Topical nalfurafine exhibits anti-inflammatory and anti-pruritic effects in a murine model of AD

Dysregulation of μ and κ opioid receptors is thought to play pivotal role in the etiology of pruritus. Opioid-induced pruritus is a well known side effect of morphine, a μ opioid receptor (MOR) agonist, and MOR antagonists such as naltrexone have been shown to be effective in reducing pruritus. The κ opioid receptor (KOR), expressed in both the central nervous system (CNS) as well as in the peripheral nervous system, mediates the opposite effects to MOR, with KOR agonists being shown to reduce both pruritus and pain [ [1] Phan N.Q. Lotts T. Antal A. Bernhard J.D. Stander S. Systemic kappa opioid receptor agonists in the treatment of chronic pruritus: a literature review. Acta Derm. Venereol. 2012; 92: 555-560 Crossref PubMed Scopus (100) Google Scholar ]. In conditions characterized by pruritus, such as atopic dermatitis (AD) or psoriasis, cutaneous KORs contribute to generation of inflammation and itch and ligands of KORs are downregulated in epidermal cells [ [2] Tominaga M. Ogawa H. Takamori K. Possible roles of epidermal opioid systems in pruritus of atopic dermatitis. J. Invest. Dermatol. 2007; 127: 2228-2235 Abstract Full Text Full Text PDF PubMed Scopus (109) Google Scholar ]. The KOR agonist nalfurafine (TRK-820) has demonstrated efficacy in treating pruritus caused by various conditions and is approved in Japan for treatment of uraemic pruritus [ [3] Inui S. Nalfurafine hydrochloride for the treatment of pruritus. Expert Opin. Pharmacother. 2012; 13: 1507-1513 Crossref PubMed Scopus (17) Google Scholar ]. In two double-blind, randomized, placebo-controlled multicentre clinical trials of patients with uraemic pruritus, intravenous nalfurafine significantly reduced itch intensity, compared with placebo [ [4] Wikstrom B. Gellert R. Ladefoged S.D. Danda Y. Akai M. Ide K. et al. Kappa-opioid system in uremic pruritus: multicenter, randomized, double-blind, placebo-controlled clinical studies. J. Am. Soc. Nephrol. JASN. 2005; 16: 3742-3747 Crossref PubMed Scopus (242) Google Scholar ]. In another clinical trial of patients with intractable uraemic pruritus, oral nalfurafine (2.5 or 5 μg/day) significantly reduced pruritus scores when compared with placebo [ [5] Kumagai H. Ebata T. Takamori K. Muramatsu T. Nakamoto H. Suzuki H. Effect of a novel kappa-receptor agonist, nalfurafine hydrochloride, on severe itch in 337 haemodialysis patients: a Phase III, randomized, double-blind, placebo-controlled study. Nephrol. Dialysis Transpl. 2010; 25: 1251-1257 Crossref PubMed Scopus (234) Google Scholar ]. Moreover, long-term durable efficacy of oral nalfurafine (5 μg/day) in a similar patient population was demonstrated in a 52-week open-label trial [ [6] Kumagai H. Ebata T. Takamori K. Miyasato K. Muramatsu T. Nakamoto H. et al. Efficacy and safety of a novel k-agonist for managing intractable pruritus in dialysis patients. Am. J. Nephrol. 2012; 36: 175-183 Crossref PubMed Scopus (104) Google Scholar ]. Side effects such as insomnia limit systemic administration. A topical formulation would be more desirable but this is not yet commercially available.