Abstract 4408: Metabolic alterations induced by ibrutinib in CLL cells as a basis for drug combinations to enhance ibrutinib therapeutic activity

The use of ibrutinib, which targets the Bruton tyrosine kinase, has significantly improved the therapeutic outcome for patients with chronic lymphocytic leukemia (CLL), especially in cases with chromosome 17p deletion, which usually respond poorly to conventional chemotherapy. However, ibrutinib is not completely curative, and a small portion of CLL patients exhibit resistance. The main objectives of this study were to investigate the effect of ibrutinib on CLL mitochondrial metabolism and to develop novel strategies to enhance the therapeutic activity of ibrutinib. Ibrutinib-induced changes in mitochondrial functions were analyzed by measuring oxygen consumption rate (OCR) as an indicator of mitochondrial respiration and extracellular acidification rate (ECAR) as a parameter of lactate production from glycolysis using an extracellular flux analyzer. In a primary CLL cell-stromal cell co-culture system (which maintains long-term CLL cell viability), ibrutinib induced little change in OCR and ECAR during the first 48-72 h. However, prolonged drug exposure (up to 7 days) in vitro caused a consistent and significant decrease in OCR compared to that in untreated co-cultured CLL cells. Importantly, CLL cells isolated from patients treated with ibrutinib for 7 days also had significantly decreased OCR compared to pre-treatment samples from the same patients, suggesting that the impact of ibrutinib on mitochondrial functions occurred in vivo and was clinically relevant. On western blot analysis, ibrutinib induced decreases in expression of certain respiratory chain components associated with AMPK activation. On electron microscopy, CLL cells treated with ibrutinib in vitro for 6 days exhibited lower numbers of mitochondria than untreated cells, which was confirmed by flow cytometry analysis of cells stained with Mitotracker Green. Functional analysis showed that ibrutinib caused CLL cells to take up less glutamine but did not affect glucose uptake, suggesting that ibrutinib has a preferential impact on glutamine metabolism. On testing the possibility that the metabolic alterations and mitochondrial dysfunction induced by ibrutinib render CLL cells vulnerable to compounds that affect mitochondria or inhibit glutamine metabolism, we found that combinations of ibrutinib with metformin (inhibitor of mitochondrial OXPHOS) or with C968 (glutaminase inhibitor) resulted in synergistic activity against CLL cells and induced massive leukemia cell death compared to either drug alone. Notably, the combination of ibrutinib and C968 was synergistic in killing CLL cells with 17p deletion. Overall, our results suggest that ibrutinib induces metabolic alterations characterized by decreased mitochondrial respiration and attenuated glutamine metabolism. Such metabolic alterations provide a biochemical basis for mechanism-based drug combination to enhance the therapeutic activity of ibrutinib. Citation Format: Helene Pelicano, Li Feng, William G. Wierda, Michael J. Keating, Peng Huang. Metabolic alterations induced by ibrutinib in CLL cells as a basis for drug combinations to enhance ibrutinib therapeutic activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4408. doi:10.1158/1538-7445.AM2017-4408