The innate immune system is the first line of defense against invading proteins, pathogens, and chemicals. The toll-like receptor 4 (TLR4)/ myeloid differentiation protein 2 (MD-2) is an important component of the innate immune system and has many functions. We showed earlier that selective antagonism of the TLR4 receptor by the opioid receptor inert enantiomers (+)-naloxone and (+)-naltrexone can result in reversal of neuropathic pain as well as potentiation of opioid analgesia. TLR4 is known to be involved in many physiological functions via mediation of glial activation including neuropathic pain, compromised acute opioid analgesia, unwanted opioid side effects (tolerance, dependence, and reward) and inflammation. Attenuation of glial activation has been shown to both alleviate exaggerated pain states induced by experimental pain models and to reduce the development of opioid tolerance. It is also known that disregulation of innate immune TLR4 signaling contributes to various diseases including neuropathic pain and drug addiction. The (+)-enantiomer of naltrexone is one of the rare TLR4 antagonists with good blood-brain barrier permeability and does not show stereoselectivity for TLR4. We synthesized (+)-norbinaltorphimine by linking 2 naltrexone units through a rigid pyrrole spacer. Interestingly, (+)-norbinaltorphimine showed 25 times better TLR4 antagonist activity than the parent (+)-naltrexone in the microglial BV-2 cell line whereas (-)-norbinaltorphimine (-)-1 lost TLR4 activity. The enantioselectivity of norbinaltorphimine was further confirmed in primary microglia, astrocytes, and macrophages. The activities of meso isomer of norbinaltorphimine and the molecular dynamic simulation results demonstrate that the stereochemistry of (+)-1 is derived from the (+)-naltrexone pharmacophore. Moreover, (+)-1 significantly increased and prolonged morphine analgesia and in vivo the efficacy of (+)-1 is long lasting. The present study provides valuable insight into structure-activity relationships in this series for development of the next generation TLR4 antagonists.
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