Here, we investigate various factors affecting the therapeutic efficacy of free doxorubicin (Free-Dox) and polyethylene glycol (PEG)-coated (PEGylated) liposomal doxorubicin (referred to as Doxil) in the ascitic J6456 lymphoma model of BALB/c mice. Free drug and liposomal drug were affected differently by the tumor burden and route of treatment administration. A delay in start of treatment from day 1 to day 5 almost completely abolished the efficacy of Free-Dox, whereas that of Doxil was only minimally reduced. Contrasting effects on the therapeutic efficacy of Free-Dox and Doxil were obtained by changing treatment administration from the i.v. to the i.p. route; the efficacy of free drug was relatively enhanced, whereas that of liposomal drug was relatively diminished. Overall, Doxil given by the systemic i.v. route was the most effective treatment in prolonging median survival and obtaining cures. Variations in the dose-schedule treatment regime confirm the superior therapeutic profile and reduced dependence on tumor burden of the PEGylated liposomal formulation over free drug. In addition, these experiments indicate that, at equal dose intensity, the dose level is more important than the frequency of administration for therapeutic activity.

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