Guardant Health Japan Secures First ESR1 Mutation Liquid Biopsy Approval for Breast Cancer Treatment Selection
Key Insights
Japan's Ministry of Health, Labour and Welfare approved Guardant360® CDx (search) as the first blood-based companion diagnostic to identify ESR1 (search) mutations in hormone receptor-positive, HER2-negative breast cancer patients.
The liquid biopsy test enables treatment selection with imlunestrant for patients who have developed resistance to aromatase inhibitor endocrine therapy.
ESR1 (search) mutations are detected in approximately 20-40% of patients with locally advanced or metastatic breast cancer previously treated with aromatase inhibitors.
Guardant Health Japan (search) Corp. announced that Japan's Ministry of Health, Labour and Welfare (MHLW) approved Guardant360® CDx (search) on October 23, 2025, as a companion diagnostic to identify ESR1 (search) mutations in patients with hormone receptor-positive, HER2-negative breast cancer with disease progression following endocrine therapy. This milestone makes Guardant360® CDx the first blood-based companion diagnostic approved in Japan for detecting ESR1 mutations, specifically for treatment consideration with imlunestrant.
Clinical Significance of ESR1 Mutations
The ESR1 (search) gene encodes estrogen receptor alpha proteins, representing one of the key hormone receptors involved in breast cancer pathogenesis. ESR1 mutations serve as recognized drivers of resistance to aromatase inhibitors, a class of endocrine therapies widely used as first-line treatment in breast cancer patients.
Clinical data demonstrates the prevalence and clinical impact of these mutations. In a randomized multicenter trial, ESR1 (search) mutations were detected in approximately 20 to 40% of patients with locally advanced or metastatic breast cancer previously treated with an aromatase inhibitor. Research indicates that ESR1 mutation prevalence increases with additional lines of therapy, highlighting the growing clinical need for mutation detection as treatment resistance develops.
Liquid Biopsy Technology Advancement
Guardant360® CDx (search) utilizes next-generation sequencing to analyze circulating tumor DNA (ctDNA), which is produced when tumors shed small pieces of their genetic material into the bloodstream. The test provides comprehensive genomic profiling results from a simple blood draw, enabling oncologists to move beyond the limitations of tissue biopsies to obtain clinically relevant information for matching patients to optimal personalized treatment.
This regulatory approval of the Guardant360® CDx (search) test, which identifies ESR1 (search) mutations using a blood sample, is expected to increase targeted treatment options for patients with hormone receptor-positive, HER2-negative breast cancer who have developed resistance to aromatase inhibitors.
Healthcare System Integration
Guardant360® CDx (search) received initial MHLW approval in March 2022 for comprehensive genomic profiling in patients with advanced solid tumors. The test is approved as a companion diagnostic for multiple cancer drugs approved in Japan. Testing under the national health insurance system is available at cancer genome core hospitals, cancer genome hub hospitals, and affiliated cancer genome medical institutions designated by MHLW.
The expanded approval for ESR1 (search) mutation detection represents a significant advancement in precision oncology for Japanese breast cancer patients, providing a non-invasive method to identify treatment resistance mechanisms and guide therapeutic decision-making in the metastatic setting.