GU81 improves that the anti-tumor efficacy of doxorubicin. MMTV-PyMT mice (age 6 weeks) were treated 19 days with vehicle alone (control), doxorubicin (dox, 2 mg/kg 1×/week via ip injection in saline), GU81 (260 μg/day via osmotic pump), or GU81 + doxorubicin (GU81 + dox). A) Tumor volume is displayed as sum of total measurable tumor burden. *, p < 0.05; **, p < 0.01; ***, p < 0.005 vs control, 2-way ANOVA. B) Total tumor weight was calculated at time of sacrifice (age 60 days). Only GU81 + doxorubicin-treated tumors were significantly smaller than control treated tumors (p ≤0.01, 1-way ANOVA).