Opportunities of the pharmacogenetic approach to personalized tamoxifen breast cancer therapy: case reports

Abstract

Tamoxifen is the selective modulator of estrogen receptors. Nowadays, it is widely used for treatment of premenopausal women with ER(+) breast cancer likewise for postmenopausal women with treatment contraindications to aromatase inhibitors. Tamoxifen is a prodrug which is metabolized by cytochrome P450 (CYP): CYP2D6, CYP3A4, CYP3A5, CYP2C9, CYP2C19 to active metabolites. There is high variability in the CYP genes therefore differences in tamoxifen metabolism, tamoxifen individual response and efficacy are observed among patients. This article presents two clinical case reports. Both patients have breast cancer luminal A subtype, similar prognosis and are administered tamoxifen but they have diverse clinical effects. Patients responded to the survey questionnaire, then samples of buccal epithelium were taken for genetic analysis of CYP2D6*4, CYP3A5*3, CYP3A4*17, CYP2C9*2,3, CYP2C19*2,3, ABCB1 gene mutations by use of real time PCR. In patient A samples were detected significant mutations in CYP2D6 (*1/*4), CYP3A5 (*3/*3) и CYP2С9 (*2/*3), but there were no mutations detected in patient B. It is interesting that patient B has had prominent tamoxifen adverse effects, such as flushes, ostealgia, faintness, after 1 month of tamoxifen therapy. Patient A has taken tamoxifen for 19 months without any adverse effects. Also there is a review in this article about clinical value of different CYP2D6, CYP3A5, CYP2C9 polymorphisms. Additionally, we make a suggestion about the role of polymorphisms in tamoxifen adverse effects and the way of solution for problems of tamoxifen resistance. We suppose that routine genetic study before tamoxifen administration would help to predict individual intolerance and increase the efficacy of treatment.