Comparative in vitro toxicity of mitoxantrone and adriamycin in human granulocyte-macrophage progenitor cells.

Mitoxantrone (MIT) has recently been introduced into cancer therapy as a possible substitute for the structurally related drug, adriamycin (ADR), because it causes less cardiotoxicity and fewer gastrointestinal side effects. However, the dose-limiting toxicity of MIT is pronounced neutropenia. The in vitro hematoxicity of both drugs in granulocyte-macrophage precursor cells (GM-CFCs) was analyzed using drug-exposure schedules analogous to the principles of the in vivo pharmacokinetics of MIT. Bone-marrow and peripheral-blood cells were exposed to 0.075-20 ng/ml MIT or ADR for 5, 20, 60, and 120 min, and for 14 days. The 14-day exposure resulted in Do values of 0.95 and 0.68 ng/ml for bone-marrow and peripheral-blood GM-CFCs subjected to MIT. Exposure to ADR resulted in Do values of 5.43 and 5.13 ng/ml, respectively. As was the case after 14-day exposure to MIT or ADR, short-term exposure again revealed that peripheral-blood GM-CFCs were more sensitive to both drugs. Moreover, at low concentrations, ADR was less toxic than MIT in both types of GM-CFCs, but was more toxic than MIT when a concentration of 20 ng/ml was used. The intracellular concentration of MIT, as measured by high-performance liquid chromatography, was constantly below 1 ng per 2 X 10(7) cells, even when it was applied at a concentration of 20 ng/ml for an exposure time of 2 h. The fact that such low concentrations of MIT are toxic for hemopoietic precursor cells may explain the myelotoxicity of this drug. However, the difference between the precursor-cell toxicity of MIT and that of ADR was small when their respective therapeutic doses were taken into consideration. Further analyses of their toxicity in stem cells and/or the microenvironment would appear to be needed.