Possible mechanism for improving the endogenous immune system through the blockade of peripheral μ-opioid receptors by treatment with naldemedine.

Background: It has been considered that activation of peripheral μ-opioid receptors (MORs) induces side effects of opioids. In this study, we investigated the possible improvement of the immune system in tumour-bearing mice by systemic administration of the peripheral MOR antagonist naldemedine.

Methods: The inhibitory effect of naldemedine on MOR-mediated signalling was tested by cAMP inhibition and β-arrestin recruitment assays using cultured cells. We assessed possible changes in tumour progression and the number of splenic lymphocytes in tumour-bearing mice under the repeated oral administration of naldemedine.

Results: Treatment with naldemedine produced a dose-dependent inhibition of both the decrease in the cAMP level and the increase in β-arrestin recruitment induced by the MOR agonists. Repeated treatment with naldemedine at a dose that reversed the morphine-induced inhibition of gastrointestinal transport, but not antinociception, significantly decreased tumour volume and prolonged survival in tumour-transplanted mice. Naldemedine administration significantly decreased the increased expression of immune checkpoint-related genes and recovered the decreased level of toll-like receptor 4 in splenic lymphocytes in tumour-bearing mice.

Conclusions: The blockade of peripheral MOR may induce an anti-tumour effect through the recovery of T-cell exhaustion and promotion of the tumour-killing system.