16-Me cyprenorphine (RX 8008M): a potent opioid antagonist with some delta selectivity.

16-Me cyprenorphine (RX 8008M) has been investigated in a number of isolated tissue preparations and found to be a pure opioid antagonist with Ke values at the delta, mu and kappa receptors of 0.73, 1.77 and 59.6 nM respectively. Comparisons of the mu, kappa and delta Ke values with a number of other antagonists in the mouse vas deferens have been made and show that the 16-Me substituent results in a marked enhancement of delta activity, making RX 8008M the most selective non-peptide delta antagonist available at the present time.