Nalfurafine hydrochloride (TRK-820 hydrochloride) is a potent selective and orally active G protein-biased kappa opioid receptor (KOR)-agonist with high translational potential^[1]. Nalfurafine hydrochloride (TRK-820 hydrochloride) enhances the therapeutic potential of MOR-targeting analgesics, has the potential for uremic pruritis treatment^[2]. In Vivo: Nalfurafine (subcutaneous injection; 0.015 mg/kg) combines with EOM-salvinorin-B  produces spinal antinociception equivalent to 5 mg/kg, it also enhances the supraspinal analgesic effect of 5 mg/kg morphine^[1].
Nalfurafine (subcutaneous injection; 4 μg/kg) causes a dose-dependent increase of the inhibition of the acetic acid-induced abdominal constriction,and the inhibition of the abdominal constriction reaches its peak 30 min after injection, gradually declined and returned to the pre-injection level 4 hr after^[2].