Nalfurafine is a κ-opioid receptor (KOR) agonist (EC50 = 0.05 nM for human receptors expressed in CHO cell membranes). [1] It is selective for κ-opioid over μ- and δ-opioid receptors (EC50s = 0.72 and 74.1 nM, respectively). Nalfurafine (≥30 μg/kg) reduces scratching behavior induced by chloroquine in mice, as well as locomotor activity when administered at a dose of 100 μg/kg.[2] In rats, nalfurafine (≥0.01 mg/kg) reduces intracranial self-stimulation, lactic acid-induced stretching behavior, and scratching behavior induced by intradermal administration of serotonin . [3] Nalfurafine is also an orexin 1 receptor (OX1R) antagonist (Ki = 250 nM).[4]

Reference:
[1]. Watanabe, Y., Kitazawa, S., Fujii, H., et al. Design, synthesis, and structure-activity relationship of novel opioid κ receptor selective agonists: α-Iminoamide derivatives with an azabicyclo[2.2.2]octene skeleton. Bioorg. Med. Chem. Lett 24(21), 4980-4983 (2014).
[2]. Tarrasón, G., Carcasona, C., Eichhorn, P., et al. Characterization of the chloroquine-induced mouse model of pruritus using an automated behavioural system. Exp. Dermatol. 26(11), 1105-1111 (2017).
[3]. Lazenka, M.L., Moerke, M.J., Townsend, E.A., et al. Dissociable effects of the kappa opioid receptor agonist nalfurafine on pain/itch-stimulated and pain/itch-depressed behaviors in male rats. Psychopharmacol. (Berl). 235(1), 203-213 (2018).
[4]. Nagase, H., Yamamoto, N., Yata, M., et al. Design and synthesis of potent and highly selective orexin 1 receptor antagonists with a morphinan skeleton and their pharmacologies. J. Med. Chem. 60(3), 1018-1040 (2017).