Choice of PD-L1 Immunohistochemistry Assay Influences Clinical Eligibility for Gastric Cancer Immunotherapy

Authors:
Chong Boon TEO^1*; Joe YEONG^2,3*; Huey Yew Jeffrey LUM^4*; Benjamin Kye Jyn TAN¹; Ryan Yong Kiat TAY¹; Yiong Huak CHAN⁵; Joan Rou-En CHOO⁶; Anand D. JEYASEKHARAN^6,7; Qing Hao MIOW⁶; Lit-Hsin LOO⁸; Wei Peng YONG^6,7+; Raghav SUNDAR^1,6,7,9,10+


Affliations:
¹Yong Loo Lin School of Medicine, National University of Singapore, Singapore
²Institute of Molecular and Cell Biology, Agency for Science, Technology and Research, Singapore
³Department of Anatomical Pathology, Singapore General Hospital, Singapore
⁴Department of Pathology, National University Health System, Singapore
⁵Biostatistics Unit, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
⁶Department of Haematology-Oncology, National University Cancer Institute, National University Health System, Singapore
⁷Cancer Science Institute of Singapore, National University of Singapore, Singapore
⁸Bioinformatics Institute, Agency for Science, Technology and Research, Singapore
⁹Cancer and Stem Cell Biology Program, Duke-NUS Medical School, Singapore, Singapore
¹⁰The N.1 Institute for Health, National University of Singapore, Singapore


Corresponding email address:
Raghav SUNDAR (mdcragh@nus.edu.sg)
Wei Peng YONG (wei_peng_yong@nuhs.edu.sg)


Publication:
Article in-press with Gastric Cancer.

Background: Immune checkpoint inhibitors (ICI) are now standard-of-care for patients with metastatic gastric cancer (GC). Programmed death ligand-1 (PD-L1) biomarker expression, assessed via immunohistochemistry (IHC) and scored using metrics such as the combined positive score (CPS), is routinely used to select patients for ICI therapy, following biomarker-based regulatory approval. However, with numerous approved ICIs, each paired with a unique PD-L1 antibody IHC assay used in their respective landmark trials, there is an unmet clinical and logistical need for harmonization. We 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 GC at the National University Hospital, Singapore. We scored 362 samples for PD-L1 CPS, tumor proportion score (TPS) and immune cells (IC) using a multiplex immunohistochemistry/immunofluorescence technique. We developed 344 samples into a tissue microarray (TMA) and used another 18 samples from GC patients treated with ICI as whole slides for orthogonal validation.

Results: The percentage of PD-L1 positive samples at clinically relevant CPS ≥1, ≥5 and ≥10 cut-offs 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, concordance was only moderate between the two assays.

Conclusion: Scoring PD-L1 CPS with the 28-8 rather than the 22C3 assay may result in higher proportion of PD-L1 positivity and higher PD-L1 scores. Clinically, this could lead to a larger number of patients eligible and approved for ICI therapy. Until stronger evidence of inter-assay concordance is found, we urge caution in treating the assays as equivalent.