2393P Ephrin signalling mediates resistance to programmed death ligand 1 (PD-L1) inhibition in urothelial carcinoma (UC)

Ephrin ligand (EFN) and receptor (EPH) signalling is increasingly implicated in tumorigenesis. Recently, EFN-B2 and PD-L1 inhibition have demonstrated impressive synergy in metastatic UC. Using data from the ABACUS trial (NCT02662309) in muscle-invasive UC, we explore the relationship between EFN/EPH expression and response to atezolizumab. The neoadjuvant ABACUS study design enabled transcriptomic and immunohistochemical (IHC) comparison of paired baseline and cystectomy samples. For multivariate disease-free survival (DFS) analysis, patients were grouped into high and low EFN/EPH using median expression levels. 83 patients were eligible for biomarker analysis. 64 had paired samples. 27 (33%) achieved complete pathological response and/or major pathological response and were deemed responders, and 17 (20%) relapsed. Following atezolizumab, there was reduced expression of EFN-A1 (log2FC:-0.9, p<0.05), EFN-A4 (log2FC:-0.8, p<0.05), EFN-A5 (log2FC:-0.8, p<0.05) and their cognate receptor EPH-A1 (log2FC:-1.7, p<0.05) as well as EFN-B2 (log2FC:-0.82, p<0.05) amongst responders, but not relapsed patients. Consequently, apart from EFN-B2, the post-treatment expression of these genes was significantly higher amongst relapsed patients than responders. High post-treatment EFN-A1 (HR: 11.9, 95%CI: 2.2 – 66.0, log-rank p<0.01), EFN-A4 (HR: 4.8, 95%CI: 0.9 – 25.3, log-rank p<0.01) and EFN-A5 (HR: 13.9, 95%CI: 2.9 – 67.9, log-rank p<0.05), and high baseline EFN-B2 (HR: 7.7, 95%CI: 2.1 – 27.7, log-rank p<0.01) expression was associated with reduced DFS. EFN-A1, EFN-A4 and EFN-A5 expression post-treatment, but not at baseline, correlated negatively with a cytotoxic T cell transcriptomic signature and granzyme B-positive CD8 T cells on IHC. EFN-B2 expression correlated positively with transcriptomic signatures of hypoxia and pathological angiogenesis. EFN-A1, EFN-A4 and EFN-A5, likely through EPH-A1 signalling, are associated with reduced immune infiltrate and survival after atezolizumab, and represent promising biomarkers and targets. High baseline EFN-B2 expression is associated with relapse, supporting the addition of EFN-B2 inhibitors to augment response rates.