It has been shown that a small, residual pool of leukemic CD34+ progenitor cells can survive in the marrow microenvironment of chronic myeloid leukemia (CML) patients after years of kinase inhibitor treatment. Bone marrow stroma contributes to the expansion and proliferation of both transformed as well as normal hematopoietic cells, and has been implicated in the long-term survival of leukemic cells. We previously demonstrated the stromal protection of leukemic cells from the anti-proliferative effects of nilotinib, and identified stromal-derived viability factors, including IL-6 and GM-CSF, as possibly mediating stromal protection of tyrosine kinase inhibitor-treated leukemic cells (Weisberg et al. Mol Cancer Ther 2008;7:1121). Additionally, we have found high leukemia burden in the tissues of nilotinib-treated mice that have significant sources of hematopoiesis-promoting stroma, suggesting that significant reservoirs for tumor growth may be tissues that are able to support normal and malignant hematopoietic stem cell development. These studies revealed a leukemia distribution pattern consistent with that observed in imatinib or nilotinib-treated patients.
One strategy for overriding stromal-mediated chemoresistance is the use of inhibitors of the stroma-derived factor (SDF-1a) receptor, CXCR4, which mediates the migration of hematopoietic cells to the bone marrow and plays a key role in leukemic cell-stromal cell interactions. Studies have shown small-molecule CXCR4 inhibitors, such as plerixafor, to be effective in mobilizing hematopoietic cells from bone marrow and enhancing chemotherapy- and tyrosine kinase inhibitor-induced apoptosis of bone marrow stroma-protected leukemic cells in vitro and in vivo. Thus, imatinib has been shown to up-regulate CXCR4, which induces CML migration to the bone marrow microenvironment and leads to stroma-mediated chemoresistance of quiescent CML progenitor cells.
Utilizing a functional in vivo assay system that allows monitoring of the growth of progressive disease, as well as baseline level (“residual” disease) resulting from treatment with a moderate-to-high dose of nilotinib, we investigated the ability of stem cell mobilization to enhance the efficacy of nilotinib by suppressing leukemia recurrence following nilotinib treatment. Specifically, 32D.p210-luc+ cells were injected into the tail-vein of mice, which were then imaged 10 days later to determine baseline bioluminescence and quantify tumor-burden. Mice were subsequently treated by oral gavage for 10 days with nilotinib (75 mg/kg qd) and reimaged (“Induction Phase” of treatment); mice were at this stage considered to have minimal residual disease, with reduced tumor burden > 2 logs. Mice were then divided into four treatment groups (“Consolidation Phase” of treatment addressing minimum residual disease) with similar mean bioluminescence (n=8 or 9 per group): Group 1: Vehicle (PEG300 po), Group 2: plerixafor (5 mg/kg sq qd), Group 3: nilotinib (75mg/kg po qd), and Group 4: Combination (plerixafor+nilotinib).
Nilotinib was highly efficacious in reducing disease burden in leukemia-engrafted mice. However, with continued treatment, animals developed resistance to nilotinib with increasing disease burden despite continued therapy. Although plerixafor had no single-agent activity, combination with nilotinib significantly delayed time to relapse, and significantly prolonged survival when compared to nilotinib monotherapy (p<0.0001). Since plerixafor had no monotherapeutic efficacy, there results demonstrate that plerixafor, at a well-tolerated dose, acts synergistically with nilotinib to suppress the growth of 32D.p210 leukemia.
We demonstrated the ability of plerixafor to delay the onset of recurring BCR-ABL-positive disease in mice carrying an extremely low tumor burden following treatment with a moderate-to-high dose of nilotinib. These results support the idea of using stem cell mobilization in conjunction with targeted tyrosine kinase inhibition to override drug resistance and suppress or eradicate residual disease.
Manley:Novartis Pharma AG: Employment. Kung:Novartis Pharmaceuticals: Consultancy, Research Funding. Griffin:Novartis Pharmaceuticals: Consultancy, Research Funding.
Asterisk with author names denotes non-ASH members.