Recently, there has been a steady stream of good news in the field of breast cancer treatment. In May 2025, Xanzhu Biotech’s independently developed Class 1 anti-tumor drug, Pirosil Tablets, was approved for market, providing a new option for HR+/HER2- breast cancer patients. In the same month, Zhejiang Cancer Hospital issued the first domestic prescription of Inalise Tablets, a precision targeted therapy for PIK3CA-mutant breast cancer, filling a treatment gap. However, nearly 70 percent of HR+/HER2- breast cancer patients treated with the combination of CDK4/6 inhibitors and endocrine therapy still face drug resistance. In this context, precisely targeting key genes has become central to breaking the therapeutic bottleneck, with the ESR1 gene being a major focus.
The ESR1 gene encodes estrogen receptor alpha (ERα), a member of the steroid hormone receptor family. ERα primarily functions by binding specifically to estrogen, thereby regulating cellular biological behavior. In female reproductive organs such as the uterus, ovaries, and breasts, ESR1 is expressed and deeply involved in growth, development, differentiation, and functional regulation. At the molecular level, once ERα binds estrogen, it undergoes a conformational change, and the resulting complex enters the nucleus, binds to specific DNA sequences, regulates downstream gene transcription, and affects physiological processes such as cell cycle progression, cell proliferation, and apoptosis.
ESR1 holds a pivotal position in breast cancer. About 70 percent of breast cancers are ER positive, and these tumors typically rely on estrogen stimulation for growth. For ER positive patients, endocrine therapies such as tamoxifen and aromatase inhibitors are key treatment options, with generally good efficacy and relatively favorable prognosis. However, ESR1 gene mutations present a notable problem during treatment. Roughly 3 percent of untreated patients and 30 percent of patients after endocrine therapy carry activating ESR1 mutations, which are a major driver of endocrine resistance. ESR1 mutations vary, including amplifications, rearrangements, and point mutations, with point mutations being the most common. Hotspot mutations cluster in exon 8 at sites such as D538 and Y537. These mutations keep ERα persistently activated even in the absence of estrogen, causing endocrine therapy failure. In advanced disease, ESR1 mutation rates can reach 30 to 60 percent and are strongly associated with poorer prognosis and specific metastatic patterns such as lung metastasis.
Pipeline Overview of ESR1-related Drug Development
As a powerful research tool, the MCF7-ESR1-HiBiT-KI cell line integrates a HiBiT tag (an 11 amino acid peptide) that enables highly sensitive bioluminescent detection of ERα expression, providing an ideal platform for drug screening. With this model, scientists can track ERα protein stability, degradation dynamics, and drug intervention effects in real time, accelerating the development of novel degraders and offering new hope for overcoming ESR1-mutant breast cancer.
ESR1-HIBIT Cell lines list:
- MCF7-ESR1-HiBiT-KI(+/-)-1A1
- MCF7-ESR1-HiBiT-KI(+/-)-1A2
- siRNA degradation validation
- Sequencing validation of MCF7-ESR1-HiBiT cells
