Application of the Statistical Method to Convert Published PASI 50/75/90/100 into Absolute PASI Response Rate in Patients with Moderate-to-Severe Plaque Psoriasis Treated with Tildrakizumab Based on Data from the Two Pivotal Phase 3 Studies reSURFACE 1 and reSURFACE 2

Background: Absolute Psoriasis Area and Severity Index (PASI) is a key endpoint in psoriasis management. Petto et al. [Pharm Stat. 2019;18(1):4-21] developed a statistical method to estimate the proportion of patients reaching absolute PASI response given baseline PASI score and proportion of patients achieving relative improvements at predefined time points.

Objectives: To test this method on clinical data from two phase 3 tildrakizumab trials (reSURFACE 1/2) comparing estimated absolute PASI ≤1/≤2/≤3/≤5 responses with reference responses from clinical databases.

Methods: Reference PASI responses of ≤1/≤2/≤3/≤5 were extracted from clinical databases. Estimation of absolute PASI ≤1/≤2/≤3/≤5 response rates at week (W) 12 and W28 by treatment and trial were performed. Differences between estimated and reference responses were analysed. Bland-Atman limits of agreement and Passing-Bablok regression to assess variations between estimated and reference responses were performed.

Results: Differences between estimated and reference absolute PASI ≤1/≤2/≤3/≤5 responses at W12 and W28 by treatment and trial were of little clinical relevance with an overall mean difference in PASI response proportion of -2.2% (e.g., for the tildrakizumab 100-mg arm, original proportions of patients achieving PASI of ≤1/≤2/≤3/≤5 at W28 were 38.5%/52.2%/63.5%/73.9% and 39.8%/54.8%/63.6%/76.9% [reSURFACE 1 and 2, respectively] vs. estimated proportions of 33.2%/49.8%/62.5%/78.3% and 34.3%/51.6%/64.5%/79.9%). Limits of agreement were -7.1% to 1.4% at W12 and -6.8% to 4.3% at W28. Scatterplots revealed linearity that stood the cusum test in Passing-Bablok regression with slope 1.14 (95% confidence intervals: 1.06 to 1.20).

Conclusion: Good estimates of absolute PASI response rates were achieved with the application of the statistical method to tildrakizumab data reported in the phase 3 studies, in particular in the verum study arms. Our data support the method provided by Petto et al. [2019] to estimate proportions of psoriasis patients reaching absolute PASI value thresholds using relative PASI improvements.