Doxorubicin directs the accumulation of interleukin-12 induced IFN gamma into tumors for enhancing STAT1 dependent antitumor effect. 
To examine the mechanism by which doxorubicin plus interleukin-12 (IL-12) gene transfer induces enhanced therapeutic efficacy against tumors.  Tumor-bearing mice were treated with doxorubicin, IL-12-encoding plasmid DNA, doxorubicin plus IL-12-encoding plasmid DNA, or plasmid DNA control.  Doxorubicin was systemically given via i.p.  injection, and IL-12 was systemically expressed via i.m.  injection.  To show that doxorubicin enhances the accumulation of IL-12-induced IFN gamma into tumors and the signal transducer and activator of transcription 1 (Stat1)-dependent antitumor efficacy, the distribution of IFN gamma and the therapeutic end points, such as T-cell infiltration, inhibition of tumor vessel density, tumor growth inhibition, and inhibition of spontaneous tumor metastasis in wild-type and Stat1(-/-) host and tumors were determined after the treatment at the indicated time points.  In this study, a novel mechanism was unveiled.  We discovered that doxorubicin enhances the accumulation of IL-12-induced IFN gamma in tumors.  The doxorubicin-mediated accumulation of IFN gamma in tumors is caused by an increased accumulation of IFN gamma-secreting immune cells and not by a direct translocation of IFN gamma protein into tumors.  Depletion of immune cells reverses the doxorubicin-mediated accumulation of IFN gamma into tumors and reverses the inhibition of tumor vessel density induced by coadministration of doxorubicin and IL-12 DNA.  Knocking out IFN gamma signaling in the tumor host reverses the significant inhibition of tumor growth by coadministration of doxorubicin and IL-12.  The enhanced antitumor efficacy by coadministration of doxorubicin and IL-12 is dependent on the accumulation of IFN gamma in tumors.  This discovery provides a possible strategy to reduce side effects caused by IL-12.