Abstract 365: High throughput screen to evaluate combinations with ibrutinib in various B-cell malignancies
Authors: Chaturvedi Shalini , Schaffer Michael , Davis Cuc , Aquino Regina , Stepanchick Emily , Versele Matthias , Balasubramanian Sriram 
Published: July 2016

Abstract
The primary objective of this study was to discover synergies leading to mechanistic insights and novel combinations for ibrutinib, a small-molecule inhibitor of Bruton's tyrosine kinase (BTK). Ibrutinib has been approved for relapsed/refractory (R/R) and del(17p) chronic lymphocytic leukemia, and R/R mantle-cell lymphoma (MCL). BTK is part of the B-cell receptor (BCR) signaling pathway, so it is of interest to examine synergy between ibrutinib and agents that target other aspects of the BCR pathway. These include PI3K and IRAK inhibitors and apoptosis inhibitors. Combinations were evaluated in a high-throughput, tumor microenvironment-directed format.
Ibrutinib was combined with inhibitors of the BCR pathway and apoptosis, whose targets included MCL-1, BCL2, XPO1, and isoforms of PI3K, IRAK, and BRD. Histologies examined included follicular lymphoma (FL), diffuse large B-cell lymphoma (DLBCL), MCL, acute myeloid leukemia (AML), acute B-lymphoblastic leukemia, and Burkitt's lymphoma. Cell lines were screened in the presence of human marrow stromal-cell-conditioned media and B-cell receptor stimulation via anti-IgG/anti-IgM antibodies in a 72-hour cell viability ATP lite assay (384-well plate, 9×9 optimized matrix, 4 replicates). Dose response matrix screening was used to measure combination effects, which manifest as potency shifts or efficacy boosts. Combination effects can be characterized by comparing each data point to a combination reference model derived from single-agent curves using the Loewe additivity model.
Ibrutinib demonstrated varying activity across cell lines. Combination activity was classified based on synergy score raw values. The best combination among tested compounds was with ABT-199, a BCL2i. This combination showed high or medium synergy in 2/5 DLBCL, 3/4 FL, and 2/5 MCL cell lines; this was also the only agent to show good synergy in AML (3/5 lines). Interestingly, ABT-737, a BCL2i that also targets BCL-XL, has previously been shown to be synergistic with ibrutinib in DLBCL lines. Synergy was also seen with other apoptotic agents such as MCL-1i and the epigenetic BETi JQ-1 across B-NHL, but in fewer lines. Among the PI3Ki, PI3Kα/δi did not show much activity, but the PI3Kδ/γi IPI-145 was synergistic in 2/5 DLBCL, 3/4 FL cell lines, and 1/5 MCL cell lines, a pattern very similar to the PI3Kδi CAL-101 (idelalisib). Ibrutinib also combined well with IRAK1/4i and XPO1i selinexor, with high activity in 1/5 DLBCL cell lines each, and medium activity in 1/5 DLBCL, 1/4 FL, and 2/5 MCL (XPO1i) and 1/5 DLBCL, 2/4 FL, and 1/5 MCL cell lines (IRAK1/4i).
Synergy was mostly observed in B-cell malignancies, but interesting synergy was observed with ABT-199 in AML. In B-NHL, ibrutinib combined with BCL2i and PI3Ki showed the best combination activity. Ibrutinib was also synergistic with other agents in selected B-NHL lines with no observed antagonism, suggesting that further study in specific histologies is warranted.
Citation Format: Shalini Chaturvedi, Michael Schaffer, Cuc Davis, Regina Aquino, Emily Stepanchick, Matthias Versele, Sriram Balasubramanian. High throughput screen to evaluate combinations with ibrutinib in various B-cell malignancies. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 365.