Laser speckle contrast imaging, the future DBF imaging technique for TRP target engagement biomarker assays

A comparison was made between the established laser Doppler imaging (LDI) technique and the more recently developed laser speckle contrast imaging (LSCI) method to measure changes in capsaicin- and cinnamaldehyde-induced dermal blood flow (DBF) as an indicator of TRPV1 and TRPA1 activation, respectively.

Capsaicin (1000 μg/20 μl) and cinnamaldehyde (10%) solutions were applied on the forearm of 16 healthy male volunteers, alongside their corresponding vehicle solutions. Pre challenge and 10, 20, 30, 40 and 60 min post challenge application, changes in DBF were assessed with the LSCI technique, followed by LDI. The area under the curve from 0 to 60 min (AUC_0-60) post capsaicin and cinnamaldehyde application was calculated as a summary measure of the response. Correlation between the LDI and LSCI instrument was assessed using a simple linear regression analysis. Sample size calculations (SSC) were performed for future studies using either the LDI or LSCI technique.

Higher arbitrary perfusion values were obtained with LDI compared to LSCI, yet a complete discrimination between the challenge and vehicle responses was achieved with both techniques. A strong degree of correlation was observed between LDI and LSCI measurements of the capsaicin- (R = 0.84 at Tmax and R = 0.92 for AUC_0-60) and cinnamaldehyde-induced (R = 0.78 at Tmax and R = 0.81 for AUC_0-60) increase in DBF. SSC revealed that LSCI requires considerably less subjects to obtain a power of 80% (about 15 versus 27 subjects in case of capsaicin and 7 versus 13 for cinnamaladehyde).

The LSCI technique was identified as the preferred method to capture capsaicin- and cinnamaldehyde-induced changes in DBF. Besides its reduced variability, the shorter scan time provides a major advantage, allowing real-time DBF measurements.