Bioorganic & Medicinal Chemistry Letters 10 (2000) 605±607
Probes for Imidazoline Binding Sites: Synthesis and Evaluation
of a Selective, Irreversible I2 Ligand
Philippa A. Coates, a Peter Grundt, a Emma S. J. Robinson, b David J. Nutt, b
Robin Tyacke, b Alan L. Hudson, b John W. Lewis a and Stephen M. Husbands a,*
aSchool of Chemistry, University of Bristol, Bristol, UK
bPsychopharmacology Department, University of Bristol, Bristol, UK
Received 20 December 1999; accepted 27 January 2000
AbstractÐAn irreversible ligand (7) has been prepared based on the selective I2 ligand 2-BFI. Compound 7 displayed high anity
and selectivity for I2-sites and has been shown to irreversibly bind to these sites in rat brain. Compound 7 should, therefore, prove
an invaluable tool for the further elucidation of I2-site function. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
food intake in rats and also neuroprotection in models
of stroke.9,10
It is now widely recognised that mammalian imidazoline
binding sites can be divided into I1 and I2 subtypes and
recently a third class, I3 sites, have also been proposed.1
In order to better determine the role and nature of these
sites there is a need for ligands of high anity and
selectivity. To this end we have identi®ed a number of
ligands (e.g. 1±3) with exceptional selectivity for I2 sites
over both I1 sites and a2-adrenoceptors.2,3 These inclu-
ded 2-BFI (2-benzofuranyl-2-imidazoline) (1) which has
been studied in several animal models of brain function
and tritiated in order to map the distribution of I2 sites
in a number of species.4±6 Even with the availability of
these highly selective ligands it is still unclear what the
imidazoline binding sites represent and what their
pharmacological role is. While Parini's group have
demonstrated that a signi®cant population of these sites
are associated with monoamine oxidase-B (MAOB),7
Garcia-Sevilla's group have de®ned distinct binding
proteins corresponding to subgroups of I2 sites.8 This
raises the possibility that I2 sites may represent more
that one entity and may help to explain the array of
eects attributed to I2 ligands. For example, I2 eects
attributed to the a2-adrenoceptor antagonist/imidazo-
line ligand idazoxan include antidepressive eects in
man, elevation of monoamines in rat brain, increase of
In order to better de®ne the role of I2-sites we have
prepared an irreversible ligand, based on the selective
ligand, 2-BFI. Irreversible ligands allow complete
knockout of one particular site enabling the role of
individual sites to be more easily determined. Addition-
ally an irreversible ligand should function as an
antagonist at its site and allow the assessment of ecacy
of other ligands at that site i.e. the apparent ecacy of a
ligand is reduced as the receptor reserve in a preparation
is immobilised. Until now it has not been possible to
label, with any certainty, the known I2 ligands as ago-
nist, partial agonist or antagonist in character.
Previously
a
radio-iodinated photoanity ligand
([125I]AZIPI) has been prepared and shown to label at
least three subtypes of I2 sites that dier in their ligand
recognition properties, apparent molecular weight and
tissue distribution.15 However, since a photo-anity
label requires photoactivation it is not possible to use
such a ligand in vivo. Furthermore the very high reac-
tivity of the intermediate generated on photo-activation
can reduce the selectivity of the labelling. An alternative
approach is to use an isothiocyanate moiety as the
alkylating function. This group is fairly small and
usually has minimum impact on the binding anity and
selectivity of the parent ligand, though examples are
known where a change in selectivity has occurred.11 The
group is highly reactive towards amino and sulhydryl
groups, while displaying low reactivity to hydroxyl
functions, including water. Thus, there is less non-speci®c
*Corresponding author at current address: Department of Pharmacy
and Pharmacology, University of Bath, Claverton Down, Bath, BA2
7AY, UK. Tel.: +44-1225-826826; fax: +44-1225-826114; e-mail:
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00062-7