Targeting Bone Marrow Microenvironment for Treatment of High-Risk B-Cell Acute Lymphoblastic Leukemia

Introduction

B-cell acute lymphoblastic leukemia (B-ALL) is the most common form of cancer in children, accounting for approximately 20% of all pediatric cancers. Although treatment of B-ALL represents one of the success stories of modern medicine with 5-year survival rates approaching 90%, some children continue to have a dismal prognosis, particularly those with high-risk genetic alterations in their ALL cells or those who relapse. The leukemia microenvironment is an attractive novel therapeutic target, especially for children with high-risk leukemia as dose-limiting toxicities of conventional chemotherapeutic agents have prevented further improvements in survival.

Aims

In this study, we investigate whether therapeutically targeting the bone marrow (BM) osteoclasts can improve treatment outcome for B-ALL and the mechanisms of crosstalk between osteoclasts and leukemia cells.

Methods

We have established syngeneic and patient-derived xenograft (PDX) models for high-risk B- ALL that recapitulate the clinical symptoms of bone loss. For in vitro experiments, leukemia cells were cultured with osteoclast-derived conditioned media (CM) for 3 days.

Results

To evaluate the therapeutic potential of targeting osteoclasts during leukemia development, we administered zoledronic acid (2ug 5 days/week for 2 weeks), a bisphosphonate inhibitor of osteoclastic bone resorption, alone and in combination with imatinib (100mg/kg daily for 28 days) or dasatinib (10mg/kg daily for 28 days) in our BCR-ABL1 ⁺ B-ALL syngeneic mouse model. We showed that zoledronic acid treatment significantly reversed leukemia-induced bone loss by attenuating osteoclast activity. Zoledronic acid alone was found to enhance event-free survival in leukemia-bearing mice and further improvement was shown in combination with imatinib (p<0.05) or dasatinib (p<0.01). In addition, we administered zoledronic acid alone and in combination with vincristine, dexamethasone, and L-asparaginase (VXL) in a relapsed B-ALL PDX model. We further demonstrated that zoledronic acid alone or in combination with VXL improved survival compared to mice treated with vehicle (p<0.001) or VXL alone (p<0.01), respectively. In the clinical setting, we assessed the feasibility and safety of incorporating zoledronic acid early into leukemia therapy in three children with B-ALL. The patients were undergoing induction or consolidation chemotherapy and were administered zoledronic acid at the same time. Zoledronic acid was well tolerated without any significant adverse effects, indicating the feasibility of safely administering zoledronic acid in combination with chemotherapy in children with ALL.

To gain mechanistic insight into the effect of osteoclasts on leukemia cells, we compared the molecular profile of the leukemia cells cultured with CM and control media using RNA sequencing. There were 30 differentially expressed genes identified at the 5% false discovery rate with a log fold-change value ≥1 or ≤-1. KEGG pathway analysis revealed a significant difference in cell cycle pathway in leukemia cells cultured with CM compared to control. Biological process analysis showed a positive regulation of B-cell proliferation and negative regulation of lymphocyte apoptotic process in leukemia cells cultured with CM compared to control. To functionally examine the effect of osteoclasts on leukemia cells, we showed that osteoclast-derived CM significantly increased B-ALL cell proliferation, as demonstrated by increased cell number (p<0.001), as well as reduced cell percentages in G0/G1 phases and increased S+G2/M phases (p<0.05) via cell cycle analyses. Using annexin V and DAPI staining, we further demonstrated that there were more live cells and less apoptotic and dead cells when the leukemia cells were cultured with CM compared to control (p<0.001). Together, our findings suggest that soluble factors produced by osteoclasts promote leukemia cell proliferation and survival.

Conclusion

Our study reveals osteoclasts play a role in leukemogenesis and leukemia-induced bone loss, and that targeting osteoclasts can provide dual clinical benefit in terms of bone health and improved survival for children with high-risk B-ALL.