Abstract 5985: Cyclin dependent kinase 9 (CDK9) inhibition increased efficacy of programmed cell death protein 1 (PD-1) blockade for hepatocellular carcinoma (HCC) through decreasing programmed death-ligand 1 (PD-L1) expression

Abstract

Background: PD-1 blockade has been approved as treatment for advanced HCC. We previously demonstrated the efficacy of CDK9 inhibition in reducing the expression of inducible proteins and as a treatment for HCC (Shao et al. Oncology 2022). We thus examined the influence of CDK9 inhibition on the expression of PD-L1 in HCC and the potential of improving the efficacy of PD-1 blockade with the combination of CDK9 inhibitors. Methods: We first examined the influence of specific CDK9 inhibitors, AZD4573 and atuveciclib, on interferon-γ (IFN-γ) induced PD-L1 expression of human HCC cell lines HuH7 and Hep3B. Overexpression of CDK9 and siRNA to CDK9 were used to confirm the findings. Similar experiments were repeated in the mouse HCC cell line BNL. To test the in vivo efficacy, we orthotopically implanted BNL cells in the subcapsular area of BALB/c mice, which were then treated with vehicle, CDK9 inhibitors, anti-PD-L1 antibodies, or their combination. To compare with tumor sizes, all mice were sacrificed after 14 days of treatment. To compare survival, the mice were only sacrificed if they met the criteria of animal euthanasia. Results: Both HuH7 and Hep3B cells had minimal PD-L1 expression, which however could be induced using IFN-γ. AZD4573 and atuveciclib decreased the IFN-γ induced PD-L1 expression of HuH7 and Hep3B cells in a dose dependent manner. The flow cytometry results confirmed that membrane PD-L1 expression was also decreased by CDK9 inhibition. Overexpression of CDK9 decreased the influence of these CDK9 inhibitors on PD-L1 expression. CDK9 knockdown using siRNA reduced the IFN-γ induced PD-L1 expression. AZD4573 and atuveciclib also reduce the PD-L1 expression in BNL cell lines. In the orthotopic animal model, the tumors of the sacrificed mice were significantly smaller in mice treated with atuveciclib and an anti-PD-L1 antibody than mice treatment with either alone. The survival of mice treated with the combination was also longer than mice treated with each therapy alone. Conclusion: We demonstrated that CDK9 inhibition could reduce the IFN-γ induced PD-L1 expression of HCC cells. Combination of CDK9 inhibitors and anti-PD-L1 antibodies was more effective than either therapy alone. Citation Format: Ching-Tso Chen, Yu-Yun Shao, Hung-Wei Hsu, Rita R. Wo, Han-Yu Wang, Ann-Lii Cheng, Chih-Hung Hsu. Cyclin dependent kinase 9 (CDK9) inhibition increased efficacy of programmed cell death protein 1 (PD-1) blockade for hepatocellular carcinoma (HCC) through decreasing programmed death-ligand 1 (PD-L1) expression. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5985.