Pharmacology and Molecular Mechanisms of Clinically Relevant Estrogen Estetrol and Estrogen Mimic BMI-135 for the Treatment of Endocrine-Resistant Breast Cancer

Abstract

Long-term estrogen deprivation (LTED) with tamoxifen (TAM) or aromatase inhibitors leads to endocrine-resistance, whereby physiologic levels of estrogen kill breast cancer (BC). Estrogen therapy is effective in treating patients with advanced BC after resistance to TAM and aromatase inhibitors develops. This therapeutic effect is attributed to estrogen-induced apoptosis via the estrogen receptor (ER). Estrogen therapy can have unpleasant gynecologic and nongynecologic adverse events. Here, we study estetrol (E 4) and a model Selective Human ER Partial Agonist (ShERPA) BMI-135. Estetrol and ShERPA TTC-352 are being evaluated in clinical trials. These agents are proposed as safer estrogenic candidates compared with 17bestradiol (E 2) for the treatment of endocrine-resistant BC. Cell viability assays, real-time polymerase chain reaction, luciferase reporter assays, chromatin immunoprecipitation, docking and molecular dynamics simulations, human unfolded protein response (UPR) RT 2 PCR profiler arrays, live cell microscopic imaging and analysis, and annexin V staining assays were conducted. Our work was done in eight biologically different human BC cell lines and one human endometrial cancer cell line, and results were compared with full agonists estrone, E 2 , and estriol, a benchmark partial agonist triphenylethylene bisphenol (BPTPE), and antagonists 4-hydroxytamoxifen and endoxifen. Our study shows the pharmacology of E 4 and BMI-135 as less-potent full-estrogen agonists as well as their molecular mechanisms of tumor regression in LTED BC through triggering a rapid UPR and apoptosis. Our work concludes that the use of a full agonist to treat BC is potentially superior to a partial agonist given BPTPE's delayed induction of UPR and apoptosis, with a higher probability of tumor clonal evolution and resistance. SIGNIFICANCE STATEMENT Given the unpleasant gynecologic and nongynecologic adverse effects of estrogen treatment, the development of safer estrogens for endocrine-resistant breast cancer (BC) treatment and hormone replacement therapy remains a priority. The naturally occurring estrogen estetrol and Selective Human Estrogen-Receptor Partial Agonists are being evaluated in endocrineresistant BC clinical trials. This work provides a comprehensive evaluation of their pharmacology in numerous endocrineresistant BC models and an endometrial cancer model and their molecular mechanisms of tumor regression through the unfolded protein response and apoptosis.

Steroid Ligands, the Forgotten Triggers of Nuclear Receptor Action; Implications for Acquired Resistance to Endocrine Therapy

“…A possible explanation for this data is that diminished genomic activation of both ER and AR may be more permissive of non-canonical pathways. The reduced production of potent sex steroids, accompanied by a surfeit of weak precursor steroids in aging adults provides rationale for the benefit of therapeutic agents that drive genomic nuclear receptor activity (45,46).…”

Estrogen Receptor Functions and Pathways at the Vascular Immune Interface

“…Estetrol (E4) has shown such a pattern [ 118 ]: this agent is less potent than E2 and shows some tissue selectivity as it induces limited effects on the liver [ 119 ]. E4 is under clinical development for a few indications including contraception and breast cancer [ 120 , 121 ].…”

Estetrol and Mammary Gland: Friends or Foes?

“…In in vitro experiments performed on a long-term estrogen-deprived (LTED) human MCF7 breast cancer cell line [63] and in endocrine-resistant ER + breast cancer cells [61] E4 had pro-apoptotic properties similar to E2. In endocrine-resistant ER + breast cancer cells, these pro-apoptotic properties were associated with an ERα-dependent UPR [61]. Singer et al [64], reported that when E4 was administrated orally (20 mg/day) for 2 weeks in women with recently diagnosed breast cancer, immunohistochemical assessment of tumors revealed an increased number of apoptotic cells but no modulation of the proliferative marker Ki67, when compared to placebo.…”

“…However, there is a reluctance to use E2 due to the potential for adverse effects, especially thromboembolism [99][100][101][102][103][104]. The pro-apoptotic properties of E4 on endocrine-resistant breast cancer demonstrated in vitro [61] and in clinical trials [64,65], in addition to the limited effect of E4 on liver factors involved in coagulation [79], support a potential role for E4 in the treatment of advanced endocrine-resistant breast cancer in postmenopausal women.…”