Plasma Concentrations Of Tamoxifen Research Articles

Factors Associated with Plasma Levels of Tamoxifen and its Main Metabolites in Mexican Patients with Breast Cancer

Background: Tamoxifen (TAM) is commonly prescribed as adjuvant therapy in women with estrogen receptor-positive breast cancer. Unfortunately, not all patients respond adequately to this drug. This variation in pharmacological response has been associated with different factors, including genetic polymorphisms of enzymes responsible for the metabolism of TAM. Objective: To determine the concentrations of tamoxifen (TAM) and its main metabolites in Mexican women with breast cancer and to evaluate its relationship with genetic, demographic and anthropometric characteristics. Methods: Eighty-four patients with a mean age of 49.3 (± 8.8) years were included in the study. Plasma concentrations of TAM and its metabolites N-desmethyl-tamoxifen (NDT), 4- hydroxy-tamoxifen (4HT) and endoxifen (END) were determined in predose for each patient. CYP2D6 * 4, * 10 and CYP3A5 * 3 genetic polymorphisms were characterized. Demographic, anthropometric, biochemical and clinical data were recorded for each patient. Results: Plasma concentrations of 4HT and END were higher in the extensive metabolizer (EM) phenotype than in the intermediate metabolizer (IM) phenotype (p<0.05). The metabolic ratio (MR) [END+4HT]/[TAM+NDT] was lower in patients with the CYP2D6 IM phenotype than those with the EM phenotype (p= 0.014). Regarding anthropometric factors, a positive correlation was found for 4HT and the END concerning age (R = 0.256 and 0.232, respectively). The body mass index (BMI) presented a statistically significant correlation with the concentrations of NDT (R=-0.351) and 4HT (R=-0.298). Conclusion: CYP2D6 phenotype, age and BMI could help to explain part of the interindividual variability of TAM plasma levels and its metabolites in the Mexican population.

Abstract P1-07-06: Endoxifen exposure after 5 weeks of preoperative tamoxifen is predictive of long-term outcome of operable hormone receptor positive (HR+) HER2 - early breast cancer

Abstract Introduction: Endoxifen exposure, the most active metabolite of tamoxifen, may be a better predictor of tamoxifen efficacy, than CYP2D6 genotypes. However, conflicting results have been reported so far. We evaluated prospectively the impact of 5 weeks of pre-operative tamoxifen on Ki-67 levels in early-stage breast cancer (EBC) patients. We correlated CYP2D6 genotype, exposure of tamoxifen and its metabolites with long-term outcome. Patients and methods: TAM pre-op was an open-label, multicentre, phase 2 trial in which pre and postmenopausal women with WHO performance status 0-1 and HR+ HER2-, operable breast cancer were given tamoxifen (20mg per day orally) for 5 weeks before surgery. Adjuvant treatments were given as standard practice. CYP2D6 genotype was determined at initiation. Plasma concentrations of tamoxifen and its main metabolites (N-desmethyltamoxifen, Z-endoxifen, and Z-4-hydroxytamoxifen) were assessed at baseline (b), week 2 (w2) and week 5 (w5) of treatment, using a validated performed LC-MS/MS method. AUCs, used as marker of exposure, were computed with Monolix© (Lixoft, Antony, France). Tumour Ki-67 variations between baseline (Ki-67b) and w5 (Ki-67w5) were determined to assess the sensitivity to tamoxifen. We used a pre-defined 10% cut-off to dichotomize low versus high Ki-67. All these parameters were associated with disease free survival by Cox regression analyses. Results: Out of the 136 patients enrolled, 98 pts had paired Ki-67 values evaluable. With a median follow up of 78 months (IQR 58-95), 14/136 women had cancer recurrence (12 distant and 2 contralateral) with the proportion free from cancer recurrence at 5 years of 90% (95% CI 83-94). Ki-67b, Ki-67w5 and variations between baseline and week 5 were not associated with DFS. Endoxifen AUC was correlated with CYP2D6 genotype (normal/hyper metabolizer versus low metabolizer) but CYP2D6 genotype was not predictive for DFS (adjusted hazard ratio, 2.24; 95% CI, 0.29 to 17.37; P = 0.44). Endoxifen AUCs at w2 and w5 were similar in patients with low Ki67b and low Ki67w5 (low-low) (n=36), low Ki67b and high Ki67w5 (n=12), high Ki67b and low Ki67w5 (n=14) and high Ki67b and high Ki67w5 (n=30). However, low endoxifen AUC at w5 was a pejorative factor of disease free survival: 5 years PFS 81.8% vs 90.3% (HR 2.70, CI 95% [1.04-7.06], p=0.04). Conclusion: This prospective clinical study shows that endoxifen exposure at w5 of pre-operative tamoxifen is associated with DFS in patients with HR+/HER2- EBC. Translationnal studies are ongoing. N°EUDRACT 2008-007652-10. Citation Format: Jean-Sebastien Frenel, Loic Campion, Antonin Schmitt, Patricia De Cremoux, Elsa Rossignol, Celine Renaudeau, Marie Robert, Jean-Marie Bard, Christine Bobin-Dubigeon, Mario Campone. Endoxifen exposure after 5 weeks of preoperative tamoxifen is predictive of long-term outcome of operable hormone receptor positive (HR+) HER2 - early breast cancer [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 P1-07-06.

A preliminary study on the association of tamoxifen, endoxifen, and 4-hydroxytamoxifen with blood lipids in patients with breast cancer

The long-term treatment with tamoxifen can alter the lipid profile of patients with breast cancer. Only a few studies associated the plasma concentrations of tamoxifen, endoxifen, and 4-hydroxytamoxifen with blood lipids, which is relevant as the distribution of these compounds for the tissues can be changed, negatively affecting the treatment. The variations in lipids also can account for the high interindividual variation in plasma concentrations of these compounds. The aim of this preliminary study was to associate the plasma levels of tamoxifen and the active metabolites with the lipid levels. An observational study of cases was conducted in patients with breast cancer using tamoxifen in a daily dose of 20 mg. The lipids were measured by spectrophotometric methods and the plasma concentrations of tamoxifen, endoxifen, and 4-hydroxytamoxifen by high-performance liquid chromatography. A total of 20 patients were included in the study. The median plasma concentrations of tamoxifen, 4-hydroxytamoxifen and endoxifen were 62 ng/mL, 1.04 ng/mL and 8.79 ng/mL. Triglycerides levels ranged from 59 to 352 mg/dL, total cholesterol from 157 to 321 mg/dL, LDL-c from 72 mg/dL to 176 mg/dL and HDL-C from 25.1 mg/dL to 62.8 mg/dL. There were no significant associations between the plasma concentrations of tamoxifen, 4-hydroxytamoxifen, and endoxifen with the levels of triglycerides and total cholesterol. The multivariate analysis revealed a weak association between plasma concentrations of tamoxifen and the active metabolites with HDL-c, LDL-c and VLDL-c. This finding provides preliminary evidence of the low impact of lipoproteins levels in the exposure to tamoxifen, 4-hydroxytamoxifen and endoxifen.

Effect of the Single Oral Combination Treatment of Tamoxifen with Gamisoyo-san on the Pharmacokinetics Profiles of Tamoxifen

The effects of Gamisoyo-san (GMSYS) co-administration within 5 min on the pharmacokinetics (PK) of tamoxifen were observed. After 50 mg/kg of tamoxifen oral treatment, GMSYS 100 mg/kg was orally administered within 5 min to 7-wk old male SPF.VAF Outbred Crl:CD [Sprague-Dawley (SD)] rats. The plasma were collected at 30 min before administration, 30 min, 1, 2, 3, 4, 6, 8 and 24 hrs after end of GMSYS treatment, and plasma concentrations of tamoxifen were analyzed using LC-MS/MS methods. Tmax, Cmax, AUC, t1/2 and MRTinf of tamoxifen were analysis as compared with tamoxifen single administered rats. Although co-administration with GMSYS did not critically influenced on the pharmacokinetic parameters of oral tamoxifen, they induced increased trends of plasma tamoxifen concentrations, especially significant (p0.05) increases of plasma tamoxifen concentrations were demonstrated at 0.5 hr after end of co-administration with GMSYS as compared with tamoxifen single formula treated rats, at dosage levels of tamoxifen 10 mg/kg and GMSYS 100 mg/kg within 5 min. It is considered that pharmacokinetic studies should be tested like the effects of GMSYS on the pharmacokinetics of tamoxifen, when they were co-administered with prolonger intervals than Tmax of tamoxifen oral administration (about 2.5 hr-intervals), to achieve the optimal dosing regimen of GMSYS and tamoxifen co-administration.

Abstract P5-12-05: Clinical outcomes in early breast cancer patients on adjuvant tamoxifen: Impact of CYP2D6 genotype and observed endoxifen concentrations

Abstract Background: Tamoxifen is widely used in patients with hormone sensitive breast cancer in the adjuvant setting to decrease risk of recurrence and metastasis. 5-10 years of tamoxifen has demonstrated a near 50% reduction in recurrence risk. Despite marked interpatient variation in the metabolism of tamoxifen to its active metabolite endoxifen, all patients are prescribed 20 mg daily dose of tamoxifen. Current evidence suggests that patients are at risk for suboptimal benefit if measured endoxifen concentration is below 5.9 ng/ml (~15nM). However, there have been conflicting data on the clinical utility of CYP2D6 genotype testing, and measuring endoxifen plasma concentration as a predictor of breast cancer recurrence. The goal of this prospective study was to determine the impact of endoxifen levels and CYP2D6 genetic variations to clinical outcome among patients with early breast cancer. Methods: Since 2010, as part of Personalized Medicine Program in Oncology, our team has been prospectively enrolling patients on tamoxifen therapy. To date 950 patients have been enrolled and preliminary analysis has been completed in 429. After initial CYP2D6 genotype testing, plasma concentration of tamoxifen and its metabolites were quantified using liquid chromatography-tandem mass spectrometry during each clinic visit. Baseline characteristic are outlined in table 1. Primary outcome assessed was invasive disease-free survival (IDFS) defined as time since start of tamoxifen therapy till an event of interest such as loco-regional recurrence, distant metastasis, new contralateral primary breast cancer, death due to breast cancer and other causes. Patients were censored at their last encounter at our institution (London Health Sciences Centre). Results: IDFS data for 426 patients were obtained. Outcome was stratified by CYP2D6 phenotypes (ultra-rapid, extensive, intermediate and poor metabolizers) and endoxifen levels (>5.9ng/ml or <5.9ng/ml). CYP2D6 phenotype did not demonstrate a significant association for IDFS. However, Kaplan Meier analysis stratified for endoxifen concentration >vs< 5.9 ng/ml showed a clear trend towards lower IDFS (hazard ratio = 2.4, (95%CI,1.26-4.77) P=0.0002) among those with endoxifen < 5.9 ng/ml. We note this preliminary data analysis was unadjusted and will undergo further in-depth statistical analysis. Conclusion: Preliminary finding suggests endoxifen concentration, but not CYP2D6 phenotype, may be a predictor of risk for suboptimal benefit during tamoxifen therapy. Detailed statistical analysis is planned for the full cohort, including adjustment for covariates including tumor stage, menopausal status, drug interactions, and use of aromatase inhibitors. To our knowledge, this is the first study of its type to prospectively collect multiple plasma samples for tamoxifen and endoxifen level in real-world patients during tamoxifen therapy, with sufficient sample size and follow-up period for primary clinical outcome. Table 1: Baseline Characteristics of PatientsCharacteristicsCYP2D6-Phenotypes, n=429 (%)Endoxifen concentration, n=429 (%)UM/EMIMPM>5.9ng/ml(~15nm)<5.9ng/ml(~15nm)n= 256n= 153n= 20n=376n= 53Age at DiagnosisMean5049485048Range24-8928-7928-8324-8427-88SexMale3 (1)7(5)08(2)2(4)Female253 (99)146(95)20(100)368(98)51(96)Tumor statusT1121(47)70(46)9(45)177(47)23(43)T297(38)58(38)10(50)142(38)23(43)T318(7)16(10)1(5)30(8)5(9)T412(5)4(3)016(4)0unknown8(3)5(3)011(3)2(4)Nodal statusN0124(97)71(46)10(50)179(48)26(49)N195 (37)53(35)9(45)144(38)13(24)N217(7)18(12)1(5)28(7)8(15)N313(5)5(3)015(4)3(6)Unknown7(3)6(4)010(3)3(6)HistologyIDC205(80)128(84)16(80)303(81)48(91)ILC28(11)14(9)3(15)41(11)3(6)other5(2)2(1)06(2)0unknown18(7)9(6)1(5)26(7)2(4)Grade154(21)24(16)3(15)78(21)3(6)2139(54)84(55)13(65)202(54)34(64)354(21)39(25)4(20)84(3)13(24)unknown9(4)6(4)012(3)3(6)ER+251(98)151(99)20(100)369(98)53(100)-5(2)2(1)07(2)0unknown00000PR+232(90)142(93)20(100)343(91)51(96)-24(9)11(7)033(9)2(4)unknown00000Her2+55(21)25(16)5(25)75(20)10(19)-200(78)127(84)15(75)299(80)43(81)unknown1102(1)0Breast SurgeryMastectomy149(58)84(55)11(55)214(57)30(57)Lumpectomy105(41)68(44)9(45)159(42)23(43)Unknown2(1)103(1)0Axillary SurgerySLNB137(54)80(52)12(60)204(54)25(47)ALND113(44)70(46)8(40)165(44)26(49)Unknown6(2)3(2)07(2)2(4)Chemotherapy190(74)108(71)16(80)277(74)37(70)Radiation Therapy213(83)126(82)17(85)310(82)46(87)Herceptin57(22)23(15)6(30)77(16)9(17) Citation Format: Veera Durga Sravanthi Panuganty, Denise Keller, Wendy A Teft, John Gordon Lenehan, Kylea Raijann Potvin, Jawaid Younus, Theodorus Anthony Vandenberg, Diane Mary Logan, Karin Hahn, Muriel Brackstone, Phillip Stanley Blanchette, Francisco Perera, Yun-Hee Choi, Richard Brian Kim. Clinical outcomes in early breast cancer patients on adjuvant tamoxifen: Impact of CYP2D6 genotype and observed endoxifen concentrations [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 P5-12-05.

Post-marketing assessment of generic tamoxifen in Brazilian breast cancer patients.

Generic formulations of tamoxifen are commonly prescribed to oestrogen receptor-positive breast cancer patients at the Brazilian National Cancer Institute (INCA). We carried out a post-marketing surveillance of the generic tamoxifen formulation in current use at INCA, by comparing plasma concentrations of the parent drug and metabolites obtained with the generic vs the reference formulation. Thirty patients participated in an open-label, bracketed protocol, comprising 3 successive phases of 30-32days each: the generic formulation was used in phases 1 and 3 and the reference formulation in phase 2. Two blood samples were collected in the last 4days of each phase, for LC-MS/MS quantification of tamoxifen and metabolites in plasma. The median plasma concentrations (ng/mL) for the reference formulation were as follows: tamoxifen, 135.0 (CI 95% 114.2-155.8); endoxifen, 35.3 (30.0-40.8); and 4-hydroxytamoxifen, 4.8 (4.2-5.4). The endoxifen/tamoxifen plasma concentration ratio was 0.27 (0.21-0.25). ANOVA detected no statistically significant difference in plasma concentrations of tamoxifen, metabolites or the endoxifen/tamoxifen ratio among the three phases. The genetic component (rGC) of the CYP2D6-mediated conversion of tamoxifen into endoxifen, estimated using the repeated drug administration procedure across the three phases, was 0.87, pointing to an important component of genetic variability. In conclusion, this first post-marketing surveillance trial of oncologic generic drugs carried out in Brazilian patients verified the switchability between the reference and the generic tamoxifen formulation currently used at our institution. The adopted bracketed protocol adds confidence to this conclusion and may serve as a frame for future trials of post-marketing assessment of other generic drug products.

Vaginal tamoxifen for treatment of vulvar and vaginal atrophy: Pharmacokinetics and local tolerance in a rabbit model over 28 days.

Vaginally delivered tamoxifen is being developed as alternative to estrogen-based therapies for the treatment of vulvar and vaginal atrophy (VVA) symptoms in subjects at high risk for breast cancer, undergoing treatment for breast cancer with aromatase inhibitors or are breast cancer survivors. Tamoxifen (1 or 20 mg) was administered intra-vaginally to female rabbits once-daily over a 28-day period to assess its pharmacokinetics, systemic exposure and local vaginal tolerance. Plasma samples were taken to assess concentrations of tamoxifen and its metabolites 4-hydroxytamoxifen and N-desmethyltamoxifen over the first day of vaginal administration and following the last dose on Day 28. In-life observations included evaluation of the vaginal region for signs of irritation. At necropsy, vaginal irritation was assessed by the method of Eckstein which reflect collective histopathological grading of four parameters within the vagina including epithelial morphology, leukocytic infiltration, congestion, and edema. Uterine effects of vaginal tamoxifen were also assessed. Plasma concentrations of tamoxifen were higher following administration of 20 mg tamoxifen compared to 1 mg tamoxifen at both Day 1 and Day 28. The metabolite 4-hydroxytamoxifen could not be detected on Day 1 and concentrations were low at Day 28 at the 1 mg tamoxifen dose. 4-Hydroxytamoxifen concentrations were low, but detectable at Day 1 and 28 following administration of 20 mg tamoxifen. The metabolite N-desmethyltamoxifen was undetectable at the 1 mg and 20 mg doses on Day 1; it remained undetectable at the 1 mg tamoxifen dose at Day 28. N-desmethyltamoxifen was detected over the first 8 h of Day 28 then fell below the quantitation limits. There was little to no vaginal or systemic accumulation of tamoxifen following once-daily dosing for 28 days. Tamoxifen accounted for more than 85% of the total systemic exposure compared to its metabolites, 4-hydroxytamoxifen, and N-desmethyltamoxifen. There was essentially no detectable vaginal irritation evident over the course of the study. At necropsy the individual Eckstein scores (maximum score of 16) of the proximal, mid, and distal vagina of females in the 1 mg and 20 mg dose groups were generally comparable in both groups and ranged from minimal to mild magnitude (1 mg dose group: ranging from 1 to 3 in the proximal vagina, 4 to 5 in the mid vagina, and 3 to 7 in the distal vagina; 20 mg dose group: ranging from 3 to 5 in the proximal vagina, 4 to 7 in the mid vagina, and 4 to 5 in the distal vagina). Overall, tamoxifen was absorbed and metabolized following vaginal administration and vaginal irritation was minimal to none at both doses.

Clinical Trial: CYP2D6 Related Dose Escalation of Tamoxifen in Breast Cancer Patients With Iranian Ethnic Background Resulted in Increased Concentrations of Tamoxifen and Its Metabolites.

Introduction: The polymorphic enzyme cytochrome P450 2D6 (CYP2D6) catalyzes a major step in the bioactivation of tamoxifen. Genotyping of clinically relevant CYP2D6 alleles and subsequent dose adjustment is a promising approach to individualize breast cancer therapy. The aim of this study was to investigate the relationship between the plasma levels of tamoxifen and its metabolites and different CYP2D6 genotypes under standard (20 mg/day) and dose-adjusted therapy (Registration ID in Iranian Registry of Clinical Trials: IRCT2015082323734N1).Materials and Methods: Using TaqMan® assays common alleles of CYP2D6 (∗1, ∗2, ∗4, ∗5, ∗6, ∗10, ∗17, and ∗41) and gene duplication were identified in 134 breast cancer patients. Based on CYP2D6 genotypes patients with an activity score 1 (n = 15) and 0–0.5 (n = 2) were treated with tamoxifen adjusted dosage of 30 and 40 mg/day, respectively. The concentration of tamoxifen and its metabolites before and after 4 and 8 months of dose adjustment were measured using LC-MS/MS technology.Results: At baseline, (Z)-endoxifen plasma concentrations (33 ± 15.5, 28.1 ± 14, 26.6 ± 23.4, 14.3 ± 8.6, and 10.7 ± 5.5 nmol/l for EM/EM, EM/IM, EM/PM, IM/IM and PM/PM, respectively) and the metabolic ratio (Z)-Endoxifen/N-desmethyltamoxifen (0.0558 ± 0.02, 0.0396 ± 0.0111, 0.0332 ± 0.0222, 0.0149 ± 0.0026, and 0.0169 ± 0.0177 for EM/EM, EM/IM, EM/PM, IM/IM, and PM/PM, respectively) correlated with CYP2D6 genotype (Kruskal–Wallis p = 0.013 and p < 0.0001, respectively). Dose escalation to 30 and 40 mg/day in patients with a CYP2D6 activity score of 1 (n = 15) and 0–0.5 (n = 2) resulted in a significant increase in (Z)-endoxifen plasma levels (22.17 ± 24.42, 34.43 ± 26.54, and 35.77 ± 28.89 nmol/l at baseline, after 4 and 8 months, respectively, Friedman p = 0.0388) along with the plasma concentrations of tamoxifen and its other metabolites. No severe side effects were recorded during dose escalation.Conclusion: For the first time, we show the feasibility of dose escalation of tamoxifen in breast cancer patients with compromised CYP2D6 activity and Iranian ethnic background to increase the plasma concentrations of (Z)-endoxifen.

CYP2D6 genotype and endoxifen plasma concentration do not predict hot flash severity during tamoxifen therapy.

Tamoxifen is frequently prescribed to prevent breast cancer recurrence. Tamoxifen is a prodrug and requires bioactivation by CYP2D6. Tamoxifen use is often limited by adverse effects including severe hot flashes. There is paucity of prospectively collected data in terms of CYP2D6 genotype and measured tamoxifen, 4-hydroxytamoxifen and endoxifen concentrations in relation to hot flash severity during tamoxifen therapy. We conducted a longitudinal prospective study of breast cancer patients on tamoxifen (n = 410). At each visit, blood samples were collected, and patients completed a standardized hot flash survey (n = 1144) that reflected hot flash severity during the 7 days prior to the visit. Plasma concentrations of tamoxifen, 4-hydroxytamoxifen, and endoxifen were measured using liquid chromatography-tandem mass spectrometry and genotyping was carried out for CYP2D6. A linear mixed-effects regression analysis assessed the association of covariates in relation to the hot flash severity score (HFSS). Median age at first assessment was 50years with 61.9% of patients considered peri-menopausal. Most patients (92.2%) experienced hot flash symptoms with 51.0% having low HFSS (0-4) and 7.32% experiencing HFSS > 25. Age was significantly associated with hot flash severity, with patients aged 45-59 more likely to have higher HFSS. Neither duration of tamoxifen therapy nor observed tamoxifen, endoxifen and 4-hydroxy tamoxifen plasma concentration predicted hot flash severity. Genetic variation in CYP2D6 or CYP3A4 was not predictive of hot flash severity. Hot flash severity during tamoxifen therapy can not be accounted for by CYP2D6 genotype or observed plasma concentration of tamoxifen, 4-hydroxytamoxifen, or endoxifen.

Cytochrome P450 Genetic Variation Associated with Tamoxifen Biotransformation in American Indian and Alaska Native People.

Despite evidence that pharmacogenetics can improve tamoxifen pharmacotherapy, there are few studies with American Indian and Alaska Native (AIAN) people. We examined variation in cytochrome P450 (CYP) genes (CYP2D6, CYP3A4, CYP3A5, and CYP2C9) and tamoxifen biotransformation in AIAN patients with breast cancer (n = 42) from the Southcentral Foundation in Alaska and the Confederated Salish and Kootenai Tribes in Montana. We tested for associations between CYP diplotypes and plasma concentrations of tamoxifen and metabolites. Only the CYP2D6 variation was significantly associated with concentrations of endoxifen (P = 0.0008) and 4‐hydroxytamoxifen (P = 0.0074), tamoxifen's principal active metabolites, as well as key metabolic ratios. The CYP2D6 was also the most significant predictor of active metabolites and metabolic ratios in a multivariate regression model, including all four genes as predictors, with minor roles for other CYP genes. In AIAN populations, CYP2D6 is the largest contributor to tamoxifen bioactivation, illustrating the importance of validating pharmacogenetic testing for therapy optimization in an understudied population.

Variations in plasma concentrations of tamoxifen metabolites and the effects of genetic polymorphisms on tamoxifen metabolism in Korean patients with breast cancer.

Inter-individual variation in tamoxifen metabolism in breast cancer patients is caused by various genetic and clinical factors. We measured the plasma concentrations of tamoxifen and its metabolites and investigated genetic polymorphisms influencing those concentrations. We measured the concentrations of tamoxifen, endoxifen, N-desmethyltamoxifen (NDM), and 4-hydroxytamoxifen (4-OH tamoxifen) in 550 plasma specimens from 281 breast cancer patients treated with tamoxifen. Duplicate or triplicate specimens were obtained from 179 patients at 3-month intervals. In 80 patients, genotyping for tamoxifen metabolizing enzymes was performed using the DMET Plus array and long-range PCR. Plasma concentrations of tamoxifen and its metabolites showed wide variations among patients. The following genetic polymorphisms were associated with the plasma concentrations when body mass index and tamoxifen concentrations were considered as co-variables: CYP1A2 -2467delT, CYP2B6 genotype, CYP2D6 activity score (AS), and FMO3 441C>T. CYP2D6 AS and three variants in the SULT1E1 gene showed correlation with ratios of tamoxifen metabolites. CYP2D6 AS was the only variable that showed associations with both metabolite concentration and ratio: endoxifen (P < 0.001), NDM (P < 0.001), endoxifen/NDM (P < 0.001), NDM/tamoxifen (P < 0.001), and 4-OH tamoxifen/tamoxifen (P = 0.005). Serial measurements of 448 plasma concentrations in 179 patients at 3-month intervals showed wide intra-individual variation. Our study showed that genetic polymorphisms can in part determine the baseline concentrations of tamoxifen and its metabolites. However, marked intra-individual variations during follow-up monitoring were observed, and this could not be explained by genotype. Therefore, serial measurements of tamoxifen and its metabolites would be helpful in monitoring in vivo tamoxifen metabolic status.

Are plasma concentrations of tamoxifen active metabolites sufficient to ensure therapeutic efficacy for tamoxifen treated women with breast cancer in Poland?

Tamoxifen is the most commonly used drug for treating those patients with breast cancer who are oestrogen receptor positive. The main active metabolite of tamoxifen is (Z)-endoxifen whose therapeutic efficacy depends on its plasma concentration. A therapeutically effective threshold level has indeed been defined for (Z)-endoxifen above which the breast cancer relapse rate is significantly reduced. Such steady-state concentrations are conditional on gene polymorphism, principally cytochrome P450 2D6 ( CYP2D6 ), that modulates the activity of the encoded enzymes that convert tamoxifen to its active metabolites. This drug’s metabolism however may become significantly altered when other medication is concomitantly taken, such as selective serotonin reuptake inhibitors, which inhibit CYP2D6. A recent study have demonstrated that the majority of tamoxifen treated women with breast cancer in Poland, may not in fact attain the therapeutic threshold levels of (Z)-endoxifen. In such cases, personalising optimal treatment should be based on direct monitoring of steady-state plasma concentrations of tamoxifen and its metabolites, which can thereby significantly improve therapeutic efficacy.

Pronounced Interindividual But Not Intraindividual Variation in Tamoxifen and Metabolite Levels in Plasma During Adjuvant Treatment of Women With Early Breast Cancer.

Tamoxifen is still an important antihormonal treatment option for patients with breast cancer and estrogen receptor-positive tumors. More than 20% of patients relapse despite treatment. The drug is usually dosed 20 mg/d irrespective of interindividual variation in drug clearance. To study interindividual and intraindividual variation in plasma levels we measured tamoxifen and metabolite levels in plasma on 2 occasions, with at least 4 weeks in between, of 39 women (19 premenopausal and 20 postmenopausal women) on adjuvant treatment (20 mg/d) of early breast cancer. We used an ultra-performance liquid chromatography with a mass spectrometry detection method for identification and quantification of tamoxifen, N-desmethyltamoxifen, 4-OH-tamoxifen, and endoxifen. Follicle-stimulating hormone, luteinizing hormone, and estradiol levels were also measured. The plasma concentrations of tamoxifen and its metabolites showed a pronounced interindividual variation, whereas intraindividual concentrations were rather stable. Despite the same dosage, interindividual tamoxifen concentrations varied from 51 to 307 ng/mL (124 ± 57, mean ± SD) and endoxifen values showed a range from 3.2 to 19 ng/mL (10.4 ± 5.2, mean ± SD), that is, 6-fold variation for both. Large interindividual variation of tamoxifen and endoxifen with stable intraindividual levels, and too low levels of endoxifen in a considerable proportion of patients strongly support that therapeutic drug monitoring and individualized dosing could lead to optimal exposure and hopefully better outcome. A randomized outcome study between conventional dosing and therapeutic drug monitoring-guided dosing is needed to show whether this approach works.

Association of CYP2C19*2 and associated haplotypes with lower norendoxifen concentrations in tamoxifen-treated Asian breast cancer patients.

The aim was to examine the influence of CYP2C19 variants and associated haplotypes on the disposition of tamoxifen and its metabolites, particularly norendoxifen (NorEND), in Asian patients with breast cancer. Sixty-six CYP2C19 polymorphisms were identified in healthy Asians (n = 240), of which 14 were found to be tightly linked with CYP2C19*2, CYP2C19*3 and CYP2C19*17. These 17 SNPs were further genotyped in Asian breast cancer patients receiving tamoxifen (n = 201). Steady-state concentrations of tamoxifen and its metabolites were quantified using liquid chromatography–mass spectrometry. Non-parametric tests and regression methods were implemented to evaluate genotypic–phenotypic associations and haplotypic effects of the SNPs. CYP2C19 functional polymorphisms and their linked SNPs were not significantly associated with plasma concentrations of tamoxifen and its main metabolites N-desmethyltamoxifen, (Z)-4-hydroxytamoxifen and (Z)-Endoxifen. However, CYP2C19*2 and its seven linked SNPs were significantly associated with lower NorEND concentrations, MRNorEND/NDDM and MRNorEND/(Z)-END. Specifically, patients carrying the CYP2C19*2 variant allele A had significantly lower NorEND concentrations [median (range), GG vs. GA vs. AA: 1.51 (0.38–3.28) vs. 1.28 (0.30–3.36) vs. 1.15 ng ml−1 (0.26–2.45, P = 0.010)] as well as significantly lower MRNorEND/(Z)-END [GG vs. GA vs. AA: 9.40 (3.27–28.35) vs. 8.15 (2.67–18.9) vs. 6.06 (4.47–14.6), P < 0.0001] and MRNorEND/NDDM [GG vs. GA vs. AA: 2.75 (0.62–6.26) vs. 2.43 (0.96–4.18) vs. 1.75 (1.10–2.49), P < 0.00001]. CYP2C19 H2 haplotype, which included CYP2C19*2, was also significantly associated with lower NorEND concentrations (P = 0.0020), MRNorEND/NDDM (P < 0.0001) and MRNorEND/(Z)-END (P < 0.0001), indicating significantly lower formation rates of NorEND. These data highlight the potential relevance of CYP2C19 pharmacogenetics in influencing NorEND concentrations in tamoxifen-treated patients, which may influence treatment outcomes.

In-vivo evaluation of tamoxifen-loaded microspheres based on mixtures of poly (D,L-lactide-co-glycolide) and poly (D,L-lactide) polymers

Microspheres of different proportions of poly-(D,L-lactide-co-glycolide) and poly-(D,L-lactide) were formulated by spray drying as a drug-delivery system for the treatment of breast cancer with tamoxifen. These systems had been evaluated previously in vitro and showed very positive results that have led to further assessment in vivo. This work evaluates the performance of these systems in an organism by carrying out a study in female Wistar rats. Microspheres were subcutaneously injected into the back of rats for the assessment of not only the biocompatibility but also the release of the drug contained and its biodistribution. As, in vitro, these systems could release the drug under physiological conditions; different plasma concentrations of tamoxifen and one of its metabolites, 4-hydroxy-tamoxifen, were achieved depending on the polymer composition. Microspheres could reduce the accumulation of the drug in different nontarget organs and presented good biocompatibility.

Concomitant use of tamoxifen and endoxifen in postmenopausal early breast cancer: prediction of plasma levels by physiologically-based pharmacokinetic modeling.

PurposeTo overcome cytochrome P450 2D6 (CYP2D6) mediated tamoxifen resistance in postmenopausal early breast cancer, CYP2D6 phenotype-adjusted tamoxifen dosing in patients with impaired CYP2D6 metabolism and/or the application of endoxifen, the most potent tamoxifen metabolite, are alternative treatment options. To elucidate both strategies comprehensively we used a physiologically-based pharmacokinetic (PBPK) modeling approach.MethodsFirstly simulation of increasing tamoxifen dosages was performed by a virtual clinical trial including populations of CYP2D6 poor (PM), intermediate (IM) and extensive metabolizers (EM) (N = 8,000). Secondly we performed PBPK-simulations under consideration of tamoxifen use plus concomitant increasing dosages of endoxifen (N = 7,000).ResultsOur virtual study demonstrates that dose escalation of tamoxifen in IMs resulted in endoxifen steady-state plasma concentrations similar to CYP2D6 EMs whereas PMs did not reach EM endoxifen levels. Steady-state plasma concentrations of tamoxifen, N-desmethyl-tamoxifen, 4-hydroxy-tamoxifen and endoxifen were similar in CYP2D6 IMs and PMs versus EMs using once daily dosing of 20 mg tamoxifen and concomitant CYP2D6 phenotype-adjusted endoxifen dosing in IMs and PMs (1 mg/d and 3 mg/d, respectively).ConclusionIn conclusion, we suggest that co-administration of endoxifen in tamoxifen treated early breast cancer women with impaired CYP2D6 metabolism is a promising alternative to reach plasma concentrations comparable to CYP2D6 EM patients.

Effects of baicalein on the pharmacokinetics of tamoxifen and its main metabolite, 4-hydroxytamoxifen, in rats: Possible role of cytochrome p450 3A4 and P-glycoprotein inhibition by baicalein

The purpose of this study was to investigate the effects of baicalein on the pharmacokinetics of tamoxifen and its active metabolite, 4-hydroxytamoxifen, in rats. Tamoxifen and baicalein interact with cytochrome P450 (CYP) enzymes and P-glycoprotein (P-gp), and the increase in the use of health supplements may result in baicalein being taken concomitantly with tamoxifen as a combination therapy to treat orprevent cancer diseases. Pharmacokinetic parameters of tamoxifen and 4-hydroxytamoxifen were determined in rats after an oral administration of tamoxifen (10 mg/kg) to rats in the presence and absence of baicalein (0.5, 3, and 10 mg/kg). Compared to the oral control group (given tamoxifen alone), the area under the plasma concentration-time curve and the peak plasma concentration of tamoxifen were significantly increased by 47.6-89.1% and 54.8-100.0%, respectively. The total body clearance was significantly decreased (3 and 10 mg/kg) by baicalein. Consequently, the absolute bioavailability of tamoxifen in the presence of baicalein (3 and 10 mg/kg) was significantly increased by 47.5-89.1% compared with the oral control group (20.2%). The metabolite-parent AUC ratio of tamoxifen was significantly reduced, implying that the formation of 4-hydroxytamoxifen was considerably affected by baicalein. Baicalein enhanced the oral bioavailability of tamoxifen, which may be mainly attributable to inhibition of the CYP3A4-mediated metabolism of tamoxifen in the small intestine and/or in the liver, and inhibition of the P-gp efflux pump in the small intestine and/or reduction of total body clearance by baicalein.

Dose-adjustment study of tamoxifen based on CYP2D6 genotypes in Japanese breast cancer patients

CYP2D6 is a key enzyme responsible for the metabolism of tamoxifen to active metabolites, endoxifen, and 4-hydroxytamoxifen. The breast cancer patients who are heterozygous and homozygous for decreased-function and null alleles of CYP2D6 showed lower plasma concentrations of endoxifen and 4-hydroxytamoxifen compared to patients with homozygous-wild-type allele, resulting in worse clinical outcome in tamoxifen therapy. We recruited 98 Japanese breast cancer patients, who had been taking 20 mg of tamoxifen daily as adjuvant setting. For the patients who have one or no normal allele of CYP2D6, dosages of tamoxifen were increased to 30 and 40 mg/day, respectively. The plasma concentrations of tamoxifen and its metabolites were measured at 8 weeks after dose-adjustment using liquid chromatography-tandem mass spectrometry. Association between tamoxifen dose and the incidence of adverse events during the tamoxifen treatment was investigated. In the patients with CYP2D6*1/*10 and CYP2D6*10/*10, the mean plasma endoxifen levels after dose increase were 1.4- and 1.7-fold higher, respectively, than those before the increase (P < 0.001). These plasma concentrations of endoxifen achieved similar level of those in the CYP2D6*1/*1 patients receiving 20 mg/day of tamoxifen. Plasma 4-hydroxytamoxifen concentrations in the patients with CYP2D6*1/*10 and CYP2D6*10/*10 were also significantly increased to the similar levels of the CYP2D6*1/*1 patients according to the increasing tamoxifen dosages (P < 0.001). The incidence of adverse events was not significantly different between before and after dose adjustment. This study provides the evidence that dose adjustment is useful for the patients carrying CYP2D6*10 allele to maintain the effective endoxifen level.

Impact of CYP2D6, CYP3A5, CYP2C9 and CYP2C19 polymorphisms on tamoxifen pharmacokinetics in Asian breast cancer patients

To investigate the impact of genetic polymorphisms in CYP2D6, CYP3A5, CYP2C9 and CYP2C19 on the pharmacokinetics of tamoxifen and its metabolites in Asian breast cancer patients. A total of 165 Asian breast cancer patients receiving 20 mg tamoxifen daily and 228 healthy Asian subjects (Chinese, Malay and Indian; n= 76 each) were recruited. The steady-state plasma concentrations of tamoxifen and its metabolites were quantified using high-performance liquid chromatography. The CYP2D6 polymorphisms were genotyped using the INFINITI™ CYP450 2D6I assay, while the polymorphisms in CYP3A5, CYP2C9 and CYP2C19 were determined via direct sequencing. The polymorphisms, CYP2D6*5 and *10, were significantly associated with lower endoxifen and higher N-desmethyltamoxifen (NDM) concentrations. Patients who were *1/*1 carriers exhibited 2.4- to 2.6-fold higher endoxifen concentrations and 1.9- to 2.1-fold lower NDM concentrations than either *10/*10 or *5/*10 carriers (P < 0.001). Similarly, the endoxifen concentrations were found to be 1.8- to 2.6-times higher in *1/*5 or *1/*10 carriers compared with *10/*10 and *5/*10 carriers (P≤ 0.001). Similar relationships were observed between the CYP2D6 polymorphisms and metabolic ratios of tamoxifen and its metabolites. No significant associations were observed with regards to the polymorphisms in CYP3A5, CYP2C9 and CYP2C19. The present study in Asian breast cancer patients showed that CYP2D6*5/*10 and *10/*10 genotypes are associated with significantly lower concentrations of the active metabolite of tamoxifen, endoxifen. Identifying such patients before the start of treatment may be useful in optimizing therapy with tamoxifen. The role of CYP3A5, CYP2C9 and CYP2C19 seem to be minor.

Activity Levels of Tamoxifen Metabolites at the Estrogen Receptor and the Impact of Genetic Polymorphisms of Phase I and II Enzymes on Their Concentration Levels in Plasma

The therapeutic effect of tamoxifen depends on active metabolites, e.g., cytochrome P450 2D6 (CYP2D6) mediated formation of endoxifen. To test for additional relationships, 236 breast cancer patients were genotyped for CYP2D6, CYP2C9, CYP2B6, CYP2C19, CYP3A5, UGT1A4, UGT2B7, and UGT2B15; also, plasma concentrations of tamoxifen and 22 of its metabolites, including the (E)-, (Z)-, 3-, and 4'-hydroxymetabolites as well as their glucuronides, were quantified using liquid chromatography-tandem mass spectrometry (MS). The activity levels of the metabolites were measured using an estrogen response element reporter assay; the strongest estrogen receptor inhibition was found for (Z)-endoxifen and (Z)-4-hydroxytamoxifen (inhibitory concentration 50 (IC50) 3 and 7nmol/l, respectively). CYP2D6 genotypes explained 39 and 9% of the variability of steady-state concentrations of (Z)-endoxifen and (Z)-4-hydroxytamoxifen, respectively. Among the poor metabolizers, 93% had (Z)-endoxifen levels below IC90 values, underscoring the role of CYP2D6 deficiency in compromised tamoxifen bioactivation. For other enzymes tested, carriers of reduced-function CYP2C9 (*2, *3) alleles had lower plasma concentrations of active metabolites (P < 0.004), pointing to the role of additional pathways.