Endocrine therapy resistance in breast cancer

Hormonal status in breast cancer Globally, breast cancer is the second most common type of cancer, comprising 11.6% of total worldwide cancer cases in 2018. In the United Kingdom, breast cancer is the most common cancer type, affecting around 1 in 8 women. It can be loosely divided into non-invasive, where the cancer is localized to the ducts or lobules in which it originated (in-situ), or invasive, where the cancer cells have spread beyond the initial duct or lobule into the surrounding breast tissue or other part of the body. Risk factors that may predispose to breast cancer include increasing age and a family history of breast cancer. Moreover, breast cancer risk is heightened following use of the combined oral contraceptive pill and combined hormone replacement therapy. Another receptor that often plays a role in breast cancer, although it is not a hormone receptor, is the human epidermal growth factor receptor 2 (HER2). The overexpression of HER2 is determined by immunohistochemistry (IHC), or with fluorescent in situ hybridization in those equivocal cases where IHC does not provide a clear result. Different molecular subtypes of breast cancer have also been described, which loosely align with receptor status: • Luminal A (ER and/or PR positive; HER2 negative) • Luminal B (ER and/or PR positive; HER2 positive) • HER2-enriched (ER/PR negative; HER2 positive) • Basal like (triple negative). Additionally, cancers can be ER-/PR+ or ER+/PR-, but these are unnamed and relatively rare. The receptor status of a cancer is assessed for all breast cancers as it has important implications on prognosis of the patient. It also dictates the treatment given: cancers that do express ER are likely to respond to endocrine therapy, but this type of therapy will have no effect on triple-negative breast cancers. Mechanism of estrogen receptor action 75% of breast cancers are ER+. ER is a transcription factor containing a DNA-binding domain which allows binding DNA at specific sequences called estrogen response elements (EREs), defined as 5’-GGTCAnnnTGACC-3’, where "n" refers to any nucleotide. The N-terminal of the ER holds the activation function 1 (AF1) domain; the AF2 domain of the ER is contained within the ligand binding domain. The AF2 domain contains binding sites for coactivators. Estrogen is a steroid hormone and can cross the cell membrane freely. It can then bind to an ER, which is located in the cytoplasm of the cell. Upon binding, the ligand-receptor complex dimerizes with another, and this homodimer moves into the nucleus. Binding of an estrogen ligand exposes a site on the AF2 domain for coactivators to attach to the ER. == Endocrine therapy overview ==

Estrogen Estrogens are responsible for the regulation of the female reproductive system and development of secondary sexual characteristics. 17β-Estradiol is the biologically active form of the sex hormone known as estrogen. It is secreted by granulose cells of the ovarian follicle and the corpus luteum, and is the primary form of circulating form of estrogen. Blocking estrogen synthesis Firstly, some forms of endocrine therapy can be administered to block estrogen synthesis. In pre-menopausal women, this can be achieved through surgical ovarian ablation (by oophorectomy) or chemical ovarian suppression (using luteinizing hormone releasing hormone agonists, such as goserelin). In postmenopausal women, the ovaries cease to be the main source of estrogen production, with estrogen being instead synthesized from regulatory steroid hormones called androgens in tissues of the bone, fat, and breast through the activity of the enzyme aromatase. This can be blocked through the administration of aromatase inhibitors (AIs), which can be reversible inhibitors (non-steroidal) such as anastrozole and letrozole, or irreversible (steroidal) inhibitors like exemestane. == Endocrine resistance overview ==

Definition of endocrine resistance The term endocrine resistance describes a resistance to estrogen signaling suppression. However, in many scientific studies endocrine resistance refers to resistance to estrogen or estrogen receptor suppression. If a patient with ER+ breast cancer develops endocrine resistance, the endocrine therapy used to treat the cancer will no longer be effective. Approximately 30-50% of ER+ breast cancer patients will relapse as a result of endocrine resistance, proving it to be a predominant challenge in the treatment of ER+ breast cancer patients. This highlights the need for new strategies to overcome endocrine resistance and better treat ER+ breast cancer. == Endocrine resistance mechanisms ==

ESR1 Modifications Mutations in the ESR1 gene have been associated with the development of resistance to endocrine therapy. The ESR1 gene codes for estrogen receptors (ERα and ERβ). Whilst it is still not fully understood how ERβ impacts breast cancer development, ERα is known to have a role in tumor growth and cell survival. The signaling pathways associated with the GPER protein involve tyrosine kinases (from the Src family). Additionally, there is crosstalk between the GPER and EGFR signaling pathways. Tamoxifen has been shown to increase expression of GPER by binding to its receptor. The signaling pathways initiated lead not only to tamoxifen resistance, but also a higher risk of metastasis and cell growth. The overall survival rate hence decreases for patients displaying high GPER expression. PR expression PR (progesterone receptor) is a gene expressed in around 50% of ER+ breast cancer patients. Clinical studies have shown that ER+/PR+ cases are more sensitive to endocrine therapy than ER+/PR- cases, suggesting that the loss of PR expression is associated with the development of endocrine therapy resistance. PR levels have been shown to be downregulated when there is activation of growth factors via the PI3K/Akt/mTOR pathway. HER2 over-expression Over-expression of HER2 is found to be present in around 30% of metastatic breast tumors and is associated with a poorer prognosis. Studies have demonstrated that there is crosstalk between HER2 and ER signaling pathways. Specifically, studies have shown there is crosstalk between HER2 and the ER coactivator A1B1 which could enhance the estrogen agonist activity of tamoxifen bound ER. Additionally, HER2 has physical associations with the cell membrane ER and interactions between them have been shown to block ER-initiated apoptosis. == Tackling endocrine resistance ==

When tackling endocrine resistance in breast cancer, the current strategies are focused around combining hormonal agents with drugs targeting several escape pathways, as outlined in the mechanisms of endocrine resistance section. The aim is to block all the tumor survival escapes. In trials conducted with a combination of anti-HER2 agents and an aromatase inhibitor, significant clinical benefit and improved progression-free survival have been observed. Many endocrine resistant breast cancers which don’t respond to aromatase inhibitors still depend on ER signaling. Selective Estrogen Receptor Degraders (SERDs) can be used in these cases as they destabilize the ER and act as pure antagonists at the ER receptor. An example SERD is fulvestrant which binds to the ER to block its dimerization and nuclear localization. The fulvestrant-ER complex is unstable, resulting in degradation. There has also been research into the combined inhibition of the ER and EGFR. An example of a pure EGFR inhibitor is gefitinib, which has been used in phase II trials to evaluate its addition to tamoxifen in patients with HR-positive advanced breast cancer. The PI3K-Akt-mTOR signaling pathway and crosstalk with the ER signaling pathway is thought to be involved in the development of resistance to endocrine therapy. Therefore, studies have been done to assess the efficacy of using mTOR inhibitors, such as everolimus or temsirolimus, in ER-positive breast cancers. Some preclinical and clinical studies have shown the possibility of mTOR inhibitors as a first step treatment to shrink the breast cancer tumor before the main treatment of an aromatase inhibitor is given. It is agreed that more clinical trials are needed to confirm this issue. Additionally, cyclin-dependent kinase (CDK) 4/6 inhibition has shown to improve the efficacy of endocrine treatment. == References ==