A Naloxone–Steroid Hybrid Azine with Selective and Long-Acting Opioid Antagonism at Delta Receptors In Vitro

The interaction of naloxone estrone azine (N-EH) with various opioid receptor types was studied in vitro. Its potency as an antagonist of opioid effects was compared to that of naloxone on the electrically evoked contractions of mouse vas deferens (Mvd) and guinea pig ileum myenteric plexus longitudinal muscle (Gpi) preparations. N-EH was found to be 9-fold more potent than naloxone in antagonizing the effects of D-Ala²-Leu⁵-enkephalin in the Mvd and 22-fold less potent in antagonizing nor-morphine in the Gpi. In the Mvd, the recovery half-time for N-EH was longer than 1000 min. Neither compound showed agonism. The two compounds were also compared for their capacity to displace the binding of ³H-D-Ala²-Leu⁵-enkephalin, ³H-dihydromorphine, and ³H-ethylketocyclazocine to rat brain membranes under conditions where delta, mu, and kappa sites were labeled. The relative affinities were 0.70, 0.16, and 0.14 for N-EH and 0.05, 0.87, and 0.08 for naloxone, respectively. Thus, compared to naloxone, which is mu selective, N-EH is a delta-selective antagonist.