Abstract C054: Discovery of KYM-003, a potent and selective STAT3 degrader with antitumor activity in heme malignancies (2019)

Abstract Purpose: Targeted protein degradation is a novel therapeutic modality that holds the promise to target previously “undruggable” proteins by harnessing the innate ubiquitin proteasome system in cells. In this study we describe the discovery and characterization of KYM-003, a novel, selective degrader of the oncogenic transcription factor STAT3 that shows potent antitumor activity in vitro and in vivo in STAT3-dependent models. Introduction: STAT3 is a transcription factor downstream of several signaling events including the IL-6-JAK pathway. Activating mutations and aberrant STAT3 activation are found in numerous cancers and STAT3 activation has been directly linked to the promotion of cancer cell survival, proliferation and immune evasion, making it a highly attractive target for oncology. Potent and selective agents specifically and directly targeting STAT3 have remained elusive, however. Herein we report the discovery of a potent and selective STAT3 degrader, KYM-003, which displays strong anti-tumor activity in models of heme malignancies. Methods: The binary binding of KYM-003 to STAT3 and the E3 ligase were characterized in fluorescence polarization assays. An AlphaLISA immunoassay was used to measure the formation of the E3-KYM-003-STAT3 ternary complex. Ubiquitination of endogenous STAT3 was evaluated in human cell lysates in the presence of KYM-003. STAT3 levels in multiple primary cells, cell lines, and xenograft tumor tissues were quantified by immunoassays. STAT3 signaling was monitored through its target gene levels by RT-qPCR. Cell viability was monitored by CellTiter-Glo. Tumor xenograft studies were conducted by implanting human cancer cell lines into immune-compromised mice and assessment of tumor volume changes. Key data: KYM-003 degrades STAT3 via an E3 ligase-dependent mechanism. KYM-003 potently binds to and demonstrated strong cooperativity in promoting the formation of the STAT3-KYM-003-E3 ligase ternary complex. In a biochemical assay with A549 cell lysates, KYM-003 potently induced ubiquitination of endogenous STAT3. KYM-003 robustly degraded STAT3 in numerous primary cells or cell lines with DC50 &amp;lt; 100 nM. Degradation was highly selective for STAT3 vs &amp;gt;10,000 other detected proteins (including all other STAT family members) in A549 and hPBMCs. Degradation of STAT3 by KYM-003 led to strong downregulation of STAT3 targets gene expression, such as SOCS3, MYC, and PIM1 Importantly, total STAT3 and pSTAT3 levels in tumors were reduced by &amp;gt;90% for at least 24 hours after a single dose of KYM-003 and repeated dosing of KYM-003 showed dose-dependent antitumor activity in xenograft models of heme maliganancies. Conclusion: KYM-003 is a potent and selective STAT3 degrader that demonstrated strong anti-tumor activity in heme cancer models. These data support STAT3 degraders as a promising new therapeutic opportunity. Citation Format: Fred Csibi, Nan Ji, Bin Yang, Karen Yuan, Michele Mayo, Haojing Rong, Scott Rusin, Kirti Sharma, Christine Loh, Henry Li, Sharon Townson, Hari Kamadurai, Jesse Chen, Duncan Walker, Jared Gollob, Nello Mainolfi. Discovery of KYM-003, a potent and selective STAT3 degrader with antitumor activity in heme malignancies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C054. doi:10.1158/1535-7163.TARG-19-C054