Abstract B043: Mithramycin-SA analogues with reduced toxicity for the treatment of ETS transcription factor-driven tumors

Abstract

Abstract Introduction: Chromosomal translocations involving the ETS family of transcription factors are common in Ewing sarcoma, prostate cancer, and leukemia. These translocations lead in overexpression of aberrant ETS transcription factors, which drive tumorigenesis. Mithramycin (MTM) inhibits EWS-FLI1, the most common ETS-related transcription factor in Ewing sarcoma, presumably through interference at its DNA binding sites on promoter regions. However, MTM has a narrow therapeutic window marked by severe liver and hematologic toxicities. These are likely the result of interference with the ubiquitously expressed SP1 transcription factor. Here, we sought to develop analogues with specificity toward ETS transcription factors and reduced interaction with SP1. Methods: Using MTM-SA and semisynthetic approaches a series of analogues were obtained. To determine specificity toward cells expressing ETS-related chromosomal translocations, MTM-SA analogues were screened for growth inhibition in a panel of Ewing sarcoma cell lines (n=8), all expressing EWS-ETS translocations, and compared to a panel of non-Ewing cell lines (n=9) that do not express these translocations. Luciferase reporter constructs were developed to evaluate interference with EWS-FLI1 and SP1 regulated genes following MTM-SA analogue treatment. Select analogues were tested in vivo to identify the maximum tolerated dose and determine pharmacokinetics (PK). Results: In growth inhibition assays, several MTM-SA analogues resulted in > 10-fold specificity toward Ewing sarcoma cell lines vs. MTM. Luciferase reporter assays identified two analogues, MTM-SA-Tryptophan-A2 and MTM-SA-Tryptophan-A10, with 10-fold reduced inhibition of SP1 but similar inhibition of EWS-FLI1, as compared to MTM. Additionally, MTM-SA-Phe analogue has lower toxicity, despite longer plasma half-life. Conclusion: These data show that MTM-SA analogues are cytotoxic against tumor cell lines expressing aberrant ETS transcription factors. Further, their reduced interference with the ubiquitously expressed SP1 transcription factor, as well as their improved PK, may result in a wider therapeutic window. Citation Format: Markos Leggas, Joseph Eckenrode, Prithiba Mitra, Amit Jha, Shaimaa Salem, Abhisek Mandal, Jon Thorson, Jurgen Rohr. Mithramycin-SA analogues with reduced toxicity for the treatment of ETS transcription factor-driven tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B043.