New directions for ER+ breast cancer treatment: do estrogen receptor gene mutations hold the key?


Christopher Morrow and Teresa Klinowska

Growing understanding of mutations in the estrogen receptor gene (ESR1) in women with estrogen receptor positive (ER+) breast cancer is fuelling treatment innovation for those whose disease progresses after initial hormone therapy.

Newly published research carried out by IMED scientists and collaborators led by researchers at the Memorial Sloan Kettering Cancer Center in New York, has significantly extended previous knowledge about the impact of ESR1 mutations on tumour progression.1 It has also provided valuable insights into how such mutations can be addressed by current and future anti-estrogen therapies.

For over four decades AstraZeneca has been at the forefront of advances in understanding of ER biology and development of the selective estrogen receptor modulators (SERMS), aromatase inhibitors and selective estrogen receptor degraders (SERDs) which have contributed to substantial survival improvements for many women with ER+ breast cancer.2-4 Today, our research in ER biology is focusing on the unmet needs of the significant population of women with ER+ breast cancer who unfortunately remain resistant to current treatment.


ESR1 mutations – where and when?

The estrogen receptor is the key driver of ER+ breast cancer, and anti-estrogen therapy is the mainstay of treatment for the over 70% of women with breast cancer whose tumours express ER.1 This approach greatly reduces the risk of recurrence of early stage disease and improves outcomes for those with advanced ER+ breast cancer.2,5 However, a significant proportion of ER+ tumours develop resistance to anti-estrogen therapy, leading to disease progression, and recent research has suggested that mutations in the estrogen-binding domain of ESR1 promote a receptor conformation that is active, even in the absence of estrogen.1,6 In initial small studies, most of these mutations were at residues Y537 and D538. 

To gain a broader understanding of activating ESR1 mutations and their implications for therapies aimed at overcoming anti-estrogen resistance, our new research, published in Cancer Discovery, analysed ESR1 DNA sequences in the largest cohort tested to date of nearly 1000 metastatic breast tumours.1 Approximately 10% of these tumours were found to have ESR1 mutations and in all but one case these were in the domain where estrogen binds to the receptor. The most frequent mutations were D538G, Y537S, E380Q, Y537C, Y537N and L536H, and just over two-thirds of women whose tumours exhibited mutations had received previous anti-estrogen treatment with aromatase inhibitors.  


Impact of ESR1 mutations

To explore the impact of these ESR1 mutations, cells overexpressing ESR1 mutations were generated which demonstrated ESR1 mutations conferred estrogen-independent ER activity and cancer cell proliferation. The results from the overexpression studies were elegantly confirmed when researchers in Discovery Sciences at our IMED Biotech Unit used state-of-the-art CRISPR gene editing to generate a cell line expressing endogenous levels of ESR1 Y537S. Our Bioscience researchers at IMED Oncology then confirmed estrogen-independent ER activity and growth in a breast cancer cell line. Additional studies showed that ESR1 mutations capable of promoting estrogen-independent activity were able to induce ER conformational stabilisation capable of facilitating binding of the co-activating protein, steroid receptor co-activator 3. Together, these results support the ability of ESR1 mutations to induce estrogen-independent activity of the ER as a possible mechanism for resistance to estrogen deprivation, with the Y537S mutation appearing to be most active.


Relevance to current breast cancer therapy

To address the relevance of ESR1 mutations to treatment options for ER+ breast cancer, the in vitro effects of the SERM, tamoxifen, and the SERDs, fulvestrant and AZD9496, on ESR1 Y537S cells were compared. Tamoxifen and other SERMs attach to the estrogen-binding domain of ER and alter its structure and function to interrupt gene expression and cell division.7 In contrast, SERDs, such as fulvestrant and AZD9496, bind to the ER and degrade it.7

Whilst cell line work suggested that mutated ER was less sensitive to SERMs and SERDs than wild-type ER, fulvestrant and AZD9496 suppressed growth of mouse xenograft models expressing ESR1 Y537S, D538G, E380Q and S463P  mutations, , though fulvestrant was less effective than AZD9496 against xenografts with ESR1 Y537S mutations. We also examined the effects of fulvestrant and AZD9496 in a proprietary AstraZeneca/MedImmune patient derived xenograft bearing an ESR1 D538G mutation. Both agents slowed tumour growth, with slightly enhanced efficacy with AZD9496.

Preliminary clinical data have shown the potential of ctDNA ESR1 biomarker testing in women with ER+ advanced breast cancer and demonstrated a tolerable safety profile for AZD9496 and positive clinical responses including in patients previously treated with fulvestrant.8


Future directions

Estrogen deprivation and receptor antagonism and degradation are the mainstay of ER+ breast cancer therapy and, as in many areas of cancer research, growing understanding of receptor mutation biology related to treatment resistance is opening up new directions for drug discovery. Our data raise the possibility that some mutations may be more effective in promoting resistance than others, making them priority targets for future therapies.

Our initial findings with SERDs suggest their potential in ESR1 mutation-targeted therapy. But considerable research is needed to confirm the relative roles of different mutations in development of resistance to current hormone therapies and how these can be addressed by next generation SERDs as monotherapy and in combination with other novel therapies, such as CDK4/6 and PI3K/AKT inhibitors.

At AstraZeneca, we remain at the cutting edge of the rapidly evolving science of ER biology, collaborating with researchers at some of the finest laboratories in the world, and striving to optimise the design of innovative new therapies for women with advanced ER+ disease whose treatment options are currently limited.


1.     Toy W, Weir H, Razavi P et al. Activating ESR1 mutations differentially impact 1 the efficacy of ER antagonists. Cancer Discovery Dec 2016. pii: CD-15-1523. [Epub ahead of print]

2.     Early Breast Cancer Trialists' Collaborative G. Effects of chemotherapy and hormonal therapy for early breast cancer on recurrence and 15-year survival: an overview of the randomised trials. Lancet. 2005;365(9472):1687-717.

3.     Howell A, Cuzick J, Baum Met al; ATAC Trialists' Group. Results of the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial after completion of 5 years' adjuvant treatment for breast cancer. Lancet. 2005 Jan 1-7;365(9453):60-2.

4.     Robertson JF, Bondarenko IM, Trishkina E et al. Fulvestrant 500 mg versus anastrozole 1 mg for hormone receptor-positive advanced breast cancer (FALCON): an international, randomised, double-blind, phase 3 trial. Lancet. 2016 Dec 17;388(10063):2997-3005

5.     Mouridsen H, Gershanovich M, Sun Y et al. Phase III study of letrozole versus tamoxifen as first-line therapy of advanced breast cancer in postmenopausal women: analysis of survival and update of efficacy from the International Letrozole Breast Cancer Group. Journal of clinical oncology : official journal of the American Society of Clinical Oncology. 2003;21(11):2101-9.

6.     Chandarlapaty S, Chen D, He W et al. Prevalence of ESR1 Mutations in Cell-Free DNA and Outcomes in Metastatic Breast Cancer: A Secondary Analysis of the BOLERO-2 Clinical Trial. JAMA oncology. 2016;2(10):1310-687 1315.

7.     Carroll JS. Mechanisms of oestrogen receptor (ER) gene regulation in breast cancer. European Journal of Endocrinology 2016; 175, R41–R49

8.     Hamilton E, Patel M, Armstrong A et al. A phase I study of AZD9496, a novel oral, selective estrogen receptor degrader (SERD) in women with estrogen receptor positive, HER-2 negative advanced breast cancer (ABC). Presented at San Antonio Breast Cancer Symposium 2016