The Pan-PIM Kinase Inhibitor LGB321 Affects Apoptotic Pathways and Microenvironmental Interactions in CLL

Introduction

The emergence of kinase inhibitors like Ibrutinib has drastically altered treatment strategies and improved outcomes in CLL patients, but lack of cure and resistance to therapy still remain serious problems. The three PIM kinases are involved in various important disease mechanisms in CLL, with PIM1 regulating CXCR4 surface expression impacting its interaction with the microenvironment, and PIM2/3 affecting the apoptotic machinery by regulating BAD. The Pan-PIM kinase inhibitor LGB321 (Novartis) targets all three PIM kinases and therefore affects both, CLL apoposis and its interaction with the microenvironment.

Results

In the study presented here, we investigated the effect of the Pan-PIM kinase inhibitor LGB321 on CLL in vitro and in vivo. LGB321 was highly effective in inducing apoptosis in primary human CLL cells, independent of risk factors or the mutation status. Apoptosis induction correlated with reduced pBAD and BAD levels. LGB321 was also effective in the presence of protective stromal cells and could completely overcome the stroma protective effects. Furthermore, we found that Pan-PIM inhibitor treatment blocked the CXCR4/CXCL12 axis by dephosphorylating the CXCR4 receptor on Ser339, by reducing total CXCR4 protein levels, and by blocking the externalization of the CXCR4 receptor. Concordantly, LGB321 blocked CXCR4 functions like migration towards a CXCL12 gradient (P<.0001), and reduced homing of LGB321-pretreated primary CLL cells towards the bone marrow (P=.0001) of NOG mice. In vivo experiments comfirmed the efficacy of LGB321 in 4 different CLL xenograft studies. Transplantation of primary human CLL cells into NOG mice and treatment with LGB321 for 2 weeks strongly reduced WBC counts, spleen size and spleen infiltration with human CLL cells (P=.0295) in all four CLL cases, and blocked BAD as well as CXCR4 phosphorylation also in vivo.

Conclusion

Our results demonstrate, that the Pan-PIM kinase inhibitor LGB321 might be an effective treatment option for CLL patients by impairing PIM2/3 mediated CLL-cell survival, and by blocking the PIM1/CXCR4-mediated interaction of CLL cells with their protective microenvironment in vitro and in vivo. Future clinical trials should be performed to validate its efficacy in human CLL.