Abstract 2050: Cell penetrating peptide, ST101, disrupts ATF5 regulation of anti-apoptotic Bcl-2 family proteins, resulting in induction of cancer cell death in vitro and tumor growth inhibition/regression in vivo

Anti-apoptotic B cell lymphoma 2 (Bcl-2) family proteins are frequently overexpressed across a variety of tumors, resulting in tumor cell survival and resistance to therapy. Inhibition of the expression or activity of these survival factors is an attractive approach for cancer therapy. Activating transcription factor 5 (ATF5) regulates gene transcription of anti-apoptotic Bcl-2 family proteins in neural progenitor cells and a wide range of human cancer cells. Here, we describe ST101, a rationally designed, synthetic, D-amino acid, cell penetrating peptide therapeutic designed to disrupt the protein-protein interactions driving ATF5-regulated gene transcription. Exposure of HL60 promyelocytic leukemia cells and MCF7 breast adenocarcinoma cells to low micromolar concentration of ST101 resulted in a decrease in MCL-1, BCL-2 and BIRC5 (Survivin) mRNA expression at 4 and 24 hrs post exposure. Further, exposure to ST101 resulted in a dose-dependent loss of viability across a panel of human cancer cells, including MCF7, HL60, U251 glioblastoma, A375 melanoma, DU145 prostate cancer, and A549 lung adenocarcinoma, characterized by an increase in annexin V and PI staining by flow cytometry peaking 48 hrs post exposure, resulting in a median half maximal effective concentration (EC50) value of 4.0 micromolar. In contrast, normal human peripheral blood mononuclear cells and bone marrow mononuclear cells were resistant to ST101-mediated cell death, with >80 micromolar EC50 values. In mouse xenograft experiments, 25mg/kg ST101 administered three times per week for three weeks resulted in significant tumor regression in MCF7 and U251 subcutaneous tumors as well as tumor growth delay in HL60 subcutaneous tumors. Tumor growth remained significantly inhibited weeks after the last treatment in the MCF7 and U251 models. In summary, ST101 selectively kills cancer cell lines by decreasing BCL-2 family gene expression, resulting in significant reductions in tumor growth in mouse models. Taken together, these data validate ST101 as a potent peptide therapeutic candidate for a variety of solid tumor and hematologic malignancies. Citation Format: Jim A. Rotolo, Rick Ramirez, Mark Koester, Siok Leong, Lila Ghamsari, Gene Merutka, Barry J. Kappel. Cell penetrating peptide, ST101, disrupts ATF5 regulation of anti-apoptotic Bcl-2 family proteins, resulting in induction of cancer cell death in vitro and tumor growth inhibition/regression in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2050.