<unk> , or<unk> -hydroxynaltrexone (developmental code name AIKO-150), is a peripherally-selective opioid receptor antagonist related to<unk> e. It is a major active metabolite of<unk> e formed by hepatic dihydrodiol dehydrogenase enzymes. With<unk> e therapy,<unk>  is present at approximately 10- to 30-fold higher concentrations than<unk> e at steady state due to extensive first-pass metabolism of<unk> e into<unk> . In addition to being an active metabolite of<unk> e,<unk>  was itself studied for the treatment of opioid-induced constipation. It was found to be effective and well-tolerated, and did not precipitate opioid withdrawal symptoms or interfere with opioid pain relief, but development was not further pursued.<unk>  binds to the opioid receptors with affinity (Ki) values of 2.12 nM for the μ-opioid receptor (MOR), 7.24 nM for the κ-opioid receptor (KOR), and 213 nM for the δ-opioid receptor (DOR). Hence,<unk>  shows 3.5-fold selectivity for the MOR over the KOR and 100-fold selectivity for the MOR over the DOR. Relative to<unk> e,<unk>  has about half the affinity for the MOR. In contrast to<unk> e,<unk>  is a neutral antagonist of the MOR (as opposed to an inverse agonist) and can antagonize the actions of both agonists and inverse agonists at the receptor.<unk>  is said to have very limited capacity to cross the blood–brain barrier. However,<unk>  is still able to cross into the brain and produce central opioid receptor antagonism at sufficient levels. In animal studies,<unk>  showed about 10-fold separation in potency between peripheral and central opioid antagonism, whereas<unk> e showed no separation. Because it is a MOR neutral antagonist and hence does not reduce basal MOR signaling,<unk>  shows much lower potential for producing opioid withdrawal symptoms than<unk> e at doses achieving similar central opioid blockade in animal studies. Due to the very high levels of<unk>  that occur during<unk> e therapy,<unk>  may contribute to the central opioid receptor antagonism of<unk> e.
6β-Naltrexol
Nandrolone hexyloxyphenylpropionate
Bisoctrizole
5α-Dihydronorethisterone
Norbormide
Dextropropoxyphene
Nandrolone phenylpropionate
Cabergoline