Recent interest in the M1 muscarinic acetylcholine (ACh) receptor (mAChR) has led to the discovery of various selective agonists for the receptor. The novel selective agonist, 1-(1 -(2-methylbenzyl)-1,4 -bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one (TBPB), displays unprecedented functional selectivity at the M1 mAChR. This functional selectivity has been described to stem from sole interaction with an allosteric site, although the evidence for such a mechanism is equivocal. To delineate TBPB s mechanism of action, several truncated variants of TBPB were synthesised and characterised. Binding experiments with [3H]-N-methylscopolamine ([3H]-NMS) at the M1, M2, M3 and M4 mAChRs revealed radioligand displacement in a manner consistent with a competitive binding mode at the orthosteric site by TBPB and fragment derivatives. Cell-based functional assays of fragment derivatives of TBPB identified both agonistic and antagonistic moieties, one of which, VCP794, lost agonistic selectivity for the M1 mAChR. Further interaction experiments between TBPB or its antagonist fragments with ACh also indicated a mechanism consistent with competitive binding at mAChRs. However, interaction with an allosteric site by an antagonist fragment of TBPB was demonstrated via its ability to retardation radioligand dissociation. To reconcile this dual orthosteric/allosteric pharmacological behaviour, we propose that TBPB is a bitopic ligand, interacting with both the orthosteric site and an allosteric site, at the M1 mAChR. This mechanism may also be the case for other selective agonists for mAChRs, and should be taken into consideration in the profiling and classification of new novel selective agonists for this receptor family.
Keov, P., Valant, C., Devine, S., Lane, J. R., Scammells, P. J., Sexton, P., & Christopoulos, A. (2013). Reverse engineering of the selective agonist TBPB unveils both orthosteric and allosteric modes of action at the M1 muscarinic acetylcholine receptor. Molecular Pharmacology, 84(3), 425 - 437. https://doi.org/10.1124/mol.113.087320