Abstract P2-11-12: The influences of adherence to tamoxifen and CYP2D6 pharmacogenetics on plasma concentrations of the active metabolite (Z)-endoxifen in breast cancer

Abstract

Abstract One-third of early breast cancer patients with estrogen receptor (ER) positive tumors treated with the selective estrogen receptor modulator tamoxifen either relapse or die from the disease in the following decade. Improvement of tamoxifen efficacy requires a better knowledge of factors determining outcome of which the medication-taking behavior, i.e. adherence, is a strong suspect. Discontinuation and non-adherence to tamoxifen are frequent and result in increased mortality. Commonly used methods to assess drug adherence such as prescription-refill patterns infer that the medication is taken exactly as prescribed yet, partial adherence or actual use of the medication cannot be controlled. An objective surrogate of tamoxifen adherence is the monitoring of drug concentrations in the patients’ plasma including the parent drug and its active metabolite (Z)-endoxifen (endoxifen), however individual concentrations depend on pharmacogene polymorphisms particularly those of the liver enzyme cytochrome P450 2D6 (CYP2D6). Patients without CYP2D6 variants have normal enzyme function (extensive metabolizer, EM) and achieve high endoxifen concentrations compared to patients with variants conferring absent activity (poor metabolizer, PM) or reduced activity (intermediate metabolizer, IM) who have significantly lower plasma concentrations of the active metabolite. A putative threshold of 5.9 ng/mL endoxifen has been proposed suggesting that patients with endoxifen concentration above or below threshold have lower or higher risk for recurrence, assuming that low endoxifen exposure in the tumor likely results in incomplete inhibition of ER-dependent growth signaling. During tamoxifen treatment it is therefore important that patients achieve clinically relevant endoxifen levels while at the same time avoid confounders of which adherence is a prime candidate. To this end, we investigated patients’ ability to achieve relevant plasma concentrations during tamoxifen treatment in relation to their CYP2D6 metabolizer status and their tamoxifen adherence behavior. Our prospective study included 192 Brazilian breast cancer patients treated with 20 mg tamoxifen daily. Two-thirds of the patients were premenopausal. Plasma levels of tamoxifen and its metabolites were measured by LC-MS/MS at 3, 6 and 12 months of treatment. Adherence to tamoxifen was evaluated by the questionnaire-based Morisky medication adherence scale. CYP2D6 polymorphisms were determined by MALDI-TOF mass spectrometry and real-time PCR. Adherence to tamoxifen decreased from 74% at month 3 to 64% at month 12, and explained 47% of the variability of tamoxifen plasma concentrations (P<.001). Accordingly, plasma monitoring of tamoxifen is useful to inform on tamoxifen adherence. Endoxifen concentrations were associated with CYP2D6 status and adherence. While CYP2D6 explained 26.4% of endoxifen variability at 12 months, the combination with adherence explained 40% (P<.001). The influence of low adherence to not achieving therapeutic endoxifen levels was highest in patients with non-compromised CYP2D6 function (EM phenotype, RR 3.65; 95% CI: 1.48-8.99). we conclude that for patients with intact CYP2D6 function adherence to tamoxifen is critical to achieve therapeutic endoxifen plasma levels. CYP2D6 phenotyping and endoxifen plasma monitoring may help to guide EM (and IM) patients to adhere to tamoxifen which is particularly relevant for premenopausal women who are at higher risk to not adhere to tamoxifen. Citation Format: Jose Claudio Casali da Rocha, Jeanine M Nardin, Werner Schroth, Thais A Almeida, Thomas Mürdter, Solange Picolotto, Evelyn CL Vendramini, Reiner Hoppe, Jenifer P Kogin, Diandra Miqueleto, Silvia DR Moraes, Matthias Schwab, Roberto F Pecoits-Filho, Hiltrud Brauch. The influences of adherence to tamoxifen and CYP2D6 pharmacogenetics on plasma concentrations of the active metabolite (Z)-endoxifen in breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-11-12.