( )-naloxone
(+)-Naloxone (dextro-naloxone) is a drug which is the opposite enantiomer of the opioid antagonist drug (−)-naloxone. Unlike (-)-naloxone, (+)-naloxone has no significant affinity for opioid receptors, but instead has been discovered to act as a selective antagonist of Toll-like receptor 4. This receptor is involved in immune system responses, and activation of TLR4 induces glial activation and release of inflammatory mediators such as TNF-α and Interleukin-1. Relation with opioids Both active and inactive enantiomers of various opioid analgesic drugs including morphine, meperidine, fentanyl, methadone and buprenorphine, as well as some otherwise inactive metabolites like morphine-3-glucuronide, have been found to act as agonists of TLR4, and chronic use of these drugs consequently causes constant low-level release of TNF-α and IL-1β as well as other downstream effects. This is thought to be involved in various adverse properties of opioid analgesic drugs, such as loss of eff ...
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