Merck & Co. Initiates Phase 3 Trial for Promising Breast Cancer Treatment, Collaborating with Daiichi Sankyo

Merck & Co Inc (MRK) has initiated the HERTHENA-Breast04 phase 3 trial to evaluate the efficacy and safety of patritumab deruxtecan (HER3-DXd) in patients with unresectable locally advanced or metastatic hormone receptor-positive, HER2-negative breast cancer. The trial targets patients who have experienced disease progression following endocrine and CDK4/6 inhibitor therapy. Positive aspects include promising clinical activity and a significant step forward in addressing unmet needs, while negative aspects include the investigational drug's unestablished safety and efficacy. The collaboration with Daiichi Sankyo underscores Merck's commitment to expanding its oncology portfolio and enhances its research capabilities and market reach.

Merck & Co., Inc. (MRK) has recently initiated the HERTHENA-Breast04 phase 3 trial to evaluate the efficacy and safety of patritumab deruxtecan (HER3-DXd) in patients with unresectable locally advanced or metastatic hormone receptor-positive, HER2-negative breast cancer [1]. The trial aims to assess the investigational drug's potential as a meaningful new treatment option for this specific type of breast cancer, which has historically shown poor outcomes.

The HERTHENA-Breast04 trial is an open-label, randomized, phase 3 study that will enroll approximately 1,000 patients across Asia, Europe, North America, and South America. Patients will be randomized 1:1 to receive patritumab deruxtecan (5.6 mg/kg) monotherapy or physician's choice of treatment, which includes chemotherapy or HER2 directed ADC (trastuzumab deruxtecan). The primary endpoints are progression-free survival by blinded independent central review and overall survival, with secondary endpoints including objective response rate, duration of response, and safety.

The trial is based on promising clinical activity observed in earlier phase studies, including ICARUS-Breast01 and a phase 1/2 breast cancer trial published in the Journal of Clinical Oncology in June 2022 [1]. The collaboration with Daiichi Sankyo underscores Merck's commitment to expanding its oncology portfolio and enhancing its research capabilities and market reach.

However, it is essential to note that patritumab deruxtecan is still an investigational drug, and its safety and efficacy have not been established. The trial is designed to address the significant unmet need for new treatment options in this challenging disease setting.

References:
[1] Merck & Co., Inc. (2025). Merck Initiates HERTHENA-Breast04 Phase 3 Trial to Evaluate Patritumab Deruxtecan in Metastatic Breast Cancer. Retrieved from https://www.merck.com/news/herthena-breast04-phase-3-trial-of-patritumab-deruxtecan-initiated-in-patients-with-metastatic-hormone-receptor-positive-her2-negative-breast-cancer-previously-treated-with-endocrine-therapy/