Design and Synthesis of Novel Dimeric Morphinan Ligands for κ and μ Opioid Receptors

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Abstract
A novel series of morphinans were synthesized, and their binding affinity at and functional selectivity for μ, δ, and κ opioid receptors were evaluated. These dimeric ligands can be viewed as dimeric morphinans, which were formed by coupling two identical morphinan pharmacophores (cyclorphan (1) or MCL 101 (2)) with varying connecting spacers. Ligands 6 and 7 with alkyl spacers on the nitrogen position and ligands 8 and 9 in which the two morphinan pharmacophores were coupled by ether moieties at the 3-hydroxyl positions showed significant decrease in affinity at all three opioid receptors. An improvement in the affinity was achieved by introducing an ester moiety as the spacer in the dimeric morphinans. It was observed that the affinity of these ligands was sensitive to the character and length of the spacer. Compound 13 (MCL-139) with a 4-carbon ester spacer, compound 17 (MCL-144) containing a 10-carbon spacer, and compound 19 (MCL-145) with the conformationally constrained fumaryl spacer were the most potent ligands in this series, displaying excellent affinities at μ and κ receptors (Ki = 0.09−0.2 nM at μ and Ki = 0.078−0.049 nM at κ), which were comparable to the parent compound 2. Ligand 12, a compound containing only one morphinan pharmacophore and a long-chain ester group, had affinity at both μ and κ receptors almost identical to that of the parent ligand 2. In the [35S]GTPγS binding assay, ligands 13, 17, and 19 and their parent morphinans 1 and 2 stimulated [35S]GTPγS binding mediated by the μ and κ receptors. Compounds 13 and 17 were full κ agonists and partial μ agonists, while compound 19 was a partial agonist at both μ and κ receptors. These novel ligands, as well as their interesting pharmacological properties, will serve as the basis for our continuing investigation of the dimeric ligands as potential probes for the pharmacotherapy of cocaine abuse and may also open new avenues for the characterization of opioid receptors.