Nalfurafine is an antipruritic that is marketed in Japan for the treatment of uremic pruritus in individuals with chronic kidney disease undergoing hemodialysis. It acts as a potent, selective, centrally-penetrant κ-opioid receptoragonist, and is the first and currently the only selective KOR agonist to have been approved for clinical use. It has also been dubiously referred to as the "first non-narcotic opioid drug" in history.
Unlike other KOR agonists, nalfurafine does not producehallucinogenic effects in humans. 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", though apparently did not produce any psychotomimetic or dysphoric effects. In rodents, a low dose of nalfurafine was found not to produce conditioned place preference or aversion, though a high dose did induce significant place aversion. The most common side effect of low-dose nalfurafine seen in clinical trials was insomnia, with few other adverse effects observed. 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. The drug also shows lower evidence of tolerance for effects such as analgesia and sedation in animals relative to other KOR agonists. In animals, nalfurafine produces anti-scratch, antinociceptive, sedative, and diuretic effects.
Mechanism of action
Nalfurafine is an orally active, centrally acting, highly potent, selective full agonist of the κ-opioid receptor . 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. Moreover, unlike U-50488, it produces neither conditioned place aversion or preference in rodents. The drug is a 4,5-epoxymorphinan derivative, and is structurally unique relative to other KOR agonists. Nalfurafine may be a biased agonist of the KOR or a KOR subtype-selective agonist. 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, and nalfurafine furthermore shows paradoxical effectsin vivo that are not consistent with its in vitro profile. 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. 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.
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. In rodents, the drug attenuates the discriminative and :wikt:reward|rewarding effects of cocaine and the rewarding and locomotor effects of morphine, and diminishes the mecamylamine-precipitated aversive effect of nicotine withdrawal.
