01: PD-L1 testing using concentrated 22C3 antibody: Results of the Swiss cross-validation study

Background: With the approval of the PD1 Inhibitor pembrolizumab for first line treatment of PD-L1+ NSCLC, PD-L1 testing by immunohistochemistry (IHC) has become a necessity. However, the DAKO Autostainer AS48 for the FDA approved DAKO 22C3 PharmDX assay (22C3PhDX) is not available in Switzerland. The goal of this study was to cross validate the concentrated 22C3 antibody on the Ventana BenchMark Ultra and Leica Bond immunostainers. Methods: We developed protocols for the concentrated anti-PD-L1 22C3 antibody for both platforms. 22C3PhDX on the AS48 was used as standard. A tissue microarray was constructed from 23 NSCLC. Empty sections and the antibody were distributed to 16 participants to be stained on the Ventana (8) and/or Leica (12) platform. Central scoring and statistical evaluation was performed in Basel. We also tested the performance of the Ventana SP263 assay and Cell signaling E1L3N antibody in selected centers using local protocols. Results: There was strong variability of 22C3 PD-L1 tumor cell staining as a continuous variable for both platforms. Categorical PD-L1 staining (0–49% vs. 50–100%) did not significantly differ between the centers using Ventana, whereas data from Leica remained highly variable (p<0.001). Staining on Leica was generally weaker than on Ventana compared to 22C3PhDX making scoring more tiresome and requiring a higher magnification. SP263 (4 centers) was highly concordant with 22C3 on Ventana and with P22C3PhDX, whereas E1L3N (3 centers) was strongly variable. Conclusions: PD-L1 IHC using a standardized protocol for 22C3 antibody on Ventana is feasible and provides satisfactory results in most centers, and also the approved SP263 assays is confirmed as a good alternative to 22C3PhDx. 22C3 PD-L1 IHC on Bond is currently not recommended due to major staining variability between centers. Local fine-tuning of IHC protocols and participation in ring trials are critical when laboratory developed 22C3 IHC is used.