Recently, Roche announced major positive news: the U.S. Food and Drug Administration has officially accepted the New Drug Application (NDA) for its investigational oral selective estrogen receptor degrader (SERD), giredestrant, in combination with everolimus.
This application targets adult patients with ER-positive, HER2-negative, ESR1-mutant locally advanced or metastatic breast cancer who have experienced recurrence or disease progression following prior endocrine therapy. The submission is based on the pivotal Phase III evERA breast cancer study.
The data demonstrate that, compared with standard-of-care treatment, the giredestrant combination regimen delivers substantial clinical benefit:
- In the intention-to-treat (ITT) population, the risk of disease progression or death was reduced by 44%
- In the ESR1-mutant subgroup, the risk reduction reached as high as 62%
The FDA is expected to issue a final decision by December 18, 2026.
Estrogen receptor (ER)-positive breast cancer accounts for approximately 70% of all breast cancer cases. Although “CDK4/6 inhibitors combined with endocrine therapy” has become the standard treatment paradigm, around 70% of patients with HR+/HER2- disease still eventually develop resistance.
Against this backdrop, precision targeting of key driver genes has emerged as the “master key” to overcoming therapeutic resistance, with the ESR1 gene standing at the very center of this next wave of breakthroughs.
The ESR1 gene encodes estrogen receptor alpha (ERα), a member of the steroid hormone receptor family. The primary function of ERα is to specifically bind estrogen and subsequently regulate cellular biological processes. ESR1 is widely expressed in female reproductive tissues, including the uterus, ovaries, and breast, where it plays a critical role in growth, development, differentiation, and functional regulation.
At the molecular level, upon binding to estrogen, ERα undergoes a conformational change. The resulting receptor–ligand complex translocates into the nucleus, where it binds to specific DNA sequences and regulates downstream gene transcription. This process influences key physiological events such as cell cycle progression, cellular proliferation, and apoptosis.
In the context of breast cancer, ESR1 holds a central role. Approximately 70% of breast cancers are ER-positive, and the growth of these tumors is typically driven by estrogen signaling. For patients with ER-positive disease, endocrine therapies—such as tamoxifen and aromatase inhibitors—remain a cornerstone of treatment, offering substantial clinical benefit and relatively favorable prognosis.
However, ESR1 mutations represent a major challenge during treatment. Approximately 3% of treatment-naïve patients and up to 30% of patients following endocrine therapy harbor activating ESR1 mutations, which are key drivers of endocrine resistance. These mutations are heterogeneous, including gene amplification, rearrangements, and point mutations, with point mutations being the most prevalent. Mutation hotspots are primarily located in exon 8, particularly at residues D538 and Y537.
These alterations result in constitutive activation of ERα in the absence of estrogen, thereby rendering endocrine therapies ineffective. In advanced disease, the prevalence of ESR1 mutations can reach 30%–60%, and these mutations are strongly associated with poorer clinical outcomes and specific metastatic patterns, such as increased propensity for lung metastasis.
For the ESR1 target, Kyinno Biotechnology has developed a comprehensive panel of ESR1 mutant cell lines, ESR1 reporter cell lines, and ESR1-HiBiT reporter cell lines. These platforms enable multidimensional evaluation of ESR1-targeting therapeutics, including pharmacological efficacy, transcriptional activity, and protein degradation. A list of representative cell lines and example applications is provided below. Please feel free to contact us for more information.
Cell Line Panel:
- KC-5870 MCF7-ESR1-Y537S-KI
- KC-5896 MCF7-ESR1-D538G-KI
- KC-5897 MCF7-ESR1-Y537S-KI
- KC-6048 MCF7-ESR1-D538G-KI
- KC-6049 MCF7-ESR1-G521S-Y537S-KI
- KC-5513 293T-GRE-Luc2-ESR1
- KC-5420 MCF7-ESR1-HiBiT-KI (+/−)
- KC-5421 MCF7-ESR1-HiBiT-KI (+/−)
Case Study: Validation of ESR1 mutant cell lines
Validation of ESR1 Reporter Cell Lines
Validation of ESR1-HiBiT Cell Lines
