<Li> 6β - Naltrexol </Li> <P> Buprenorphine and dezocine are partial agonists of the MOR but antagonists of the KOR . Contrarily, eptazocine is an antagonist of the MOR but an agonist of the KOR; the same is also true for nalorphine and levallorphan . A variety of partial agonists or mixed agonists - antagonists of the MOR and KOR are also marketed, and include butorphanol, levorphanol, nalbuphine, pentazocine, and phenazocine . All of the aforementioned drugs may be described as opioid modulators instead of as pure antagonists . With the sole exception of nalorphine, all of the preceding are used as analgesics (by virtue of the fact that both MOR and KOR agonism independently confer pain relief). However, these opioid analgesics have atypical properties in comparison to the prototypical pure MOR full agonist opioid analgesics, such as less or no risk of respiratory depression for MOR partial agonists and antagonists, reduced or no euphoria, abuse potential, and dependence liability with MOR partial agonists / antagonists, and use - and dose - limiting side effects such as dysphoria and hallucinations with KOR agonists . In addition, by virtue of its KOR antagonism, buprenorphine (as buprenorphine / samidorphan (ALKS - 5461) or buprenorphine / naltrexone to block its MOR agonism) is under investigation for the treatment of depression and cocaine dependence, as are other KOR antagonists such as CERC - 501 (LY - 2456302) and, previously, JDTic and PF - 4455242 (both discontinued due to toxicity concerns). </P> <P> All of the centrally active opioid antagonists used widely in medicine are non-selective, either blocking multiple opioid receptors, or blocking the MOR but activating the KOR . However, for scientific research, selective antagonists are needed which can block one of the opioid receptors but without affecting the others . This has led to the development of antagonists which are highly selective to one of the four receptors: </P> <Ul> <Li> Cyprodime is a selective MOR antagonist </Li> <Li> Naltrindole is a selective DOR antagonist </Li> <Li> Norbinaltorphimine is a selective KOR antagonist </Li> <Li> J - 113,397 is a selective nociceptin receptor (NOP) antagonist </Li> </Ul>

Opioid antagonists work by blocking the actions of