Dear Editor,

I read with great interest the article by de Souwer et al. [1] in which they study the pharmacokinetics and -dynamics of tamoxifen in older women with non-metastatic breast cancer. According to their findings, older women receiving steady-state adjuvant tamoxifen treatment have higher serum concentrations of tamoxifen and its demethylated metabolites, such as endoxifen, than younger women. The authors put forth a number of theories to explain why older patients had higher endoxifen concentrations. There was a missing explanation. Endoxifen levels in older patients may also be higher as creatinine clearance gradually decreases with age and urinary elimination is a major route of endoxifen excretion [2]. This subject deserves more explanation.