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Background: Although tamoxifen is a main adjuvant therapy in estrogen positive breast cancer especially in premenopausal women, reliance on genetic polymorphisms of its CYP2D6 metabolizing enzyme and/or therapeutic drug monitoring of its active metabolite, endoxifen to determine tamoxifen-therapeutic outcomes is debatable. Objective: The goal of the study was to investigate the prevalence of CYP2D6*4 and possible association between its nonfunctional allele (A) and both tamoxifen-induced adverse effects and cancer relapse in premenopausal breast cancer patients. Method:Polymerase chain reaction -restriction fragment length polymorphism (PCR-RFLP) analysis was applied for detection of CYP2D6*4 (1846 G>A) genotypes. Results:Genotyping analysis of CYP2D6*4 showed a minimal allele frequency of 23%. Increased endometrial thickness in mm was significantly correlated with CYP2D6*4 GA and AA genotypes in comparison with GG genotypes (P< 0.01). No other relations were found between CYP2D6*4 alleles and other tamoxifen adverse effects (P > 0.9) or cancer relapse (P= 0.7). Conclusion: The prevalence of CYP2D6*4 polymorphism in Egyptian premenopausal females with breast cancer who are given tamoxifen is similar to that in Caucasians. The nonfunctional allele (A-allele) of CYP2D6*4 showed a significant association with increased endometrial thickness possibly related to tamoxifen, but no association with other drug adverse effects or cancer relapse.

Background The complex metabolic profile of tamoxifen anticancer drug and polymorphism in its metabolizing enzymes particularly CYP2D6 contribute to the high-observed inter-individual variability in its main active metabolite endoxifen. Therapeutic drug monitoring of endoxifen may play a key role in optimizing tamoxifen therapy, and control of both adverse effects and cancer recurrence. This pilot study aims to assess the clinical benefits of applying endoxifen measurement during tamoxifen therapy in patients with breast cancer. Methods Adult premenopausal breast cancer patients ≥ 18 years who received tamoxifen at a fixed dose of 20 mg daily were included. The primary endpoint was to identify the inter-subject variability in serum concentration of the drug and its metabolites especially endoxifen, through fixation of the tamoxifen dose. The secondary endpoint was to check the correlation between endoxifen metabolite concentration and the development of tamoxifen’s adverse effects and cancer recurrence. Results Sixty patients were included in the study with a mean age of 38.4  ±  0.6 years (range: 26–50). The mean concentration of tamoxifen and endoxifen was 181  ±  9.6 ng/mL and 31.49 ng/mL, respectively. The inter-individual variability in concentrations for the drug and its active metabolite as estimated by the coefficient of variation percentage was in 41% and 31%, respectively. Cancer recurrence was observed in a group of patients ( n  =  16) with an average endoxifen level of 24.48 ng/mL. Another group of patients ( n  =  25) developed different tamoxifen adverse effects including hot flashes, vaginal bleeding, endometrial thickness, and ovarian cysts with the average endoxifen level of 38.61 ng/mL. The rest of the patients ( n  =  19) who responded smoothly to the drug with no complications had an average endoxifen level of 31.37 ng/mL. Analysis of variance test showed a significant difference in endoxifen levels between the three groups ( p  =  0.002). Conclusion The measurement of the endoxifen active metabolite of tamoxifen in breast cancer patients can help dose optimization in light of the observed wide inter-individual variability in drug fixed-dose related concentration of the metabolite. Monitoring of serum concentration of endoxifen can help to reveal, reduce and control tamoxifen’s adverse effects and cancer recurrence.