A randomized phase II trial published in the New England Journal of Medicine, showed a selective kappa opioid receptor agonist caused "modestly" higher improvements in itching than a placebo in individuals with notalgia paresthetica, but it was also linked to negative side effects.

The phase II study included 125 persons with notalgia paresthetica who were enrolled in 28 sites across North America. They underwent an 8-week, 1:1 randomized trial with difelikefalin (2 mg) or a placebo twice daily. The patients were 60 years old on average. The average amount of time that notalgia paresthetica lasted was almost 8 years.

After 8 weeks significant improvements in average itching were seen and were substantially greater in the oral difelikefalin (Korsuva) group compared to placebo group. This was measured on the Worst Itch Numeric Rating Scale (WI-NRS), which extends from 0-to-10 scale shows from no itch to the worst itch imaginable. 

The majority of patients were classified as experiencing severe itch, with mean baseline WI-NRS ratings in each group being 7.6. On a scale of 0 to 60, both groups baseline Skindex-10 scores were 38.6, and the mean scores on the 100-point Sleep Disturbance Subscale were 40.3 in the research arm and 38.7 in the placebo arm.

The average Skindex-10 total score change between baseline and week 8 between the difelikefalin and placebo groups did not differ significantly for secondary outcomes (-20.1 vs -18.7 points). Both groups at week 8 experienced comparable declines on the sleep disturbance subscale (-15.7 vs -16.3, respectively).

The study concluded that more in-depth research is required to analyze the advantages and disadvantages of difelikefalin in the treatment of notalgia paresthetica.

(Source: https://www.medpagetoday.com/dermatology/generaldermatology/103021