The PIM Kinase Inhibitor K00135 Sensitizes Primary Acute Lymphoblastic Leukaemia (ALL) Cells to DNA-Damaging Agents

Abstract 3238

Relapsed ALL is the most common cause of death in children, with only 30% being cured despite intensified treatment and stem cell transplantation. We previously showed that a subset of paediatric ALL tumours have a defect in the apoptotic response to DNA damage in vitro which correlates with a poor clinical response in vivo. Proviral integration site for Moloney murine leukaemia virus (PIM) proteins are a family of serine/threonine kinases which mediate prosurvival signals in response to cytokines, growth factors, hypoxia and DNA damage. PIM kinases have been implicated in several haematological malignancies such as chronic myeloid leukaemia (CML), chronic lymphocytic leukaemia (CLL), acute myeloid leukaemia (AML), and T-ALL. However, the role of PIM kinases and their contribution to treatment resistance in a large cohort of primary paediatric ALL has not been comprehensively addressed. To address the frequency of PIM overexpression in ALL we used quantitative real-time PCR to quantify the levels of PIM-1 and PIM-2 in an unselected cohort of 86 primary ALL tumours compared to the PIM-expressing lymphoid cell line K562. We observed high PIM-1 levels in only 2/86 (2.3%) ALLs, whereas PIM-2 was highly expressed in 20/86 (23%) primary tumours. PIM-2 overexpression was detected in tumours with a range of chromosome translocations or hyperdiploidy. In representative samples, the level of PIM-2 protein expression correlated with abundance of mRNA transcript. We then investigated the role of PIM overexpression on apoptosis using the highly specific PIM inhibitor, imidazo[1,2-b]pyridazine K00135, to sensitize tumour cells with high PIM expression to ionising radiation (IR)-induced apoptosis. Pretreatment with K00135 led to sensitization of representative primary tumour cells with high PIM-2 expression to 5Grays IR at an half maximal lethal concentration (LC50) of 10μM, and killing was mediated by caspase-dependant apoptosis. Furthermore, consistent with the role of PIM-2 in the regulation of c-Myc protein stability (Zhang Y, Wang Z, Li X, Magnuson NS. 2008; Oncogene; 27;4890-4819), K00135-induced killing coincided with the downregulation of c-Myc protein. We are currently addressing the impact of PIM inhibition in an ALL xenograft model. Constitutive expression of PIM-2 may be an underlying mechanism of treatment resistance in a subset of paediatric ALL. Pharmacological inhibition of PIM-2 may prove to be a useful therapeutic option for poorly responding ALL in children in the future.