BY13 is an SRC-3 PROTAC degrader with a DC50 of 0.031 μM. It selectively obstructs the ER signaling pathway by downregulating ERα levels, showing greater selectivity over the androgen receptor (AR). BY13 effectively addresses endocrine resistance in breast cancer by inducing cell cycle arrest at the G1 phase and triggering apoptosis. Additionally, it surpasses Fulvestrant in efficacy and significantly inhibits the growth of resistant breast tumors in LCC2 xenograft mouse models, exhibiting no noticeable toxicity.

SRC-3:0.031 μM (DC50)

BY13 reduces SRC-3 and ERα protein levels in MCF-7 cells in a dose-dependent manner, with 0.1-10 μM reducing them by 71% and 85% after 24 hours. At 0.01-10 μM for 36 hours, BY13 effectively inhibits the proliferation of wild-type, mutant, and resistant breast cancer cells, achieving IC50 values of 0.003-0.35 μM for MCF-7, LCC2, and MCF-7 D538G/Y537S/EGFR cells, and degrades SRC-3 and ERα in mutant cells, particularly in MCF-7 Y537S cells at 0.1 μM. When applied at 0.1-10 μM over 3-48 hours, BY13 effectively reduces SRC-3 and ERα protein levels in LCC2 cells, outperforming Fulvestrant and achieving maximum degradation at 36 hours with marginal further increase over time. At 0.01-20 μM for 36 hours, BY13 significantly decreases SRC-3 (DC50 0.031 μM) and ERα protein levels in MCF-7 cells, showing preference for the SRC-1 subtype over SRC-2. BY13 at 0.01-5 μM for 24 hours downregulates AR protein levels in MCF-7 cells, exhibiting a weaker effect compared to ERα, and moderately inhibits AR overexpression in LNCaP cells (IC50 1.43 μM). At 1 μM for 36 hours, BY13 significantly reduces SRC-3 protein levels in MCF-7 cells via the ubiquitin-proteasome system (UPS), similarly observed in LCC2 cells. Additionally, BY13 at 1 μM for 6 hours under temperatures of 40-76°C enters tumor cells, binds directly to SRC-3, and significantly enhances SRC-3 protein thermal stability in MCF-7 cells. At 10 μM for 6 hours, BY13 induces the spatial proximity of SRC-3 and CRBN in MCF-7 cells, promoting the formation of the SRC-3-BY13-CRBN ternary complex. BY13 at 0.01-10 μM for 36 hours increases SRC-3 mRNA expression while reducing ERα mRNA levels in LCC2 cells as concentration increases. Between 1-20 μM over 48 hours, BY13 induces significant apoptosis in breast cancer cells, enhancing both early and late apoptosis in MCF-7 and LCC2 cells. With doses of 5-20 μM for 48 hours, BY13 causes dose-dependent G1 phase arrest in LCC2 cells, with a noticeable increase in cell proportion relative to the S phase. BY13, at concentrations between 0.01-100 μM, displays adequate metabolic stability and acceptable safety, with IC50 values for CYP3A4 and hERG channels of 2.73 and 1.1 μM, respectively.

BY13 (3-10 μM/kg, intraperitoneal injection every other day for 23 days) exhibits potent anti-endocrine resistance activity by targeting the degradation of SRC-3 and ERα in the LCC2 xenograft mouse model. It demonstrates high safety and improves the poor prognosis of endocrine-resistant breast cancer.