Development of AITC-induced dermal blood flow as a translational in vivo biomarker of TRPA1 activity in human and rodent skin.

BACKGROUND AND PURPOSE: Develop a translational assay of Transient Receptor Potential Ankyrin 1 (TRPA1) activity for use as a preclinical and clinical biomarker. EXPERIMENTAL APPROACH: Allyl isothiocyanate (AITC), capsaicin, or citric acid were applied to ears of wildtype and Trpa1-knock out (Trpa1 KO) rats, and changes in dermal blood flow (DBF) were measured by laser speckle contrast imaging. In humans, the DBF, pain and itch responses to 5-20% AITC applied to the forearm were measured and safety was evaluated. Reproducibility of the DBF, pain, and itch responses to topically applied 10 and 15% AITC was assessed at two visits separated by 13-15 days. DBF changes were summarized at 5-minute intervals as areas under the curve (AUC) and maxima. Intraclass correlation coefficient (ICC) was calculated to assess arm-arm and period-period reproducibility. KEY RESULTS: AITC- and citric acid- induced DBF were significantly reduced in Trpa1 KO rats compared to wildtype (90± 2% and 65± 11% reduction, respectively), whereas capsaicin response did not differ. In humans, each AITC concentration significantly increased DBF compared to vehicle with the maximal increase occurring 5 minutes post-application. Ten and 15% AITC were selected as safe and effective stimuli. AUC from 0-5 minutes was the most reproducible metric of AITC-induced DBF across arms (ICC = 0.92) and periods (ICC = 0.85). Subject-reported pain was more reproducible than itch across visits (ICC=0.76 vs 0.17, respectively). CONCLUSION AND IMPLICATIONS: AITC-induced DBF is a suitable target engagement biomarker of TRPA1 activity for preclinical and clinical studies of TRPA1 antagonists.