On June 2, 2026 SEED Therapeutics, Inc. ("SEED"), a clinical-stage biotechnology company pioneering rationally designed molecular glue degraders, reported the presentation of a trial-in-progress poster at the 2026 American Society of Clinical Oncology (ASCO) (Free ASCO Whitepaper) Annual Meeting highlighting the first-in-human Phase 1 study of ST-01156, SEED’s oral, selective RBM39 molecular glue degrader. The presentation underscores SEED’s transition from platform validation to clinical execution, with ST-01156 advancing across multiple RBM39-dependent cancers, with development informed by mechanism-based preclinical data and real-time pharmacokinetic and pharmacodynamic assessments.

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ST-01156 is designed to degrade RNA-binding motif protein 39 (RBM39), a regulator of RNA splicing programs that govern oncogenes essential for tumor survival. The ASCO (Free ASCO Whitepaper) poster outlines the scientific rationale, Phase 1 dose-escalation design, and planned expansion strategy for ST-01156which is currently being evaluated in a first-in-human Phase 1 dose-escalation study (NCT07197554) in patients with advanced solid malignancies.

Highlights At A Glance

Clinical-stage lead asset: ST-01156 is currently being evaluated in an open-label Phase 1 study in patients with advanced solid malignancies.
Oral, brain-penetrant molecular glue degrader: ST-01156 was rationally optimized for selective RBM39 degradation, potency, metabolic stability, and drug-like properties.
Integrated biomarker and dose-selection strategy: The study incorporates real-time RBM39 target engagement in peripheral blood mononuclear cells (PBMCs), together with safety and pharmacokinetics, to support recommended Phase 2 dose (RP2D) selection.
Broad oncology expansion potential: Planned expansion cohorts include Ewing sarcoma, advanced hepatocellular carcinoma, KRAS-mutant cancers, and other RBM39-dependent tumors, including biliary tract carcinoma, endometrial carcinoma, and DNA damage repair–aberrant cancers.
Preclinical proof-of-concept across multiple models: SEED has demonstrated preclinical antitumor activity in response to RBM39 degradation, including tumor regression, with complete regression observed in Ewing sarcoma, neuroblastoma, and KRAS-mutant colorectal cancer models.

Together, these findings support SEED’s strategy to advance ST-01156 from dose escalation into patient-enriched expansion cohorts where emerging RBM39 biology informs indication selection and clinical proof-of-concept.

Scientific Rationale and ASCO (Free ASCO Whitepaper) Poster Takeaways

RBM39 is an RNA-binding protein that regulates cancer-relevant RNA splicing programs, including pathways involved in tumor proliferation, survival, DNA damage response, and oncogenic fusion proteins. ST-01156 is designed to act as a molecular glue degrader by recruiting RBM39 to DCAF15, an E3 ligase adapter, leading to RBM39 degradation through the ubiquitin-proteasome system. By eliminating RBM39, ST-01156 has the potential to disrupt multiple cancer-driving pathways that are difficult to address with conventional targeted therapies.

The ASCO (Free ASCO Whitepaper) poster highlights preclinical and clinical-development findings supporting SEED’s strategy:

Ewing sarcoma rationale: ST-01156 demonstrated tumor regression in an A673 Ewing sarcoma xenograft model, with complete regression at higher dose levels. Separately, treatment with ST-00937, a non-deuterated precursor of ST-01156, showed complete elimination of RBM39 and the EWS-FLI1 fusion protein in tumor lysates.
Neuroblastoma and KRAS-mutant cancer activity: ST-01156 produced complete tumor regression in an SH-SY5Y neuroblastoma xenograft model, while ST-00937 demonstrated complete regression in an HCT-116 KRAS G13D-mutant colorectal cancer xenograft model.
Mechanism-based expansion strategy: The clinical development plan includes expansion cohorts in Ewing sarcoma, advanced hepatocellular carcinoma, KRAS-mutant cancers, and other RBM39-dependent tumors, including biliary tract carcinoma, endometrial carcinoma, and tumors with DNA damage repair aberrations.
Integrated dose-selection approach: The Phase 1 study is designed to determine the optimal dose and recommended Phase 2 dose using safety, pharmacokinetic, and pharmacodynamic data, including real-time measurement of RBM39 target engagement in PBMCs.

"ST-01156 is designed to address a biologically important and difficult-to-drug target through selective RBM39 degradation. The ASCO (Free ASCO Whitepaper) presentation highlights a disciplined clinical strategy that integrates safety, pharmacokinetics, and real-time target engagement to guide dose selection and expansion into cancers with strong mechanistic rationale. We believe this approach positions ST-01156 to generate early meaningful clinical proof-of-concept across multiple RBM39-dependent tumor types," said Dr. James Tonra, PhD, President, and Chief Scientific Officer of SEED.

"The advancement of ST-01156 into first-in-human clinical evaluation is an important milestone for SEED and a validation of our RITE3 technology. Our goal is not only to discover molecular glues, but to rationally design degraders with clear target biology, translational biomarkers, and a defined clinical development path. ST-01156 reflects that strategy and represents a meaningful step toward unlocking disease drivers that have historically been considered undruggable," said Dr. Lan Huang, PhD, Co-Founder, Chairman, and Chief Executive Officer of SEED.

Clinical Development Status

ST-01156 is being evaluated in an ongoing, open-label Phase 1 multiple ascending dose (MAD) study in patients with advanced solid malignancies. The study is designed to enroll approximately 30 to 50 patients, with ST-01156 administered orally once daily for five days every seven days, with the option to adapt to a continuous once-daily schedule based on emerging data.

The primary objectives are to characterize safety and tolerability and determine the optimal dose and recommended Phase 2 dose. Secondary objectives include pharmacokinetics, RBM39 target engagement in PBMCs, and preliminary antitumor activity. Per protocol, SEED plans to evaluate ST-01156 in mechanism-based back-fill cohorts within this MAD study, including Ewing sarcoma, advanced hepatocellular carcinoma, KRAS-mutant cancers, and other RBM39-dependent tumors. Data from these cohorts are intended to inform the design of the protocol’s subsequent Phase 1 expansion phase.

ASCO 2026 Poster Presentation Details:

Title: First-in-Human Clinical Evaluation of ST-01156, an Optimized and Selective Degrader of RNA-Binding Motif 39 (RBM39): A Phase 1 Study in Advanced Solid Malignancies with a Focus on RBM39-Dependent Cancers
Presenter/Authors: Eric K. Rowinsky, Gregory M. Cote, George D. Demetri, Robert G. Maki, Suzanne George, Daneng Li, Alain C. Mita, Monica M. Mita, Jordi Rodon Ahnert, Dan Lu, Dong Liu, Lan Huang, James Tonra
Session: Developmental Therapeutics—Molecularly Targeted Agents and Tumor Biology
Abstract Number: TPS3164

(Press release, Seed Therapeutics, JUN 2, 2026, View Source [SID1234666400])