Biomarker testing in advanced or metastatic hormone receptor-positive, human epidermal growth factor receptor 2 negative breast cancer: expert recommendations

Biomarker testing is central to precision oncology, enabling effective use of targeted therapies in hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (mBC). Recommendations for current and future biomarker testing and diagnostic workflows were developed using a three-step approach: pre-meeting survey, descriptive analysis, and steering committee discussion. A panel of 14 experts from Latin America, Asia Pacific, the Middle East, and Africa developed recommendations using an expert opinion-based approach without a formal consensus methodology. Recommendations with >90% agreement were considered panel recommendations. Some recommendations are expert-driven and not yet standard-of-care. High agreement was observed for biomarker testing at diagnosis for estrogen receptor (ER), progesterone receptor (PR), HER2, and BReast CAncer gene (BRCA)1/2. For endocrine-resistant patients, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) testing is recommended at diagnosis; otherwise, during or after first-line treatment. AKT serine/threonine kinase 1 (AKT1) and phosphatase and tensin homolog (PTEN) testing can occur either during first-line treatment or at progression, while estrogen receptor 1 (ESR1) testing is advised at progression. Tumor tissue (preferably from a re-biopsy) is the preferred sample for PIK3CA, AKT1, and PTEN testing; if unavailable, primary tissue or circulating tumor DNA (ctDNA) may be considered. Blood is recommended for germline BRCA1/2 or PALB2 testing, while ctDNA is preferred for ESR1 detection. Next-generation sequencing (NGS) is the preferred method for most biomarkers, with polymerase chain reaction as an alternative where NGS is unavailable; PTEN may also be assessed using immunohistochemistry. Above 90% of experts proposed future biomarker testing at diagnosis for ER, PR, HER2, germline BRCA1/2, PALB2, PIK3CA, AKT1, PTEN, mismatch repair, microsatellite instability, tumor mutational burden, and neurotrophic tyrosine receptor kinase in all patients with HR+, HER2-negative mBC. This article offers a practical guide for biomarker testing in HR+, HER2-negative mBC.