The compounds of structural formulae (I) and (II) and use of the compounds and salts and hydrates thereof, as therapeutic agents, is disclosed. A compound of formula (I) or (II) is an agonist for the &mgr; and &dgr; opioid receptors, an antagonist for the &kgr; opioid receptor, and has high affinity at all three receptors. A compound of formula (I) or (II) has utility in a variety of therapeutic and research areas where &kgr; opioid receptor antagonism is beneficial, including the treatment of opiate addiction or pain, or in a method of stimulating an immune system of a human.