Novel (Z)-endoxifen-related new chemical entities exhibit potent anti-cancer activity in ERα+ breast cancer.

Approximately 80% of breast cancers express estrogen receptor alpha (ERα), yet ERα-positive disease remains associated with high recurrence and late relapse. (Z)-Endoxifen (ENDX), the most abundant active metabolite of tamoxifen, is a potent, orally bioavailable, selective estrogen receptor modulator currently in development for breast health indications. During its synthesis, structurally related compounds are generated as byproducts termed here as new chemical entities (NCEs): AT416E, AT416Z, AT402E, AT402Z, and AT300, which have not been previously studied. We evaluated these NCEs alongside ENDX using multiple ERα-positive breast cancer cell lines, including models harboring clinically relevant ESR1 mutations. Activity was assessed using 2D and 3D proliferation, apoptosis, cell cycle profiling, migration, invasion, ERα transcriptional activity, RNA sequencing and synergy with abemaciclib. Several NCEs exhibited potent anti-estrogenic and anti-cancer effects. In various settings, these compounds matched or exceeded ENDX's activity, highlighting their therapeutic promise. RNA sequencing analysis revealed both shared and compound-specific transcriptional programs. Some NCEs also combined favorably with abemaciclib, producing additive to synergistic effects comparable to or greater than ENDX. Together, our findings support further investigation of select NCEs for ERα-positive breast cancer, particularly for tumors with ESR1 activating mutations and as potential second- or third-line options for recurrent disease.