Evaluation of some anthracycline antibiotics in an in vivo model for studying drug-induced human leukemia cell differentiation

Human acute promyelocytic leukemia cells (HL-60) were induced to undergo terminal differentiation by treatment in vivo with marcellomycin. This was accomplished by cloning HL-60 cells in 0.3% agar in diffusion chambers, which were subsequently implanted in the peritoneal cavities of mice. These animals were given injections i.v. with anthracycline antibiotics, and the chambers were transferred to a second recipient 24 hr later. After an additional 8 days, the chambers were removed, the cloning efficiency was determined, and colonies were scored for the presence of differentiated cells based on their ability to reduce nitro blue tetrazolium (NBT). Marcellomycin produced dose-de-pendent decreases in cloning efficiency and increases in the number of differentiated cells present in chambers. At all dose levels of marcellomycin tested, three types of colonies were observed; these were colonies consisting entirely of undifferentiated cells and approximately equal numbers of colonies consisting of solely differentiated cells and those with mixtures of both differentiated and undifferentiated cells. An 8-fold increase in colonies containing differentiated cells (both pure and mixed) was observed after a single injection of marcellornycin (10 mg/kg), a dose which reduced cloning efficiency by 40%. At that dosage level, aclacinomycin A also induced differentiation, while doxorubicin was ineffective, a finding consistent with the effects of these anthracyclines on HL-60 cells in suspension culture. In addition to the functional changes accompanying differentiation, commitment was characterized by a limitation in proliferative potential. Thus, the average size of uniform NBT-positive colonies was approximately 16 cells, and few clones of NBT-positive cells greater than 32 cells in number were observed; this contrasted with NBT-negative clones which contained up to 100 cells. This finding suggests that HL-60 cells undergo an average of four and a maximum of five divisions upon commitment to granulocytic differentiation. The in vivo system described may be useful in further evaluation of differentiation-inducing agents for therapeutic potential after an initial in vitro screen to identify active compounds.