Nearly 75% of breast tumors express estrogen receptor (ER), and will be treated with endocrine therapy, such as selective estrogen receptor modulator (SERM), tamoxifen, or aromatase inhibitors. Despite their proven success, as many as 40-50% of ER+ tumors fail to respond to endocrine therapy and eventually recur as aggressive, metastatic cancers. Therefore, preventing and/or overcoming endocrine resistance in ER+ tumors remains a major clinical challenge. Deregulation or activation of the nuclear factor kappa B (NF kappa B) pathway has been implicated in endocrine resistance and poor patient outcome in ER+ tumors. As a consequence, one option to improve on existing anti-cancer treatment regimens may be to introduce additional anti-NF kappa B activity to endocrine therapy drugs. Our approach was to design and test SERM-fumarate co-targeting hybrid drugs capable of simultaneously inhibiting both ER, via the SERM, raloxifene, and the NF kappa B pathway, via fumarate, in breast cancer cells. We find that the hybrid drugs display improved anti-NF kappa B pathway inhibition compared to either raloxifene or fumarate. Despite some loss in potency against the ER pathway, these hybrid drugs maintain anti-proliferative activity in ER+ breast cancer cells. Furthermore, these drugs prevent clonogenic growth and mammosphere formation of ER+ breast cancer cells. As a proof-of-principle, the simultaneous inhibition of ER and NF kappa B via a single bifunctional hybrid drug may represent a viable approach to improve the anti-inflammatory activity and prevent therapy resistance of ER-targeted anti-cancer drugs.