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ABCB1 Modulation Does Not Circumvent Drug Extrusion from Primitive Leukemic Progenitor Cells and May Preferentially Target Residual Normal Cells in Acute Myelogenous Leukemia

Authors' Affiliation: Department of Hematology and Central Hematology Laboratory, University Medical Center Nijmegen, Nijmegen, the Netherlands

Requests for reprints: Marc H.G.P. Raaijmakers, Department of Hematology, University Medical Center Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, the Netherlands. Phone: 31-24-3614762; Fax: 31-24-3542080; E-mail: h.raaijmakers{at}hemat.umcn.nl.

Purpose: Acute myelogenous leukemia (AML) is a disease originating from normal hematopoietic CD34⁺CD38^– progenitor cells. Modulation of the multidrug ATP-binding cassette transporter ABCB1 has not resulted in improved outcome in AML, raising the question whether leukemic CD34⁺CD38^– cells are targeted by this strategy.

Experimental Design: ABCB1-mediated transport in leukemic CD34⁺CD38^– cells compared with their normal counterparts was assessed by quantitating the effect of specific ABCB1 modulators (verapamil and PSC-833) on mitoxantrone retention [defined as efflux index (EI), intracellular mitoxantrone fluorescence intensity in the presence/absence of inhibitor].

Results: ABCB1 was the major drug transporter in CD34⁺CD38^– cells in normal bone marrow (n = 16), as shown by the abrogation of mitoxantrone extrusion by ABCB1 modulators (EI, 1.99 ± 0.08). Surprisingly, ABCB1-mediated drug extrusion was invariably reduced in CD34⁺CD38^– cells in AML (n = 15; EI, 1.21 ± 0.05; P < 0.001), which resulted in increased intracellular mitoxantrone retention in these cells (mitoxantrone fluorescence intensity, 4.54 ± 0.46 versus 3.08 ± 0.23; P = 0.004). Active drug extrusion from these cells occurred in the presence of ABCB1 modulators in the majority of samples, pointing in the direction of redundant drug extrusion mechanisms. Residual normal CD34⁺CD38^– cells could be identified by their conserved ABCB1-mediated extrusion capacity.

Conclusion: ABCB1-mediated drug extrusion is reduced in leukemic CD34⁺CD38^– progenitor cells compared with their residual normal counterparts. Redundant drug transport mechanisms confer mitoxantrone transport from leukemic progenitors. These data argue that ABCB1 modulation is not an effective strategy to circumvent drug extrusion from primitive leukemic progenitor cells and may preferentially target residual normal progenitors in AML.

Commentary

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