Abstract 1730: Up-regulation of C/EBPα inhibits suppressive activity of myeloid cells and potentiates antitumor response in mice and cancer patients

Tumor associated macrophages (TAM) and myeloid-derived suppressor cells (MDSC) are widely implicated in the suppression of immune responses in cancer and are associated with negative clinical outcomes. We report that in mouse tumor models, therapeutic up-regulation of the transcriptional factor C/EBPα with small activating RNA (MTL-CEBPA) blocks the suppressive activity of monocytic (M) MDSC and TAM without affecting polymorphonuclear (PMN) MDSC. MTL-CEBPA treatment demonstrated antitumor activity that was dependent on T cells. Combination of MTL-CEBPA and anti-PD1 antibody or with PMN-MDSC targeted therapy resulted in marked antitumor effect. A phase I trial was conducted in 36 patients with advanced hepatocellular carcinoma (HCC). Marked decrease in M-MDSC and the expression of genes and proteins involved in immune suppressive activity of these cells were observed. Combined treatment with MTL-CEBPA and sorafenib resulted in a 27% objective radiological response and three complete responses in patients with virally associated HCC. Tissue biopsies from these patients demonstrated decrease in M2 polarized macrophages within the tumors. Thus, therapeutic up-regulation of C/EBPα inhibited suppressive myeloid cells that resulted in potent antitumor effect in mice and with encouraging clinical response in cancer patients. We are currently recruiting to a multi-centre study of MTL-CEBPA in combination with a PD-1 inhibitor (pembrolizumab) in adult patients with advanced solid tumors (TIMEPOINT - ClinicalTrials.gov Identifier: NCT04105335). Citation Format: Ayumi Hashimoto, Debashis Sarker, Vikash Reebye, Sheba Jarvis, Mikael H. Sodergren, Andrew Kossenkov, Nina Raulf, Jenni Vasara, Pinelopi Andrikakou, Tim Meyer, Kai-Wen Huang, Ruth Plummer, Cheng Ean Chee, Duncan Spalding, Madhava Pai, David J. Pinato, Rohini Sharma, Bristi Basu, Daniel Palmer, Yuk-Ting Ma, Robert Habib, Anna Martirosyan, Naouel Elasri, Adeline Reynaud, John Rossi, Mark Cobbold, Nagy Habib, Dmitry Gabrilovich. Up-regulation of C/EBPα inhibits suppressive activity of myeloid cells and potentiates antitumor response in mice and cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1730.