Abstract LB192: MTL-CEBPA in combination with pembrolizumab converts an immune desert to an inflamed TME in solid tumors resistant to checkpoint blockade

Despite significant advances in outcomes with immunotherapy, most cancer patients do not benefit from currently approved immune checkpoint inhibitors (ICI). The reasons for ICI resistance are multi-faceted and suggest that additional immunomodulation is required to improve outcomes. MTL-CEBPA is a novel immunotherapy based on RNA activation that upregulates expression of a master myeloid transcription factor, CEBPA. The small activating RNA for CEBPA is encapsulated within a NOV340 liposome that targets the myeloid cell lineage. MTL-CEBPA has shown favorable safety and promising clinical activity in combination with tyrosine kinase inhibitors (Sorafenib) in hepatocellular carcinoma (NCT-02716012) [Hashimoto et al, CCR 2021; Sarker et al, CCR 2020]. We recently reported preliminary clinical data from the ongoing multi-center phase 1 TIMEPOINT study (NCT-04105335) evaluating the safety, pharmacokinetics, immunomodulation, and clinical activity of MTL-CEBPA in combination with pembrolizumab in patients with solid tumors who have exhausted standard therapy. This demonstrated a favorable safety profile and initial clinical activity [Plummer et al, JITC 2021]. Here we report the findings from a biomarker pharmacodynamic analysis of paired baseline and cycle 2 tumor sample biopsies in 23 patients from the TIMEPOINT trial. Brightplex® IHC and digital pathology analyses of the samples for myeloid and T cell panels were undertaken, alongside gene expression (Nanostring I/O 360). Prior to study treatment, nine patients out of 23 had an immune cold tumor microenvironment (TME) at baseline as measured by the Immunosign®21 score. Following the combination of MTL-CEBPA with pembrolizumab, seven of these patients converted to an inflamed TME by Immunosign®21 (P=0.008). This change in the TME was associated with infiltration of CD8 and cytotoxic T cells (CD8+, GrzB+, Ki-67+) (P=0.1). GSEA analysis indicated that a Tstem-like signature was enriched post-treatment. A Brightplex® IHC analysis of myeloid cells in these patients indicated that, post treatment, there was a significant influx of HLA-DR+ myeloid cells into the TME (P=0.04). We also observed a significant increase in the expression of CXCL9, 10, and 11. The remaining 14 patients had an inflamed TME at baseline. Here, we also observed an increase in HLA-DR+ cells, T cells, and chemokines, though to a lesser extent. Further, however, in these inflamed tumors—which have significantly greater infiltration of myeloid-de