Unlike other KOR agonists, nalfurafine does not produce hallucinogenic effects in humans.[6][7] Single intramuscular injections of up to 30 μg are well tolerated by humans, whereas a dose of 40 μg produced "moderate behavioral/psychological side effects" (possibly referring to sedation), though apparently did not produce any psychotomimetic or dysphoric effects.[14] In rodents, a low dose of nalfurafine (10–40 μg/kg) was found not to produce conditioned place preference or aversion, though a high dose (80 μg/kg) did induce significant place aversion.[6] The most common side effect of low-dose nalfurafine seen in clinical trials was insomnia (observed in 10–15% of patients), with few other adverse effects observed.[2][8] In addition, tolerance to the antipruritic effects of nalfurafine was not found after treatment of patients with the drug for one year, and nalfurafine has shown no evidence of either physical nor psychological dependence in humans.[8] The drug also shows lower evidence of tolerance for effects such as analgesia and sedation in animals relative to other KOR agonists.[6][15] In animals, nalfurafine produces anti-scratch, antinociceptive, sedative, and diuretic effects.[6]
Mechanism of action
Nalfurafine is an orally active, centrally acting, highly potent, selective full agonist of the κ-opioid receptor (KOR) (Ki = 75 pM; EC50 = 25 pM).[6] As touched on above, nalfurafine shows atypical properties as a KOR agonist relative to other drugs. Notably, it does not completely substitute for the prototypical KOR agonist U-50488 in rodents, indicating qualitative differences in the discriminative effects of the two compounds.[6] Moreover, unlike U-50488, it produces neither conditioned place aversion or preference in rodents.[16] The drug is a 4,5-epoxymorphinan derivative, and is structurally unique relative to other KOR agonists.[16] Nalfurafine may be a biased agonist of the KOR or a KOR subtype-selective agonist.[14] Indeed, it has been found to act as a biased agonist of the KOR, preferring activation of β-arrestin signaling in vitro, but paradoxically, β-arrestin appears to be responsible for KOR agonist-induced aversion,[17] and nalfurafine furthermore shows paradoxical effects in vivo that are not consistent with its in vitro profile.[18] As such, more research is needed to clarify the distinct mechanisms and effects of this drug.
Nalfurafine has been found in vitro to bind to the μ-opioid receptor and to possess weak partial agonist activity at this site, albeit with much lower affinity relative to the KOR.[19] However, in vivo, nalfurafine has shown no indications of MOR agonism or antagonism in animals or humans, including no evidence of rewarding or reinforcing effects or physical dependence.[19]
Research
Nalfurafine has been found to be effective in a variety of animal models relevant to drug abuse, addiction, and dependence, and may represent a novel potential treatment for these maladies.[6] In rodents, the drug attenuates the discriminative and rewarding effects of cocaine and the rewarding and locomotor effects of morphine, and diminishes the mecamylamine-precipitated aversive effect of nicotine withdrawal.[6]
^Nakao K, Mochizuki H (May 2009). "Nalfurafine hydrochloride: a new drug for the treatment of uremic pruritus in hemodialysis patients". Drugs of Today. 45 (5): 323–329. doi:10.1358/dot.2009.45.5.1377595. PMID19584962.
^"Winfuran". European Medicines Agency - Human medicines. Archived from the original on 2016-08-19.
^Endoh T, Matsuura H, Tajima A, Izumimoto N, Tajima C, Suzuki T, et al. (1999). "Potent antinociceptive effects of TRK-820, a novel kappa-opioid receptor agonist". Life Sciences. 65 (16): 1685–1694. doi:10.1016/s0024-3205(99)00417-8. PMID10573186.
^Suzuki T, Izumimoto N, Takezawa Y, Fujimura M, Togashi Y, Nagase H, et al. (January 2004). "Effect of repeated administration of TRK-820, a kappa-opioid receptor agonist, on tolerance to its antinociceptive and sedative actions". Brain Research. 995 (2): 167–175. doi:10.1016/j.brainres.2003.09.057. PMID14672806. S2CID25094224.