SELLAS Life Sciences to Present In Vivo Preclinical Data Demonstrating Statistically Significant Survival Benefit of SLS009 in T-Cell Prolymphocytic Leukemia at the European Society for Medical Oncology (ESMO) Congress 2025

Rhea-AI Summary

SELLAS Life Sciences (NASDAQ: SLS) will present preclinical in vivo data showing that its selective CDK9 inhibitor SLS009 (tambiciclib) prolonged survival in a T-cell prolymphocytic leukemia (T-PLL) patient-derived xenograft (PDX) model at ESMO Congress 2025 (Oct 17–21, 2025).

In the PDX model, SLS009 monotherapy and SLS009 plus venetoclax yielded median overall survival of 7.4 weeks and 7.9 weeks versus 4.4 weeks for venetoclax alone (statistically significant, p<0.05), with better control of circulating T-PLL cells and acceptable tolerability for the combination.

Positive

• SLS009 monotherapy median survival 7.4 weeks
• SLS009+venetoclax median survival 7.9 weeks
• Venetoclax comparator median survival 4.4 weeks
• Survival differences reached statistical significance (p<0.05)
• Combination regimen reported as well tolerated
• Data derived from a patient-derived xenograft (PDX) model mirroring human T-PLL

Negative

• Findings limited to a preclinical PDX model, not human clinical data
• No clinical efficacy, safety, or dosing data in patients disclosed

Insights

Hematology Translational Scientist

Preclinical PDX data show SLS009 extends survival versus venetoclax and reduces circulating T-PLL cells; results are statistically significant and clinically relevant for translation.

In a patient-derived xenograft model of relapsed/refractory T-PLL, selective CDK9 inhibition with SLS009 produced single-agent activity and improved survival when combined with venetoclax; median survivals reported were 7.4 and 7.9 weeks for SLS009 alone and the combination versus 4.4 weeks for venetoclax monotherapy, with statistical significance (p<0.05). The model reproduced key human clinicopathological features, and the combination showed tolerability in the study, strengthening the biological plausibility of a CDK9/BCL2 therapeutic interaction in this indication.

Dependencies and risks include the preclinical nature of the evidence and the usual limits of PDX models in predicting human outcomes; clinical safety, pharmacokinetics, and dose scheduling remain unknown from the disclosed data. Watch for the detailed poster at ESMO on October 18, 2025 and any forthcoming disclosure of study design elements (group sizes, statistical plan), dose exposures, and early human data such as a first-in-human or dose-escalation study within the next 6–18 months to assess translatability and clinical impact.

Preclinical data support SLS009 as a monotherapy or in combination with venetoclax for the treatment of T-PLL, a rare and highly aggressive form of mature T-cell leukemia

NEW YORK, Oct. 13, 2025 (GLOBE NEWSWIRE) -- 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, today announced that preclinical efficacy of its highly selective CDK9 inhibitor, SLS009 (tambiciclib), as a monotherapy and in combination with venetoclax in T-cell prolymphocytic leukemia (T-PLL), will be presented at the European Society for Medical Oncology (ESMO) Congress 2025, being held October 17 – 21, 2025, in Berlin, Germany.

The results highlight an in vivo patient-derived xenograft (PDX) model of relapsed/refractory T-PLL that reproduced key human clinicopathological features of the disease. In this model, SLS009 demonstrated meaningful single-agent activity as well as in combination with the BCL2 inhibitor venetoclax. Both SLS009 monotherapy and the combination prolonged overall survival (7.4 weeks and 7.9 weeks, respectively) compared to venetoclax alone (4.4 weeks), a difference that was statistically significant (p<0.05), and SLS009 achieved better control of circulating T-PLL cells in the peripheral blood relative to other treatments. The combination regimen was also well tolerated in the study.

“These results are highly encouraging and provide important preclinical evidence that selective CDK9 inhibition with SLS009 may play a critical role in the treatment of T-PLL, an aggressive leukemia with very limited treatment options,” said Dr. Dragan Cicic, Senior Vice President, Chief Development Officer at SELLAS. “Notably, SLS009 alone and in combination with venetoclax prolonged survival more effectively than venetoclax monotherapy, while demonstrating favorable tolerability. This reinforces the potential of SLS009 to improve outcomes in T-PLL and broadens its therapeutic relevance across additional hematologic malignancies that require novel treatment approaches.”

“The T-PLL patient-derived xenograft model we developed is highly robust and closely mirrors the behavior and progression of the disease in humans,” said Dr. Francisco Vega, Professor and Head of the Lymphoma Section in the Department of Hematopathology at the University of Texas MD Anderson Cancer Center (MDACC) and lead author of the study. “This breakthrough allows for more rapid and meaningful translation into the clinic, greatly accelerating drug development by enabling us to evaluate novel therapies with strong clinical potential, such as SLS009, without waiting years for early clinical trial results.”

Poster Presentation Details:

Title: CDK9 Inhibition Enhances Venetoclax Activity and Prolongs Survival in a T-PLL Patient-Derived Xenograft Model

Session Date and Time: Saturday, October 18, 2025, 12:00 PM – 12:45 PM CEST; poster on display from 9:00 AM – 5:00 PM CEST

Session Title: Haematological Malignancies

Location: Poster Area Hall 25

Lead Author: Francisco Vega, MD, PhD, Hematopathology Department, University of Texas MD Anderson Cancer Center, Houston, TX

Abstract #: FPN: 1284P

About SELLAS Life Sciences Group, Inc.

SELLAS is a late-stage clinical biopharmaceutical company focused on the development of novel therapeutics for a broad range of cancer indications. SELLAS’ lead product candidate, GPS, is licensed from Memorial Sloan Kettering Cancer Center and targets the WT1 protein, which is present in an array of tumor types. GPS has the potential as a monotherapy and combination with other therapies to address a broad spectrum of hematologic malignancies and solid tumor indications. The Company is also developing SLS009 (tambiciclib) - potentially the first and best-in-class differentiated small molecule CDK9 inhibitor with reduced toxicity and increased potency compared to other CDK9 inhibitors. Data suggests that SLS009 demonstrated a high response rate in AML patients with unfavorable prognostic factors including ASXL1 mutation, commonly associated with poor prognosis in various myeloid diseases. For more information on SELLAS, please visit www.sellaslifesciences.com.

Forward-Looking Statements

This press release contains forward-looking statements. All statements other than statements of historical facts are “forward-looking statements,” including those relating to future events. In some cases, forward-looking statements can be identified by terminology such as “plan,” “expect,” “anticipate,” “may,” “might,” “will,” “should,” “project,” “believe,” “estimate,” “predict,” “potential,” “intend,” or “continue” and other words or terms of similar meaning. These statements include, without limitation, statements related to the GPS clinical development program, including the REGAL study and the timing of future milestones related thereto. These forward-looking statements are based on current plans, objectives, estimates, expectations, and intentions, and inherently involve significant risks and uncertainties. Actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks and uncertainties with oncology product development and clinical success thereof, the uncertainty of regulatory approval, and other risks and uncertainties affecting SELLAS and its development programs as set forth under the caption “Risk Factors” in SELLAS’ Annual Report on Form 10-K filed on March 20, 2025 and in its other SEC filings. Other risks and uncertainties of which SELLAS is not currently aware may also affect SELLAS’ forward-looking statements and may cause actual results and the timing of events to differ materially from those anticipated. The forward-looking statements herein are made only as of the date hereof. SELLAS undertakes no obligation to update or supplement any forward-looking statements to reflect actual results, new information, future events, changes in its expectations, or other circumstances that exist after the date as of which the forward-looking statements were made.

Investor Contact

John Fraunces

Managing Director

LifeSci Advisors, LLC

jfraunces@lifesciadvisors.com

FAQ

What did SELLAS (SLS) announce about SLS009 at ESMO 2025 on October 13, 2025?

SELLAS announced preclinical in vivo data showing SLS009 alone and with venetoclax prolonged survival in a T-PLL PDX model, with results presented at ESMO 2025.

How much did SLS009 improve survival versus venetoclax in the T-PLL PDX model?

SLS009 monotherapy had median survival of 7.4 weeks and SLS009+venetoclax 7.9 weeks versus 4.4 weeks for venetoclax alone.

Were the survival improvements for SLS009 statistically significant in the SLS announcement?

Yes; the reported survival differences were statistically significant (p<0.05).

Did SELLAS report tolerability for the SLS009 plus venetoclax combination?

Yes; the company reported the combination regimen was well tolerated in the preclinical study.

When and where will SELLAS present the SLS009 T-PLL data at ESMO 2025?

Poster presentation is on October 18, 2025, 12:00 PM–12:45 PM CEST, poster on display 9:00 AM–5:00 PM CEST in Poster Area Hall 25.

Does the SELLAS release include human clinical results for SLS009 in T-PLL?

No; the announcement describes preclinical PDX model results and does not present human clinical data.