The POWER Trial: Personalised Dose Optimisation With Adjuvant Tamoxifen Therapy in Breast cancER

In Sweden, approximately 7000 women are diagnosed with hormone-sensitive breast cancer annually. According to international and national guidelines, most of these women are recommended anti-hormonal therapy for five to ten years to improve prognosis. Tamoxifen, one of the most widely used anti-hormonal agents globally, reduces the risk of recurrence by 40% and breast cancer mortality by 30%. Tamoxifen is a pro-drug that undergoes hepatic metabolism to form endoxifen and other active metabolites. Variability in metabolic capacity affects therapeutic efficacy: poor metabolisers produce insufficient endoxifen and other active metabolites, risking therapeutic failure, while ultrarapid metabolisers generate excessive amounts, leading to intolerable adverse effects. Today, 30-50% of patients discontinue treatment prematurely due to severe side effects, resulting in suboptimal outcomes. Currently, tamoxifen is uniformly prescribed at a daily dose of 20 mg, and so far, no clinical trials have tested whether individualised dosing could enhance adherence and improve survival outcomes. The primary objective is to evaluate whether individualised tamoxifen dosing reduces discontinuation rates and enhances patient outcomes in breast cancer treatment.