Zoledronate Inhibits Leukemia Growth in Bone Marrow and Synergizes with Imatinib Mesylate Against Ph⁺ Primary Leukemic Cells.

Imatinib mesylate has drastically changed Philadelphia chromosome positive (Ph⁺) leukemia treatment. However, remissions induced in advanced phase Ph⁺ leukemias tend to be short-lived even treated with imatinib. Combined therapy of imatinib with an agent which inhibits downstream signaling of BCR/ABL such as Ras is intriguing. Phase I studies of farnesyl transferase inhibitors (FTIs), which targeted farnesylation of Ras, with imatinib against Ph⁺ leukemias demonstrated limited combination effects. We previously reported that the third-generation bisphosphonates (BPs) zoledronate (ZOL) synergized with imatinib mesylate to inhibit Ph⁺ leukemia. BPs which inhibit both farnesylation and geranylgeranylation of Ras proteins may be more potent than FTIs. However, three important questions have remained unanswered to promote this attractive combination to a clinical trial. Concentrations of 20-30 μM ZOL are required in vitro to induce apoptosis in leukemic cell lines. However, peak serum concentrations after infusion of 4 mg ZOL were 1–2 μM. Therefore, it is important to investigate how ZOL achieves effective concentrations in vivo. The second is whether ZOL augments the effects of imatinib against not only cell lines but also primary cells from Ph⁺ leukemia patients. In addition, the safety of this combination therapy must be established, because the dose of ZOL used in our previous study was high compared with clinical doses administered for inhibition of bone lesions. To answer these questions, we herein investigated about the combination therapy of ZOL plus imatinib using NOD/SCID mice engrafted with primary Ph⁺ leukemic cells with or without mutations in the ATP binding site. ZOL inhibited the geranylgeranylation of Ras proteins of leukemic cells in bone marrow but not in peripheral blood, indicating that sufficient concentration of ZOL could be achieved in bone marrow. ZOL synergized with imatinib to enhance survival of mice engrafted with primary Ph⁺ leukemic cells without mutations and this combination was well-tolerated without any severe adverse effects. These findings suggest that ZOL inhibits leukemia growth in bone marrow and the combination of ZOL plus imatinib is a potential therapy for Ph⁺ leukemia patients.