FLUOROURACIL SELECTIVELY SPARES ACUTE MYELOID-LEUKEMIA CELLS WITH LONG-TERM GROWTH ABILITIES IN IMMUNODEFICIENT MICE AND IN CULTURE

A subset of leukemic cells is assumed to maintain long-term growth of acute myeloid leukemia (AML) in vivo. Characterization of these AML pr ogenitor cells may further define growth properties of human leukemia. In vitro incubations with 5-fluorouracil (5-FU) have been used for en richment of normal primitive hematopoietic stem cells. By analogy to n ormal hematopoiesis, it was hypothesized that primitive leukemic stem cells might be kinetically more inactive than colony-forming cells (co lony-forming units-AML [CFU-AML]). To examine this hypothesis, conditi ons were established for incubation with 5-FU that eliminated all CFU- AML. These conditions selected a 5-FU-resistant AML fraction that was evaluated for its capacity for long-term growth by transplantation int o mice with severe combined immunodeficiency (SCID) and long-term cult ure in the quantitative cobblestone area-forming cell (CAFC) assay. Tr ansplantation of the 5-FU-resistant fraction of four cases of AML into SCID mice resulted in growth of AML, Whereas no CFU-AML survived, 31% to 82% of primitive (week-6) CAFC were recovered from the 5-FU-treate d cells, Hematopoietic cells proliferating in the CAFC assay were show n to be leukemic by cytologic, cytogenetic, or molecular analysis, The reduction of AML growth as determined by outgrowth of AML in SCID mic e was in the same order of magnitude as the primitive (week-6) CAFC re duction, This indicates that both assays measure closely related cell populations and that the CAFC assay can be used to study long-term gro wth of AML, These results show a hierarchy of AML cells that includes 5-FU-resistant progenitors. These cells are characterized as primitive (week-6) CAFC and at leukemia-initiating cells in SCID mice. (C) 1996 by The American Society of Hematology.