Antipruritic effect of pretreatment with topical capsaicin 8% on histamine- and cowhage-evoked itch in healthy volunteers: a randomized, vehicle-controlled, proof-of-concept trial.

BACKGROUND: Chronic itch is difficult to treat. Low-concentration topical capsaicin (0·006-0·05%) has previously been applied in itch therapy but evidence on its efficacy is contradictory.

OBJECTIVES: This vehicle-controlled, double-blinded study investigated the effect of topical capsaicin 8% after 1- and 24-h application on evoked itch, neurogenic inflammation and itch-associated dysaesthesia.

METHODS: Sixteen healthy volunteers (aged 22 ± 0·5 years, nine female) were treated with capsaicin for 1 h and 24 h, and vehicle for 24 h on each volar forearm. Subsequently, histamine (1%, administered prick test lancets) and cowhage (40-45 spicules) were applied to the pretreated areas. Evoked itch and pain intensities were recorded for 10 min using a visual analogue scale (0-10 cm), while sensitivity to touch-evoked itch was evaluated using von Frey filaments before and after itch provocations. Neurogenic inflammation was assessed using perfusion imaging.

RESULTS: In the vehicle areas peak itch responses to histamine and cowhage were 4·67 ± 0·58 and 5·15 ± 0·71, respectively. Capsaicin pretreatment reduced peak itch responses to histamine and cowhage after 24-h pretreatment to 1·41 ± 0·58 (P = 0·003) and 0·81 ± 0·18, (P < 0·001), respectively. Capsaicin pretreatment for 1 h reduced only cowhage-induced itch (P = 0·023). Furthermore, 24-h capsaicin pretreatment abolished punctuate hyperknesis and lowered histamine-induced neurogenic inflammation but did not affect weal reactions.

CONCLUSIONS: Topical capsaicin 8% pretreatment for 24 h reduced histaminergic and nonhistaminergic itch by about 75%, while a significant reduction (≈60%) was achieved for only nonhistaminergic itch in a standard 1-h treatment. Further investigations are needed to elucidate the clinical potential of high-concentration capsaicin as an antipruritic.