Choice of PD-L1 immunohistochemistry assay influences clinical eligibility for gastric cancer immunotherapy.

Ryan Yong Kiat Tay Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore info_outline Ryan Yong Kiat Tay, Joe Poh Sheng Yeong, Jeffrey Lum, Chong Boon Teo, Benjamin Kye Jyn Tan, Yiong Huak Chan, Joan RE Choo, Anand Jeyasekhran, Qing Hao Miow, Lit-Hsin Loo, Wei-Peng Yong, Raghav Sundar

Authors Ryan Yong Kiat Tay Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore info_outline Ryan Yong Kiat Tay, Joe Poh Sheng Yeong, Jeffrey Lum, Chong Boon Teo, Benjamin Kye Jyn Tan, Yiong Huak Chan, Joan RE Choo, Anand Jeyasekhran, Qing Hao Miow, Lit-Hsin Loo, Wei-Peng Yong, Raghav Sundar Organizations Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore, Singapore General Hospital, Singapore, Singapore, Department of Pathology, National University Hospital, National University Health System, Singapore, Singapore, Department of Haematology-Oncology, National University Cancer Institute, Singapore, Singapore, NUH, Singapore, Singapore, Bioinformatics Institute, Agency for Science, Technology and Research, Singapore, Singapore, National University Cancer Institute Singapore, Singapore, Singapore, National University Health System, Singapore, Singapore Abstract Disclosures Research Funding Other Background: Immune checkpoint inhibitors (ICI) are now standard-of-care treatment for patients with metastatic gastric cancer (GC). To guide patient selection for ICI therapy, programmed death ligand-1 (PD-L1) biomarker expression is routinely assessed via immunohistochemistry (IHC). Regulatory approval for ICIs is granted based on PD-L1 expression status, scored using metrics such as the combined positive score (CPS). However, with an increasing number of approved ICIs, each paired with a different PD-L1 antibody IHC assay used in their respective landmark trials, there is an unmet clinical and logistical need for harmonization. We thus investigated the interchangeability between the Dako 22C3, Dako 28-8 and Ventana SP-142 assays in GC PD-L1 IHC. Methods: In this cross-sectional study, samples were obtained via biopsy or resection of gastric cancer at the National University Hospital, Singapore. We scored 362 GC samples for PD-L1 CPS, tumor proportion score (TPS) and immune cells (IC) using a multiplex immunohistochemistry/immunofluorescence technique. 344 samples were developed into a tissue microarray (TMA), while 18 samples were used as whole slides for orthogonal validation. The samples selected for whole slide analysis were obtained from GC patients treated with ICI therapy. Results: The percentage of PD-L1 positive samples at clinically relevant CPS ≥1, ≥5 and ≥10 cut-offs (Table) for the 28-8 assay were approximately two-fold higher than that of the 22C3 (CPS≥1: 70.3% vs 49.4%, p<0.001; CPS≥5: 29.1% vs 13.4%, p<0.001; CPS≥10: 13.7% vs 7.0%, p=0.004). The mean CPS score on 28-8 assay was nearly double that of the 22C3 (6.39 ±14.5 vs 3.46±8.98, p<0.001). At the clinically important CPS≥5 cut-off, there was only moderate concordance between the 22C3 and 28-8 assays. Conclusions: Our findings suggest that scoring PD-L1 CPS with the 28-8 assay may result in higher proportion of PD-L1 positivity and higher PD-L1 scores compared to assessment with the 22C3 and other assays. Clinically, this could lead to a larger number of patients eligible and approved for ICI therapy. If assays are viewed and used interchangeability, a substantial number of patients may be inaccurately denied or granted treatment with ICIs based on the assay chosen. As such, until stronger evidence of inter-assay concordance is found, we urge caution in treating the assays as equivalent. The percentage of PD-L1 positive samples at clinically relevant CPS ≥1, ≥5 and ≥10 cut-offs. Assay CPS ≥1 CPS ≥5 CPS ≥10 22C3 49.4% 13.4% 7.0% 28-8 70.3% 29.1% 13.7% SP-142 49.4% 19.8% 9.6%