JNK1: An emerging therapeutic target in Chronic Lymphocytic Leukemia (CLL) downstream of the B-cell receptor, which overcomes ibrutinib resistance

Background: B‐cell receptor (BCR) inhibitors like ibrutinib are highly effective in the treatment of CLL, but frequent occurrence of resistance mutations limits treatment duration and causes relapses in many patients. The C‐Jun NH2‐terminal protein kinases 1 (JNK 1) belongs to the family of serine/threonine kinases, is involved in cell proliferation, survival and differentiation, and is located downstream of the BCR in CLL. Aims: Therefore, we aimed to validate JNK1 as crucial mediator of BCR signaling in CLL. Further, we wanted to validate JNK1 as a therapeutic target in ibrutinib‐resistant CLL. Methods: Patient derived xenograft mice from CLL patients and ibrutinib‐resistant CLL patients, and Eμ‐TCL1 tg mice were daily treated with the JNK1 inhibitor SP600125 using oral gavage. After 2 and 3 weeks of treatment, mice were sacrificed, and spleen and bone marrow cells were harvested and stained for CLL cells (BD). Results: In this study, we found that JNK1 is overexpressed and highly activated especially in IgVH unmutated B‐CLL lymphocytes with adverse prognosis, and overactive BCR signaling. JNK inhibition with the kinase inhibitors SP600125 and CC‐930, the substrate binding inhibitor protein L‐JNKi or siRNA mediated silencing all lead to apoptosis induction in IgVH unmutated primary human CLL samples. JNK1 inhibition was effective even in the presence of protective stroma cells, and resulted in down‐regulation of the anti‐apoptotic proteins BCL2 and MCL1, and the oncogenic transcription factor c‐JUN. By performing knockdown experiments and kinase inhibition of essential upstream BCR‐signaling mediators in CLL like SYK, BTK and PI3K, we identified JNK1 as a crucial downstream kinase in the BCR signaling pathway. In vivo treatment studies in patient derived xenograft mouse models and the Eμ‐TCL1 tg mouse model with the JNK1 inhibitor SP600125 confirmed our results and strongly reduced CLL cell numbers in the peripheral blood and spleen of both mouse models. JNK1 inhibition was even effective in a CLL xenograft model of a patient being resistant to ibrutinib treatment. In addition to direct effects on CLL cells, we found that JNK1 inhibitors in vivo also improve T‐cell responses, and reduce the frequency of immunosuppressive Tregs, while increasing the activity of CD4+ and CD8+ T‐cells. Summary/Conclusion: In conclusion, our studies revealed JNK1 as a promising novel therapeutic target in CLL downstream of the BCR, which can overcome ibrutinib resistance, and additionally improve immune responses against CLL cells.