Characterisation of bis(4-aminoquinoline)s as α_1A adrenoceptor allosteric modulators

The development of sub-type selective α₁ adrenoceptor ligands has been hampered by the high sequence similarity of the amino acids forming the orthosteric binding pocket of the three α₁ adrenoceptor subtypes, along with other biogenic amine receptors. One possible approach to overcome this issue is to target allosteric sites on the α₁ adrenoceptors. Previous docking studies suggested that one of the quinoline moieties of a bis(4-aminoquinoline), comprising a 9-carbon methylene linker attached via the amine groups, could interact with residues outside of the orthosteric binding site while, simultaneously, the other quinoline moiety bound within the orthosteric site. We therefore hypothesized that this compound could act in a bitopic manner, displaying both orthosteric and allosteric binding properties. To test this proposition, we investigated the allosteric activity of a series of bis(4-aminoquinoline)s with linker lengths ranging from 2 to 12 methylene units (designated C2-C12). A linear trend of increasing [³H]prazosin dissociation rate with increasing linker length between C7 and C11 was observed, confirming their action as allosteric modulators. These data suggest that the optimal linker length for the bis(4-aminoquinoline)s to occupy the allosteric site of the α_1A adrenoceptor is between 7 and 11 methylene units. In addition, the ability of C9 bis(4-aminoquinoline) to modulate the activation of the α_1A adrenoceptor by norepinephrine was subsequently examined, showing that C9 acts as a non-competitive antagonist. Our findings indicate that the bis(4-aminoquinolines) are acting as allosteric modulators of orthosteric ligand binding, but not efficacy, in a bitopic manner.