HPK1 Inhibition and Venetoclax Combination Suppressed AML By Enhancing Cytotoxic T-Cell Response to Leukaemia

Background: Immune escape is one of the major causes of treatment failure in cancer therapy and modulation of immune cell function has become an emerging therapeutic strategy. Haematopoietic progenitor kinase 1 (HPK1) is a negative regulator of T cell signaling and its inhibition promotes T cell function. Venetoclax, a BCL2 inhibitor, has become a standard of care for unfit or elderly patients with acute myeloid leukaemia (AML) and may enhance anti-leukaemic effector T-cell function. We hypothesize that HPK1 inhibitor (HPK1i) may synergize with venetoclax and improve treatment outcome in AML by co-activating cell-death pathway and T cell immunity. Methods: Human T cells were activated by anti-CD3/CD28 antibodies in vitro. Proliferation, cytokine production, T-cell subsets and exhaustion were examined by flow cytometry after 3-day treatment of HPK1i. Recipient mice engrafting with MLL-AF9 AML were treated with HPK1i singly or in combination with venetoclax. Leukaemic burden, apoptosis and T cell functions and exhaustion were examined. Results: In vitro, HPK1i treatment of activated human T-cells induced cellular proliferation and expression of TNF-α and INF-γ. It increased effector memory T-cell (CD62L-CD45RA-) and decreased naïve T-cell population (CD62L+CD45RA+). Furthermore, it reduced exhaustion markers including PD-1 and TIM3. In vivo, HPK1i and venetoclax combination significantly decreased the burden and induced apoptosis of leukaemic cells on 18th DPT (days post transplantation), and prolonged survival of non-irradiated mice that were transplanted with donor mouse MLL-AF9 AML cells. Cytotoxic T-cells were activated, associated with increase in TNF-α and INF-γ expression and increase in the effector memory T cell subset. There was a decrease in naïve T cell population and T cell exhaustion marker PD-1. Single cell transcriptomes of serial bone marrow samples from mice treated with the combination were being analyzed and will be presented in the meeting. Conclusion: HPK1i and venetoclax reduced the leukaemic burden and enhanced the anti-leukemic ability of T cells in MLL-AF9 AML mouse model. Acknowledgements This research is supported by the Centre for Oncology and Immunology under the Health@InnoHK Initiative funded by the Innovation and Technology Commission, Theme-based Research Scheme (T12-702/20-N), Hong Kong Jockey Club Charities Trust, Li Shu Fan Medical Foundation, Croucher Foundation, S.K.Yee Medical Foundation (260940161, 260940149, 260940187), Tang King Yin Research Fund, Mr. Ying Wan Leung Research Fund, Madam Madeline Tong Lai-Sheung Cancer Research Fund (A.Y.H.L), The Government of Hong Kong SAR, China.