Inhibition of Galactosylceramide Synthesis Abolishes the Resistance of Mammary Carcinoma Tumors to Doxorubicin Therapy

Background: Drug resistance remains a significant obstacle in cancer therapy, particularly in patients with advanced-stage disease. In addition to developing more effective chemotherapeutic agents, considerable research has focused on compounds capable of reversing drug resistance. These agents, commonly referred to as drug resistance modulators, are not chemotherapeutic drugs themselves but can restore sensitivity in resistant cancer cells when administered in combination with chemotherapy. Previous studies have shown that the accumulation of galactosylceramide (GalCer) in breast cancer (BC) cells, resulting from the overexpression of galactosylceramide synthase (UGT8), increases resistance to anticancer drugs compared with GalCer-negative counterparts. Since drug resistance continues to be a major challenge in BC therapy, we propose the use of small-molecule inhibitors targeting UGT8 to suppress GalCer biosynthesis, thereby enhancing the sensitivity of BC cells to conventional chemotherapeutics. Methods: We previously demonstrated that murine 4T1 mammary carcinoma cells overexpressing murine UGT8 (UGT8a) exhibit increased resistance to doxorubicin (DOX), similar to human BC cells, in contrast to control 4T1 cells lacking UGT8 expression. As proof of concept for the hypothesis that UGT8 inhibition can sensitize BC cells to chemotherapy, we treated 4T1.UGT8a cells with DOX, both in vitro and in vivo, either alone or in combination with a free or liposomal formulation of the UGT8 inhibitor 3-(methylcarbamoyl)-7-(trifluoromethyl)thieno[3,2-b]pyridin-5-yl)-piperidin-4-yl-(S)-1-(trifluoromethoxy)propan-2-ylcarbamate (Inhibitor 19). Results: Our results showed that the liposomal formulation, but not free Inhibitor 19, significantly inhibited GalCer synthesis, which enhanced the antitumor efficacy of DOX in DOX-resistant, GalCer-producing mammary carcinoma cells overexpressing UGT8a. These findings confirm that, in breast cancers characterized by high UGT8 expression and GalCer accumulation, the use of small-molecule inhibitors may improve the outcomes of chemotherapy.