Endoxifen E-isomer hydrochloride is the hydrochloride salt of the (E)-stereoisomer of endoxifen, the primary active metabolite of tamoxifen responsible for most of its therapeutic effects in breast cancer treatment. It is primarily utilized in oncology research and investigational therapies for estrogen receptor-positive (ER+) breast cancer. As a potent selective estrogen receptor modulator (SERM), it potently inhibits the growth of ER+ breast cancer cells by antagonizing estrogen receptor signaling. This compound supports therapeutic strategies to reduce cancer recurrence risk and improve survival rates. It is also explored in personalized medicine, tailored to patient genetics (e.g., CYP2D6 polymorphisms) and metabolism. In research, it aids in studying estrogen receptor signaling mechanisms and resistance to hormonal therapies.