Endocrine Therapy Plus CDK4/6 in First or Second Line for Hormone (SONIA) Receptor Positive Advanced Breast Cancer

Combining cyclin-dependent kinases 4 and 6 (CDK 4/6) inhibitors with endocrine therapies in hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer has shown to result in substantial improvements in progression-free survival. There is however no evidence that this combination strategy leads to an improved overall survival. Furthermore, no specific subgroups that will or will not benefit from the combination of drugs have been identified yet. This means the optimal strategy for deploying CDK 4/6 inhibitors in clinical practice is not yet known. Since CDK 4/6 inhibitors are costly and can have toxic effects, it is important to determine the optimal treatment strategy to avoid both over- and undertreatment. The SONIA-trial is an investigator-initiated, multicenter, randomized phase III study. The primary objective of this study is to evaluate if treatment with a non-steroidal aromatase inhibitor combined with CDK 4/6 inhibition in first line followed at progression by fulvestrant in second line (strategy A) improves progression-free survival compared to treatment with a non-steroidal aromatase inhibitor in first line followed at progression by fulvestrant combined with CDK4/6 inhibition in second line (strategy B). The primary end point is progression-free survival after two lines (PFS2), secondary end points include overall survival, quality of life, safety and biomarker analyses.