The IC50 values (Table 1) revealed that (i) modification of OA with hydroxyimino and lactone moieties (HIMOXOL and Br-HIMOLID, respectively) provoked a higher cytotoxicity effect in studied breast cancer cells, and (ii) blocking ER or EGFR receptors in breast cancer cells (MCF7/ER− and MDA-MB-231/EGFR− cells, respectively) decreased the sensitivity of both studied derivatives relative to the maternal compound, OA, while it provoked increased cytotoxicity of the two derivatives. Here, ESR1 is linked to breast cancer.