
serotonin, and noradrenaline.6,7 For about 20 years, all efforts
to identify the gene encoding the H3R were in vain. As a result,
drug discovery efforts were seriously frustrated as no recom-
binant cell lines for HTS approaches were available. A
breakthrough came in 1999 when Lovenberg and co-workers
reported the cloning of the cDNA encoding the H3R.8 This
breakthrough sparked fierce H3R research in academia and
industry alike.9,10 Soon, the crucial finding of high H3R
constitutive activity was disclosed, leading to reclassification
of H3R ligands into inverse agonists, antagonists, and agonists.11,12
Many therapeutic areas in which H3R may play pivotal roles
have since been unraveled, having placed the H3R in a prominent
position within the drug discovery community. More specifi-
cally, evidence has been rapidly accumulating that H3R antago-
nists or inverse agonists potentially play crucial roles in
These exciting preclinical studies were to a great extent
enabled by many effective antagonists and inverse agonists that
emerged from HTS campaigns within pharmaceutical companies
following the cloning of the receptor.16 Similar development
of agonists during this quest has been less widely pursued
despite early successes with agonists such as 1 (BP 2-94)17 and
2 (SCH50971)18,19 in the 1990s (Scheme 1).20 The main reason
for this was undoubtedly the rather limited evidence for potential
therapeutic areas of H3R agonists when compared to antagonists
or inverse agonists. Until recently, such evidence has been
largely confined to the areas of ischemic arrhythmias, migraine,
and asthma.15 Some of these indications have borne fruit as, in
2006, the agonist NR-methylhistamine (3) was the first H3R
ligand ever to complete phase III studies, namely for migraine
prophylaxis.21 However, during the last two years, the list of
potential therapeutic areas for H3R agonists has been expanded
with three new interesting areas: obesity,22 diabetes mellitus,22
and liver cholestasis.23 Recent evidence also shows that H3R-
KO mice develop more severe symptoms in experimental
allergic encephalomyelitis, the autoimmune model of multiple
sclerosis, thereby suggesting that H3R agonists could be of
therapeutic value in multiple sclerosis.24 It is reasonable to
suggest that all this will cause H3R agonist research to gather
more momentum in the near future.
From a fundamental point of view, a serious consequence of
the lower availability of agonist studies is that the structural
requirements for receptor activation are still not fully understood.
Many models for H3R binding and/or activation involve
acid-base interaction of a protonated basic group in the ligand
side chain to an aspartate residue in TM3 (D114),28–30 which
is highly conserved in GPCRs for biogenic neurotransmitter
amines. Indeed, many known and widely used H3R agonists
have basic groups in their side chains that presumably bind this
aspartate residue, see, for example, histamine itself, 3, RAMH
(4) and methimepip25 (5) (Scheme 1). However, a handful of
reports has challenged this popular hypothesis by disclosing H3R
agonists that have a less basic side chain (i.e., immethridine
* To whom correspondence should be addressed. Phone: 31-20-5987600.
Fax: 31-20-5987610. E-mail: i.de.esch@few.vu.nl.
‡
Leiden/Amsterdam Center for Drug Research, Division of Medicinal
Chemistry, Faculty of Exact Sciences, VU University Amsterdam.
†
UCB.
a Abbreviations: CADD, computer-assisted computational drug design;
cAMP, cyclic adenosine monophosphate; CHO, Chinese hamster ovary;
DAST, diethylaminosulfur trifluoride; DIC, N,N′-diisopropylcarbodiimide;
DMAP, 4-dimethylaminopyridine; DMF, dimethylformamide; GPCR, G
protein-coupled receptor; HBA, hydrogen-bond acceptor; HBD, hydrogen-
bond donor; HTS, high throughput screen; IBS, imidazole-binding site; KO,
knockout; P1, N,N-dimethylsulfamoyl; RAMH, R-R-methylhistamine; r.t.,
room temperature; TEA, triethylamine; TFA, trifluoroacetic acid; TM,
transmembrane.
10.1021/jm7014149 CCC: $40.75
2008 American Chemical Society
Published on Web 04/24/2008

4-Benzyl-1H-imidazoles as Histamine H3R Agonists
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 2945
Scheme 1. Selected H3R Agonists and Their Affinity/Activity Data as Extracted from the Literature9,17,19,20,25–27
Scheme 2. Structures and Affinities of HTS Hits
Acetylation of benzylalcohols 34a,b and reduction of the acetate
group in 35a,b by hydrogenolysis afforded key intermediates
36a,b, which after acidic deprotection gave 12 and 22. For the
synthesis of bromide-substituted compounds, the potential
liability of the arylbromide in the acetate reduction and perhaps
in the Grignard exchange prompted the design of an alternative
route. Here, the Grignard intermediate was generated from P1-
protected 4-iodoimidazole 37 (see Scheme 4).26 After a
magnesium-copper exchange, the resulting cuprate was reacted
with 4-bromo-benzylbromide to install the essential benzylic
C-C bond of 38. Following basic deprotection, target 39 was
obtained but only in moderate yield (46%) over two steps.
Therefore, in an attempt to improve the yield for the para
counterpart 23, we reverted to classical imidazole functional-
ization on P1-protected imidazole 40, as utilized by us before.36
Key to that approach is the one-pot protection, metalation, and
subsequent functionalization of the imidazole nucleus by
repeated addition of n-BuLi and electrophiles. Acidic depro-
tection of intermediate 41 afforded target 23 in a yield much
improved with respect to that of 39.
Precursors 23, 38, and 39 afforded excellent entry into more
elaborate functionalizations of the phenyl groups (Scheme 5).
Using standard Suzuki conditions, the bromide group in 38 was
exchanged for a 4-pyridyl or phenyl group by means of the
boronic acid (for 42a) or a cyclic boronic diol ester (for 42c) in
moderate to excellent yield. Given the known stability problems
of 2-pyridine boronic acid,37 we performed the C-C coupling
for 42b under Stille conditions using the corresponding stannane
43. In all cases, final deprotection of the P1 group to targets
14-16 was achieved using KOH in EtOH.
6)26 or do not have a basic group in the side chain at all,27,31–34
an example of which is proxifan (7). More remarkably, some
of these compounds, i.e., 8 (GT-2331)33 and 9 (VUF5523),27
have strictly aliphatic side chains lacking any heteroatom
(Scheme 1).
To shed more light on the structural determinants for H3R
activation and in view of the emerging reports on possible
therapeutic uses for H3R agonists, we wish to disclose our
findings in the development of rigid H3R agonists without basic
or highly polar groups in the side chain. Starting from two HTS
hits (10 and 11, Scheme 2), the side chain was systematically
altered with predominantly lipophilic substituents (12-31). This
exercise has not only afforded several novel H3R agonists with
affinities and activities in the single digit nM range but has also
provided molecular clues on how agonists without a basic side
chain are able to activate H3R. Last, our results also reveal a
hitherto unexplored functional group in H3R research.
Chemistry
Noncombinatorial Chemistry. Nitrile-substituted com-
pounds 12 and 22 were obtained using a Grignard addition route
described by us previously (Scheme 3).27 4(5)-Iodo-1H-imida-
zole (32) was protected with a trityl group to give 33. This was
reacted with EtMgBr in DCM/Et2O, and the resulting imida-
zolyl-Grignard reagent was added to the aldehyde group of the
appropriately substituted cyanobenzaldehyde in good yields. The
peculiar use of DCM and Et2O as solvent in the initial Grignard
exchange between 33 and EtMgBr is crucial, as more coordinat-
ing solvents led to rapid rearrangement of the intermediate
4-imidazolyl Grignard reagent to the undesired 2-isomer.35
The bromide substituents in 39 and 23 were anticipated to
provide excellent handles for alkyne substitution under classical
Sonogashira couplings (Pd catalysis, Cu catalysis, TEA).
Anticipating that a free imidazole group would be detrimental
to this reaction because of its high affinity for Cu ions, we first
installed electron-withdrawing groups on the imidazole ring
(tosyl or P1 group) in order to minimize this complexation.
Indeed, compounds 44 and 38 underwent smooth Sonogashira
couplings with 3,3-dimethyl-1-butyne or phenylacetylene to
afford protected alkynes 45 and 46a,b. Subsequent deprotection

2946 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Scheme 3. Synthetic Pathways for Compounds 12 and 22a
Wijtmans et al.
a Key: (a) Tritylchloride, TEA, THF, reflux, 1.5 h (64%). (b) [1] EtMgBr, Et2O, DCM, r.t., 30 min [2] RPhCHO, r.t., 1 d (34a: 85%, 34b: 70%). (c) Ac2O,
DMAP, TEA, DCM, r.t., 1 h (35a: 89%, 35b: 87%). (d) 5% Pd/C, EtOH, H2, r.t., 3 h (36a: 62%, 36b: 78%). (e) HCl, EtOH, reflux, 1 h (12: 79%, 22: 78%).
Scheme 4. Synthetic Pathway for Intermediate 39 and Compound 23a
a Key: (a) [1] EtMgBr, Et2O, DCM, r.t., 80 min [2] CuCN.2LiCl [3] 3-BrPhCH2Br, r.t., 3 d (46%). (b) KOH, H2O, EtOH, reflux, 1 d (quantitative yield).
(c) N,N-dimethylsulfamoylchloride, TEA, DCM, r.t., 1 d (91%). (d) [1] n-BuLi, THF, -65 °C, 15 min. [2] t-BuMe2SiCl, r.t., 1 h. [3] n-BuLi, THF, -65 °C,
30 min. [4] 4-BrPhCH2Br, r.t., 1 d. (e) 30% aq HBr, reflux, 1 d (yield >90% for steps (d) and (e) combined).
Scheme 5. Synthetic Pathway for Compounds 14-16a
a Key: (a) For 42a,c: ArB(OR)2, DMF/H2O, Na2CO3, Pd(PPh3)4, 90 °C, 16 h. For 42b: 2-tributylstannylpyridine (43), DMF, Pd(PPh3)4, 90 °C, 16 h (42a:
91%, 42b: 30%, 42c: 26%). (b) KOH, H2O, EtOH, reflux, 1 d (14: 58%, 15: 98%, 16: 73%).
Scheme 6. Synthetic Pathway for Compounds 18-21 and 24a
a Key: (a) TsCl, DCM, TEA, r.t., 4 d (94%). (b) RC≡CH, TEA, DMF, CuI, Pd(PPh3)4, 70 °C, 3 h, then: r.t., 1 d (45: 49%, 46a: 56%, 46b: 39%, 20: 63%,
21: 36%). (c) MeOH, 1 M NaOH, heat, 2 h (27%). (d) KOH, H2O, EtOH, reflux, 1 d (18: 36%, 19: 95%).
of these gave 24, 18, and 19 (Scheme 6). To our surprise, we
later found that the imidazole group could be left unprotected
in the Sonagashira coupling, and this protocol was used to
couple bromide 39 with 1-ethynylnaphthalene and 1-pentyne
rendering 20 and 21. An attempt to cleave the tert-butyl group
in 18 using TFA unexpectedly afforded the ketone 17, which

4-Benzyl-1H-imidazoles as Histamine H3R Agonists
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 2947
Scheme 7. Synthetic Pathway for Compound 17a
ring by structurally distinct groups to explore the general
propensity of substituted 4-benzyl-1H-imidazoles to activate
H3R and (B) substitution of the oxazoline ring with additional
substituents or replacement by the closely related oxazines.
a Key: TFA, H2O, 75 °C, 4 h (47%).
was also included in our assays (Scheme 7). While hydration
of alkynes with aqueous acid is known to be a sluggish process
in the absence of coreagents such as Hg salts, we assume it is
the electron donating character of both the tert-butyl and the
phenyl substituent that allows this hydration to proceed relatively
mildly through stabilization of the intermediate vinyl cation.38
The noncombinatorial synthesis of compounds 10 and 26
involved preparation of a Grignard reagent from an appropriately
substituted arylbromide 47a,b followed by a subsequent addition
to N-trityl-protected imidazole-carbaldehyde 48 (Scheme 8).
Treatment of the intermediate alcohols 49a,b with H2/Pd under
acidic conditions induced both reduction of the alcohol and
cleavage of the trityl-protecting group to arrive at the target
compounds.
Combinatorial Chemistry. Hit optimization of oxazoline 10
was done by a combinatorial chemistry approach. A total of 42
amino alcohols were converted into their corresponding oxazo-
lines or oxazines as outlined in Scheme 9 for representative
members 27-31. Nitrile 36a was hydrolyzed to the carboxylic
acid 50, which, without purification, was carried on to an acidic
deprotection step rendering key precursor 51. The imidazole
group was again protected, this time using the more stable
4-methoxyphenylsulfonyl group, and the resulting molecule 52
was coupled to a pentafluorophenol-derivatized resin to give
an activated carboxylic acid linked to solid support (53).
Aminolysis of 53 with a variety of different amino alcohols
afforded a library of amides (general structure 54), which were
subsequently cyclized using DAST39 and deprotected with base
to deliver the desired library.
Approach A: Exploring the 4-Benzyl-1H-imidazole Core.
The oxazoline substituent in hit 10 was replaced by other
functional groups to get a better understanding of what variations
are tolerated on the general 4-benzyl-1H-imidazole agonist core.
This explorative strategy involved substituents such as nitriles,
phenyls, and pyridines. We also included a series of compounds
in which the benzyl linker was extended by one acetylene unit
to further limit the flexibility of the side chain. Such a double-
constraint strategy was used in optimizing H3R affinity in the
GT-2331 series, where a tandem cyclopropane and acetylene
constraint worked excellently.42,43 H3R affinity was assayed
using radiolabeled [3H]-NR-methylhistamine displacement on
membranes from CHO cells stably expressing the H3R. Com-
pounds with pKi values over 6.5 were also tested for their
functional effects (Gi-mediated inhibition of cAMP production).
The effects of the meta substituents are presented in Table
1. A cyano-substituent (12) did not improve upon the affinity
of unsubstituted compound 13. However, attachment of a phenyl
or 2-pyridyl group (14 and 15) increased the affinity equally
by about 0.5 log unit with respect to that of 13, while the
4-pyridyl group (as in 16) is significantly less tolerated than
the 2-pyridyl or phenyl group. All this indicates that the pyridine
N-atom in 15 plays no important role in receptor affinity and
that a more exposed polar group (as in 16) is actually disfavored.
It is of interest to note that this trend is the opposite of what
has been observed in the immethridine series (6), which differs
in that it lacks the middle phenyl group.26,27 That is, replacement
of the pyridine N-atom in immethridine by a CH group led to
an almost 1000-fold drop in affinity. This opposite trend is a
strong indication that the aryl side chain of the present
compounds is targeting a different, more lipophilic pocket of
the hH3R than the side chain from immethridine. In line with
this hypothesis was the affinity observed for 17, where the
incorporation of a lipophilic 3,3-dimethyl-1-butanoyl group was
beneficial.
Gratifyingly, the double-constraint approach using the alkyne-
extended system proved successful in the meta system. A variety
of lipophilic endgroups comprising a tert-butyl, phenyl, or propyl
group (18, 19, 21) was able to push the hH3R affinity in the
single digit nM range. Increasing the size of the group, however,
was detrimental: a dramatic 1000-fold drop in affinity was
observed for the 1-naphthyl derivative 20 with respect to phenyl
derivative 19. Taken together, these findings clearly indicate
that the benzylalkyne linker in conjunction with a moderately
sized lipophilic group induces efficient recognition of the
postulated lipophilic pocket to achieve high H3R affinities.
The 4-benzyl-1H-imidazole-containing ligands described by
Schering-Plough40,41 (i.e., 55) all contain substituents on the
para position. For purposes of comparison, we selected a concise
set of compounds from Table 1 and tested the corresponding
para-substituted compounds (Table 2). The large positional
effect observed between our hits 10 and 11 proved to be a
general effect: meta substitution is also preferred over para
Results and Discussion
HTS Hits and Design Strategy. At the start of this project,
a HTS of the UCB compound database was initiated using a
[3H]-NR-methylhistamine displacement assay. Two of the hits
that emerged from this screen were compounds 10 and 11
(Scheme 2). These compounds have moderate to high affinity
for hH3R (pKi hH3R ) 8.4 and 5.6, respectively). Their
structures are composed of an imidazole ring linked to a
dimethyloxazoline unit through a benzyl linker, i.e., are based
on a 4-benzyl-1H-imidazole template. Compound 10 behaves
as an efficient agonist on hH3R (vide infra). Two interesting
observations can be made with regard to the structure of this
agonist. First, although the 4-benzyl-1H-imidazole template has
occasionally been explored in a systematic H3R SAR study,40
the resulting lead compound (55) was reported to behave as an
antagonist on hH3R (human saphenous vein assay and guinea
pig ileum electrical field stimulated contraction assay).41 In fact,
only one H3R agonist with a 4-benzyl-1H-imidazole core is
known, i.e., 13.27 Second, the oxazoline unit (a cyclic imidate)
is not frequently encountered in medicinal chemistry. Its claim
to fame is the use as a protecting group in organic synthesis.
We launched a project in which both the 4-benzyl-1H-imidazole
and oxazoline moieties were explored as a means to achieve
efficient agonistic activity on hH3R. Toward this end, we
followed two main approaches: (A) replacement of the oxazoline

2948 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Wijtmans et al.
Scheme 8. Representative Synthetic Pathway for Compounds 10 and 26a
a Key: (a) [1] Mg, THF [2] aldehyde 48, r.t., 0.5-13 h (49a: 71%, 49b: 57%). (b) 80-135 psi H2, 10% Pd/C, AcOH, 66 °C, 4 h (26: 65%, 10: 19%).
Scheme 9. Representative Combinatorial Synthetic Pathway for Compounds 27-31a
a (a) KOH, EtOH/H2O, reflux, 18 h. (b) 1.0 M HCl, reflux, 1 h (55% over steps (a) and (b)). (c) 4-Methoxyphenylsulfonyl chloride, TEA, DCM, 0
°C-r.t., overnight (28%). (d) PS-TFP resin, DIC, DMAP, DCM/DMF, r.t., 18 h. (e) Amino alcohol H2NC(R1R2)(CR3R4)nC(R5R6)OH, DCM, DMF, r.t.,
18 h. (f) DAST, DCM, r.t., 1.5 h. (g) [1] 1 N NaOH, MeOH, r.t., overnight. [2] Preparative LC-MS.
substitution in two other tested couples (24 vs 18, 22 vs 12). In
fact, para substitution with neither a small bromide, amino, or
cyano substituent nor with a tert-butylacetylene group improved
at all upon the affinity of unsubstituted compound 13.
Approach B: Exploring the Oxazoline Substituent. Taking
into account that a meta-positioned oxazoline unit leads to a
highly efficient compound with unique structural properties, we
unleashed a combinatorial chemistry approach to optimize the
oxazoline substructure and explore the general effect of a meta-
positioned cyclic imidate substituent (Scheme 10). This was
achieved by preparing compounds containing oxazolines (gen-
eral structure 56a) or their six-membered counterparts (oxazines,
general structure 56b).
A total of 42 amino alcohols were selected based on their
chemical diversity and with the aid of a pharmacophore model.44
Of the library of 42 oxazolines and oxazines prepared and tested,
six compounds had appreciable affinity for hH3R (Table 3),
while the others displayed inferior pKi values, for example,
compound 31. It appears that dimethylsubstitution at C1 (see
Scheme 10 for enumeration) is most beneficial for affinity, as
indicated by 10 and 26 having excellent pKi values of 8.44 and
8.90, respectively. A further increase in substituent size at C1,
as in 27, leads to a reduction in affinity. Likewise, monosub-
stitution at C1 with larger substituents (29 and 30) as well as
disubstitution at C2 (28 and 31) are less favored. Of overall
importance here is that the discussed six hits consist of both
five-membered oxazolines and six-membered oxazines contain-
ing strictly lipophilic substituents. This suggests that the presence
of a meta-positioned cyclic imidate moiety and of a small,
suitably oriented lipophilic unit close to the N-atom on this
imidate ring are both beneficial for appreciable affinity. Interest-
ingly, when comparing approaches A and B, the cyclic imidate
group can be considered a somewhat rigid linker between the
phenyl group and a lipophilic end group.
Functional Activities on hH3R and gpH3R. While exploring
the effect of structural modifications on the affinities of the
ligands, we also closely monitored functional activities of our
compounds at the Gi-coupled H3R. Initial one-point screening
of ligand activities in approach A was accomplished by
determining inhibition of forskolin-stimulated cAMP produc-
tion (see Tables 1 and 2). This screen revealed that, without
exception, all compounds measured were agonists on the hH3R.
The same conclusion was reached for the compounds for
approach B listed in Table 3, albeit that a different type of screen
was used (GTPγS assay, see Table 4). Selected candidates from
both approaches A and B were investigated in more detail for
their functional properties on hH3R (Table 4). All compounds
display pEC50 values (GTPγS assay) that correspond acceptably
with their pKi values. The highest functional activities were
observed for compounds 10, 18, and 26, having excellent pEC50
values of 7.8, 8.1, and 8.7, respectively. The full dose-response
curves for these three compounds (Figure 1) indicate that all
behave as full agonists on hH3R (R ≈ 1). Clearly, the
compounds described here represent a significant addition to
the growing list of efficient hH3R agonists that lack basic side
chains.27,31–34
Compounds 26, 18, 30, and 17 were also tested on guinea
pig H3R (gpH3R) by measuring their inhibitory effect on
electrically evoked contractile response of isolated guinea pig
intestine. The twitch contraction observed on the guinea pig
isolated ileal myenteric plexus in response to electrical field
stimulation is principally due to the release of acetylcholine from

4-Benzyl-1H-imidazoles as Histamine H3R Agonists
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 2949
Table 1. Affinities and Activities of Meta-Substituted
4-Benzyl-1H-imidazole on hH3R (Approach A)
Table 2. Affinities and Activities of Para-Substituted
4-Benzyl-1H-imidazole on hH3R (Approach A)
a Displacement of [3H]-NR-methylhistamine on membranes from CHO
cells stably expressing the hH3R (n ) 3, SEM shown). b Defined as the
percentage of reduction in cAMP levels in CHO cells stably expressing
the human H3R induced by treatment of 10-5 M of compound relative to
untreated cells (n ) 3, SEM shown). c Not determined. d Reported by us
before.27 Reference values for H3R inverse agonist clobenpropit and for
H3R agonist immepip are shown in Table 1.
Scheme 10. General Structures of the 42 Library Members
Prepared by Combinatorial Chemistrya
a Enumeration of the imidate ring carbons is also depicted.
0.05 (n ) 22) and 6.02 ( 0.06 (n ) 12), respectively, implying
at least 40-fold lower potencies than thioperamide. Increasing
concentrations of compounds 26 and 18 induced a rightward
shift as well with, however, a significant decrease of the maximal
amplitude of the concentration-response curve to RAMH
(Figure 2). This noncompetitive behavior is not due to residual
agonist activity of these compounds, as up to 10-6.5 M, neither
18 nor 26 decreases twitch contraction by itself. The pA2 values
of 26 and 18 amount to 8.4 ( 0.10 (n ) 16) and 7.1 ( 0.06 (n
) 11), respectively, and as such these compounds are equipotent
or even 6-fold more potent than thioperamide.
a Displacement of [3H]-NR-methylhistamine on membranes from CHO
cells stably expressing the hH3R (n ) 3, SEM shown). b Defined as the
percentage of reduction in cAMP levels in CHO cells stably expressing
the human H3R induced by treatment of 10-5 M of compound relative to
untreated cells (n ) 3, SEM shown). c Not determined. Reference values
for H3R inverse agonist clobenpropit and for H3R agonist immepip are also
given.
The antagonist behavior observed on the guinea pig ileal
myenteric plexus contrasts with the agonist profile observed in
the GTPγS assay. However, such differences are not uncommon
to occur. In fact, Harper et al. very recently disclosed a novel
type of assay which predicts that certain imidazole-containing
H3R ligands, previously classified as antagonists based on
isolated tissue bioassays, may possess residual agonism ef-
ficacy.51 Indeed, discrepancies between activity assays on hH3R
and gpH3R have already been reported for a wide variety of
H3R imidazole-based agonists. This list includes compounds
with a basic side chain such as imbutamine,27 impentamine,12,27
and a three-carbon chain homologue of the agonist imetit12 as
well as agonists without a basic side chain as exemplified by
8,27,52 n-alkyl substituted imidazoles (9),27 and burimamide.27
These results together with our current findings further strengthen
Harper’s notion. Discrepancies in functional activities of H3R
the cholinergic nerve terminals.45,46 In this model, potent H3R
agonists such as RAMH, immethridine, immepip, or imetit
induce a concentration-dependent decrease of the electrically
induced twitch contraction through the activation of the hista-
minergic H3 presynaptic receptor47–50 present on this preparation
(pD2 ) 7.2, 7.5, 8.0, and 7.9, respectively).26 Under the same
conditions, increasing concentrations of an H3R antagonist such
as thioperamide induced a rightward shift without any decrease
of the maximal amplitude of the concentration-response curve
to RAMH, suggesting the existence of a competitive inhibition
between thioperamide and RAMH on the histamine H3 receptors
present on this preparation (pA2 ) 7.6 ( 0.07 (n ) 30), Schild
plot slope of 1.07). A similar result was obtained with increasing
concentrations of compounds 30 and 17 (not shown). The
potencies (pA2 values) of these compounds amount to 5.65 (

2950 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Table 3. Affinities of the Best Library Compounds (Approach B)
Wijtmans et al.
Figure 1. Full curves for functional activity on hH3R of compounds
10, 26, and 18 as well as for reference agonist histamine and reference
antagonist thioperamide. n ) 2-6.
a Displacement of [3H]-NR-methylhistamine on membranes from CHO
cells stably expressing the hH3R. n ) 2-5, SEM is shown. Reference values
for H3R inverse agonist clobenpropit and for H3R agonist immepip are
shown in Table 1.
Table 4. Functional Properties of Selected Compounds as Measured by
the Effect of the Compound on [35S]GTPγS Binding to Membranes
Expressing H3R Receptorsa
cmpd
pEC50 ( SEM
10
14
17
18
23
26
30
7.8 ( 0.33
6.7 ( 0.11
7.1 ( 0.20
8.1 ( 0.05
6.2 ( 0.23
8.7 ( 0.10
7.1 ( 0.30
a n ) 2-6, SEM is shown.
ligands are largely assay-dependent and seem to be consistent
throughout structurally diverse types of imidazole-containing
molecules.
Figure 2. Effect of compounds 18 and 26 on guinea pig isolated paced
ileal myenteric plexus stimulated with RAMH. Shown on the y-axis is
the Emax value, which represents the maximum inhibitory effect of
contraction. The inset shows the tested compound and the log value of
the applied concentration.
Pharmacophore Modeling. A CADD approach was used
to explain the observed binding and functional properties of
our compounds, which are all relatively rigid and have no basic
groups except for the imidazole. The key observation is that
addition of an additional hydrophobic side chain is tolerated
and in some cases increases the affinity toward the hH3R, all
indicative of an important interaction with a lipophilic pocket.
Interestingly, several molecular modeling studies reveal two
distinct lipophilic pockets available for antagonist binding.28,44,53,54
For our investigations on this matter, the starting geometries of
H3R ligands, among which the well-known antagonist 57
(ciproxifan) and antagonist/partial agonist 58 (GR175737)33,55
were generated and partially optimized using Sybyl6.8 (struc-
tures shown in Figure 3a). Conformational spaces were gener-
ated using our internal procedure CONFEX as has been
described previously.56 The conformational spaces generated
were used as starting points to find common pharmacophoric
features using DISCOtech and FlexS (Sybyl6.8). For clarity,
only 57 and 58 are represented in Figure 3b. Compounds 10
and 18 were subsequently fitted onto the generic pharmacophore
to ensure maximal coverage of the pharmacophoric features.
Comparing the pharmacophoric characteristics of 10 and 18 with

4-Benzyl-1H-imidazoles as Histamine H3R Agonists
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 2951
Figure 3. (a) Structures of 57 and 58. (b) Superposition of compounds 10 and 18 to a generic pharmacophore model for imidazole-based H3R
antagonists showing 57 and 58 colored by atom. The pharmacophore features the presence of a basic moiety (IBS), an aromatic system as well as
an additional HBA and lipophilic moiety. Compound 10 and 18 are represented with the carbon atoms colored in yellow and orange, respectively.
Histamine is represented with the carbon atoms colored in green. For clarity, all hydrogen atoms have been omitted. Also shown is a 2D schematic
representation.
Table 5. Additional Pharmacological Data on Selected Compoundsa
those of 57 and 58 suggests that one of the hydrophobic pockets
cmpd
hH4R (pKi)b
CYP2D6 (IC50, µM)
CYP3A4 (IC50, µM)
could be available for binding by agonists such as 10 and 18 as
well (Figure 3b). The latter two compounds share the aromatic,
delocalized π-electron system as well as an additional polar
group and peripheral lipophilic moiety found in various H3R
antagonists.57 The model predicts that filling of the hydrophobic
pocket by the peripheral lipophilic group of 10 and 18 places
their imidazole rings in overlap with the primary amine of
histamine, which is located near the highly conserved and crucial
aspartate D114 residue (Figure 3b). Consequently, while the
imidazole ring of histamine has been shown to interact with
E206 at the IBS, we suggest that our constrained agonists, which
only contain one basic moiety, interact through their imidazole
moiety with D114 instead. Indeed, the existence of only one
specific imidazole binding site for all imidazole-containing
histamine H3R ligands and the role of the interaction between
the imidazole ring and E206 are challenged by mutagenesis
experiments, indicating that E206A mutation does not signifi-
cantly modify the KD value of iodoproxyfan,30 which suggests
the possibility of alternative binding modes. We therefore
hypothesize that the alternative imidazole-D114 interactions,
observed for 10 and 18, allow for receptor activation in analogy
to receptor activation through D114 by a basic group of many
known dibasic agonists (vide supra). More specifically, the
interaction of the agonists with D114 is suggested to have a
pronounced effect on the conformation of this residue, which
has already been associated with receptor activation.28
11
17
18
19
21
26
17 ( 1%c
8.25 ( 0.05
8.25 ( 0.05
n.d.
n.d.
8.08 ( 0.03
0.543 ( 0.008
1.926 ( 0.012
1.785 ( 0.044
0.488 ( 0.007
0.644 ( 0.015
0.292 ( 0.003
2.522 ( 0.079
1.104 ( 0.027
1.860 ( 0.049
2.763 ( 0.142
1.639 ( 0.093
0.873 ( 0.022
a Left column: affinity of test compounds for hH4R (n ) 2-4, SEM is
shown). Right columns: inhibition of cytochrome P450 isoforms CYP2D6
and CYP3A4 by test compounds as established by dextromethorphan
O-demethylase and midazolam 1′-hydroxylase assays, respectively. n ) 11,
SEM is shown. b Displacement of [3H]-histamine on membranes from CHO
cells stably expressing the hH4R. c Percentage inhibition of [3H]-histamine
binding at 10 µM test compound.
Such moderate H3R/H4R selectivity levels are regularly en-
countered with imidazole-containing ligands, as these two
receptors share the highest homology within the histamine
receptor family.58,59 Furthermore, in recent years, it has become
clear that another potential liability of many imidazole-contain-
ing compounds is their binding to cytochrome P450 isoforms.60,61
This may induce distorted metabolism and/or drug-drug
interactions. A clear illustration of the latter is the case of H3R
imidazole-ligands ciproxifan and thioperamide, which cause
significant drug-drug interactions in rats when coadministered
with the antipsychotic drugs risperidone and haloperidol, likely
through cytochrome P450 interactions.62 We tested a selected
set of compounds on two important isoforms of cytochrome
Receptor Selectivity and Cytochrome P450 Binding. The
three high-affinity ligands 17, 18, and 26 were evaluated further
for their selectivity on the full panel of histamine receptors.
None showed appreciable inhibition of H1R or H2R (<20% and
<40% inhibition at 10 µM, respectively). However, all three
exhibit significant affinity for the human H4 receptor (Table 5).
P450: CYP2D6, which is estimated to be involved in the
metabolism of 30% of marketed drugs despite its low abundancy
of ca. 2%,61 and CYP3A4, which is the most abundant isoform
and involved in the metabolism of ca. 60% of all clinically
relevant drugs. Unfortunately, both isoforms were potently
inhibited by our test set (Table 5) with IC50 values ranging from

2952 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Wijtmans et al.
(9) Leurs, R.; Bakker, R. A.; Timmerman, H.; de Esch, I. J. P. The
histamine H3 receptor: from gene cloning to H3 receptor drugs. Nat.
ReV. Drug DiscoVery 2005, 4, 107–120.
(10) Celanire, S.; Wijtmans, M.; Talaga, P.; Leurs, R.; de Esch, I. J. P.
Histamine H3 receptor antagonists reach out for the clinic. Drug
DiscoVery Today 2005, 10, 1613–1627.
(11) Morisset, S.; Rouleau, A.; Gbahou, X. L.; Tardivel-Lacombe, J.; Stark,
H.; Schunack, W.; Ganellin, C. R.; Schwartz, J. C.; Arrang, J. M.
High constitutive activity of native H3 receptors regulates histamine
neurons in brain. Nature 2000, 408, 860–864.
0.5-2.8 µM. This is very likely a result of the presence of the
imidazole ring.
Despite the undesired H4R- and cytochrome P450-binding of
the compounds, both approaches A and B have left us with some
interesting H3R agonists as exemplified by the highly potent
oxazoline-derivative 26 (pEC50 ) 8.7). These compounds may
fulfill a valuable role as tools in H3R research.
Conclusion
(12) Wieland, K.; Bongers, G.; Yamamoto, Y.; Hashimoto, T.; Yamatodani,
A.; Menge, W. M. B. P.; Timmerman, H.; Lovenberg, T. W.; Leurs,
R. Constitutive activity of histamine H-3 receptors stably expressed
in SK-N-MC cells: Display of agonism and inverse agonism by H-3
antagonists. J. Pharmacol. Exp. Ther. 2001, 299, 908–914.
(13) Esbenshade, T. A.; Fox, G. B.; Cowart, M. D. Histamine H3 receptor
antagonists: preclinical promise for treating obesity and cognitive
disorders. Mol. InterV. 2006, 6, 77–78.
(14) Bonaventure, P.; Letavic, M.; Dugovic, C.; Wilson, S.; Aluisio, L.;
Pudiak, C.; Lord, B.; Mazur, C.; Kamme, F.; Nishino, S.; Carruthers,
N.; Lovenberg, T. Histamine H3 receptor antagonists: from target
identification to drug leads. Biochem. Pharmacol. 2007, 73, 1084–
1096.
(15) Wijtmans, M.; Leurs, R.; de Esch, I. J. P. Histamine H3 receptor ligands
break ground in a remarkable plethora of therapeutic areas. Expert
Opin. InVest. Drugs 2007, 16, 1–19.
(16) Letavic, M. A.; Barbier, A. J.; Dvorak, C. A.; Carruthers, N. I. Recent
medicinal chemistry of the histamine H3 receptor. Prog. Med. Chem.
2006, 44, 181–206.
(17) Rouleau, A.; Garbarg, M.; Ligneau, X.; Mantion, C.; Lavie, P.;
Advenier, C.; Lecomte, J. M.; Krause, M.; Stark, H.; Schunack, W.;
Schwartz, J. C. Bioavailability, antinociceptive and antiinflammatory
properties of BP 2-94, a histamine H3 receptor agonist prodrug.
J. Pharmacol. Exp. Ther. 1997, 281, 1085–1094.
(18) McLeod, R. L.; Aslanian, R.; Prado, M. D.; Duffy, R.; Egan, R. W.;
Kreutner, W.; McQuade, R.; Hey, J. A. SCH50971, an orally active
histamine H3 receptor agonist, inhibits central neurogenic vascular
inflammation and produces sedation in the guinea pig. J. Pharmacol.
Exp. Ther. 1998, 287, 43–50.
(19) Shih, N. Y.; Aslanian, R.; Lupo, A. T.; Orlando, S.; Piwinski, J. J.;
Green, M. J.; Ganguly, A. K.; West, R.; Tozzi, S.; Kreutner, W.; Hey,
J. A. Trans-4-methyl-3-imidazoyl pyrrolidine as a potent, highly
selective histamine H-3 receptor agonist in vivo. Bioorg. Med. Chem.
Lett. 1998, 8, 243–248.
(20) de Esch, I. J. P.; Belzar, K. J. Histamine H-3 receptor agonists. Mini-
ReV. Med. Chem. 2004, 4, 955–963.
In the current study, we have systematically explored the use
of a 4-benzyl-1H-imidazole core and an oxazoline side chain
in pursuit of hH3R agonists. Classical synthetic means as well
as combinatorial chemistry approaches led to the preparation
of 55 compounds. Meta substitution of the phenyl group was
generally preferred over para substitution. The most potent
compounds possess an alkyne moiety (18, 21) or a cyclic imidate
moiety (10, 26) with a peripheral lipophilic group. Interestingly,
all compounds proved to be agonists for the hH3R. Yet, a second
basic unit in the side chain, often associated with H3R agonism,
was lacking in all these compounds. We rationalized these
binding and functional results by molecular modeling studies,
which reveal that the peripheral lipophilic group of the agonists
occupies a hydrophobic pocket previously known to play a role
in antagonist binding. This in turn positions the imidazole group
near aspartate residue D114, allowing receptor activation.
The clinical potential of the present compounds is unfortu-
nately compromised by substantial binding to cytochrome P450
isoforms and moderate selectivity for hH3R over hH4R. Our
findings nevertheless contribute valuable lessons about the exact
determinants for hH3R affinity and activity and provide a
framework for explaining H3R agonism by an ever growing list
of ligands not having any polar groups in the side chain. Such
a framework will prove useful in responding to the steadily
increasing list of therapeutic indications for H3R agonists. Most
notably, the oxazoline unit represents an unexplored type of
structure in H3R agonist research and opens new venues in the
H3R antagonist area as well.63
(21) Milla´n-Guerrero, R. O.; Isais-Milla´n, R.; Benjam´ın, T.-H.; Tene, C. E.
N-R-Methylhistamine safety and efficacy in migraine prophylaxis:
phase III study. Can. J. Neurol. Sci. 2006, 33, 195–199.
(22) Yoshimoto, R.; Miyamoto, Y.; Shimamura, K.; Ishihara, A.; Takahashi,
K.; Kotani, H.; Chen, A. S.; Chen, H. Y.; MacNeil, D. J.; Kanatani,
A.; Tokita, S. Therapeutic potential of histamine H3 receptor agonist
for the treatment of obesity and diabetes mellitus. Proc. Natl. Acad.
Sci. U.S.A. 2006, 103, 13866–13871.
Supporting Information Available: Synthesis and spectral
characterization of all compounds, all procedures for biological
measurements, and selected copies of spectra for key compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(23) Francis, H.; Franchitto, A.; Ueno, Y.; Glaser, S.; DeMorrow, S.; Venter,
J.; Gaudio, E.; Alvaro, D.; Fava, G.; Marzioni, M.; Vaculin, B.; Alpini,
G. H3 histamine receptor agonist inhibits biliary growth of BDL rats
by downregulation of the cAMP-dependent PKA/ERK1/2/ELK-1
pathway. Lab. InVest. 2007, 87, 473–487.
(24) Teuscher, C.; Subramanian, M.; Noubade, R.; Gao, J. F.; Offner, H.;
Zachary, J. F.; Blankenhorn, E. P. Central histamine H-3 receptor
signaling negatively regulates susceptibility to autoimmune inflam-
matory disease of the CNS. Proc. Natl. Acad. Sci. U.S.A. 2007, 104,
10146–10151.
(25) Kitbunnadaj, R.; Hashimoto, T.; Poli, E.; Zuiderveld, O. P.; Menozzi,
A.; Hidaka, R.; de Esch, I. J. P.; Bakker, R. A.; Menge, W. M.;
Yamatodani, A.; Coruzzi, G.; Timmerman, H.; Leurs, R. N-Substituted
piperidinyl alkyl imidazoles: discovery of methimepip as a potent and
selective histamine H3 receptor agonist. J. Med. Chem. 2005, 48, 2100–
2107.
(26) Kitbunnadaj, R.; Zuiderveld, O. P.; Christophe, B.; Hulscher, S.;
Menge, W. M. P. B.; Gelens, E.; Snip, E.; Bakker, R. A.; Celanire,
S.; Gillard, M.; Talaga, P.; Timmerman, H.; Leurs, R. Identification
of 4-(1H-imidazol-4(5)-ylmethyl)pyridine (immethridine) as a novel,
potent, and highly selective histamine H-3 receptor agonist. J. Med.
Chem. 2004, 47, 2414–2417.
References
(1) Hough, L. B. Genomics meets histamine receptors: new subtypes, new
receptors. Mol. Pharmacol. 2001, 59, 415–419.
(2) Oda, T.; Morikawa, N.; Saito, Y.; Masuho, Y.; Matsumoto, S.
Molecular cloning and characterization of a novel type of histamine
receptor preferentially expressed in leukocytes. J. Biol. Chem. 2000,
275, 36781–36786.
(3) Zhang, M.; Venable, J. D.; Thurmond, R. L. The histamine H4 receptor
in autoimmune disease. Expert Opin. InVest. Drugs. 2006, 15, 1443–
1452.
(4) Lim, H. D.; Smits, R. A.; Leurs, R.; de Esch, I. J. P. The emerging
role of the histamine H-4 receptor in anti-inflammatory therapy. Curr.
Top. Med. Chem. 2006, 6, 1365–1373.
(5) Arrang, J. M.; Garbarg, M.; Schwartz, J. C. Autoinhibition of brain
histamine release mediated by a novel class (H3) of histamine receptor.
Nature 1983, 302, 832–837.
(6) Schlicker, E.; Kathmann, M. Modulation of in vitro neurotransmission
in the CNS and in the retina via H3 heteroreceptors. In The Histamine
H3 Receptor: A Target for New Drugs; Leurs, R., Timmerman, H.,
Eds.; Elsevier: Amsterdam, 1998; pp 13-16.
(7) Arrang, J. M.; Garbarg, M.; Schwartz, J. C. Autoregulation of histamine
release in brain by presynaptic H3-receptors. Neuroscience 1985, 15,
553–562.
(27) Kitbunnadaj, R.; Hoffmann, M.; Fratantoni, S. A.; Bongers, G.; Bakker,
R. A.; Wieland, K.; el Jilali, A.; de Esch, I. J. P.; Menge, W. M. P. B.;
Timmerman, H.; Leurs, R. New high affinity H-3 receptor agonists
without a basic side chain. Bioorg. Med. Chem. 2005, 13, 6309–6323.
(28) de Esch, I. J. P.; Timmerman, H.; Menge, W. M. P. B.; Nederkoorn,
P. H. J. A qualitative model for the histamine H-3 receptor explaining
(8) Lovenberg, T. W.; Roland, B. L.; Wilson, S. J.; Jiang, X.; Pyati, J.;
Huvar, A.; Jackson, M. R.; Erlander, M. G. Cloning and functional
expression of the human histamine H3 receptor. Mol. Pharmacol. 1999,
55, 1101–1107.

4-Benzyl-1H-imidazoles as Histamine H3R Agonists
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 2953
agonistic and antagonistic activity simultaneously. Arch. Pharm. 2000,
333, 254–260.
muscle preparations from guinea pig ileum. Br. J. Pharmacol. 1973,
48, P362–P363.
(46) Daniel, E. E.; Kwan, C. Y.; Janssen, L. Pharmacological techniques
for the in vitro study of intestinal smooth muscles. J. Pharmacol.
Toxicol. 2001, 45, 141–158.
(47) Trzeciakowski, J. P. Inhibition of guinea pig ileum contractions
mediated by a class of histamine-receptor resembling the H-3 subtype.
J. Pharmacol. Exp. Ther. 1987, 243, 874–880.
(48) Hew, R. W. S.; Hodgkinson, C. R.; Hill, S. J. Characterization of
histamine H-3 receptors in guinea pig ileum with H-3 selective ligands.
Br. J. Pharmacol. 1990, 101, 621–624.
(49) Taylor, S. J.; Kilpatrick, G. J. Characterization of histamine-H-3
receptors controlling nonadrenergic noncholinergic contractions of the
guinea pig isolated ileum. Br. J. Pharmacol. 1992, 105, 667–674.
(50) Menkveld, G. J.; Timmerman, H. Inhibition of electrically evoked
contractions of guinea pig ileum preparations mediated by the
histamine H3-receptor. Eur. J. Pharmacol. 1990, 186, 343–347.
(51) Harper, E. A.; Shankley, N. P.; Black, J. W. Correlation of apparent
affinity values from H3-receptor binding assays with apparent affinity
(pKapp) and intrinsic activity (alpha) from functional bioassays. Br. J.
Pharmacol. 2007, 151, 128–143.
(52) Tedford, C. E.; Hoffmann, M.; Seyedi, N.; Maruyama, R.; Levi, R.;
Yates, S. L.; Ali, S. M.; Phillips, J. G. High antagonist potency of
GT-2227 and GT-2331, new histamine H-3 receptor antagonists, in
two functional models. Eur. J. Pharmacol. 1998, 351, 307–311.
(53) Lorenzi, S.; Mor, M.; Bordi, F.; Rivara, S.; Rivara, M.; Morini, G.;
Bertoni, S.; Ballabeni, V.; Barocelli, E.; Plazzi, P. V. Validation of a
histamine H-3 receptor model through structure-activity relationships
for classical H-3 antagonists. Bioorg. Med. Chem. 2005, 13, 5647–
5657.
(29) Yao, B. B.; Hutchins, C. W.; Carr, T. L.; Cassar, S.; Masters, J. N.;
Bennani, Y. L.; Esbenshade, T. A.; Hancock, A. A. Molecular
modeling and pharmacological analysis of species-related histamine
H-3 receptor heterogeneity. Neuropharmacology 2003, 44, 773–786.
(30) Uveges, A. J.; Kowal, D.; Zhang, Y. X.; Spangler, T. B.; Dunlop, J.;
Semus, S.; Jones, P. G. The role of transmembrane helix 5 in agonist
binding to the human H3 receptor. J. Pharmacol. Exp. Ther. 2002,
301, 451–458.
(31) Sasse, A.; Stark, H.; Ligneau, X.; Elz, S.; Reidemeister, S.; Ganellin,
C. R.; Schwartz, J. C.; Schunack, W. (Partial) agonist/antagonist
properties of novel diarylalkyl carbamates on histamine H-3 receptors.
Bioorg. Med. Chem. 2000, 8, 1139–1149.
(32) Pelloux-Leon, N.; Fkyerat, A.; Piripitsi, A.; Tertiuk, W.; Schunack,
W.; Stark, H.; Garbarg, M.; Ligneau, X.; Arrang, J. M.; Schwartz,
J. C.; Ganellin, C. R. Meta-substituted aryl(thio)ethers as potent partial
agonists (or antagonists) for the histamine H-3 receptor lacking a
nitrogen atom in the side chain. J. Med. Chem. 2004, 47, 3264–3274.
(33) Wulff, B. S.; Hastrup, S.; Rimvall, K. Characteristics of recombinantly
expressed rat and human histamine H-3 receptors. Eur. J. Pharmacol.
2002, 453, 33–41.
(34) Gbahou, F.; Rouleau, A.; Morisset, S.; Parmentier, R.; Crochet, S.;
Lin, J. S.; Ligneau, X.; Tardivel-Lacombe, J.; Stark, H.; Schunack,
W.; Ganellin, C. R.; Schwartz, J. C.; Arrang, J. M. Protean agonism
at histamine H3 receptors in vitro and in vivo. Proc. Natl. Acad. Sci.
U.S.A. 2003, 100, 11086–11091.
(35) Turner, R. M.; Lindell, S. D.; Ley, S. V. A facile route to imidazol-
4-yl anions and their reaction with carbonyl compounds. J. Org. Chem.
1991, 56, 5739–5740.
(36) Vollinga, R. C.; Menge, W. M. P. B.; Leurs, R.; Timmerman, H.
Homologs of histamine as histamine H-3 receptor antagonists: a new
potent and selective H-3 antagonist, 4(5)-(5-aminopentyl)-1H-imida-
zole. J. Med. Chem. 1995, 38, 266–271.
(37) Lutzen, A.; Hapke, M. Synthesis of 5-substituted 2,2′-bipyridines from
substituted 2-chloropyridines by a modified Negishi cross-coupling
reaction. Eur. J. Org. Chem. 2002, 2292–2297.
(38) Allen, A. D.; Chiang, Y.; Kresge, A. J.; Tidwell, T. T. Substituent
effects on the acid hydration of acetylenes. J. Org. Chem. 1982, 47,
775–779.
(54) Schlegel, B. The human histamine H3-receptor: a molecular modeling
study of a G-protein coupled receptor, Ph.D. Thesis, Heinrich-Heine-
Universita¨t, Dusseldorf, Germany, 2005.
(55) Clitherow, J. W.; Beswick, P.; Irving, W. J.; Scopes, D. I. C.; Barnes,
J. C.; Clapham, J.; Brown, J. D.; Evans, D. J.; Hayes, A. G. Novel
1,2,4-oxadiazoles as potent and selective histamine H-3 receptor
antagonists. Bioorg. Med. Chem. Lett. 1996, 6, 833–838.
(56) Moureau, F.; Neuwels, M.; Dogimont, C.; Goldstein, S.; Massingham,
R. Conformational analysis of pseudo-peptides: The case of FK888,
a potent and selective substance P receptor antagonist. Lett. Pept. Sci.
1998, 5, 155–158.
(57) Celanire, S.; Lebon, F.; Stark, H. Drug Discovery: From Hits to Clinical
Candidates. In The Third Histamine Receptor: SelectiVe Ligands as
Potential Therapeutic Agents in CNS Disorders; Vohora, D. S. , Ed.;
Taylor & Francis CRC Press Inc.: Boca Raton, FL, 2007; in press.
(58) Lim, H. D.; v. Rijn, R. M.; Ling, P.; Bakker, R. A.; Thurmond, R. L.;
Leurs, R. Evaluation of histamine H1-, H2-, and H3-receptor ligands
at the human histamine H4 receptor: identification of 4-methylhistamine
as the first potent and selective H4 receptor agonist. J. Pharmacol.
Exp. Ther. 2005, 314, 1310–1321.
(59) Gbahou, F.; Vincent, L.; Humbert-Claude, M.; Tardivel-Lacombe, J.;
Chabret, C.; Arrang, J. M. Compared pharmacology of human
histamine H-3 and H-4 receptors: structure-activity relationships of
histamine derivatives. Br. J. Pharmacol. 2006, 147, 744–754.
(60) Yang, R.; Hey, J. A.; Aslanian, R.; Rizzo, C. A. Coordination of
histamine H3 receptor antagonists with human adrenal cytochrome
P450 enzymes. Pharmacology 2002, 66, 128–135.
(61) Vaccaro, W. D.; Sher, R.; Berlin, M.; Shih, N. Y.; Aslanian, R.;
Schwerdt, J. H.; McCormick, K. D.; Piwinski, J. J.; West, R. E.;
Anthes, J. C.; Williams, S. M.; Wu, R. L.; She, H. S.; Rivelli, M. A.;
Mutter, J. C.; Corboz, M. R.; Hey, J. A.; Favreau, L. Novel histamine
H3 receptor antagonists based on the 4-[(1H-imidazol-4-yl)methyl]pi-
peridine scaffold. Bioorg. Med. Chem. Lett. 2006, 16, 395–399.
(62) Zhang, M.; Ballard, M. E.; Pan, L. P.; Roberts, S.; Faghih, R.; Cowart,
M.; Esbenshade, T. A.; Fox, G. B.; Decker, M. W.; Hancock, A. A.;
Rueter, L. E. Lack of cataleptogenic potentiation with non-imidazole
H-3 receptor antagonists reveals potential drug-drug interactions
between imidazole-based H-3 receptor antagonists and antipsychotic
drugs. Brain Res. 2005, 1045, 142–149.
(39) Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D. R. Synthesis
of functionalized oxazolines and oxazoles with DAST and Deoxo-
Fluor. Org. Lett. 2000, 2, 1165–1168.
(40) Aslanian, R.; Mutahi, M. W.; Shih, N. Y.; McCormick, K. D.; Piwinski,
J. J.; Ting, P. C.; Albanese, M. M.; Berlin, M. Y.; Zhu, X. H.; Wong,
S. C.; Rosenblum, S. B.; Jiang, Y. H.; West, R.; She, S.; Williams,
S. M.; Bryant, M.; Hey, J. A. Identification of a novel, orally
bioavailable histamine H3 receptor antagonist based on the 4-benzyl-
(1H-imidazol-4-yl) template. Bioorg. Med. Chem. Lett. 2002, 12, 937–
941.
(41) McLeod, R. L.; Rizzo, C. A.; West, R. E.; Aslanian, R.; McCormick,
K.; Bryant, M.; Hsieh, Y.; Korfmacher, W.; Mingo, G. G.; Varty, L.;
Williams, S. M.; Shih, N. Y.; Egan, R. W.; Hey, J. A. Pharmacological
characterization of the novel histamine H3-receptor antagonist N-(3,5-
dichlorophenyl)-N′-[[4-(1H-imidazol-4-ylmethyl)phenyl]-methyl]-
urea (SCH79687). J. Pharmacol. Exp. Ther. 2003, 305, 1037–1044.
(42) Tedford, C. E.; Phillips, J. G.; Gregory, R.; Panlowski, G. P.; Fadnis,
L.; Khan, M. A.; Ali, S. M.; Handley, M. K.; Yates, S. L. Development
of trans-2-[lH-imidazol-4-yl] cyclopropane derivatives as new high-
affinity histamine H-3 receptor ligands. J. Pharmacol. Exp. Ther. 1999,
289, 1160–1168.
(43) Ali, S. M.; Tedford, C. E.; Gregory, R.; Handley, M. K.; Yates, S. L.;
Hirth, W. W.; Phillips, J. G. Design, synthesis, and structure--activity
relationships of acetylene-based histamine H-3 receptor antagonists.
J. Med. Chem. 1999, 42, 903–909.
(44) de Esch, I. J. P.; Mills, J. E. J.; Perkins, T. D. J.; Romeo, G.; Hoffmann,
M.; Wieland, K.; Leurs, R.; Menge, W. M. P. B.; Nederkoorn, P. H. J.;
Dean, P. M.; Timmerman, H. Development of a pharmacophore model
for histamine H-3 receptor antagonists, using the newly developed
molecular modeling program SLATE. J. Med. Chem. 2001, 44, 1666–
1674.
(63) Celanire, S.; Talaga, P.; Leurs, R.; Denonne, F.; Timmerman, H.;
Lebon, F. Compounds comprising an oxazoline or thiazoline moiety,
processesformakingthemandtheiruses.WorldPatentWO2006103057.
2006.
(45) Ambache, N.; Killick, S. W.; Zar, M. A. Antagonism by burimamide
of inhibitions Induced by histamine in plexus-containing longitudinal
JM7014149