Anticancer agents are used for cancer therapy. Studies on the biological response to treatment with an agent facilitate its effective use. Eribulin mesylate (eribulin) is an anticancer agent. In this study, we found that c-Fos is upregulated in response to eribulin treatment in the triple-negative breast cancer cell lines MDA-MB-231 and HCC70, which have low eribulin sensitivity. c-Fos expression was not upregulated in other cell lines investigated, including high eribulin-sensitive cells. We hypothesized that c-Fos upregulation is involved in low eribulin sensitivity and thus used the c-Fos inhibitor, T-5224. In MDAMB-231 and HCC70 cells, combined treatment with eribulin and T-5224 showed a stronger anticancer effect than treatment with eribulin alone in cell growth assays, cell death assays and a mouse xenograft tumor model, whereas T-5224 alone showed no anticancer effect. These results suggest that T-5224 may enhance the anticancer effect of eribulin. Our findings contribute to the improvement of cancer therapy. (C) 2020 Elsevier Inc. All rights reserved.