Abstract P1-07-08: Endoxifen predicts early recurrence in breast cancer: A Brazilian prospective study
Authors: Almeida Thais , Schroth Werner , Nardin Jeanine Marie , Mürdter Thomas , Picolotto Solane , Hoppe Reiner , Kogin Jenifer Primon , Gaio Elisa Daniele , Dasenbrock Angela , Skrsypcsak Raquel Cristina , Noronha Lucia , Brauch Hiltrud , Casali-da-Rocha José Claudio 
Published: March 2022

Tamoxifen is widely used in adjuvant therapy for breast cancer patients, but up to one-third of patients recur within 15 years after surgery even with its regular use. Low plasma levels of (Z)-endoxifen, the most active tamoxifen metabolite, have been studied as a possible predictor of treatment failure. Here, we evaluated plasma (Z)-endoxifen as a predictor of response to adjuvant treatment in breast cancer patients. Methods: One hundred forty nine patients were selected from a prospective cohort of consecutive newly diagnosed breast cancer patients recruited at a national referral center for treatment of cancer in Curitiba, Southern Brazil. Inclusion criteria were breast cancer women on stage I - III, positive hormone receptor tumors, operable, undergoing or not (neo)adjuvant chemotherapy, and signed for adjuvant tamoxifen for at least five years. At diagnosis, peripheral blood samples were collected for CYP2D6 genotyping for *1 *2, *3, *4, *5, *6, *7, *8, *9, *10, *17, *29 and *41 and subsequent metabolic activity scores according to phenotypes assigned for each allele (poor metabolizer PM; intermediate, IM; extensive, EM and ultrarapid metabolizer, UM). After the tamoxifen initiation, patients were evaluated at months 3, 6, and 12 for adherence and drug interactions, and plasma (Z)-endoxifen quantification. They were followed from April 2014 to April 2019. The study's primary objective was to evaluate whether (Z)-endoxifen plasma levels predict early recurrence in patients on adjuvant tamoxifen. The secondary aim was to evaluate an association between (Z)-endoxifen levels and clinical and phenotypic CYP2D6 metabolism variables. A Cox model estimated hazard ratios (HR) comparing recurrence free survival (RFS) according to (Z)-endoxifen plasma levels. The survival was compared using the Kaplan-Meier method and the log-rank test. Results: The median follow-up period was 52.3 months and at that point, there were 20 recurrences of 149 patients. At 3 months there were no cases of low adherence to tamoxifen. The (Z)-endoxifen levels were established by quartiles, with the first lower quartile being used as a cutoff (14.72nM). High (Z)-endoxifen levels at 3 months (&gt;14.72nM) were detected in 75.8% (n = 113) of cases and low levels (≤14.72nM) in 24.2% (n = 36). There was no statistically significant difference between the groups with low or high (Z)-endoxifen levels regarding clinical and pathological variables, but there was a higher CYP2D6 PM/IM frequency in the low (Z)-endoxifen levels group (p&lt;0.001). High (Z)-endoxifen levels were associated with lower RFS compared to low (Z)-endoxifen levels (HR 2.5, 95% CI 1.00-6.22, p=0.041). Low (Z)-endoxifen levels increased the recurrence risk even when adjusted for staging and molecular subtypes (HR 2.55, 95% CI 1.00-6.48, p&lt;0.001) or staging and CYP2D6 phenotype (UM/EM vs IM/PM; HR 4.38, 95% CI 1.66-11.59, p=0.003), or prior chemotherapy exposure (HR 2.65, 95% CI 1.06-6.61, p=0.036). Conclusion: This is the first Brazilian prospective trial evaluating the association of plasma (Z)-endoxifen levels, with the clinical outcome in breast cancer women treated with adjuvant tamoxifen, controlled by predictable confounding factors of plasma (Z)-endoxifen levels, such as tamoxifen adherence, and drug interaction with CYP2D6 inhibitors. In our study, a low plasma level of (Z)-endoxifen (≤ 14.72nM) after 3 months of tamoxifen treatment was a predictor of early recurrence in patients under adjuvant tamoxifen (p=0.041). A longer follow-up and a larger cohort may be needed to confirm the (Z)-endoxifen cutoff value that is a predictor of long-term recurrence.

Citation Format: Thais Almeida, Werner Schroth, Jeanine Marie Nardin, Thomas Mürdter, Solane Picolotto, Reiner Hoppe, Jenifer Primon Kogin, Elisa Daniele Gaio, Angela Dasenbrock, Raquel Cristina Skrsypcsak, Lucia Noronha, Hiltrud Brauch, José Claudio Casali-da-Rocha. Endoxifen predicts early recurrence in breast cancer: A Brazilian prospective study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-07-08.