PF157 THERAPEUTIC TARGETING OF CYTOKINE INDUCED PIM1 ACTIVATION IN T‐CELL ACUTE LYMPHOBLASTIC LEUKEMIA AND LYMPHOMA
Authors: R. De Smedt, S. Peirs, M. Van Trimpont, J. Morscio, J. Roels, L. Reunes, B. Lintermans, T. Lammens, N. Van Roy, B. De Moerloose, Y. Benoit, J.P. Meijerink, S. Goossens, P. Van Vlierberghe
Journal: HemaSphere (2019)

Background: PIM1 is an oncogenic kinase that recently emerged as an interesting novel therapeutic target for the treatment of T‐cell acute lymphoblastic leukemia and lymphoma (T‐ALL/T‐LBL). Indeed, aberrant levels of PIM1 have been identified in primary T‐ALL/T‐LBLs with T‐cell receptor driven PIM1 translocations or activating mutations targeting IL7R/JAK/STAT signaling. However, based on these cell‐intrinsic mechanisms, the absolute number of T‐ALL/T‐LBL patients that might benefit from PIM inhibition remains limited. Aims: Besides intrinsic molecular genetic defects, non‐cell autonomous mechanisms might also be able to drive therapeutically relevant aberrant signaling in T‐ALL/T‐LBL. Indeed, recent work revealed that interleukin‐7 (IL7) induced JAK/STAT signaling could be targeted by JAK inhibitors, irrespective of the cell‐intrinsic IL7R/JAK/STAT mutational status of the T‐ALL/T‐LBL patient sample. With this in mind, we here aimed to study the potential mechanisms as well as the therapeutic relevance of cytokine induced PIM1 activation in human T‐ALL/T‐LBL. Methods: We performed ex vivo stimulation experiments on patient derived xenograft cells from T‐ALL and T‐LBL samples using a panel of cytokines. Phospho‐STAT5 levels were analyzed by flow cytometry and PIM1 levels were determined by qPCR and western blot analysis. T‐ALL or T‐LBL patients samples that showed cytokine induced PIM1 activation were subsequently used for preclinical in vivo evaluation of the PIM inhibitor PIM447 in immunocompromised mice both as monotherapy as well as in combination with glucocorticoids. Results: Here, we show that specific hematopoietic cytokines, such as interleukin‐3 (IL3), IL7, stem cell factor (SCF) and FLT3 ligand, are able to induce PIM1 expression in specific primary T‐ALL and T‐LBL patient samples. As expected, this induction capacity is based on the pattern of cytokine receptor expression (IL3R, IL7R, KIT or FLT3) on the leukemic blasts and is therefore linked to the mature stage of the respective leukemic T‐ALL/T‐LBL sample. Indeed, IL7 mediated effects were observed in a broad panel of T‐ALL/T‐LBLs, whereas IL3, SCF and FLT3LG driven effects on PIM1 induction were more restricted to immature T‐ALLs. Interestingly, we subsequently used patient derived xenografts to show that cytokine induced PIM1 activation renders T‐ALL/T‐LBL cells susceptible to in vivo treatment with PIM447, a PIM inhibitor that is currently in clinical trials for the treatment of acute myeloid leukemia. Of note, these in vivo anti‐leukemic effects upon PIM inhibition were also observed for primary T‐ALL/T‐LBL patient samples that initially displayed low PIM1 levels at diagnosis. In line with this notion, paired analysis of diagnostic and xenografted material from these T‐ALL/T‐LBL samples revealed a profound increase in PIM1 expression upon xenotransplantation. Finally, we also confirmed that in cytokine induced T‐ALL/T‐LBL samples, PIM447 treatment displays a strong synergy with glucocorticoids. Summary/Conclusion: In conclusion, we show that non‐cell autonomous mechanisms can cause aberrant PIM1 activation in primary T‐ALL/T‐LBL through IL3, IL7, SCF or FLT3LG stimulation. We show that cytokine induced PIM1 activation renders leukemic blasts susceptible for in vivo PIM inhibition with synergistic anti‐leukemic effects observed for the combination with glucocorticoids. Therefore, our results suggest that the patient population that might benefit from combined PIM/glucocorticoid inhibition therapy extends beyond T‐ALL/T‐LBL samples that display cell‐intrinsic PIM1 activation.