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Programmed death ligand-1 (PD-L1) expression as determined by immunohistochemistry (IHC) is potentially predictive of clinical outcome. The aim of this study was to assess the concordance of reported PD-L1 IHC assays and investigate factors influencing variability. Consecutive sections from 20 non-small cell lung cancers (NSCLCs) comprising resection, core biopsy, cytology and pleural fluid samples underwent IHC with 5 different antibody/autostainer combinations: 22C3/Link48, 28-8/BOND-MAX, E1L3N/BOND-MAX, SP142/BenchMark and SP263/BenchMark. PD-L1 RNA levels were assessed using RNAscope. The frequency of positive cases using scoring thresholds from clinical trials was 72%, 33%, 61%, 56%, and 33% for the 5 IHC protocols respectively, and 33% for RNAscope. Pairwise agreement on the classification of cases as positive or negative for PD-L1 expression ranged from 61%-94%. On a continuous scale, the lowest correlation was between 28-8/BOND-MAX and SP142/BenchMark (R2=0.25) and highest was between 22C3/Link48 and E1L3N/BOND-MAX (R2=0.71). When cases were ordered according to tumor cell (TC)%, a similar ranking of cases across IHC protocols could be observed, albeit with different quanta and limits of detection. Single-slide OPAL 7-color fluorescence IHC analysis revealed a high degree of co-localization of staining from the 5 PD-L1 antibodies. Using SP142 antibody in a BOND-MAX protocol led to increased TC% quanta, while retaining a similar ranking of samples according to TC%. The results of this study highlight tumor PD-L1 status can vary significantly according to IHC protocol. Protocol-dependent staining intensities and nominated thresholds for positivity contribute to this variability, while the antibody used appears to be less of a factor.

// Ashleigh C. McEvoy 1 , Leslie Calapre 1 , Michelle R. Pereira 1 , Tindaro Giardina 2 , Cleo Robinson 2, 3, 4 , Muhammad A. Khattak 3, 5 , Tarek M. Meniawy 3, 6 , Antonia L. Pritchard 7 , Nicholas K. Hayward 7 , Benhur Amanuel 2, 3 , Michael Millward 3, 6 , Melanie Ziman 1, 4 and Elin S. Gray 1 1 School of Medical and Health Sciences, Edith Cowan University, Joondalup, Western Australia, Australia 2 Anatomical Pathology, PathWest, QEII Medical Centre, Nedlands, Western Australia, Australia 3 School of Medicine and Pharmacology, The University of Western Australia, Crawley, Western Australia, Australia 4 School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia 5 Department of Medical Oncology, Fiona Stanley Hospital, Murdoch, Western Australia, Australia 6 Department of Medical Oncology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia 7 QIMR Berghofer Medical Research Institute, Herston, Brisbane, QLD, Australia Correspondence to: Elin S. Gray, email: e.gray@ecu.edu.au Keywords: droplet digital PCR (ddPCR), TERT , melanoma, cancer Received: June 06, 2017 Accepted: July 25, 2017 Published: August 18, 2017 ABSTRACT Background: Currently mainly BRAF mutant circulating tumor DNA (ctDNA) is utilized to monitor patients with melanoma. TERT promoter mutations are common in various cancers and found in up to 70% of melanomas, including half of BRAF wild-type cases. Therefore, a sensitive method for detection of TERT promoter mutations would increase the number of patients that could be monitored through ctDNA analysis. Methods: A droplet digital PCR (ddPCR) assay was designed for the concurrent detection of chr5:1,295,228 C>T and chr5:1,295,250 C>T TERT promoter mutations. The assay was validated using 39 melanoma cell lines and 22 matched plasma and tumor samples. In addition, plasma samples from 56 metastatic melanoma patients and 56 healthy controls were tested for TERT promoter mutations. Results: The established ddPCR assay detected TERT promoter mutations with a lower limit of detection (LOD) of 0.17%. Total concordance was demonstrated between ddPCR and Sanger sequencing in all cell lines except one, which carried a second mutation within the probe binding-site. Concordance between matched plasma and tumor tissue was 68% (15/22), with a sensitivity of 53% (95% CI, 27%-79%) and a specificity of 100% (95% CI, 59%-100%). A significantly longer PFS (p=0.028) was evident in ctDNA negative patients. Importantly, our TERT promoter mutations ddPCR assay allowed detection of ctDNA in 11 BRAF wild-type cases. Conclusions: The TERT promoter mutation ddPCR assay offers a sensitive test for molecular analysis of melanoma tumors and ctDNA, with the potential to be applied to other cancers.