Abstract 5666: Tambiciclib (SLS009), a CDK9 inhibitor, promotes apoptosis and suppresses MCL-1 levels in AML cell lines.

Abstract Background: Tambiciclib (SLS009) is a potent, selective CDK9 inhibitor that has shown promising results in Phase I clinical trials in hematological malignancies (NCT04588922). CDK9 is a critical regulator of transcription elongation through RNA polymerase II. Its inhibition results in downregulation of protein synthesis, thus decreasing the expression of short half-life molecules, such as MCL-1, MYC, E2F, and survivin, which leukemia cells, especially acute myeloid leukemia (AML) cells, depend on for survival. Here, we demonstrate the mechanistic effects of tambiciclib exposure correlated with apoptosis and cell death in AML cell lines. Methods: We exposed the following cell lines to various concentrations of tambiciclib for 6 to 8 hours followed by washout: THP1 (p53 mutant, MLL-AF9 rearranged, NRAS mutant), NOMO1 (p53 mutant, MLL-AF9 rearranged, ASXL1 mutant), MOLM-13 WT (MLL-AF9 rearranged, NRAS mutant, FLT3 mutant), and MOLM-13 TP53 KO (MLL-AF9 rearranged, NRAS mutant, FLT3 mutant, p53 knockout). We performed viability assays on these cell lines using CellTiterGlo and examined protein expression using western immunoblotting and intracellular flow cytometry. Additionally, we stained cells with Annexin-V and a fixable viability dye before intracellular staining to correlate how protein expression levels change as cells undergo apoptosis and cell death. Results: Treatment of AML cell lines with tambiciclib demonstrated cytotoxicity in the low nanomolar range and a dose-dependent decrease in cell viability after an 8-hour exposure. Repeated 8-hour exposures of tambiciclib consistently decreased IC50s, warranting further investigation into the mechanism through which this occurs. Our results show that when cells are exposed to 50 nM or 100 nM SLS009 for 6 hours, active caspase-3 levels increase and MCL-1 and survivin levels decrease. These changes in caspase-3 and MCL-1 expression are observed as early as 6 hours and are even more pronounced 24 hours after the completion of treatment. We observed that lower levels of MCL-1 and survivin correlate with increased apoptosis, suggesting that tambiciclib may lower the threshold for AML cell death and therefore may be useful in combination with existing therapeutic agents for leukemia treatment. Conclusions: These results suggest that AML cell lines undergo cell death and apoptosis at low nanomolar concentrations of tambiciclib. Preliminary analysis of apoptotic molecules and pathways suggests a mechanism involving MCL-1 and survivin. With ongoing investigations into the dose and schedule of tambiciclib, our future directions include optimizing the use of tambiciclib in combination therapy to leverage the mechanistic effects of CDK9 inhibition and provide a novel therapeutic approach to treating patients with high-risk AML. Citation Format: Elizabeth C. Trull, Philip C. Amrein. Tambiciclib (SLS009), a CDK9 inhibitor, promotes apoptosis and suppresses MCL-1 levels in AML cell lines abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5666.