Tamoxifen resistance is a major roadblock in the treatment of patients with breast cancer. Ribonucleotide reductase M2 (RRM2) was found to be involved in acquired resistance of breast cancer cells (BCCs) to tamoxifen. Here, we used GW8510, which has been identified as a potential RRM2 inhibitor, to evaluate the effect of RRM2 inhibition on reversing resistance of BCCs to tamoxifen and investigate its mechanisms. We showed that RRM2 overexpression played a key role in the development of acquired tamoxifen resistance in BCCs through downregulation of autophagy level. Combination treatment with tamoxifen and GW8510 significantly inhibited survival of the tamoxifen-resistant BCCs through induction of autophagic cell death compared to either of the two drugs. Furthermore, combination of tamoxifen and GW8510 resulted in marked growth inhibition of tamoxifen-resistant BBC xenograft tumor in vivo compared to tamoxifen or GW8510 alone. In conclusion, tamoxifen in combination with GW8510 can overcome acquired tamoxifen resistance in BCCs and may be a rational therapeutic approach against breast cancer with high RRM2 expression. (C) 2020 Elsevier Inc. All rights reserved.