Evaluation of tumour microenvironment identifies immune correlates of response to combination immunotherapy with margetuximab (M) and pembrolizumab (P) in HER2+ gastroesophageal adenocarcinoma (GEA)

Abstract

Background: Despite improvements in treatments, the 5-year survival of GEA patients (pts) is disappointing. Individual molecular subtypes display preferential responses to PD-1 blockade. M, an investigational Fc-optimized anti-HER2 monoclonal antibody, is being tested in combination with P in HER2+GEA post trastuzumab. We present gene expression data from archival FFPE biopsies. Methods: 55 pt samples were assessed by NanoString PanCancer IO360TM assay. Immune signature scores are presented as fold changes (FC) and analyzed by unpaired t-test. Associations examined include baseline IHC PD-L1 (pos vs neg) and HER2 expression (IHC3+ vs 2+), ERBB2 mRNA, inflamed tumor microenvironment (TME), radiographic response, and GC vs GEJ. Results: ERBB2 mRNA was increased in pts with HER2 IHC3+vs 2+(5.6 FC, p<0.001); IHC3+ was also associated with an inflamed TME (higher IFN-γ signaling, cell proliferation, PD-L2 and inflammatory chemokine expression). ERBB2 expression significantly correlated with response; pts with CR/PR had a 5.13 FC ERBB2 expression compared to PD (p<0.001). Importantly, ERBB2 expression was associated with antitumor activity (ROC-AUC=0.754), with a 3.1 FC between SD/PR/CR vs PD (p=0.004). TME gene signatures, including PD-1/PD-L1, CTLA4, cytotoxic CD56dim NK cells and DR5 abundance, also trended higher in responders. GC tumors expressed higher levels of ERBB2 (5.25 FC, p<0.001), with a trend towards amore inflamed TME (higher PD-L1, IFNγ), and had increased clinical response-this differed from GEJ tumors, which showed lower ERBB2 expression, higher expression of IFITM1, MYC and STAT3, and less clinical responses. Lastly, in PD-L1pos vs PD-L1neg samples, PD-L1 gene expression (1.3 FC, p=0.013), IFN-γ signaling (1.5 FC, p<0.001), LAG3 expression (1.4 FC, p=0.045), IDO1 expression (1.7 FC, p=0.031), inflammatory chemokines (1.7, p=0.005), and tumor inflammation signature (1.5 FC, p=0.0064) were all significantly elevated. Conclusions: Our study documents for the first time ERBB2 expression and inflamed TME in GEA, which can help differentiate immunologically between GC and GEJ tumors.