A series of norbinaltorphimine congeners (2-12) which contain different groups at the N17'-position have been synthesized in order to evaluate the role of N17' in conferring kappa opioid antagonist selectivity at opioid receptor sites. The compounds that contain a basic N17' nitrogen (2-9) were found to be selective kappa antagonists. Amidation of N17' afforded congeners 10-12 with feeble kappa antagonist potency and low selectivity. The fact that potent antagonism and selectivity were observed only when members of the series contain a basic N17' nitrogen suggests that it interacts with extracellular domains of the kappa receptor that contain acidic amino acid residues. The N-terminal domain and extracellular loop 2, both of which contain acidic residues, are candidates for this interaction and may be components of the kappa address subsite of the receptor.

A series of 2',3'-disubstituted pyrrolomorphinans (5a-i) were synthesized to determine the role of steric hindrance at mu and kappa receptors in promoting delta opioid receptor antagonist selectivity. In smooth muscle preparations, five members of the series (5a-c,e,f) possessed Ke values in the range 2-15 nM and were delta selective. Since the unsubstituted analogue 4 possessed delta antagonist potency of similar magnitude, but was not delta selective, it is suggested that the 2',3'-substitution confers delta selectivity by hindering the interaction of the pharmacophore at mu and kappa receptors, while not affecting delta receptors.

The presence of a molecular scaffold to orient a basic group is important for potent and selective kappa opioid antagonist selectivity. An attempt to determine how the geometry of the scaffold affects this selectivity has led to the synthesis of a bivalent ligand (5) whose linker constrains the N17' basic nitrogen (the "address") to a position that is 6.5 A from N17' in the kappa antagonist norBNI (1) when these molecules are superimposed. The fact that compound 5 was found to be a highly selective and potent mu-selective antagonist supports the idea that the position of N17' in 5 precludes effective ion pairing with the nonconserved residue Glu297 on outer loop 3 of the kappa opioid receptor. The high mu receptor binding affinity and in vitro pharmacological selectivity of 5 coupled with its presumed low central nervous system bioavailability suggest that it may be a useful antagonist for the investigation of peripheral mu opioid receptors.

A series of amino acid conjugates (2-6) of naltrindole (1) were synthesized from 7'-carboxynaltrindole (7) in order to obtain delta antagonists that would have minimal access to the central nervous system (CNS) upon peripheral administration. All of the ligands (2-7) were tested in smooth muscle preparations and found to be potent and selective delta opioid antagonists. Receptor binding showed 2-7 to be highly delta-selective, with Ki ratios (mu/delta, kappa/delta) ranging from 127 to 38,000. Two of the more selective conjugates, the glycinate 2 and aspartate 3, were evaluated by the iv and icv routes in mice, and they afforded very high iv/icv dose ratios (112,766 and 46,667, respectively) consistent with poor CNS penetration. The in vivo testing revealed that 2 and 3 are delta 1-selective antagonists, in contrast to naltriben and related ligands which are delta 2-selective. The fact that the binding data are not consistent with the in vivo data suggests that the origin of the selectivity of naltrindole congeners may be related to selective access to tissue compartments in the CNS rather than to binding affinity differences between delta opioid receptor subtypes.