The impact of endoxifen-guided tamoxifen dose reductions on endocrine side-effects in patients with primary breast cancer

S. M. Buijs*, E. Oomen de Hoop, C. L. Braal, M. M. van Rosmalen, J. C. Drooger, Q. C. van Rossum-Schornagel, M. B. Vastbinder, S. L.W. Koolen, A. Jager, R. H.J. Mathijssen

*Corresponding author for this work

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BACKGROUND: Tamoxifen is important in the adjuvant treatment of hormone-sensitive breast cancer and substantially reduces recurrence; however, almost 50% of patients are non-compliant mainly due to side-effects. The aim of this study was to investigate whether endoxifen-guided tamoxifen dose reduction could lead to fewer side-effects.

MATERIALS AND METHODS: Effects of tamoxifen dose reduction were investigated in patients with bothersome side-effects and endoxifen levels ≥32 nM and compared to patients with side-effects who remained on tamoxifen 20 mg. Endocrine symptoms and health-related quality of life (HR-QOL) were assessed after 3 months with the Functional Assessment of Cancer Therapy-Endocrine Symptoms (FACT-ES) questionnaire.

RESULTS: Tamoxifen dose was reduced in 20 patients, 17 of whom were assessable for side-effect analyses. A clinically relevant improvement of >6 points was observed in endocrine symptoms and HR-QOL in 41% and 65% of the patients, respectively. In total, there was a significant and clinically relevant improvement in endocrine symptoms [5.7, 95% confidence interval (CI) -0.5-11.5] and HR-QOL (8.2, 95% CI 0.9-15.4) after dose reduction. This was not seen in patients whose doses were not reduced (n = 60). In 21% of patients, endoxifen dropped slightly below the 16-nM threshold (12.8, 15.5, 15.8, 15.9 nM).

CONCLUSIONS: Endoxifen-guided dose reduction of tamoxifen significantly improved tamoxifen-related side-effects and HR-QOL. Nearly 80% of patients remained above the most conservative endoxifen threshold.

Original languageEnglish
Article number100786
JournalESMO Open
Issue number1
Publication statusPublished - Feb 2023

Bibliographical note

Funding Information:
This work was supported by an unrestricted MRACE grant ( Erasmus MC , The Netherlands) [grant number 2017-108 ].

Publisher Copyright:
© 2023 The Authors


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