CD44 Monoclonal Antibody-Enhanced Clearance Of Chronic Myeloid Leukemia Stem Cells From The Malignant Niche

Leukemia stem cells (LSC) in chronic myeloid leukemia (CML) are important in disease progression and relapse. Conventional therapy with tyrosine kinase inhibitors (TKIs), although very effective at reducing bulk CML cells, frequently fail to eliminate LSC residing in the protective bone marrow niche. Thus, there is an unmet need for combination therapy that simultaneously targets bulk disease and eliminates LSC in order to prevent disease progression and relapse. Stem cell niches are often rich in hyaluronic acid. CD44, the main receptor for hyaluronic acid, and some of its splice variants are frequently overexpressed on cancer stem cells, including LSC. In this study we aimed to evaluate the in vivo anti-LSC activity of CD44 monoclonal antibody (mAb) RG7356.

For anti-CD44 LSC inhibition studies, immunodeficient RAG2^-/-γ_c^-/- neonatal mice were intrahepatically transplanted with human BC LSC from imatinib responsive and resistant patients. Once engraftment was verified, mice were treated with control IgG1 (30mg/kg/week), dasatinib (25mg/kg/day), CD44 mAb (RG7356) (30mg/kg/week) or a combination of CD44 mAb (RG7356) and dasatinib. The mice were sacrificed after two weeks of therapy and bone marrow, spleen, peripheral blood and myeloid sarcomas were analyzed by FACS to assess CML LSC burden. In the imatinib responsive patient, CD44 mAb single agent therapy showed an 85% reduction of LSC in bone marrow, 98% reduction in spleen and 91% reduction in peripheral blood. CD44 mAb therapy thus showed an efficacy equal to that of dasatinib, which showed an 87%, 98% and 98% reduction in bone marrow, spleen and peripheral blood respectively. Combination therapy (CD44 mAb + dasatinib) completely eradicated any trace of CML LSC in all tissues analyzed. In the imatinib resistant patient, dasatinib very effectively reduced the number of myeloid sarcomas and blasts but did not inhibit LSC survival in bone marrow. In contrast, CD44 mAb single agent therapy effectively reduced the number of CML LSC to 51% in bone marrow, 96% in spleen and 93% in peripheral blood.

This study demonstrates that a human CD44 specific mAb, RG7356, significantly reduces CML LSC survival. Notably, TKI resistance of CML LSC, can be overcome by treatment with a human CD44 specific mAb, RG7356, as it sensitizes CML LSC residing in malignant niches to dasatinib. From these results, RG7356 appears to be an excellent antibody for future combination clinical studies aimed at eradicating CML.

Runza:employee of Roche Diagnostics GmbH: Employment. Weigand:Roche Diagnostics GmbH: Employment. Jamieson:J&J: Research Funding; Sanofi: Consultancy; Roche: Research Funding.