Novel Kappa Opioid Receptor Antagonist from a Fungus , Ctenomyces serratus ATCC 15502

(μ receptor agonist) and naloxone (nonselective opioid receptor antagonist) are widely used as an analgesic and an antidote, respectively1-4). However, morphine has unwanted side effects such as tolerance, dependency or respiratory depression, and its usage is strictly limited. Naloxone is reported to have a relatively-short plasma half-life of 60-90 minutes5). In the circumstances there is a clear medical need for more efficacious drugs with fewer side effects. Considerable pharmacological, biochemical6-9) and molecular biological10-20) studies of opioid receptors have suggested that there are at least four major classes