On July 15, 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 that is has met all primary endpoints in its Phase 2 trial of SLS009 (tambiciclib), a highly selective CDK9 inhibitor, in relapsed/refractory acute myeloid leukemia (r/r AML) (Press release, Sellas Life Sciences, JUL 15, 2025, View Source [SID1234654383]).
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The Phase 2 clinical trial of SLS009 is an open-label, single-arm, multi-center study designed to evaluate the safety, tolerability, and efficacy of SLS009 in combination with venetoclax and azacitidine at two dose levels, 45 mg and 60 mg. In the 60 mg dose cohort, patients were treated with either a 60 mg dose once per week or a 30 mg dose two times per week. The trial was expanded to include ASXL1-mutated AML patients as well as patients with myelodysplasia-related cytogenetic abnormalities other than ASXL1 mutations. The target response rate for this Phase 2 trial, at the optimal dose level, was at least 20% and a target median survival of at least 3 months. The primary endpoint for the trial was overall response rate (ORR), and key secondary endpoints included overall survival (OS), safety, and tolerability.
The trial met all endpoints, demonstrating strong efficacy and favorable safety and tolerability with robust anti-tumor activity. Based on these data, the Company plans to advance SLS009 into a randomized trial that will expand into the newly diagnosed AML populations where earlier intervention may enhance therapeutic outcomes, as well as patients refractory to venetoclax and azacitidine, with the study to support a potential New Drug Application (NDA) with the FDA.
"We are excited to report that our Phase 2 trial met all key endpoints, with clinical responses and survival outcomes that exceed targeted expectations and historical benchmarks," said Angelos Stergiou, MD, ScD h.c., President and Chief Executive Officer of SELLAS. "AML remains an area of urgent unmet medical need, particularly for patients with relapsed or refractory disease, where standard treatments are often ineffective and poorly tolerated. What sets SLS009 apart is its consistent efficacy across a broad range of molecular subtypes. The remarkable response rates of 44% among AML MR patients, 50% among ASXL1-mutated AML MR, and 50% among M4/M5 patients at the optimal 30 mg BIW dose far exceed the targeted 20% benchmark. We saw a clear survival benefit with median OS reaching 8.8 months in patients refractory to venetoclax-based regimens in the cohort of patients with median 1 prior line of therapy, surpassing the historical median of 2.4 months and 4.1 months in cohorts with median 2 lines of prior therapy, versus 1.8 months reported in similar patient population. The treatment was also well-tolerated, with no dose-limiting toxicities across any treatment arm, validating both the biological selectivity and safety profile of our approach. We believe these data strongly support the potential of SLS009 to meaningfully extend life in patients with otherwise limited options, and we look forward to sharing these findings in more detail in the future. With the expected Phase 3 REGAL study final analysis by year-end, our galinpepimut-S (GPS) immunotherapy and SLS009 are complementary therapies that together enable us to hopefully address AML patients across the treatment spectrum — from early intervention to maintenance."
Key Phase 2 Results:
Patients Characteristics:
54 evaluable r/r AML patients who previously failed venetoclax-based therapies were enrolled and treated with SLS009, and venetoclax/azacitidine; patients were enrolled across all five cohorts. Among the 54 treated patients, 47 had AML MR (87%) and 23 had ASXL1 mutations (43%).
47 out of 54 had AML MR (acute myeloid leukemia with myelodysplasia-related changes).
Among AML MR patients,17 had myelomonocytic/myelomonoblastic subtype of AML (M4 and M5), representing 31% of all patients.
All patients had adverse risk cytogenetics except one patient who had intermediate risk cytogenetics.
The median age of all patients was 69.
Median number of prior lines of therapy was 2.
Efficacy:
The results exceeded the pre-specified ORR threshold of 20%, demonstrating robust clinical activity and supporting advancement into late-stage development.
The ORR in all evaluable patients was 33% across all cohorts and dose levels and 40% for the 30mg BIW dose level.
At the 30 mg BIW dose, among AML MR patients, the ORR was 44%.
The highest efficacy was observed among patients with ASXL1 mutations, with an ORR of 50% (9/18) at 30 mg BIW dose levels and M4/M5 patients with 50% (6/12) ORR.
The mOS surpassed the historical benchmark of best available therapy of 2.4 months1 for patients who received one prior line of therapy and 1.8 months for those who received more than one prior line of therapy.
The mOS for patients treated with 30mg BIW, with a median of 1 prior line of therapy, was 8.8 months, while the mOS in AML MR patients reached 8.9 months vs. 2.4 months with best available therapy.
The mOS for cohorts with a median of 2 prior lines of therapies was 4.1 months vs.1.8 months with best available therapy.
Safety:
The addition of SLS009 to the venetoclax/azacitidine regimen was well tolerated and did not result in increased toxicities compared to ven/aza alone. No dose-limiting toxicities were observed across all dose levels.
Front Line Trial Planning Underway Following FDA Guidance
Following a productive end of Phase 2 meeting, the FDA recommended that SELLAS proceeds into a trial to include newly diagnosed, first-line AML patients eligible for venetoclax/azacitidine (aza/ven) therapy, where the agency believes clinical benefit might be greatest.
The randomized 80-patient trial is currently in preparation and is expected to begin enrollment by Q1 2026. The trial will include two groups:
Predictive biomarker cohort: Newly diagnosed patients unlikely to benefit from standard aza/ven therapy based on molecular profiling
Early resistance cohort: Patients who initiate treatment with aza/ven but demonstrate confirmed lack of any response after two treatment cycles
This precision approach allows SELLAS to target subpopulations with high unmet need and greatest potential for benefit.
"These SLS009 results represent an important advancement for patients with r/r AML, where treatment options remain limited and outcomes are often poor," said Dr. Yair Levy, Director of Hematologic Malignancies Research at Texas Oncology Baylor University Medical Center. "The response rates and survival outcomes are particularly compelling, especially given the consistency of responses across high-risk molecular subtypes and the favorable safety profile. What’s especially encouraging is the opportunity to now explore this therapy in the first-line setting, where outcomes are often dictated by how patients respond to initial treatment. The FDA’s recognition of this unmet need and its support for a trial in newly diagnosed patients reflects SLS009’s potential to address a critical gap in AML care."
"Following constructive FDA guidance, we are preparing the trial focused on newly diagnosed AML patients as well as those early refractory to venetoclax and azacitidine," said Dragan Cicic, MD, Chief Development Officer of SELLAS. "The study will include two groups – one comprising patients predicted not to benefit from standard aza/ven, based on cytogenetic risk factors, and a second comprising patients who begin aza/ven treatment but demonstrate confirmed resistance after two cycles. We believe earlier intervention with SLS009 may offer greater clinical benefit before patients’ bone marrow reserve is depleted by disease or prior therapies, and before the disease evolves into more resistant and aggressive forms. Data from other recent clinical trials suggests meaningful differences in response rates between newly diagnosed and relapsed/refractory patients, reinforcing the importance of this strategic approach. In addition, our ongoing collaboration with one of the nation’s most prestigious cancer centers continues to generate insights in genomics, proteomics, and transcriptomics, which will refine patient selection and our precision medicine strategy and help us unlock the full potential of SLS009 as we prepare to enter pivotal development."