On June 2, 2025 SELLAS Life Sciences Group, Inc. (NASDAQ: SLS) ("SELLAS" or the "Company"), a late-stage clinical biopharmaceutical company focused on the development of novel therapies for a broad range of cancer indications, reported preclinical efficacy of SLS009 (tambiciclib) in ASXL1 mutated colorectal cancer lines. The data are featured in a presentation, entitled "In vitro efficacy of CDK9 inhibitor tambiciclib (SLS009) in ASXL1 mutated colorectal cancer cell lines" at the 2025 American Society of Clinical Oncology (ASCO) (Free ASCO Whitepaper) Annual Meeting, taking place May 30- June 3, 2025, in Chicago, Illinois (Press release, Sellas Life Sciences, JUN 2, 2025, View Source [SID1234653622]).

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In a panel of cell lines, SLS009 demonstrated potent anti-proliferative activity:

In 50% (4/8) of ASXL1 mutant cell lines showed an IC50<100 nM, compared to 0% (0/4) of ASXL1 wild-type lines
Among cell lines harboring ASXL1 frameshift mutations (FSMs), 75% (3/4) responded with IC50 <100 nM versus only 12.5% (1/8) in cell lines without FSMs
All cell lines (3/3) with ASXL1 FSMs in the 637-638 protein region responded to treatment with SLS009
In cell lines with IC50 <100 nM, 75% (3/4) also demonstrated IC99 values below 100 nM, indicating steep dose response curve
Importantly, effective concentrations were significantly lower than those achieved in patients treated at the recommended phase 2 dose determined to be safe, suggesting a broad therapeutic window.
"These results provide strong rationale for continued advancement of SLS009 as a potential treatment for ASXL1-mutated cancers," said Dr. Dragan Cicic, Senior Vice President, Chief Development Officer at SELLAS. "The ability to selectively target ASXL1-driven tumors at concentrations well below the known safety threshold opens the door for tolerable and effective therapy. Based on the findings, we believe that ASXL1 mutation status could serve as a potential biomarker for response to SLS009 inhibition, which may allow us to further refine patient selection and improve outcomes. We look forward to presenting these results at ASCO (Free ASCO Whitepaper)."

Poster presentation details:

Title: In vitro efficacy of CDK9 inhibitor tambiciclib (SLS009) in ASXL1 mutated colorectal cancer cell lines
Session Date and Time: Monday, June 2, 2025, 1:30 PM-4:30 PM CDT
Session Title: Developmental Therapeutics—Molecularly Targeted Agents and Tumor Biology
Location: Hall A – Posters and Exhibits
Abstract #: 3121
Poster Board #: 436

SLS009 is currently being investigated in a Phase 2 open-label, single-arm, multi-center study designed to evaluate the safety, tolerability, and efficacy of SLS009 in combination with venetoclax and azacitidine including AML patients with ASXL1 mutations. Initial clinical safety and efficacy data are available. In addition, the study aims to identify biomarkers for the target patient population and enrichment for further trials. For more information on the study, visit clinicaltrial.gov identifier NCT04588922.