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Hormonal therapy plays a vital part in the treatment of estrogen receptor–positive (ER +) breast cancer. ER can be activated in a ligand-dependent and independent manner. Currently available ER-targeting agents include selective estrogen receptor modulators (SERMs), selective estrogen receptor degraders (SERDs), and aromatase inhibitors (AIs). Estrogen receptor mutation (ESR1 mutation) is one of the common mechanisms by which breast cancer becomes resistant to additional therapies from SERMs or AIs. These tumors remain sensitive to SERDs such as fulvestrant. Fulvestrant is limited in clinical utilization by its intramuscular formulation and once-monthly injection in large volumes. Oral SERDs are being rapidly developed to replace fulvestrant with the potential of higher efficacy and lower toxicities. Elacestrant is the first oral SERD that went through a randomized phase III trial showing increased efficacy, especially in tumors bearing ESR1 mutation, and good tolerability. Two other oral SERDs recently failed to achieve the primary endpoints of longer progression-free survival (PFS). They targeted tumors previously treated with several lines of prior therapies untested for ESR1 mutation. Initial clinical trial data demonstrated that tumors without the ESR1 mutation are less likely to benefit from the SERDs and may still respond to SERMs or AIs, including tumors previously exposed to hormonal therapy. Testing for ESR1 mutation in ongoing clinical trials and in hormonal therapy for breast cancer is highly recommended. Novel protein degradation technologies such as proteolysis-targeting chimera (PROTACS), molecular glue degrader (MGD), and lysosome-targeting chimeras (LYTACS) may result in more efficient ER degradation, while ribonuclease-targeting chimeras (RIBOTAC) and small interfering RNA (siRNA) may inhibit the production of ER protein.

Resistance to endocrine therapies in hormone receptor-positive breast cancer is challenging. We aimed to assess the next-generation oral selective oestrogen receptor degrader (SERD) and complete oestrogen receptor antagonist, camizestrant, versus the first-approved SERD, fulvestrant, in post-menopausal women with oestrogen receptor-positive, HER2-negative, advanced breast cancer. SERENA-2 is an open-label, randomised, phase 2 trial that is being conducted at 74 study centres across Asia, Europe, the Middle East, and North America. Female patients aged 18 years or older who were post-menopausal with histologically or cytologically confirmed metastastic or locoregional oestrogen receptor-positive, HER2-negative breast cancer, an Eastern Cooperative Oncology Group or WHO performance status of 0 or 1, and disease recurrence or progression on at least one line of endocrine therapy, and no more than one previous endocrine therapy in the advanced setting. Patients were initially randomly assigned (1:1:1:1) to receive oral camizestrant once daily at 75 mg, 150 mg, or 300 mg (until the 300 mg group was closed), or fulvestrant intramuscularly at 500 mg (per label). Randomisation was managed through an interactive web-based system and stratified by previous treatment with CDK4/6 inhibitors and presence of liver and/or lung metastases. The primary objective was to determine clinical efficacy of camizestrant versus fulvestrant at each dose level using the primary endpoint of investigator-assessed progression-free survival, per Response Evaluation Criteria in Solid Tumours (version 1.1), assessed by intention to treat in all randomly assigned patients (full analysis set). No formal statistical comparison for the efficacy analysis of the camizestrant 300 mg dose versus fulvestrant was to be performed. Safety analyses included all randomly assigned patients who received at least one dose of study treatment. This study is registered with ClinicalTrials.gov, NCT04214288, and is ongoing. Between May 11, 2020, and Aug 10, 2021, 240 patients were randomly assigned to receive camizestrant 75 mg (n=74), 150 mg (n=73), 300 mg (n=20), or fulvestrant (n=73), and were included in the full analysis set. All patients received at least one dose of study drug. Median follow-up was 16·6 months (IQR 12·9–19·4) for the camizestrant 75 mg group, 16·3 months (12·9–18·3) for the camizestrant 150 mg group, and 14·7 months (12·7–20·1) for the fulvestrant 500 mg group. Median progression-free survival was 7·2 months (90% CI 3·7–10·9) with camizestrant 75 mg, 7·7 months (5·5–12·9) with camizestrant 150 mg, and 3·7 months (2·0–6·0) with fulvestrant. The hazard ratio for camizestrant 75 mg versus fulvestrant was 0·59 (90% CI 0·42–0·82; p=0·017), and the hazard ratio for camizestrant 150 mg versus fulvestrant was 0·64 (0·46–0·89; p=0·0090). Treatment-related adverse events occurred in 39 (53%) of 74 patients in the camizestrant 75 mg group, 49 (67%) of 73 patients in the camizestrant 150 mg group, 14 (70%) of 20 patients in the camizestrant 300 mg group, and 13 (18%) of 73 patients in the fulvestrant group. No single grade 3 or worse treatment-emergent adverse event occurred in more than two (3%) patients in any group. Serious treatment-emergent adverse events occurred in six (8%) patients in the camizestrant 75 mg group, seven (10%) patients in the camizestrant 150 mg group, two (10%) patients in the camizestrant 300 mg group, and four (5%) patients in the fulvestrant group. No treatment-related deaths occurred. Camizestrant at 75 and 150 mg showed a significant benefit in progression-free survival versus fulvestrant. These results support further development of camizestrant for the treatment of oestrogen receptor-positive, HER2-negative breast cancer. AstraZeneca.

Patients with HER2-positive breast cancer who have received two or more previous therapies for advanced disease have few effective treatment options. The monarcHER trial aimed to compare the efficacy of abemaciclib plus trastuzumab with or without fulvestrant with standard-of-care chemotherapy of physician's choice plus trastuzumab in women with advanced breast cancer. This phase 2, three-group, open-label trial was done across 75 hospitals, clinics, and medical centres in 14 countries. Eligible patients were women aged 18 years or older, who had hormone receptor-positive, HER2-positive advanced breast cancer with unresectable, locally advanced, recurrent or metastatic disease, Eastern Cooperative Oncology Group performance status of 0 or 1, and who had previously received at least two HER2-targeted therapies for advanced disease. Patients were randomly assigned 1:1:1 to the abemaciclib, trastuzumab, and fulvestrant (group A), abemaciclib and trastuzumab (group B), or standard-of-care chemotherapy and trastuzumab (group C). Oral abemaciclib 150 mg 12 hourly was administered on days 1–21 of a 21-day cycle, intravenous trastuzumab 8 mg/kg on cycle 1 day 1, followed by 6 mg/kg on day 1 of each subsequent 21-day cycle, and intramuscular fulvestrant 500 mg on days 1, 15, and 29 and once every 4 weeks thereafter. Standard-of-care chemotherapy was administered as specified by the product label. Randomisation was by a computer-generated random sequence by means of an interactive web-response system and stratified by number of previous systemic therapies for advanced breast cancer and measurable versus non-measurable disease. The primary endpoint was investigator-assessed progression-free survival in the intention-to-treat population, first testing group A versus group C and, if this result was significant, then group B versus group C. Safety was assessed in all patients who had received at least one dose of study treatment. This trial is registered at ClinicalTrials.gov (NCT02675231) and is ongoing for long-term survival follow-up. Between May 31, 2016, and Feb 28, 2018, 325 patients were screened, of whom 237 eligible patients were enrolled and randomly assigned to groups A (n=79), B (n=79), and C (n=79). Median follow-up was 19·0 months (IQR 14·7–25·1). The study met its primary endpoint, showing a significant difference at the prespecified two-sided α of 0·2 in median progression-free survival between group A (8·3 months, 95% CI 5·9–12·6) and group C (5·7 months, 5·4–7·0; HR 0·67 [95% CI 0·45–1·00]; p=0·051). No difference was observed between median progression-free survival in group B (5·7 months, 95% CI 4·2–7·2) and group C (HR 0·94 [0·64–1·38]; p=0·77). The most common grade 3–4 treatment-emergent adverse event in groups A, B, and C was neutropenia (21 [27%] of 78 patients, 17 [22%] of 77, and 19 [26%] of 72). The most common serious adverse events were: in group A, pyrexia (three [4%]), diarrhoea (two [3%]), urinary tract infection (two [3%]), and acute kidney injury (two [3%]); in group B, diarrhoea (two [3%]) and pneumonitis (two [3%]); and in group C, neutropenia (four [6%]) and pleural effusion (two [3%]). Two deaths were attributed to treatment: one due to pulmonary fibrosis in group B and one due to febrile neutropenia in group C. The combination of abemaciclib, fulvestrant, and trastuzumab significantly improved progression-free survival versus standard-of-care chemotherapy plus trastuzumab while showing a tolerable safety profile. Our results suggest that a chemotherapy-free regimen might potentially be an alternative treatment option for patients with hormone receptor-positive, HER2-positive advanced breast cancer. Eli Lilly and Company.

Endometriosis is a frequent and chronic inflammatory disease with impacts on reproduction, health and quality of life. This disorder is highly estrogen-dependent and the purpose of hormonal treatments is to decrease the endogenous ovarian production of estrogens. High estrogen production is a consistently observed endocrine feature of endometriosis. mRNA and protein levels of estrogen receptors (ER) are different between a normal healthy endometrium and ectopic/eutopic endometrial lesions: endometriotic stromal cells express extraordinarily higher ERβ and significantly lower ERα levels compared with endometrial stromal cells. Aberrant epigenetic regulation such as DNA methylation in endometriotic cells is associated with the pathogenesis and development of endometriosis. Although there is a large body of data regarding ERs in endometriosis, our understanding of the roles of ERα and ERβ in the pathogenesis of endometriosis remains incomplete. The goal of this review is to provide an overview of the links between endometriosis, ERs and the recent advances of treatment strategies based on ERs modulation. We will also attempt to summarize the current understanding of the molecular and cellular mechanisms of action of ERs and how this could pave the way to new therapeutic strategies.

Vepdegestrant is an oral proteolysis-targeting chimera (PROTAC) estrogen receptor (ER) degrader that directly harnesses the ubiquitin-proteasome system. In this phase 3, open-label, randomized trial, we enrolled patients with ER-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer who had received one previous line of cyclin-dependent kinase 4 and 6 inhibitor therapy plus one line of endocrine therapy (and up to one additional line of endocrine therapy). Patients were randomly assigned in a 1:1 ratio to receive vepdegestrant at a dose of 200 mg orally once every day of each 28-day cycle or fulvestrant at a dose of 500 mg, administered intramuscularly, on day 1 and day 15 of cycle 1 and on day 1 of subsequent cycles, with randomization stratified according to -mutation status and presence or absence of visceral disease. The primary end point was progression-free survival as assessed by blinded independent central review among the patients with mutations and among all the patients who underwent randomization. Progression-free survival was estimated with Kaplan-Meier methods and hazard ratios with a stratified Cox proportional-hazards model. A total of 624 patients underwent randomization; 313 were assigned to receive vepdegestrant, and 311 to receive fulvestrant. Among the 270 patients with mutations, the median progression-free survival was 5.0 months (95% confidence interval [CI], 3.7 to 7.4) with vepdegestrant and 2.1 months (95% CI, 1.9 to 3.5) with fulvestrant (hazard ratio, 0.58 [95% CI, 0.43 to 0.78]; P<0.001). Among all the patients, the median progression-free survival was 3.8 months (95% CI, 3.7 to 5.3) with vepdegestrant and 3.6 months (95% CI, 2.6 to 4.0) with fulvestrant (hazard ratio, 0.83 [95% CI, 0.69 to 1.01]; P = 0.07). Adverse events of grade 3 or higher occurred in 23.4% of the patients in the vepdegestrant group and in 17.6% of the patients in the fulvestrant group. Adverse events led to treatment discontinuation in 2.9% and 0.7% of the patients, respectively. Among patients with ER-positive, HER2-negative advanced breast cancer, vepdegestrant was associated with significantly longer progression-free survival than fulvestrant in the subgroup with mutations but not in the full patient population. (Funded by Pfizer and Arvinas Estrogen Receptor; VERITAC-2 ClinicalTrials.gov number, NCT05654623.).

Background Endocrine resistance is a major challenge in treating patients with ER+ /HER2− metastatic breast cancer (MBC) necessitating a switch from endocrine therapy to more toxic therapies. Mutations in ESR1 constitute a key mechanism of resistance to endocrine therapy in ER+ /HER2− BC. Therapies that overcome endocrine resistance are needed. Palazestrant is a novel oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) belonging to a new class of ER-targeting agents that completely blocks estrogen-induced transcriptional activity, regardless of ESR1 mutation status. This first-in-human, open-label, multicenter, phase 1/2 dose-escalation/expansion study was designed to determine the recommended phase 2 dose (RP2D) and to evaluate safety, pharmacokinetics, and antitumor activity of palazestrant in patients with ER+ /HER2− MBC with disease progression on prior treatment. Methods Adults with ER+ /HER2‒ MBC who received ≥ 1 prior line of endocrine therapy for advanced disease and ≤ 2 prior chemotherapy regimens for metastatic disease were eligible. Patients received once-daily oral palazestrant (30–300 mg) in 28-day cycles until progression or intolerable toxicity. Results This study enrolled 146 patients. No dose-limiting toxicities were observed at doses up to 300 mg/day palazestrant. Confirmed partial responses were observed with 60 and 120 mg/day palazestrant. Both doses showed similar and tolerable safety profiles, favorable pharmacokinetics, and steady-state plasma concentrations above the predicted threshold for complete ER inhibition. Greater clinical benefit at palazestrant 120 mg/day (46%) versus 60 mg/day (19%) led to selection of 120 mg/day as RP2D and study expansion dose. At 120 mg/day, the median progression-free survival was 4.8 months (95% CI, 3.5–7.1) overall and 5.6 months (95% CI, 4.8–NE) among patients with cancers with ESR1 mutations. Most treatment-emergent adverse events (TEAEs) were grade 1–2. The most common TEAEs were nausea (62.8%), vomiting (29.1%), and fatigue (25.6%). The most common grade ≥ 3 TEAE was transient neutropenia (10.5%) managed by dose interruption and reduction. Conclusions Palazestrant demonstrated a manageable safety profile, with antitumor activity observed in patients with heavily pretreated cancers with wild-type and ESR1 -mutated BC. These data support the ongoing phase 3 study evaluating palazestrant in patients with ER+ /HER2 − MBC. Trial registration ClinicalTrials.gov, NCT04505826 . Registered August 6, 2020.

The selective estrogen-receptor degrader imlunestrant plus abemaciclib led to a median progression-free survival of 9.4 months among patients with ER-positive, HER2-negative breast cancer (vs. 5.5 months with imlunestrant alone).

In metastatic hormone receptor-positive breast cancer, ESR1 mutations are a common cause of acquired resistance to the backbone of therapy, estrogen deprivation by aromatase inhibition. How these mutations affect tumor sensitivity to established and novel therapies are active areas of research. These therapies include estrogen receptor-targeting agents, such as selective estrogen receptor modulators, covalent antagonists, and degraders (including tamoxifen, fulvestrant, and novel agents), and combination therapies, such as endocrine therapy plus CDK4/6, PI3K, or mTORC1 inhibition. In this review, we summarize existing knowledge surrounding the mechanisms of action of ESR1 mutations and roles in resistance to aromatase inhibition. We then analyze the recent literature on how ESR1 mutations affect outcomes in estrogen receptor-targeting and combination therapies. For estrogen receptor-targeting therapies such as tamoxifen and fulvestrant, ESR1 mutations cause relative resistance in vitro but do not clearly lead to resistance in patients, making novel agents in this category promising. Regarding combination therapies, ESR1 mutations nullify any aromatase inhibitor component of the combination. Thus, combinations using endocrine alternatives to aromatase inhibition, or combinations where the non-endocrine component is efficacious as monotherapy, are still effective against ESR1 mutations. These results emphasize the importance of investigating combinatorial resistance, challenging as these efforts are. We also discuss future directions and open questions, such as studying the differences among distinct ESR1 mutations, asking how to adjust clinical decisions based on molecular surveillance testing, and developing novel therapies that are effective against ESR1 mutations.

ER-positive, HER2-negative breast cancer, which accounts for about 70% of all breast cancers, is heterogeneous. Antiestrogen therapy is the cornerstone of systemic therapy, and its efficacy depends on such factors as grade, Ki-67 labeling, and progesterone receptor expression.

In women with hormone-receptor–positive metastatic breast cancer that had progressed after endocrine therapy, palbociclib plus fulvestrant was associated with progression-free survival of more than 9 months, as compared with less than 4 months with fulvestrant alone. Approximately 80% of breast cancers express estrogen receptors, progesterone receptors, or both. Endocrine therapies are the mainstay of treatment for these hormone-receptor–positive cancers, substantially reducing the relapse rate after presentation with early-stage cancer. 1 Despite advances in endocrine therapy, many women have a relapse during or after completing adjuvant therapy. The care of these women remains a considerable clinical challenge. Single-agent treatment with an aromatase inhibitor or tamoxifen has shown limited clinical benefit. 2 , 3 The selective estrogen-receptor degrader fulvestrant has modest activity in this population of patients, 4 , 5 and the development of effective therapies that can reverse resistance to endocrine therapy . . .