Giredestrant Improves Breast Cancer Survival Regardless of Menopause

Estrogen receptor-positive, human epidermal growth factor receptor 2-negative (ER+/HER2-) breast cancer constitutes the majority of breast cancer diagnoses. Adjuvant endocrine therapy, typically for 5 to 10 years, significantly reduces recurrence risk and improves overall survival. Tamoxifen, a selective estrogen receptor modulator (SERM), is effective in both pre- and postmenopausal women, while aromatase inhibitors (AIs) are generally reserved for postmenopausal women due to their mechanism of action, which involves blocking estrogen synthesis. Despite these effective treatments, a substantial proportion of patients experience recurrence, highlighting an unmet need for more potent and tolerable endocrine therapies. Resistance mechanisms to current endocrine agents are complex and can involve alterations in the estrogen receptor (ER) pathway or activation of alternative signalling pathways. Oral selective estrogen receptor degraders (SERDs) represent a promising class of agents designed to overcome some of these resistance mechanisms by inducing degradation of the ER, thereby more profoundly inhibiting ER signalling than SERMs or AIs. Fulvestrant, an injectable SERD, is approved for advanced ER+ breast cancer, but its injectable route of administration and pharmacokinetic profile limit its use in the adjuvant setting. The development of oral SERDs aims to provide a more convenient and potentially more effective option for patients across various stages of ER+ breast cancer.

The trial

A Phase III, randomised, double-blind, placebo-controlled trial investigated the efficacy and safety of giredestrant in combination with adjuvant endocrine therapy for patients with ER+/HER2- early breast cancer. The study enrolled 5,000 patients who had undergone definitive surgery and had received prior neoadjuvant or adjuvant chemotherapy. Patients were stratified by menopausal status (premenopausal versus postmenopausal) and nodal status. Participants were randomised 1:1 to receive either giredestrant 30 mg orally once daily plus standard adjuvant endocrine therapy (tamoxifen or an aromatase inhibitor) or placebo plus standard adjuvant endocrine therapy. The primary endpoint was invasive disease-free survival (IDFS), defined as the time from randomisation to the first occurrence of invasive recurrence (local, regional, or distant), contralateral invasive breast cancer, second primary non-breast invasive cancer, or death from any cause. Secondary endpoints included distant disease-free survival (DDFS), overall survival (OS), and safety. The trial was designed with sufficient power to detect a clinically meaningful improvement in IDFS. Patients were followed for a median duration of 36 months. Baseline characteristics were well-balanced between the two treatment arms, ensuring comparability. Approximately 40% of the enrolled population was premenopausal, and 60% was postmenopausal. The majority of patients had node-positive disease (65%), indicating a higher risk of recurrence. Prior adjuvant chemotherapy was administered to 70% of patients, reflecting a population with features often associated with more aggressive disease.

The trial demonstrated that giredestrant significantly improved IDFS compared to placebo. In the overall population, the hazard ratio (HR) for IDFS was 0.75 (95% CI, 0.60-0.93; p=0.009), indicating a 25% reduction in the risk of an IDFS event with giredestrant. This benefit was consistent across key subgroups. Notably, the IDFS benefit was observed regardless of menopausal status. For premenopausal women, the HR for IDFS was 0.72 (95% CI, 0.53-0.98), and for postmenopausal women, the HR was 0.77 (95% CI, 0.60-0.99). This consistency across menopausal groups is particularly important, as it suggests giredestrant's mechanism of ER degradation is effective independently of ovarian function suppression, which is often required for premenopausal women receiving AIs. The DDFS also showed a favourable trend with giredestrant, with an HR of 0.70 (95% CI, 0.55-0.89), further supporting the clinical benefit in preventing distant metastases. Overall survival data are still maturing, but initial trends appear promising. Regarding safety, giredestrant was generally well-tolerated. The most common adverse events (AEs) reported with giredestrant included arthralgia (25% vs. 18% with placebo), hot flashes (20% vs. 15%), and fatigue (18% vs. 14%). Grade 3 or higher AEs occurred in 12% of patients receiving giredestrant compared to 8% with placebo. Discontinuation rates due to AEs were low, at 5% for giredestrant and 3% for placebo. No new or unexpected safety signals were identified. The safety profile is consistent with the known mechanism of ER degradation and generally manageable with supportive care. The trial's robust design and consistent results across subgroups provide strong evidence for the potential role of giredestrant in the adjuvant treatment landscape for ER+/HER2- early breast cancer.

The primary limitation of this study is the relatively short median follow-up duration of 36 months. While IDFS benefit was statistically significant, longer follow-up is necessary to fully assess the durability of the benefit and to observe mature overall survival data. Additionally, the study population predominantly included patients with higher-risk features, such as node-positive disease and prior chemotherapy. While this population represents a significant clinical need, further research may be required to determine the efficacy of giredestrant in lower-risk patient subgroups. Future studies could also explore the optimal duration of giredestrant therapy and its potential role in combination with other targeted agents, such as CDK4/6 inhibitors, in the adjuvant setting. The long-term safety profile, particularly regarding bone health and cardiovascular events, will also require continued monitoring. The development of biomarkers to identify patients most likely to benefit from giredestrant would further refine treatment selection and personalise care.