Despite that new therapeutic strategies are currently being developed to treat the different tumor types; chemotherapy continues to be widely used for advanced breast cancer treatment. On the flip side, this therapy is often nonspecific and induces significant side effects. Thus, the development of drug delivery methods such as liposomes aimed to increase tumor specific effects, increasing therapeutic efficacy and sustained and controlled release of the therapeutic drug reducing systemic side effects has gained relevance. Microspheres can be administered locally either for drug delivery or they can also be used as embolic agents. At the Atomic Center Bariloche, sulfonated polyvinyl alcohol microspheres (SPVA; 35 µm) were designed and loaded with doxorubicin (Doxo-SPVA). To test the effect of these doxorubicin-loaded microspheres in an in vivo scenario we selected the luminal C4-HI tumor model which grows in BALB/c mice and was previously used to compare the effect of free doxorubicin vs doxorubicin pegylated liposomes. Fourteen days after subcutaneous tumor transplantation, female mice were treated with Doxo-SPVA (2 mg, locally), free doxorubicin (40 µg, locally), empty SPVA (2mg, locally), or remained untreated (n=5-7/group). Mice were treated once a week for 3 weeks. Tumors were measured with a caliper twice a week. One week after the third dose was administered tumors were excised and weighed. The tumors treated with Doxo-SPVA microspheres were significantly smaller than untreated mice (p<0.05). The therapeutic effect was similar to that obtained with the systemic dose proving the efficacy of the local administration. Future experiments will focus on studying whether the slow and continuous release of doxorubicin from the Doxo-SPVA microspheres produces an improved therapeutic effect in the long term compared to free or pegylated liposome doxorubicin.