The New England Journal of Medicine recently published a research article on a drug trial of difelikefalin (who comes up with these names?) to treat Notalgia paresthetica. Notalgia paresthetica is an allergic mimic; it is actually a form of localized neuropathy. It is sometimes called the itch that rashes. It is characterized by chronic recurrent localized itching (usually unilateral) in the interscapular paravertebral area. (i.e., the shoulder blade).
It is felt to be due to dysfunction of the cutaneous branches of thoracic spinal nerves. Because the itch is so intense it leads to excessive scratching which in turn leads to the skin both thickening and darkening. It is resistant to therapies such as antihistamines and topical anti-itch creams and topical steroid creams.
Difelikefalin is a selective kappa opioid receptor agonist. It has been known for many years that the sense of itch is carried in our sensory pain fibers. Think back to a skin injury or cut that was painful at the onset but itched as it was healing. Opioids of all forms have been used to treat pain. The selective kappa receptor on pain fibers has been shown to suppress itch when it is stimulated. Hence the development of Difelikefalin.
The drug study had a small enrollment (126 people) but the 8-week trial showed excellent results. The main side effects were nausea and constipation. A larger study is underway to try and obtain FDA approval.