Nalmefene enhances the remyelinating ability of nalfurafine when administered therapeutically at very low doses during experimental autoimmune encephalomyelitis

Objectives: Multiple sclerosis is an immune-mediated inflammatory disease of the central nervous system. Recently, kappa opioid receptor (KOR) agonists were found to promote remyelination and recovery in models of demyelination. However, the side effects of many full KOR agonists limit clinical use. Activation of KOR and mu opioid receptors (MOR) often has opposing effects. Therefore, it is plausible that activating KOR and antagonising MOR may confer a beneficial increase in therapeutic efficacy at lower doses. Methods: This study investigated the hypothesis that combining the clinically available full KOR agonist nalfurafine (NalF) with a MOR antagonist/partial KOR agonist nalmefene (NalM) would provide greater than additive benefits in the murine model of immune-driven demyelination, experimental autoimmune encephalomyelitis (EAE). Results: We found that NalM alone reduced EAE disease severity when administered therapeutically although NalM at the most effective doses did not enhance myelination or reduce lesions in the spinal cord. Moreover, a combination of NalF/NalM at suboptimal doses significantly reduced lesion size and promoted myelination in the spinal cord suggesting a better-than-additive effect on lesion resolution. This finding was supported by a bioinformatic analysis response that highlighted the myelination benefits of the suboptimal dose combination while the optimal dose combination of NalF/NalM stimulated increased immunomodulatory activity. Conclusions: Findings from this study indicate that the remyelinating and immunomodulatory effects of the KOR agonist NalF can be significantly enhanced through interactions with the MOR antagonist NalM in a distinct dose-dependent manner.