Univalent and bivalent ligands of butorphan: characteristics of the linking chain determine the affinity and potency of such opioid ligands.

Bivalent morphinan compounds containing ester linkers were synthesized and their binding affinities at the mu, delta, and kappa opioid receptors determined. Addition of methyl groups adjacent to the hydrolytically labile ester linkage increased stability while only partially affecting binding affinity. The resulting bivalent ligands with optimized spacer length and structure show potent binding profiles with the most potent compound (4b), having K(i) values of 0.47 nM for both the mu and kappa opioid receptors, and 4a, having K(i) values of 0.95 and 0.62 nM for the mu and kappa receptors, respectively. Both 4a and 4b were partial agonists at the kappa and micro receptors in the [(35)S]GTPgammaS binding assay.