Design and Synthesis of Naltrexone-Derived Affinity Labels with Nonequilibrium Opioid Agonist and Antagonist Activities. Evidence for the Existence of Different μ Receptor Subtypes in Different Tissues

A series of β-funaltrexamine (2, β-FNA) analogues (3-14) were synthesized that contain a variety of electrophilic groups attached at the 6/3-position of the opiate. The opioid agonist and antagonist activities of these ligands were evaluated in the guinea pig ileum (GPI) and mouse vas deferens (MVD) in vitro assays. Several of the compounds behaved like β-FNA in that they exhibited reversible agonist activity at k opioid receptors and irreversible antagonist activity at p opioid receptors. The rank order of irreversible antagonism for a series of related Michael acceptors did not parallel their intrinsic chemical reactivity, confirming that the degree of covalent binding is in part dependent on the spatial disposition of the electrophilic center relative to the receptor nucleophile (secondary recognition). The maleimidoacetamide 8 behaved very differently from β-FNA in that it exhibited considerably greater irreversible μ antagonism in MVD relative to the p blockage in the GPI. This suggests that different proportions of p receptor subtypes exist in the two tissues. Several of the agents tested, including some nonreactive control compounds, displayed an unusual type of persistent k agonist activity in the GPI. This activity, which was reversed by addition of naloxone, reappeared upon washing. Receptor models have been presented to explain this effect. A few of the reactive ligands displayed a true nonreversible k agonist activity, suggesting a covalent association with the receptor. Of note in this regard was the propiolamide 6, which appeared to be an irreversible mixed agonist-antagonist at k and μ receptors.