Abstract 5098: B7-H3 blockade in combination with natural killer cell activity enhancers including NKTR-255 and venetoclax synergistically induces cytotoxicity in B7-H3+ AML cells

Abstract Background: Acute myeloid leukemia (AML) is characterized by clonal proliferation of malignant myeloid blasts in the bone marrow. We have recently reported that blocking B7-H3 using a novel monoclonal antibody (mAB), T-1A5 enhances natural killer (NK) cell mediated cytotoxicity against AML cells. IL-15 is an essential component for NK cell activity. Recombinant human IL-15 (rIL-15), however, degrades rapidly and therefore may provide a suboptimal stimulus to NK cells. Studies have shown that BCL2 inhibitor, venetoclax (ABT-199), enhances NK cell induced antibody dependent cellular cytotoxicity (ADCC) against cancer cells. However, the combined effect of B7-H3 blocking and BCL2 inhibition remains unexplored in AML. We hypothesized that blocking B7-H3 using T-1A5 in the presence of novel investigational polymer-engineered IL-15 receptor agonist, NKTR-255, and the BCL2 inhibitor synergistically induces NK cell mediated ADCC in AML cells. Methods: CD16 and NKG2D activation markers measured by flow cytometry in AML patient and healthy donor derived NK cells. The NKTR-255 was used to generate NK cells, and their cytotoxicity was compared to that of NK cells generated with rIL-15 using IncuCyte. To investigate these NK cells’ cytotoxic activity against AML cells, we treated B7-H3+ AML cell lines (OCI-AML3 and MV4-11) with anti-B7-H3 chimeric mAb ChT-1A5 in the presence and absence of NK cells generated using NKTR-255 or rIL-15. To assess the potential synergy of BCL2 and B7-H3 in NK cell mediated anti-leukemic activity against AML cells, we treated B7-H3+ cells with T-1A5 with or without ABT-199in the presence or absence of NK cells. Results: The expression of NK cell activation markers were significantly lower in NK cells derived from AML patients than healthy donors. To assess the effect of NKTR-255 on NK cell activity, we generated NK cells using NKTR-255 and compared them with NK cells generated using rIL-15 using the standard 14-day culture protocol. We observed similar proliferation and NK cell numbers between NKTR-255 and rIL-15. However, NK cells stimulated with NKTR-255 showed significantly enhanced (more than 2-fold) cytolytic activity against B7-H3+ AML cell lines compared with NK cells stimulated with rIL-15. Moreover, when combined with ChT-1A5, NKTR-255 significantly enhanced NK cell mediated ADCC activity in B7-H3+ AML cells compared to rIL-15. Interestingly, we observed that the combination of T-1A5 and venetoclax synergistically induced an anti-leukemic effect in B7-H3+ AML cell lines when compared with either drug alone or isotype IgG1 in a dose-dependent manner. Conclusion: Our data demonstrate that blockade of the immune checkpoint protein, B7-H3 with the IL-15 receptor agonist, NKTR-255, or the BCL2 inhibitor, ABT-199, synergistically induces NK cell mediated cytotoxicity against B7-H3+ AML cells. Citation Format: Anudishi Tyagi, A. Mario Marcondes, Willem Overwijk, Venkata Lokesh Battula. B7-H3 blockade in combination with natural killer cell activity enhancers including NKTR-255 and venetoclax synergistically induces cytotoxicity in B7-H3+ AML cells. [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 5098.