Discovery and Characterization of PVTX-321 as a Potent and Orally Bioavailable Estrogen Receptor Degrader for ER+/HER2– Breast Cancer

Estrogen receptor α (ERα) is a key therapeutic target in ER+/HER2– breast cancer, but ESR1 mutations drive resistance to endocrine therapies. Heterobifunctional degraders (HBDs) targeting ERα offer a promising strategy to overcome this resistance. Here, we report PVTX-321 ( 16a ), a potent ER HBD derived from a novel spirocyclic cereblon ligand and an ERα binder. PVTX-321 achieves a DC 50 of 0.15 nM in MCF-7 cells and acts as a strong antagonist (IC 50 = 59 nM), suppressing proliferation in ERα+ cell lines, including mutant variants (Y537S, D538G). It demonstrates favorable oral bioavailability, dose-dependent ERα degradation in vivo and induces tumor regression at 10 mg/kg (QD) in MCF-7 xenografts. With minimal CYP inhibition and a strong preclinical safety profile, PVTX-321 is a promising candidate for advancing ER+/HER2– breast cancer treatment.