Endoxifen Plasma Concentrations Research Articles

Nonlinear Mixed-Effects Model of Z-Endoxifen Concentrations in Tamoxifen-Treated Patients from the CEPAM Cohort.

Tamoxifen is widely used in patients with hormone receptor-positive breast cancer. The polymorphic enzyme CYP2D6 is primarily responsible for metabolic activation of tamoxifen, resulting in substantial interindividual variability of plasma concentrations of its most important metabolite, Z-endoxifen. The Z-endoxifen concentration thresholds below which tamoxifen treatment is less efficacious have been proposed but not validated, and prospective trials of individualized tamoxifen treatment to achieve Z-endoxifen concentration thresholds are considered infeasible. Therefore, we aim to validate the association between Z-endoxifen concentration and tamoxifen treatment outcomes, and identify a Z-endoxifen concentration threshold of tamoxifen efficacy, using pharmacometric modeling and simulation. As a first step, the CYP2D6 Endoxifen Percentage Activity Model (CEPAM) cohort was created by pooling data from 28 clinical studies (> 7,000 patients) with measured endoxifen plasma concentrations. After cleaning, data from 6,083 patients were used to develop a nonlinear mixed-effect (NLME) model for tamoxifen and Z-endoxifen pharmacokinetics that includes a conversion factor to allow inclusion of studies that measured total endoxifen but not Z-endoxifen. The final parent-metabolite NLME model confirmed the primary role of CYP2D6, and contributions from body weight, CYP2C9 phenotype, and co-medication with CYP2D6 inhibitors, on Z-endoxifen pharmacokinetics. Future work will use the model to simulate Z-endoxifen concentrations in patients receiving single agent tamoxifen treatment within large prospective clinical trials with long-term survival to identify the Z-endoxifen concentration threshold below which tamoxifen is less efficacious. Identification of this concentration threshold would allow personalized tamoxifen treatment to improve outcomes in patients with hormone receptor-positive breast cancer.

The interplay between tamoxifen and endoxifen plasma concentrations and coagulation parameters in patients with primary breast cancer

BackgroundTamoxifen is an effective treatment for primary breast cancer but increases the risk for venous thromboembolism. Tamoxifen decreases anticoagulant proteins, including antithrombin (AT), protein C (PC) and tissue factor (TF) pathway inhibitor, and enhances thrombin generation (TG). However, the relation between plasma concentrations of both tamoxifen and its active metabolite endoxifen and coagulation remains unknown. MethodsTamoxifen and endoxifen were measured in 141 patients from the prospective open-label intervention TOTAM-study after 3 months (m) and 6 m of tamoxifen treatment. Levels of AT and PC, the procoagulant TF, and TG parameters were determined at both timepoints if samples were available (n = 53–135 per analysis). Levels of coagulation proteins and TG parameters were correlated and compared between: 1) quartiles of tamoxifen and endoxifen levels, and 2) 3 m and 6 m of treatment. ResultsAt 3 m, levels of AT, PC, TF and TG parameters were not associated with tamoxifen nor endoxifen levels. At 6 m, median TF levels were lower in patients in the 3rd (56.6 [33] pg/mL), and 4th (50.1 [19] pg/mL) endoxifen quartiles compared to the 1st (lowest) quartile (76 [69] pg/mL) (P=0.027 and P=0.018, respectively), but no differences in anticoagulant proteins or TG parameters were observed. An increase in circulating TF levels (3 m: 46.0 [15] versus 6 m: 54.4 [39] pg/mL, P < 0.001) and TG parameters was observed at the 6 m treatment timepoint, while AT and PC levels remained stable. ConclusionsOur results indicate that higher tamoxifen and endoxifen levels are not correlated with an increased procoagulant state, suggesting tamoxifen dose escalation does not further promote hypercoagulability.

Pharmacogenetics and pharmacokinetics of tamoxifen in a Zimbabwean breast cancer cohort.

Tamoxifen is the most used hormonal therapy for oestrogen receptor-positive breast cancer. CYP2D6 is the main enzyme in the metabolic pathway of tamoxifen to endoxifen. Variations in endoxifen plasma concentrations are associated with CYP2D6 polymorphisms. This study aimed to determine the association between the CYP2D6 polymorphisms and endoxifen plasma concentrations in a cohort of Zimbabwean breast cancer patients (n= 40). TaqMan genotyping and copy number assays were done to determine CYP2D6 genotypes. Tamoxifen and metabolites were quantitated using LC-MS/MS. The population had high frequencies of the CYP2D6 reduced function alleles, *17 (15%) and *29 (18%). The median endoxifen concentration was 4.78 ng/mL, and in 55% of the patients, mostly intermediate metabolizers were below the endoxifen therapeutic threshold of 5.97 ng/mL. The CYP2D6 phenotypes and activity scores were significantly associated with endoxifen plasma concentrations (P= 0.0151) and with endoxifen to N-desmethyl-tamoxifen ratios (P= 0.0006).

Abstract PD15-12: PD15-12 A pre-surgical window trial of oral tamoxifen versus transdermal 4-hydroxytamoxifen gel in women with estrogen receptor positive duct carcinoma in situ (DCIS)

Abstract Background: Adjuvant oral tamoxifen (TAM) benefits women with DCIS, but toxicity concerns have limited its acceptance. Transdermal therapy with 4-hydroxy tamoxifen (4-OHT) gel applied to the breast skin is a possible solution. Previous pilot data suggest equivalent anti-proliferative efficacy of TAM and 4-OHT gel, but minimal systemic exposure with transdermal therapy. We report a prospective double blinded randomized phase 2 trial comparing TAM to 4-OHT gel in women with DCIS. Methods: 107 women with estrogen receptor positive (≥10%) DCIS were randomized to TAM (20 mg/day + placebo gel) or 4-OHT gel (2mg 4-OHT gel/breast, bilaterally + oral placebo), for 4-10 weeks prior to surgery. The primary endpoint was reduction in DCIS Ki67 labeling index (LI). Secondary endpoints included the 12-gene DCIS Score assay (Exact Sciences), breast tissue and plasma concentrations of 4-OHT and endoxifen, TAM-responsive circulating proteins, and patient reported symptoms (Breast Eight Symptom Scale). We estimated that 80 evaluable participants would provide 80.5% power to establish non-inferiority of 4-OHT, defined as relative Ki67-LI decline &amp;gt;35% and absolute decline &amp;gt;2.6%, with one-sided 𝛼=0.10. Non-inferiority of 4-OHT gel for Ki67-LI reduction was tested using an ANCOVA model. Statistical comparisons within- and between-arms were calculated with paired t-test and Welch Two Sample t-test, respectively. Results: 72 of 87 women adhered to the protocol, and were evaluable for the primary endpoint (39 TAM and 33 4-OHT gel). Mean treatment duration was 47 days for TAM and 44 days for 4-OHT gel (p=0.2). The median absolute decline in Ki67 labeling index was significant in the oral TAM (-3.7%, p&amp;lt; 0.001) but not in 4-OHT gel arm (-1.3%, p=0.2) (p=0.002). Ki67 results following menopausal stratification also favored the TAM arm: (-1.3%; p=0.06 in 37 premenopausal women and -3.7%; p=0.02 in 35 postmenopausal women). Similarly, DCIS score showed a significantly greater reduction in the TAM (-14, p&amp;lt; 0.001) but not in the 4-OHT gel arm (-4, p=0.1). Tissue 4-OHT concentrations were non-significantly higher in the TAM arm and were similar between superficial and deep sampling locations (superficial 6.1 and 4.2 ng/g for TAM and 4-OHT gel, respectively, p= 0.55; deep 5.7 and 3.8 ng/g, respectively, p= 0.06), whereas plasma 4-OHT concentration was markedly lower in the gel group (2 ng/mL and 0.24 ng/mL for TAM and 4-OHT gel, respectively, P &amp;lt; 0.001). Endoxifen was abundant in plasma (11 ng/mL) and deep tissue (13 ng/g) of the TAM arm, but present in trace amounts in the 4-OHT gel arm (undetectable in plasma and 0.31 ng/g in tissue; p &amp;lt; 0.001). Circulating TAM responsive markers (insulin like growth factor 1, sex hormone binding globulin, von Willebrand factor, and protein S total) and vasomotor symptoms were significantly and unfavorably modulated by TAM, but not by 4-OHT gel therapy. Conclusions: The non-inferiority of transdermal 4-OHT gel to Tam in terms of anti-proliferative effect in DCIS lesions was not demonstrated at the doses used for this study. DCIS Score analysis gave similar results. Tissue 4-OHT concentration in 4-OHT gel and Tam-treated subjects was roughly similar. However, endoxifen exposure was higher with oral TAM therapy and may partially explain the observed differences in major endpoints. In future studies, use of higher 4-OHT gel doses, longer duration of treatment, or different formulation may overcome these. Citation Format: Oukseub Lee, Xinlei Mi, Yanfei Xu, Luis Blanco, Azza M. Akasha, Kelly Benante, Shanshan Zhang, Carissa LaBoy, Thomas Helland, Melissa Pilewskie, Amy Degnim, Zahraa Al-Hilli, Amanda L. Amin, E Shelley Hwang, Joseph M. Guenther, Simon Steinar Hustad, Demirkan B. Gursel, Masha Kocherginsky, Gunnar Mellgren, Eileen Dimond, Marjorie Perloff, Brandy M. Heckman-Stoddard, Seema Khan. PD15-12 A pre-surgical window trial of oral tamoxifen versus transdermal 4-hydroxytamoxifen gel in women with estrogen receptor positive duct carcinoma in situ (DCIS) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD15-12.

Cross-Ancestry Genome-Wide Association Study Defines the Extended CYP2D6 Locus as the Principal Genetic Determinant of Endoxifen Plasma Concentrations.

The therapeutic efficacy of tamoxifen is predominantly mediated by its active metabolites 4-hydroxy-tamoxifen and endoxifen, whose formation is catalyzed by the polymorphic cytochrome P450 2D6 (CYP2D6). Yet, known CYP2D6 polymorphisms only partially determine metabolite concentrations in vivo. We performed the first cross-ancestry genome-wide association study with well-characterized patients of European, Middle-Eastern, and Asian descent (n=497) to identify genetic factors impacting active and parent metabolite formation. Genome-wide significant variants were functionally evaluated in an independent liver cohort (n=149) and in silico. Metabolite prediction models were validated in two independent European breast cancer cohorts (n=287, n=189). Within a single 1-megabase (Mb) region of chromosome 22q13 encompassing the CYP2D6 gene, 589 variants were significantly associated with tamoxifen metabolite concentrations, particularly endoxifen and metabolic ratio (MR) endoxifen/N-desmethyltamoxifen (minimal P=5.4E-35 and 2.5E-65, respectively). Previously suggested other loci were not confirmed. Functional analyses revealed 66% of associated, mostly intergenic variants to be significantly correlated with hepatic CYP2D6 activity or expression (ρ=0.35 to -0.52), and six hotspot regions in the extended 22q13 locus impacting gene regulatory function. Machine learning models based on hotspot variants (n=12) plus CYP2D6 activity score (AS) increased the explained variability (~ 9%) compared with AS alone, explaining up to 49% (median R2 ) and 72% of the variability in endoxifen and MR endoxifen/N-desmethyltamoxifen, respectively. Our findings suggest that the extended CYP2D6 locus at 22q13 is the principal genetic determinant of endoxifen plasma concentration. Long-distance haplotypes connecting CYP2D6 with adjacent regulatory sites and nongenetic factors may account for the unexplained portion of variability.

Impact of organic anion transporting polypeptide, P-glycoprotein, and breast cancer resistance protein transporters on observed tamoxifen and endoxifen concentration and adverse effects.

Drug transporters are important determinants of drug disposition and response. Tamoxifen is an antiestrogen for breast cancer therapy known for adverse drug reactions (ADRs). In this study, the involvement of OATP transporters in tamoxifen and endoxifen transport was studied in vitro while the impact of single nucleotide variation (SNV) in OATP and efflux transporters P-glycoprotein ( ABCB1 ) and Breast Cancer Resistance Protein ( ABCG2 ) on ADRs during tamoxifen therapy were assessed. Patients receiving tamoxifen for breast cancer, who were CYP2D6 normal metabolizers were enrolled ( n = 296). Patients completed a survey that captured ADRs and a blood sample was collected. Tamoxifen and endoxifen plasma concentration were measured, while DNA was genotyped for SNVs in ABCB1, ABCG2, SLCO1A2, SLCO1B1 , and SLCO2B1 . HEK293T cells were used to determine the extent of OATP-mediated transport of tamoxifen and endoxifen. Common SNVs of ABCB1, ABCG2, SLCO1A2 , and SLCO1B1 were not associated with tamoxifen or endoxifen concentration. However, tamoxifen concentration was significantly higher in carriers of SLCO2B1 c.935G>A (129.8 ng/mL) compared to wildtype (114.9 ng/mL; P = 0.036). Interestingly, subjects who carried SLCO1A2 c.38A>G reported significantly less dizziness ( P = 0.016). In-vitro analysis demonstrated increased cellular accumulation of tamoxifen in cells overexpressing OATP1A2 and 1B1, but endoxifen uptake was not effected in OATP overexpressing cells. We showed that OATP1A2 , a transporter known to be expressed at the blood-brain barrier, is capable of tamoxifen transport. Additionally, OATP1A2 c.38A>G was associated with reduced ADRs. Taken together, our findings suggest genetic variation in OATP transporters may be an important predictor of tamoxifen ADRs.

178 (PB-091) Poster - The impact of therapeutic drug monitoring-based tamoxifen dose-reductions on endocrine side effects in patients with primary breast cancer

Background: Adjuvant treatment with tamoxifen in hormone sensitive breast cancer substantially reduces recurrence. Unfortunately, many patients experience side effects which lead to almost 50% of non-compliance. Recent studies suggest fewer side effects after dose-reduction of tamoxifen but this is hardly studied in the adjuvant setting, and was not based on endoxifen plasma concentrations before.

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.

Clinical CYP2D6 Genotyping to Personalize Adjuvant Tamoxifen Treatment in ER-Positive Breast Cancer Patients: Current Status of a Controversy

Simple SummaryTamoxifen is an important adjuvant endocrine therapy in estrogen receptor (ER)-positive breast cancer patients. It is mainly catalyzed by the enzyme CYP2D6 into the most active metabolite endoxifen. Genetic variation in the CYP2D6 gene influences endoxifen formation and thereby potentially therapy outcome. However, the association between CYP2D6 genotype and clinical outcome on tamoxifen is still under debate, as contradictory outcomes have been published. This review describes the latest insights in both CYP2D6 genotype and endoxifen concentrations, as well CYP2D6 genotype and clinical outcome, from 2018 to 2020.Tamoxifen is a major option for adjuvant endocrine treatment in estrogen receptor (ER) positive breast cancer patients. The conversion of the prodrug tamoxifen into the most active metabolite endoxifen is mainly catalyzed by the enzyme cytochrome P450 2D6 (CYP2D6). Genetic variation in the CYP2D6 gene leads to altered enzyme activity, which influences endoxifen formation and thereby potentially therapy outcome. The association between genetically compromised CYP2D6 activity and low endoxifen plasma concentrations is generally accepted, and it was shown that tamoxifen dose increments in compromised patients resulted in higher endoxifen concentrations. However, the correlation between CYP2D6 genotype and clinical outcome is still under debate. This has led to genotype-based tamoxifen dosing recommendations by the Clinical Pharmacogenetic Implementation Consortium (CPIC) in 2018, whereas in 2019, the European Society of Medical Oncology (ESMO) discouraged the use of CYP2D6 genotyping in clinical practice for tamoxifen therapy. This paper describes the latest developments on CYP2D6 genotyping in relation to endoxifen plasma concentrations and tamoxifen-related clinical outcome. Therefore, we focused on Pharmacogenetic publications from 2018 (CPIC publication) to 2021 in order to shed a light on the current status of this debate.

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 (&amp;gt;5.9ng/ml or &amp;lt;5.9ng/ml). CYP2D6 phenotype did not demonstrate a significant association for IDFS. However, Kaplan Meier analysis stratified for endoxifen concentration &amp;gt;vs&amp;lt; 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 &amp;lt; 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&amp;gt;5.9ng/ml(~15nm)&amp;lt;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.

Hormonal status affects plasma exposure of tamoxifen and its main metabolites in tamoxifen-treated breast cancer patients

BackgroundTamoxifen is considered a prodrug of its active metabolite endoxifen, which is dependent on the CYP2D6 and CYP3A enzymes. Tamoxifen pharmacokinetic variability influences endoxifen exposure and, consequently, its clinical outcome. This study investigated the impact of hormonal status on the pharmacokinetics of tamoxifen and its metabolites in TAM-treated breast cancer patients.MethodsTAM-treated breast cancer patients (n = 40) previously believed to have CYP3A activity within the normal range based on oral midazolam and phenotyped as CYP2D6 normal metabolizers using oral metoprolol were divided into two groups according to premenopausal (n = 20; aged 35–50 years) or postmenopausal (n = 20; aged 60–79 years) status. All patients were treated with 20 mg/day tamoxifen for at least three months. Serial plasma samples were collected within the 24 h dose interval for analysis of unchanged tamoxifen, endoxifen, 4-hydroxytamoxifen and N-desmethyltamoxifen quantified by LC-MS/MS. CYP activities were assessed using midazolam apparent clearance (CYP3A) and the metoprolol/alfa-hydroxymetoprolol plasma metabolic ratio (CYP2D6). CYP3A4, CYP3A5 and CYP2D6 SNPs and copy number variation were investigated using TaqMan assays.ResultsPostmenopausal status increased steady-state plasma concentrations (Css) of tamoxifen (116.95 vs 201.23 ng/mL), endoxifen (8.01 vs 18.87 ng/mL), N-desmethyltamoxifen (485.16 vs 843.88 ng/mL) and 4-hydroxytamoxifen (2.67 vs 4.11 ng/mL). The final regression models included hormonal status as the only predictor for Css of tamoxifen [β-coef ± SE, p-value (75.03 ± 17.71, p = 0.0001)] and 4-hydroxytamoxifen (1.7822 ± 0.4385, p = 0.0002), while endoxifen Css included hormonal status (8.578 ± 3.402, p = 0.02) and race (11.945 ± 2.836, p = 0.007). For N-desmethyltamoxifen Css, the final model was correlated with hormonal status (286.259 ± 76.766, p = 0.0007) and weight (− 8.585 ± 3.060, p = 0.008).ConclusionThe premenopausal status was associated with decreased endoxifen plasma concentrations by 135% compared to postmenopausal status. Thus, the endoxifen plasma concentrations should be monitored mainly in the premenopausal period to maintain plasma levels above the efficacy threshold value.Trial registrationRBR-7tqc7k.

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.

Improved Prediction of Endoxifen Metabolism by CYP2D6 Genotype in Breast Cancer Patients Treated with Tamoxifen.

Purpose: Prediction of impaired tamoxifen (TAM) to endoxifen metabolism may be relevant to improve breast cancer treatment, e.g., via TAM dose increase. The polymorphic cytochrome P450 2D6 (CYP2D6) strongly determines an individual’s capacity for endoxifen formation, however, CYP2D6 phenotype assignments inferred from genotype widely differ between studies. Thus, we modeled plasma endoxifen predictability depending on variable CYP2D6 genotype groupings.Methods: CYP2D6 diplotype and metabolite plasma concentrations were assessed in 908 pre- and post-menopausal estrogen receptor (ER)-positive, TAM treated early breast cancer patients of Caucasian (N = 678), Middle-Eastern Arab (N = 77), and Asian (N = 153) origin. Robust coefficients of determination (R2) were estimated for endoxifen (E) or metabolic ratio endoxifen/desmethyl-TAM (E/DMT) as dependent and different CYP2D6 phenotype assignments as independent variables. Allele activity scores (ASs) were modified with respect to a reduced ∗10 allele activity. Predictability of endoxifen plasma concentrations above the clinical threshold of 5.9 ng/mL was investigated by receiver operating characteristic (ROC) analysis.Results: CYP2D6 diplotypes (N = 898) were strongly associated with E and E/DMT independent of age (P < 10-15). Across all ethnicities, 68–82% inter-patient variability of E/DMT was explained by CYP2D6 diplotype, while plasma endoxifen was predictable by 39–58%. The previously used codeine specific phenotype classification showed worse prediction for both endpoints particularly in Asians (median R2< 20%; P < 10-9). Downgrading of ∗10 activity slightly improved the explanatory value of metabolizer phenotype (P < 0.002). Endoxifen plasma concentrations above the clinical threshold of 5.9 ng/mL were achieved in 82.3% of patients and were predictable (96% sensitivity, 57% specificity) by CYP2D6 diplotypes with AS > 0.5, i.e., omitting PM/PM and PM/IM patients.Conclusion: The CYP2D6 explanatory power for active drug level assessment is maximized by TAM-specific phenotype assignments while a genotype cutoff that separates PM/PM and PM/IM from the remaining patients may improve clinical benefit via increased endoxifen concentrations.

Abstract 5033: Optimized CYP2D6 phenotype assignment for plasma endoxifen prediction in breast cancer patients treated with tamoxifen

Abstract Tamoxifen (Tam), a standard therapy for estrogen receptor (ER)-positive breast cancer, is bioactivated to its active metabolite endoxifen by the cytochrome P450 enzyme CYP2D6. CYP2D6 polymorphisms constituting extensive (EM), intermediate (IM) and poor (PM) metabolizer phenotypes are main determinants of variable endoxifen plasma concentrations and supposed to influence therapeutic efficacy. Genotype-guided prediction of TAM metabolizer status has long been debated to be useful for clinical dose adjustment in patients with impaired metabolism. As this requires accurate genotype-phenotype assignments, we tested several CYP2D6 phenotype classifications including the codeine metabolism-based scoring implemented by the ‘Clinical Pharmacogenetics Implementation Consortium’ (Crews et al. 2014, CPT) for their predictive value of active metabolite concentrations. We applied linear modelling of square-root transformed plasma endoxifen or log-transformed metabolic ratio (MR) endoxifen/desmethyl-TAM depending on CYP2D6 phenotype classification in a cohort of 936 pre- and postmenopausal TAM treated breast cancer patients of European, Middle Eastern Arabic, and Asian descent (Muerdter et al CPT 2011; Saladores et al. TPJ 2015). CYP2D6 diplotypes were defined by PM (*3, *4, *5, *6, *7), IM (*9, *10, *41), EM (*1, *2, *35) or UM (duplicated EM) alleles. CYP2D6 phenotypes were defined by various diplotype groupings and an adapted activity score regarding the *10 allele. Models were adjusted for CYP2C9 and CYP3A5 genotypes, and robust coefficients of determination (R2) with 95% confidence intervals from 10.000 bootstrap replicates were calculated using R package lmrob. Our analyses indicate, that the commonly used codeine-based CYP2D6 phenotype assignment poorly predicts CYP2D6 mediated endoxifen formation. This is evident from a low predictability (&amp;lt;20%) for both metabolite endpoints particularly in Asians, most likely due to a misclassification of abundant IM/IM (*10) diplotypes as EM. Importantly, the active metabolite-to-precursor MR appears to be the endpoint that is more closely linked to CYP2D6 enzyme activity, as it explains up to 68-82% of MR variability. In contrast, absolute endoxifen concentrations were only modestly predictable (38-57%) by CYP2D6 diplotype or other phenotype groupings. Moreover, a reduced activity for the *10 compared to other IM alleles appears to improve endoxifen predictabilty by phenotype. Personalized TAM treatment decisions based on plasma endoxifen prediction should rely on TAM specific CYP2D6 genotype-guided phenotype assignments that quantitatively capture the allele-dosage effects and that include endoxifen-to-precursor MR as the endpoint most directly linked to CYP2D6. Citation Format: Werner Schroth, Stefan Winter, Michel Eichelbaum, Thomas Muerdter, Matthias Schwab, Hiltrud Brauch. Optimized CYP2D6 phenotype assignment for plasma endoxifen prediction in breast cancer patients treated with tamoxifen [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5033. doi:10.1158/1538-7445.AM2017-5033

Abstract 2029: A high ratio of tamoxifen metabolite E to tamoxifen is associated with an increased risk of breast cancer recurrences in premenopausal women

Abstract Tamoxifen (Tam), a standard therapy for estrogen receptor (ER)-positive breast cancer, is excessively metabolized mainly by enzymes of the CYP450 family to anti-estrogenic and estrogenic compounds. Whereas in breast cancer cells the anti-estrogenic metabolites endoxifen and 4-hydroxy-Tam inhibit the ER, Tam bisphenol (Bis) and both isomers (E)- and (Z)- of metabolite E (Met E) show full estrogenic properties, with (E)-Met E being the most potent ER agonist. Contrary to known clinical effects of ER antagonists, the influence of Tam Bis, (E)-, and (Z)-Met E on treatment outcome is not thoroughly explored. Therefore we analyzed whether interindividual differences in plasma levels of estrogenic Tam metabolites interact with Tam therapy in vivo. Plasma concentrations of estrogenic metabolites Bis, (E)-, and (Z)-Met E and their metabolic ratios (MR) to Tam were quantified in 306 premenopausal breast cancer patients mainly of European origin who were treated with 20 mg/day of Tam (Eccles D, et al. BMC Cancer 2007; 7: 160) using recently published LC-MS/MS methods (Johänning et al. Anal Bioanal Chem 2015; Mürdter et al. Clin Pharmacol Ther 2011). The endpoints recurrence-free interval (RFI) and event-free survival (EFS) were analyzed in hormone-receptor positive patients (N = 296) by Kaplan-Meyer and multivariate Cox regression adjusted for age, nodal status, tumor size, grade and plasma concentrations of endoxifen (as MR of desmethyl-Tam to endoxifen). Plasma concentrations were as follows: Bis 13-832 pM (median 187 pM), (E)-Met E 15-1029 pM (median 213 pM), (Z)-Met E 128-2484 pM (median 889 pM), and Tam 47-1061 nM (median 360 nM). Patients with higher (E)-Met E to Tam ratios showed a higher risk of breast cancer recurrence when classified into quartiles: compared to patients of the lower quartile, patients had shorter RFI when they grouped to the interquartile (P = 0.037) and the upper quartile (P = 0.005). There was a trend of higher recurrence risk for patients belonging to the upper quartile compared to the interquartile (P = 0.063). These findings were confirmed in multivariate Cox analyses: patients belonging to the upper quartile had a significantly increased hazard ratio (HR) of 2.77 (95% confidence interval (CI) 1.34-5.71; P = 0.006) and patients belonging to the interquartile showed a trend for increased HR of 1.75 (CI = 0.88-3.46; P = 0.1). For EFS, a linearly increased HR was confirmed for patients with higher (E)-Met E to Tam ratios (HR = 1.44, CI = 1.06-1.97; P = 0.021). Bis, (Z)-Met E and their MRs to Tam showed no significantly altered risk of cancer recurrence or death. These findings suggest that higher formation rates of the most potent estrogenic Tam metabolite (E)-Met E influence Tam outcome in breast cancer patients and may be linked to therapeutic failure. Citation Format: Janina Johänning, Diana M. Eccles, Bryony Eccles, Michel Eichelbaum, Matthias Schwab, Hiltrud Brauch, Thomas Mürdter, Werner Schroth. A high ratio of tamoxifen metabolite E to tamoxifen is associated with an increased risk of breast cancer recurrences in premenopausal women. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2029.

Boronic prodrug of endoxifen as an effective hormone therapy for breast cancer.

As a prodrug, tamoxifen is activated by the P450 enzyme CYP2D6 that is responsible for converting it to the active metabolites, 4-hydroxytamoxifen and endoxifen. Patients with genetic polymorphisms of CYP2D6 may not receive the full benefit of tamoxifen therapy. There is increasing evidence that poor metabolizer patients have lower plasma concentrations of endoxifen and suffer worse disease outcome, although some clinical studies reported no correlation between CYP2D6 polymorphism and tamoxifen therapy outcome. Endoxifen is currently undergoing clinical trials as a potentially improved and more potent SERM (Selective Estrogen Receptor Modulator) for endocrine therapy that is independent of CYP2D6 status in patients. However, direct administration of endoxifen may present the problem of low bioavailability due to its rapid first-pass metabolism via O-glucuronidation. We have designed and synthesized ZB483, a boronic prodrug of endoxifen suitable for oral administration with greatly enhanced bioavailability by increasing the concentration of endoxifen in mouse blood. Our study demonstrated that ZB483 potently inhibited growth of ER+ breast cancer cells in vitro and was efficiently converted to endoxifen in cell culture media by oxidative deboronation. This metabolic conversion is equally efficient in vivo as indicated in the pharmacokinetic study in mice. Moreover, when administered at the same dose, oral ZB483 afforded a 30- to 40-fold higher plasma level of endoxifen in mice than oral administration of endoxifen. The significantly enhanced bioavailability of endoxifen conferred by the boronic prodrug was further validated in an in vivo efficacy study. ZB483 was demonstrated to be more efficacious than endoxifen in inhibiting xenograft tumor growth in mice at equal dosage but more so at lower dosage. Together, these preclinical studies demonstrate that ZB483 is a promising endocrine therapy agent with markedly enhanced bioavailability in systemic circulation and superior efficacy compared to endoxifen.

Abstract OT2-1-05: Prospective multicenter study evaluating the effect of impaired tamoxifen metabolization on efficacy in breast cancer patients receiving tamoxifen in the neo-adjuvant or metastatic setting - The CYPTAM-BRUT 2 trial

Abstract Background Tamoxifen is commonly used to treat and prevent hormone receptor positive breast cancers. This drug is metabolized into more active metabolites by liver enzymes such as cytochrome P450 (CYP) enzymes. Endoxifen is considered to be the principal active metabolite of tamoxifen. As CYP enzymes are highly polymorphic in humans, endoxifen plasma levels are modulated by the patient’s genotype. It, however, is not yet clear if lowered endoxifen plasma levels have an effect on tamoxifen efficacy. This is the first prospective study where the association between endoxifen plasma concentrations, multiple CYP-genotypes and clinical outcome in postmenopausal patients treated with tamoxifen is investigated. Trial Design CYPTAM-BRUT 2 is a prospective multi-center open label, single-arm, non-randomized observational study. Postmenopausal women with measurable, estrogen receptor positive breast cancer receiving tamoxifen as neo-adjuvant or as first-line metastatic treatment are included in this study. The objective treatment response and clinical benefit are observed to investigate the efficacy of 20 mg tamoxifen daily. Patients are allowed to have started tamoxifen before inclusion but not more than three months. Further, if more than twelve months have passed after completion of the adjuvant therapy prior endocrine therapy in the adjuvant setting is allowed. Patients receiving neo-adjuvant tamoxifen will be assessed no more than four months after starting with tamoxifen. The primary endpoint is a statistical association between steady-state endoxifen plasma concentrations and the objective response rate (ORR) after 3-6 months of tamoxifen, under the assumption that the relationship is linear with an odds ratio (OR) of 1.49 per 10 nmol/L. Using available data on endoxifen concentrations, this OR is chosen to reflect an improvement from 10% ORR in the lowest endoxifen quartile to 30% in the highest endoxifen quartile when the overall ORR is around 18%. To have 90% power at a 5% significance level, 180 patients have to be included into the study. The main secondary study endpoint is the relation between endoxifen plasma concentrations and clinical benefit (CR+PR+SD at 6 months). The study has to include 270 patients to detect a statistically significant association with endoxifen with 90% power at a 5% significance level, assuming an OR of 1.28 per 10 nmol/L. This OR is chosen to reflect an improvement of clinical benefit at 6 months from 30% in the lowest endoxifen quartile to 50% in the highest endoxifen quartile (overall clinical benefit around 39%). For both endpoints the RECIST criteria are used. Other endpoints are progression-free survival, tolerability of tamoxifen treatment and the association between CYP2D6 genotype and clinical outcome. Patient accrualPatients from 22 participating centers in Belgium and Switzerland are included in this trial. In May 2014, the predefined sample size of 270 patients was reached. Follow-up of the last patients will continue until all required data are obtained (i.e blood samples and response evaluation). Citation Format: Kathleen Van Asten, Lynn Jongen, Anne-Sophie Dieudonné, Anneleen Lintermans, Chantal Blomme, Olivier Brouckaert, Diether Lambrechts, Hans Wildiers, Marie-Rose Christiaens, Dirk Timmerman, Ben Van Calster, Jan Decloedt, Patrick Berteloot, Didier Verhoeven, Markus Joerger, Khalil Zaman, Vincent Dezentjé, Patrick Neven. Prospective multicenter study evaluating the effect of impaired tamoxifen metabolization on efficacy in breast cancer patients receiving tamoxifen in the neo-adjuvant or metastatic setting - The CYPTAM-BRUT 2 trial [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr OT2-1-05.

SY10-1 - Pharmacokinetic Issues on Cancer Pharmacotherapy

Efficacy of cancer pharmacotherapy depends upon drug exposure to the body and responsiveness of the tumor to the drug treatment. Drug exposure is determined by pharmacokinetics and dose, whereas responsiveness is intrinsically variable by tumor heterogeneity. Recently, a number of molecular targeting agents are used as promising therapy for cancers, and most of them are coupled with particular diagnostics which can predict treatment response. These diagnostics examine somatic mutations of individual cancers, and are used to distinguish responders and non-responders before choosing right patients and right drugs.On the other hand, lower drug exposure to the body also leads to insufficient efficacy. Although most of pharmacokinetic studies in the past were related to the toxicity of oncologic agents, a number of important evidences have been published recently on exposure-effect relationships for molecular targeting, chemotherapeutic and hormonal agents. For example, efficacy of tyrosine kinase inhibitor, imatinib in chronic myelogenous leukemia (CML) or gastrointestinal stromal tumor (GIST) depended on plasma concentration of imatinib. Therefore, therapeutic drug monitoring (TDM) of imatinib has been introduced into clinical practice in Japan. Usefulness of TDM was also reported to improve the efficacy and safety of continuous 5-fluorouracil therapy. Individual variation in the capability of hepatic activation creates inter-individual variation in efficacy of the tamoxifen treatment. CYP2D6 is a key enzyme responsible for converting tamoxifen to active metabolites, endoxifen and 4-hydroxytamoxifen. The breast cancer patients who carry decreased-functional alleles of CYP2D6 showed lower plasma concentrations of endoxifen compared to patients with normal functional allele, probably resulting in worse clinical outcome in tamoxifen therapy. Knowledge of the exposure-response relationships becomes quite important for improving cancer pharmacotherapy.

Abstract LB-229: A phase 1 open-label fixed-sequence two-period crossover study of the effect of multiple doses of tamoxifen on palbociclib (PD-0332991) pharmacokinetics in healthy male volunteers

Abstract Introduction: Tamoxifen and one of its primary active metabolites, 4‑hydroxy‑tamoxifen, are known to induce CYP3A4 in vitro, and in vivo coadministration of tamoxifen with letrozole and anastrozole (both CYP3A4 substrates) has resulted in decreased exposures (AUC) of each by 37% and 27%, respectively. The primary route of oxidative metabolism of palbociclib (PD‑0332991) is through CYP3A4, thus coadministration of tamoxifen and palbociclib could result in decreased exposure of palbociclib. The primary objective of this study is to estimate the relative exposure of single 125 mg palbociclib oral doses alone and in the presence of steady‑state concentrations of tamoxifen and its active metabolites (4‑hydroxy‑tamoxifen, N‑desmethyl‑tamoxifen, and endoxifen (N‑desmethyl‑4-hydroxytamoxifen)) in healthy male subjects in order to inform appropriate palbociclib dosing when administered in combination with tamoxifen in the upcoming Phase 3 PENELOPEB study. Methods: This was an open label, 2‑period fixed‑sequence study of the effect of multiple oral doses of tamoxifen on palbociclib pharmacokinetics in 25 healthy male volunteers. Each subject received a single 125 mg oral dose of palbociclib alone, followed by a single 125 mg dose of palbociclib on Day 22 of a 27-day tamoxifen loading dose regimen (60mg QD for 4 days, followed by 20mg QD for 23 days). Serial blood sampling for palbociclib pharmacokinetics was performed up to 144 hours post-palbociclib dose in both periods. Tamoxifen pre-dose blood sampling was performed on specified visits to document achievement of steady-state tamoxifen and metabolite concentrations. Plasma concentrations of palbociclib, tamoxifen, 4‑hydroxy‑tamoxifen, N‑desmethyl‑tamoxifen, and endoxifen were each measured using validated LC/MS/MS (liquid chromatography coupled with mass spectrometry) methods. Palbociclib plasma pharmacokinetic (PK) parameters were estimated using standard non-compartmental methods. Discussion: There were no significant changes in the palbociclib plasma PK parameters AUCinf (area under the curve from time zero to infinity) and Cmax (maximal plasma concentration) in the presence of steady-state tamoxifen when compared to palbociclib alone. Tamoxifen, 4‑hydroxy‑tamoxifen, N‑desmethyl‑tamoxifen, and endoxifen all achieved steady-state prior to administration of palbociclib in period 2. In general, palbociclib and the combination of palbociclib and tamoxifen were well tolerated with no reported serious adverse events; 1 subject discontinued due to generalized itchiness which resolved with a single dose of fexofenadine. Citation Format: Justin T. Hoffman, Melissa O'Gorman, Cho-Ming Loi, Anna Plotka, Tanya Boutros, Leonid Kirkovsky, Corrado Gallo Stampino, Diane Wang. A phase 1 open-label fixed-sequence two-period crossover study of the effect of multiple doses of tamoxifen on palbociclib (PD-0332991) pharmacokinetics in healthy male volunteers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-229. doi:10.1158/1538-7445.AM2014-LB-229