Daunorubicin (DNR) is one of the most important cytotoxic agents in the treatment of acute myeloid leukemia (AML). Its use is usually limited by drug-induced cardiotoxicity depending on the cumulative dose administered. Liposomal encapsulation of DNR (DaunoXome, DNX) seems to reduce the risk of this severe side effect. To investigate the toxicity of DNX in heavily pretreated patients, we conducted a phase I trial, including patients (pts) older than 60 years with relapsed or refractory AML. DNX was used at doses of 40, 60, 75 and 90 mg/m(2), biweekly. Fourteen patients with a median age of 69 years (range, 63-77) were enrolled. A total of 49 courses of DNX were administered [3 pts at 40 mg/m(2) (for a total of 13 courses), 5 at 60 mg/m(2) (20 courses), 4 at 75 mg/m(2) (12 courses), and 2 at 90 mg/m(2) (4 courses)]. The mean cumulative dose of DNX administered was 340 mg (range, 120-1200). A 20% decline in the left ventricular ejection fraction (LVEF) without clinical signs and symptoms of heart failure was noted in 2 patients after a cumulative DNX dose of 480 mg, both with pre-existing heart disease. Even at the highest cumulative doses of DNX, no further decline in LVEF was noted. Nausea, vomiting, alopecia and mucositis were absent. All patients had significant myelosuppression requiring transfusion support. During treatment, 3 patients showed a 25% reduction of leukemic blasts in the bone marrow, 3 patients had to be excluded due to AML progression after the 2nd DNX course, and 7 patients died during the first 6 weeks of treatment. We conclude from these data that DNX offers a less toxic alternative to DNR and other anthracyclines. Using DNX dosages of 40 to 90 mg/m(2) biweekly seems to have little anti-leukemic activity in a patient population heavily pretreated with anthracyclines.