Abstract Relapsed/refractory DLBCL is an aggressive B-cell malignancy with limited treatment options. BTK inhibitors have demonstrated preclinical and clinical activity in DLBCL of the activated B-cell (ABC) subset, but responses are limited and not durable. Therefore, an acalabrutinib (BTK inhibitor) combination screen was conducted in a panel of 9 DLBCL cell lines to identify synergistic and active combinations. The in vitro combination of acalabrutinib plus capivasertib (AKT inhibitor) demonstrated significant combination benefit in the ABC-DLBCL cell lines TMD8 (Loewe synergy score 9.3) and OCI-LY10 (Loewe synergy score 3.2). Capivasertib and acalabrutinib monotherapy gave 0% and 85% tumor growth inhibition (TGI) in TMD8 and 5% and 79% in OCI-LY10 tumour xenograft models respectively. The combination gave tumour regressions of 99% in TMD8 and 72% in OCI-LY10. To explore a potential mechanism of action, RNAseq analysis was performed on the TMD8 cell line and TMD8 xenograft tumors treated with the monotherapies and combination. 24-hour monotherapy treatment of TMD8 cells with acalabrutinib altered the expression of 7390 genes while capivasertib altered 398 genes (absolute fold change ≥ 1.25 and adjusted p-value < 0.05), with enrichment for genes regulated by NFkB signalling detected. Pathway analysis following combination treatment revealed significant decrease in expression of the G2M checkpoint pathway genes compared to acalabrutinib alone (Adjusted p-value = 0.007) as well as decreased expression of CDK1, AURKA and MYC, and significant shifts in the TNFa signalling via NFkB pathway (Adjusted p-value = 0.02). The NFkB signalling node was examined in more detail in TMD8 tumor samples treated with the combination. Lower expression of NFKBIA, NFKBID, EGR2 and BCL2A1 was evident in the combination group, compared to control tumors (p<0.01 for all). Interestingly, the combination strongly increased the expression of MS4A2 (CD20) compared to control (Adjusted p-value = 0.002), capivasertib monotherapy (Adjusted p-value = 0.004) and acalabrutinib monotherapy (Adjusted p-value = 0.02). Therefore, we tested the addition of Rituxan to acalabrutinib + capivasertib combination in the TMD8 xenograft model. The triple combination produced durable complete regressions in 5/5 mice after cessation of treatment whereas tumors regrew after cessation of treatment of the acalabrutinib + capivasertib doublet. These data suggest that the combination of BTK and AKT inhibition may enhance anti-tumor activity in ABC DLBCL, with the addition of anti-CD20 giving more durable tumour control. Citation Format: Kathleen Burke, Justine Roderick-Richardson, Natasha Narang, Brandon Willis, Hannah Dry, Lillian Castriotta, Alan Rosen, Jay Mettetal, Simon Barry, Andrew Bloecher. Combination activity of acalabrutinib and capivasertib in diffuse large B-cell lymphoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1024.