Ibrutinib resistance in the Eµ-TCL-1 mouse model of Chronic Lymphocytic Leukemia
Haniyeh Yazdanparast1, Yashna Paul1, Marc Zapatka1, Stephan Stilgenbauer2, Thorsten Zenz3, Peter Lichter1, Martina Seiffert1
1German Cancer Research Center, Heidelberg; 2University of Ulm, Ulm; 3National Center for Tumor Diseases, Heidelberg
The introduction of Ibrutinib, a Bruton's tyrosine kinase (BTK) inhibitor that blocks B-cell receptor signaling, represents a paradigm shift in the treatment of chronic lymphocytic leukemia (CLL). Under continuous treatment, however, a growing number of resistant patients with highly aggressive disease, were identified. The underlying mechanism of resistance is so far only partly understood. We here present resistance development in the Eµ-TCL-1 adoptive transfer mouse model of CLL under Ibrutinib treatment. After an initial response, we observed loss of therapeutic efficacy in these mice with an increase of Ki-67-positive leukemic cells in the blood and lymphoid organs. Re-transplantation of therapy-resistant tumor cells leads to uncontrolled leukemia development under Ibrutinib treatment, demonstrating acquired, cell-intrinsic resistance of these tumors. Moreover, we observed phenotypical alterations of Ibrutinib-resistant leukemia cells, including reduced expression of CD19, CD38, and CD45, all of which are involved in B-cell receptor signaling. To characterize the underlying resistance mechanisms, we performed whole exome and RNA sequencing of tumor cells. As we did not detect any genetic mutations in BTK or other genes in the B-cell receptor pathway, and no recurrent genetic aberrations in any genes, we focused our analyses on transcriptional changes in the resistant leukemia cells. Thereby, we identified several pathways and cellular functions that are significantly different in Ibrutinib-sensitive and -resistant cells and might represent novel therapeutic targets for patients that relapse under Ibrutinib treatment. In vitro testing of 63 clinically approved drugs identified 25 compounds with cytotoxic activity for the resistant leukemia cells. Further, 4 drugs were identified that showed a differential activity for Ibrutinib-sensitive and -resistant cells. Future work will now focus on deciphering molecular mechanisms of Ibrutinib resistance, including transcriptome data of Ibrutinib-resistant CLL patients. In addition, preclinical trials of promising compounds will be performed in the Eµ-TCL-1 adoptive transfer mouse model of CLL.