Transformation of a κ-opioid receptor antagonist to a κ-agonist by transfer of a guanidinium group from the 5′- to 6′-position of naltrindole

The importance of the indole scaffold of GNTI 3 in directing its address (5′-guanidinium group) to associate with the Glu297 residue of the κ-opioid receptor was investigated by the synthesis and biological evaluation of its 4′- (4a), 6′- (4b), and 7′- (4c) regioisomers. The finding that only the 5′-regioisomer (GNTI) possessed potent κ-opioid antagonist activity and high affinity at κ-receptors illustrates the importance of the 5′-position in orienting the guanidinium group to the proper recognition locus (Glu 297) for potent κ-antagonist activity. The discovery that the 6′-regioisomer of GNTI was a potent κ-agonist, together with the results of site-directed mutagenesis studies that are consistent with association between the 6′-guanidinium group and Glu297, suggest that the transition from an inactive to an active state of the κ-receptor involves a conformational change of TM6. We propose that association of the 6′-guanidinium group of 4b with Glu297 promotes axial rotational motion of transmembrane helix VI which leads to receptor activation via a conformational change of inner loop 3.