Assessment of digital perfusion as a surrogate outcome in Raynaud's phenomenon clinical trials

Objectives: Measurement of digital perfusion, sometimes coupled with a cold challenge, has been widely used as an objective outcome in trials evaluating drug therapies in RP, in addition to patient-reported outcomes or to establish the proof-of-concept in preliminary studies. However, whether digital perfusion is a valid surrogate for clinical outcomes in RP trials has never been explored. The principal aim of this study was to evaluate the potential surrogacy of digital perfusion, by combining individual-level and trial-level data. Methods: We used individual data from a series of n-of-1 trials, and trial data from a network meta-analysis. We estimated individual-level surrogacy through coefficients of determination between digital perfusion and clinical outcomes (R-ind(2)). We further calculated the coefficients of determination between treatment effect on the clinical outcomes and on digital perfusion, at the individual level (R-TEind(2)) and at the trial level (R-trial(2)), using non-weighted linear regression, with their 95% CI calculated through bootstrapping. Results: Results from 33 patients and 24 trials were included in the final analysis. At the individual level, there was no correlation between digital perfusion and clinical outcomes at rest and in response to various cooling tests (the highest R-ind(2) was 0.03 [-0.07, 0.09]), and R-TEind(2) was also very low 0.07 (0, 0.29). At the trial level, the highest value of R-trial(2) was 0.1 (0, 0.477). Conclusions: Digital perfusion, at rest or in response to a cold challenge, and whatever the method used, does not fulfil the criteria of a valid surrogate for existing patient-reported outcomes in RP trials.