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© Copyright 2004 The British Pharmacological Society

063P University of Bath
Summer Meeting July 2004

Preliminary evaluation of the affinity of isoquinoline-containing compounds at imidazoline1 binding sites

 

Abu Ghazaleh, H, Lalies, MD, Tyacke, RJ, *Husbands, SM, *Rees, C, *Hudson, N, *Kwan, J, *Purchase, L, Nutt DJ & Hudson, AL. Psychopharmacology Unit, University of Bristol, Bristol, BS1 3NY. *Department of Pharmacy and Pharmacology, University of Bath, Bath, BA2 7AY

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Abu Ghazaleh H
Lalies MD
Tyacke RJ
Husbands SM
Rees C
Hudson N
Kwan J
Purchase L
Nutt DJ
Hudson AL

The imidazoline-1 binding site (I1BS) displays some functional and pharmacological similarities to the
2-adrenoceptor ( 2-AR). The antihypertensive drugs moxonidine and rilmenidine are proposed to lower blood pressure at the level of the brainstem by an action on I1BS (Bousquet et al., 1999). Whether this effect is solely mediated by I1BS stimulation remains controversial, since evidence suggests the involvement of 2-AR in eliciting a hypotensive response (Szabo, 2002). We have previously reported BU98008 (1-(4,5-dihydro-1H-imidazol-2-yl)isoquinoline) to show selectivity for I1BS (Robinson et al., 2003). The present study investigated the affinities and selectivities of a newly synthesised series of BU98008 derivatives for I1BS and 2-ARs, namely BU04003 (1-[1-benzyl-4,5-dihydro-1H-imidazol-2-yl]isoquinoline), BU04004 (1-[3H-pyrrol-2-yl]isoquinoline), BU04005 (1-isoquinolin-1-ylmethanamine) and BU04006 (1-[5-methyl-4,5-dihydro-1H-imidazol-2-yl]isoquinoline).

Kidneys and brains were obtained from male Wistar rats (250-275g) and membranes prepared according to the methods of Robinson et al. (2003). In binding studies the displacing ligands were assessed over a concentration range: 1nM – 100µM. Brain membranes ( 2-AR) or kidney membranes (I1BS) were incubated with [3H]RX821002 (1nM) or [3H]clonidine (3nM + 10µM rauwolscine to mask 2-ARs binding) to label 2-AR and I1BS respectively (room temperature, 30 min). Non-specific binding for
I 1BS was defined with 10µM clonidine, and for 2-AR, 10µM rauwolscine. Assays were terminated by rapid filtration and radioactivity was determined by liquid scintillation counting. Data were analysed by iterative non-linear regression using GraphPad Prism version 3.03 for Windows.

 

Compound

IC50 (µM)

I1BS ± s.e.mean (n)

Ki (µM)

2-AR ± s.e.mean (n)

BU04003

4.35 ± 0.25 (3)

10.72 ± 1.40 (3)

BU04004

2.08 ± 1.01 (3)

139.33 ± 73.88 (3)

BU04005

0.62 ± 0.17 (3)

7.52 ± 3.41 (3)

BU04006

1.27 ± 0.39 (3)

235.65 ± 196.57 (3)

BU98008

0.083 ± 0.008 (3)

4.04 ± 0.49 (4)

Table1. Affinities of isoquinoline compounds for I1BS and 2-ARs.

None of the compounds displayed a higher affinity for I1BS than the parent compound BU98008 although they did show moderate affinity for this site (Table 1). However, BU04006 did exhibit higher selectivity for I1BS over 2-ARs compared with BU98008. In conclusion, in view of the binding profile of BU04004 and BU04006 these compounds need to be characterised for potential hypotensive activity in vivo.

Bousquet, P. et al., (1999) Ann. N. Y. Acad. Sci., 881, 272-278
Robinson, E.S.J. et al. (2003) Ann.N.Y.Acad. Sci., 1009, 283-287
Szabo, B. (2002) Pharmacol Ther., 93, 1-35

We thank the Wellcome Trust for their financial support