Biologic effects of prolonged melphalan treatment of murine long-term bone marrow cultures and interleukin 3-dependent hematopoietic progenitor cell lines.

The mechanism of alkylating agent-induced leukemia is unknown. For the determination of whether chronic alkylating agent treatment of hematopoietic stem cells in vitro was detectably leukemogenic, murine long-term bone marrow cultures (LTBMC) and clonal interleukin 3 (IL-3)-dependent multipotential hematopoietic progenitor cell lines [B6SUtA clone (cl) 27 and Ro cl 3-1] derived from LTBMC were chronically pulse treated in vitro with the alkylating agent melphalan [L-phenylalanine mustard (L-PAM)]. Weekly treatment of C3H/HeJ or CD-1 Swiss mouse LTBMC with 3 X 10(-6)M L-PAM significantly decreased cumulative production of nonadherent granulocytes and granulocyte-macrophage progenitor cells responsive to L-cell or WEH1-3 cell colony-stimulating factor compared to the production seen in untreated control cultures; it also significantly reduced the hematopoietic longevity (13 wk compared to greater than 20 wk for untreated control cultures). Weekly, twice weekly, or daily (3 X 10(-6)M) L-PAM treatment of IL-3-dependent cell lines induced gradual L-PAM adaptation in the absence of a detectable change in the maximum binding capacity of 125I-labeled IL-3. No leukemogenic variants of line B6SUtA cl 27 were detectably induced. However, 3 stably expressed marker chromosomes were induced after 12 months of L-PAM treatment of line B6SUtA cl 27. Thus IL-3-dependent hematopoietic progenitor cells slowly adapt to L-PAM when in suspension culture in vitro. Physiologic expression of drug toxicity in LTBMC may prevent this hematopoietic cell gradual adaptation.