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STAT3 (signal transducers and activators of transcription 3) is a transcription factor and a member of the STAT protein family that is activated through a variety of different cytokine and growth factor receptors via JAKs, as well as through oncogenic fusion proteins and gain-of-function (GoF) mutations in STAT3 itself. STAT3 hyperactivation and GoF mutations are found in numerous cancers, including clinically aggressive hematologic malignancies with high unmet medical need, such as peripheral T cell lymphomas (PTCLs) (Andersson et al., 2020). We have previously shown that a potent and selective STAT3 heterobifunctional degrader, KTX-201, strongly represses cell growth in models of STAT3-dependent heme malignancies (Csibi et al., 2019). Herein, we report on the cellular mechanisms underlying the anti-tumor effect of STAT3 degradation in PTCL and provide a model for the relationship between pharmacokinetics/ pharmacodynamics (PK/PD) and activity of KTX-201 in vivo. The relationship between STAT3 degradation by KTX-201, anti-tumor mechanism of action and in vivo activity were investigated in anaplastic large T cell lymphoma (ALCL) models, a subset of PTCLs. In vitro, a decrease of STAT3 by 90% for 48hr was required for ALCL cells to commit to death. To identify anti-tumor mechanism(s) of KTX-201 at the systems level, we performed a time-resolved analysis of the proteomic changes of SU-DHL-1 cells undergoing growth inhibition mediated by KTX-201 at GI95. We measured the abundance of 10,000 proteins and confirmed selective degradation of STAT3 by KTX-201 after 8h of treatment. Significant changes in several marker proteins known to be involved in STAT3-mediated proximal signaling in ALCL including SOCS3, Myc and Granzyme B were observed after 16h. Functional annotation analysis of proteins identified pathways that were significantly enriched in at least one time point. Using unsupervised hierarchical clustering of annotations, we found that proteins that increased in abundance over 48h of exposure to KTX-201 were associated with markers of apoptosis and those that decreased in abundance by 24h and 48h were associated with cytokine signaling and cell cycle, respectively. Based on these data, this study identifies inhibition of cytokine signaling, G1 cell cycle arrest and induction of apoptosis as key anti-tumor mechanisms associated with KTX-201 consistent with observed cell phenotypes. STAT3 degradation in tumor was characterized in mice bearing SU-DHL-1 tumors following single dose IV administration. The STAT3 PD response in tumor was correlated with exposures in tumor. At the dose of 25 mg/kg weekly where complete tumor regression was achieved, KTX-201 achieves >90% STAT3 degradation at 24h post dosing in SUDHL1 xenografts. STAT3 degradation was maintained at 90% at 4 days post dosing. The results from the PK/PD study suggests that STAT3 degradation in tumor of >90% is necessary for anti-tumor efficacy in vivo of KTX-201, but only for a limited duration, such as 4 days out of a weekly dosing cycle. Collectively, our data demonstrate that significant STAT3 degradation for a limited time during dosing interval with KTX-201 in ALCL promotes early changes in key signaling nodes involved with proliferation and cytokine stimulation, followed by profound changes in apoptotic proteins. By integrating mechanistic biology with a deep understanding of PK/PD and efficacy, this study provides a foundation for the clinical development of STAT3 degraders using intermittent dosing regimen for treatment of PTCL and other STAT3-dependent heme malignancies. Disclosures Rong: Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Sharma:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Csibi:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company, Ended employment in the past 24 months. Yang:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Rusin:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Shi:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Dey:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Karnik:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Mayo:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Yuan:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Chutake:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. McDonald:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Zhu:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Ji:Kymera Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Liu:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Li:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Walker:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Gollob:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Mainolfi:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company. Desavi:Kymera Therapeutics: Current Employment, Current equity holder in publicly-traded company.

Targeted protein degradation mediated by small molecule degraders represents a new and exciting therapeutic modality to target difficult-to-drug oncogenic proteins including transcription factors. These molecules bind to both the target protein and an E3 ligase, enabling the formation of a ternary complex that leads to ubiquitination and subsequent degradation of the target protein by the proteasome. STAT3 (signal transducers and activators of transcription 3) is a transcription factor and a member of the STAT protein family. In response to cytokines and growth factors, STAT3 is phosphorylated by receptor-associated serine/threonine kinases, and phosphylated STAT3 (pSTAT3) then forms dimers that translocate into the nucleus, binds to DNA, and regulate transcription. STAT3 is frequently mutated and activated in numerous cancers including clinically aggressive hematologic malignancies with high unmet medical need. Mechanistically, aberrant activation of STAT3 has been directly linked to the promotion of cancer cell survival, proliferation and immune evasion, thus 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 heterobifunctional degrader, KYM-003, which displays strong anti-tumor activity in models of STAT3-dependent heme malignancies. KYM-003 degrades STAT3 via an E3 ligase-dependent mechanism. It strongly binds to STAT3 and a E3 ligase, leading to the formation of the a productive ternary complex, which leads to ubiquitination of STAT3 and subsequent proteasomal degradation. KYM-003 robustly degraded STAT3 in a number of primary cells or cell lines with DC50 < 100 nM. Degradation was highly selective for STAT3 vs >10,000 other detected proteins (including all other STAT family members) in cell lines and human PBMCs. Degradation of STAT3 by KYM-003 led to significant downregulation of STAT3 target genes, such as SOCS3, MYC, and PIM1. Importantly, total STAT3 and pSTAT3 levels in tumors were reduced by >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 malignancies. Collectively, our data demonstrates that KYM-003 is a potent and selective STAT3 degrader that exhibited strong anti-tumor activity in vitro and in vivo. These data support STAT3 degraders as a new and exciting therapeutic opportunity in heme malignancies. Disclosures Csibi: kymera Therapeutics: Employment, Equity Ownership. Ji:Kymera Therapeutics: Employment, Equity Ownership. Yang:Kymera Therapeutics: Employment, Equity Ownership. Yuan:Kymera Therapeutics: Employment, Equity Ownership. Mayo:kymera Therapeutics: Employment, Equity Ownership. Rong:Kymera Therapeutics: Employment, Equity Ownership. Rusin:Kymera Therapeutics: Employment, Equity Ownership. Sharma:kymera Therapeutics: Employment, Equity Ownership. Loh:Kymera Therapeutics: Employment, Equity Ownership. Li:Kymera Therapeutics: Employment, Equity Ownership. Townson:Kymera Therapeutics: Employment, Equity Ownership. Chen:kymera therapeutics: Employment, Equity Ownership. Kamadurai:Kymera Therapeutics: Employment, Equity Ownership. Walker:Kymera Therapeutics: Employment, Equity Ownership. Gollob:Kymera Therapeutics: Employment, Equity Ownership. Mainolfi:Kymera Therapeutics: Employment, Equity Ownership.