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Cancer Research and Treatment > Accepted Articles
doi: https://doi.org/10.4143/crt.2023.1285    [Accepted]
Endoxifen Concentration is Associated with Recurrence-Free Survival in Hormone-Sensitive Breast Cancer Patients
Beomki Lee1,* , Seok Jin Nam2, Seok Won Kim2, Jonghan Yu2, Byung-Joo Chae2, Se Kyung Lee2, Jai Min Ryu2, Jeong Eon Lee2 , Soo-Youn Lee1
1Department of Laboratory Medicine and Genetics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
2Division of Breast Surgery, Department of Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

*Present address: Graduate School of Medical Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Korea
Correspondence  Jeong Eon Lee ,Tel: 82-2-3410-0260, Fax: 82-2-3410-6982, Email: jeongeon.lee@samsung.com
Soo-Youn Lee ,Tel: 82-2-3410-1834, Fax: 82-2-3410-2719, Email: suddenbz@skku.edu
Received: December 5, 2023;  Accepted: June 16, 2024.  Published online: June 18, 2024.
The metabolism of tamoxifen is influenced by various cytochrome p450 enzymes, including CYP2D6 and CYP2C19, leading to variations in the levels of endoxifen, even with the same tamoxifen dosage. However, the clinical significance of endoxifen on the prognosis of breast cancer patients remains controversial. This study aimed to elucidate the relevance of endoxifen level to recurrence-free survival censored with tamoxifen discontinuation (RFSt), representing the RFS for tamoxifen itself, of breast cancer patients and determine a suitable cutoff for prognostication.
Materials and Methods
The study included 478 breast cancer patients, and tamoxifen and its metabolites, including endoxifen, were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). An optimal cutoff was determined with maximally selected rank statistics. Survival analysis and Cox regression were conducted based on this cutoff.
An endoxifen level of 21.00 ng/mL was the optimal cutoff for prognostication. Survival analysis revealed a statistically significant difference in RFSt between the low endoxifen group (≤ 21.00 ng/mL) and high endoxifen group (> 21.00 ng/mL) (log-rank test, p=0.032). The 10-year probability of RFSt was 83.2% (95% CI, 77.0–89.9%) and 88.3% (95% CI, 83.3–93.5%) in the low and high endoxifen groups, respectively. Multivariable Cox proportional hazards regression indicated endoxifen concentration as a significant factor affecting prognosis, which was adjusted with other clinical characteristics.
Endoxifen could serve as a marker for appropriate tamoxifen treatment, and an endoxifen cutoff of 21.00 ng/mL could be advantageous in prognostication. Based on this cutoff, therapeutic drug monitoring would benefit patients displaying a suboptimal concentration.
Key words: Breast Neoplasms, Endoxifen, Prognosis, Recurrence-free survival, Tamoxifen
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