Comparative long-term study of the toxicities of free and liposome-associated doxorubicin in mice after intravenous administration.

The toxicities of free doxorubicin (F-DOX) and liposome-associated doxorubicin (L-DOX) were investigated in inbred BALB/c and outbred Sabra mice treated iv with 5, 7.5, and 10 mg doxorubicin (DOX)/kg body weight every 2 weeks up to 8 injections and observed for 6 months. Sonicated liposomes containing phosphatidylcholine, phosphatidylglycerol, and cholesterol were used. The lethal effect was reduced in mice treated with L-DOX as compared to mice treated with F-DOX. At a dose of 7.5 mg DOX/kg, 100% of mice receiving the L-DOX survived a cumulative dose of 60 mg/kg administered over 98 days, while 92% of mice receiving the F-DOX died. Two distinct patterns of death were observed: an acute phase type occurring early after injection of high doses of DOX and apparently related to gastrointestinal toxicity and a delayed phase type requiring a long latency after initial drug exposure and characterized by a complex pattern of abnormalities. Delivery of DOX by liposomes effectively protected against both types of lethal effects. Reduced toxicity of L-DOX resulted in reduced body and organ weight losses, reduced severity of pathologic changes, and fewer blood biochemical alterations. The pathological damage to the heart muscle found in mice treated with L-DOX was less severe than with F-DOX, and in some cases it was reversible. Nephrotoxicity was extremely frequent and severe among F-DOX-treated mice, while it was totally insignificant among L-DOX-treated mice. Hyperlipidemia, hypoglycemia, and glycogen-depleted hepatocytes were characteristic findings in mice treated with F-DOX. Altogether, the data obtained in this study indicate that liposomes significantly diminish the toxicity of DOX with the use of an intermittent schedule of chemotherapy. In addition to changes in tissue distribution as a mechanism of reduced toxicity, it is proposed that DOX associated with liposomal lipids interacts less efficiently than the free drug with target intracellular phospholipids.