The acute and chronic cardiotoxicity and cytotoxicity of the novel doxorubicin (DXR) derivative 4'-amino-3'-hydroxy-DXR were compared with those of 4'-deoxy-DXR and DXR. In the acute cardiotoxicity study, the ECG and hemodynamic changes recorded in anesthetized rats that had been treated i.v. with 10 mg/kg 4'-amino-3'-hydroxy-DXR or 8.6 mg/kg 4'-deoxy-DXR were significantly less severe than those caused by 13 mg/kg DXR. In the chronic cardiotoxicity study, rats received 3 weekly i.v. injections of 3 mg/kg DXR, 3 mg/kg 4'-amino-3'-hydroxy-DXR, or 2 mg/kg 4'-deoxy-DXR during the first 14 days of the study and were observed for an additional 35-day period. DXR induced severe cardiomyopathy that was characterized by ECG changes in vivo (S alpha T-segment widening and T-wave flattening) and by impairment of the contractile responses (Fmax, +/- dF/dtmax) to adrenaline of hearts isolated from treated animals. 4'-Deoxy-DXR caused a progressive enlargement of the S alpha T segment in vivo and a significant impairment of the -dF/dtmax value in vitro, which were less severe than those produced by DXR. The least cardiotoxic drug was 4'-amino-3'-hydroxy-DXR, which induced minor ECG changes without causing significant alterations in the contractile responses of isolated hearts to adrenaline. On the basis of the drug concentration required to inhibit 50% of the colony formation (IC50) of cell lines in vitro, 4'-amino-3'-hydroxy-DXR was less active than 4'-deoxy-DXR but at least twice as active as DXR against human cancer and murine transformed cell lines. These data indicate that 4'-amino-3'-hydroxy-DXR is significantly less cardiotoxic and more cytotoxic than DXR.