Β-Funaltrexamine
β-Funaltrexamine is an irreversible opioid antagonist that was used to create the first crystal structure of the μ-opioid receptor. It is selective for antagonism of the MOR over the δ-opioid receptor and κ-opioid receptor. Chemically, it is a naltrexone derivative with a methyl-fumaramide group in the 6-position. In addition to its MOR irreversible antagonism, β-FNA is a reversible agonist of the κ-opioid receptor and produces KOR-mediated analgesic effects in animals. This has limited its usefulness and contributed to the development of methocinnamox as a more selective functionally irreversible antagonist of the MOR with no significant opioid agonistic actions.