Lipid association improves the therapeutic index of lomustine [1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea] to suppress 36B-10 tumor growth in rats.

The therapeutic efficacy and tumor accumulation of a liposome formulation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), an effective agent used in the treatment of malignant brain tumors, was examined in an animal tumor model. Pharmacokinetic studies in normal and tumor-bearing rats indicated that a 2-fold greater plasma exposure was achieved with liposome-formulated CCNU compared with the free drug. In Fisher rats bearing s.c. tumors 36B-10, tumor growth was delayed substantially when liposomal CCNU was delivered compared with free-drug treatment. In single-dose treatments of 20, 35, and 50 mg/kg, tumor progression after each dose was reduced approximately 2-fold with liposomal compared with free CCNU (four animals in each treatment group). Multiple-dose treatments (given as three weekly doses with eight animals in each treatment group) with cumulative doses of 80 and 100 mg/kg of free and liposomal CCNU also resulted in a 2-fold reduction in tumor progression when compared with free-drug treatment. When drug levels in tumors relative to plasma were examined, it was observed that tumor drug concentrations did not exceed those found in plasma after administration of free CCNU; after administration of liposomal CCNU, however, tumor concentrations exceeded those in plasma by nearly 10-fold. These results suggest that the increased efficacy of liposome-formulated CCNU may be attributable to enhanced drug accumulation in tumor tissues.