Tamoxifen Metabolism Research Articles

Solanidine Metabolites as Diet-Derived Biomarkers of CYP2D6-Mediated Tamoxifen Metabolism in Breast Cancer Patients.

Tamoxifen is an important antiestrogen for the treatment of hormone receptor-positive breast cancer and undergoes bioactivation by CYP2D6 to its active metabolite endoxifen. Genetic variation in CYP2D6 has been linked to endoxifen levels during tamoxifen therapy. Recent studies have suggested solanidine, a glycoalkaloid phytochemical in potatoes, undergoes CYP2D6-mediated metabolism to 4-OH-solanidine (m/z 414) and 3,4-seco-solanidine-3,4-dioic acid (SSDA; m/z 444). Using a retrospective cohort of 1,032 breast cancer patients on tamoxifen therapy, we examined the association of solanidine metabolites with CYP2D6 activity and its correlation with tamoxifen metabolism. Solanidine, 4-OH-solanidine, or SSDA was detected in 99.7% (N = 1,029) of plasma samples. Decreased solanidine metabolite ratios were found in CYP2D6 intermediate and poor metabolizers (P < 0.0001). Patients on CYP2D6 strong inhibitors had a 77.6% and 94.2% decrease in 4-OH-solandine/solanidine (P < 0.0001) and SSDA/solanidine (P < 0.0001), respectively. The ratio of endoxifen to tamoxifen was highly correlated with both 4-OH-solandine/solanidine (ρ = 0.3207, P < 0.0001) and SSDA/solanidine (ρ = 0.5022, P < 0.0001) ratios. Logistic regression modeling was used to determine that 4-OH-solanidine/solanidine and SSDA/solanidine ratios below 2.1 and 0.8, respectively, predicted endoxifen concentrations of <16 nM. In conclusion, solanidine, 4-OH-solanidine, and SSDA are diet-derived biomarkers of CYP2D6 activity. Moreover, in patients on tamoxifen therapy, 4-OH-solanidine/solanidine and SSDA/solanidine predicted endoxifen levels including the inhibitory effects of concomitantly prescribed CYP2D6-interacting medications. Accordingly, 4-OH-solanidine/solanidine or SSDA/solanidine ratio has the potential to be particularly useful prior to initiation of tamoxifen or for determining the impact of CYP2D6 drug interactions, as well as prior to switching from an aromatase inhibitor to tamoxifen.

Endoxifen Concentration is Associated with Recurrence-Free Survival in Hormone-Sensitive Breast Cancer Patients.

The metabolism of tamoxifen is influenced by various cytochrome p450 enzymes, including CYP2D6 and CYP2C19, leading to variations in the levels of endoxifen, even with the same tamoxifen dosage. However, the clinical significance of endoxifen on the prognosis of breast cancer patients remains controversial. This study aimed to elucidate the relevance of endoxifen level to recurrence-free survival censored with tamoxifen discontinuation (RFSt), representing the RFS for tamoxifen itself, of breast cancer patients and determine a suitable cutoff for prognostication. The study included 478 breast cancer patients, and tamoxifen and its metabolites, including endoxifen, were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). An optimal cutoff was determined with maximally selected rank statistics. Survival analysis and Cox regression were conducted based on this cutoff. An endoxifen level of 21.00 ng/mL was the optimal cutoff for prognostication. Survival analysis revealed a statistically significant difference in RFSt between the low endoxifen group (≤ 21.00 ng/mL) and high endoxifen group (> 21.00 ng/mL) (log-rank test, p=0.032). The 10-year probability of RFSt was 83.2% (95% CI, 77.0-89.9%) and 88.3% (95% CI, 83.3-93.5%) in the low and high endoxifen groups, respectively. Multivariable Cox proportional hazards regression indicated endoxifen concentration as a significant factor affecting prognosis, which was adjusted with other clinical characteristics. Endoxifen could serve as a marker for appropriate tamoxifen treatment, and an endoxifen cutoff of 21.00 ng/mL could be advantageous in prognostication. Based on this cutoff, therapeutic drug monitoring would benefit patients displaying a suboptimal concentration.

A Genome-Wide Association Study of Endoxifen Serum Concentrations and Adjuvant Tamoxifen Efficacy in Early-Stage Breast Cancer Patients.

Tamoxifen is part of the standard of care of endocrine therapy for adjuvant treatment of breast cancer. However, survival outcomes with tamoxifen are highly variable. The concentration of endoxifen, the 30-100 times more potent metabolite of tamoxifen and bioactivated by the CYP2D6 enzyme, has been described as the most relevant metabolite of tamoxifen metabolism. A genome-wide association study (GWAS) was performed with the objective to identify genetic polymorphisms associated with endoxifen serum concentration levels and clinical outcome in early-stage breast cancer patients receiving tamoxifen. A GWAS was conducted in 608 women of the CYPTAM study (NTR1509/PMID: 30120701). Germline DNA and clinical and survival characteristics were readily available. Genotyping was performed on Infinium Global Screening Array (686,082 markers) and single nucleotide polymorphism (SNP) imputation by using 1000 Genomes. Relapse-free survival during tamoxifen (RFSt) was defined the primary clinical outcome. Endoxifen serum concentration was analyzed as a continuous variable. Several genetic variants reached genome-wide significance (P value: ≤5 × 10-8). Endoxifen concentrations analysis identified 430 variants, located in TCF20 and WBP2NL genes (chromosome 22), which are in strong linkage disequilibrium with CYP2D6 variants. In the RFSt analysis, several SNP were identified (LPP gene: rs77693286, HR 18.3, 95% CI: 15.2-21.1; rs6790761, OR 18.2, 95% CI: 15.5-21.1). Endoxifen concentrations have a strong association with the chromosome 22, which contains the CYP2D6 gene.

Influence of CYP2D6 and its polymorphic forms on the metabolism of tamoxifen in therapy of luminal forms of breast cancer

Influence of CYP2D6 and its polymorphic forms on the metabolism of tamoxifen in therapy of luminal forms of breast cancer

Impact of CYP2D6 polymorphisms on the efficacy of tamoxifen in Iraqi women with breast cancer

Objectives: The aim of the present study was to investigate the impact of the CYP2D6 genetic polymorphism on clinical outcome in Iraqi breast cancer patients who were candidates for Tamoxifen therapy. Methods: Comprehensive CYP2D6 genotyping was performed in 140 Iraqi women with breast cancer who were women on adjuvant treatment with tamoxifen. Breast cancer patients recruited into the study were divided into two groups: seventy breast cancer women who had no history of recurrence at the time of sampling and had a long time on tamoxifen without recurrence and seventy breast cancer women who had recurrence at the time of sampling after one year of treatment with tamoxifen therapy. Recurrence free survival (RFS) was determined in the recruited patients. Results: Multiple genetic variants of the gene encoding the CYP2D6 enzyme were detected with significant differences in their frequencies and percentages in both recurrent and non-recurrent groups of breast cancer patients. The findings of this study suggest that interindividual variation in clinical outcome may be related to genetic variation in CYP2D6 enzyme, which is characterized by variable RFS periods. Conclusions: This study revealed that the CYP2D6 enzyme of breast cancer patients who participated in this study is highly polymorphic. The CYP2D6 gene of study participants exhibited different allelic combinations with variable frequencies. The multiple genetic variants (alleles) of the gene encoding the CYP2D6 enzyme exhibited significant differences in their frequencies and percentages in both recurrent and non-recurrent groups of breast cancer patients. .The study revealed variable Recurrence free survival (RFS) with highly polymorphic gene. Our study quotes the presence of increased CYP2D6 enzyme polymorphism is associated with variable clinical response.

Influence of polymorphism of enzymes of the UDP family-glucuronyl transferases on the biotransformation of tamoxifen in the therapy of luminal forms of breast cancer

Annotation. Tamoxifen (TAM) (1-[4-(2-dimethylaminoethoxy)-phenyl]-1,2-diphenylbut-1(Z)-ene) is a non-steroidal selective estrogen receptor modulator (SERM), which is recognized as the "gold standard" of hormone therapy for estrogen-dependent breast cancer (BC). It is known that adjuvant treatment with TAM increases recurrence-free survival and overall survival in patients with hormone-receptor-positive breast cancer. Also, tamoxifen manifests itself as a partial estrogen agonist, which can be associated with the development of complications such as endometrial cancer, venous thromboembolism, etc. The presence of resistance and relapses during TAM therapy, which reach up to 30%, remains an actual problem. Therefore, studying the mechanisms underlying the individualization of both therapeutic effect and toxicity associated with TAM remains an important challenge. In the detoxification of both TAM and its active metabolites, glucuronidation processes, which belong to the second phase of biotransformation of xenobiotics and actively take place in the liver as well as in the mammary gland, play an important role, and therefore the study of this process can contribute to the understanding of the interindividual variability of the therapeutic effect and toxicity of TAM. The aim – to analyze the data of the scientific literature on the study of the influence of glucuronyltransferase (UGT) enzymes and their polymorphic forms on the biotransformation of TAM and its active metabolites in the treatment of hormone-receptor-positive breast cancer. A retrospective analysis of the literature of scientific databases Scopus, Web of Science, PubMed., MedLines for 2013-2023 was carried out. It is possible to draw the following conclusions that UGT isozymes are responsible for the conjugation and detoxification of tamoxifen and its metabolites in the form of glucuronides 4-OH-tamoxifen-N-glucuronide, 4-OH-tamoxifen-O-glucuronide and endoxifen-O-glucuronide. UGT1A8, UGT1A10, UGT2B7, UGT2B15 and UGT2B17 isoforms played the greatest role in glucuronidation of tamoxifen and its active metabolites, but UGT1A4 was recognized as the main one. Depending on the content of active TAM metabolites and their glucuronides in the blood plasma, it can be stated that carriers of the UGT2B15 Lys523Thr and UGT2B17del alleles demonstrated increased enzyme activity, and individuals with one variant UGT2B15 523Thr allele can even be considered superactive metabolizers of 4-OH-tamoxifen-O- glucuronide and endoxifen-glucuronide. Also, high levels of 4-OH-tamoxifen-N-glucuronide were observed in carriers of the allele of the UGT2B17del genotype. Carriers of the above alleles have high activity of glucuronidation processes and low levels of active metabolites of TAM, which calls into question the rationality of prescribing TAM as hormone therapy. In contrast, patients with UGT1A4 48Val, UGT2B7 268Tyr alleles, or with wild-type genotypes for UGT2B17 nodel and UGT2B15 523Lys, will have high levels of active metabolites and are the group of choice for tamoxifen therapy in estrogen-receptor-positive breast cancer because they will have a low rate of glucuronidation and detoxification. However, in order to create a system of clinical algorithms for the formation of tamoxifen-sensitive groups of patients, further detailed study of other possibilities of the biotransformation system in the metabolism of tamoxifen is required.

Effects of Genetic Polymorphisms of Drug Metabolizing Enzymes and co-Medications on Tamoxifen Metabolism in Black South African Women with Breast Cancer.

Clinical outcomes of tamoxifen (TAM) treatment show wide interindividual variability. Comedications and genetic polymorphisms of enzymes involved in TAM metabolism contributes to this variability. Drug-drug and drug-gene interactions have seldom been studied in African Black populations. We evaluated the effects of commonly co-administered medicines on TAM pharmacokinetics in a cohort of 229 South African Black female patients with hormone-receptor positive breast cancer. We also investigated the pharmacokinetic effects of genetic polymorphism in enzymes involved in TAM metabolism, including the variants CYP2D6*17 and *29, which have been mainly reported in people of African descent. TAM and its major metabolites, N-desmethyltamoxifen (NDM), 4-OH-tamoxifen, and endoxifen (ENDO), were quantified in plasma using the liquid chromatography-mass spectrometry. The GenoPharm open array was used to genotype CYP2D6, CYP3A5, CYP3A4, CYP2B6, CYP2C9, and CYP2C19. Results showed that CYP2D6 diplotype and CYP2D6 phenotype significantly affected endoxifen concentration (P < 0.001 and P < 0.001). CYP2D6*17 and CYP2D6*29 significantly reduced the metabolism of NDM to ENDO. Antiretroviral therapy had a significant effect on NDM levels and the TAM/NDM and NDM/ENDO metabolic ratios but did not result in significant effects on ENDO levels. In conclusion, CYP2D6 polymorphisms affected endoxifen concentration and the variants CYP2D6*17 and CYP2D6*29 significantly contributed to low exposure levels of ENDO. This study also suggests a low risk of drug-drug interaction in patients with breast cancer on TAM.

Development and validation of a liquid chromatography-mass spectrometry assay for quantification of Z- and E- isomers of endoxifen and its metabolites in plasma from women with estrogen receptor positive breast cancer

The selective estrogen receptor modifier tamoxifen (TAM) is widely used for the treatment of women with estrogen receptor positive (ER+ ) breast cancer. Endoxifen (ENDX) is a potent, active metabolite of TAM and is important for TAM’s clinical activity. While multiple papers have been published regarding TAM metabolism, few studies have examined or quantified the metabolism of ENDX. To quantify ENDX and its metabolites in patient plasma samples, we have developed and validated a rapid, sensitive, and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantitative determination of the E- and Z-isomers of ENDX (0.5–500 ng/ml) and the ENDX metabolites norendoxifen (1–500 and 0.5–500 ng/ml E and Z, respectfully), ENDX catechol (3.075–307.5 and 1.92–192 ng/ml E and Z, respectfully), 4′-hydroxy ENDX (0.33–166.5 and 0.33–333.5 ng/ml E and Z, respectfully), ENDX methoxycatechol (0.3–300 and 0.2–200 ng/ml E and Z, respectfully), and ENDX glucuronide (2–200 and 3–300 ng/ml E and Z, respectfully) in human plasma. Chromatographic separation was accomplished on a HSS T3 precolumn attached to an Poroshell 120 EC-C18 analytical column using 0.1 % formic acid/water and 0.1 % formic acid/methanol as eluents followed by MS/MS detection. The analytical run time was 6.5 min. Standard curves were linear (R2 ≥ 0.98) over the concentration ranges. The intra- and inter-day precision and accuracy, determined at high-, middle-, and low-quality control concentrations for all analytes, were within the acceptable range of 85 % and 115 %. The average percent recoveries were all above 90 %. The method was successfully applied to clinical plasma samples from a Phase I study of daily oral Z-ENDX.

Factors Influencing Pharmacokinetics of Tamoxifen in Breast Cancer Patients: A Systematic Review of Population Pharmacokinetic Models.

Tamoxifen is useful in managing breast cancer and it is reported to have significant variability in its pharmacokinetics. This review aimed to summarize reported population pharmacokinetics studies of tamoxifen and to identify the factors affecting the pharmacokinetics of tamoxifen in adult breast cancer patients. A systematic search was undertaken in Scopus, Web of Science, and PubMed for papers published in the English language from inception to 20 August 2022. Studies were included in the review if the population pharmacokinetic modeling was based on non-linear mixed-effects modeling with a parametric approach for tamoxifen in breast cancer patients. After initial selection, 671 records were taken for screening. A total of five studies were selected from Scopus, Web of Science, PubMed, and by manual searching. The majority of the studies were two-compartment models with first-order absorption and elimination to describe tamoxifen and its metabolites' disposition. The CYP2D6 phenotype and CYP3A4 genotype were the main covariates that affected the metabolism of tamoxifen and its metabolites. Other factors influencing the drug's pharmacokinetics included age, co-medication, BMI, medication adherence, CYP2B6, and CYP2C19 genotype. The disposition of tamoxifen and its metabolites varies primarily due to the CYP2D6 phenotype and CYP3A4 genotype. However, other factors, such as anthropometric characteristics and menopausal status, should also be addressed when accounting for this variability. All these studies should be externally evaluated to assess their applicability in different populations and to use model-informed dosing in the clinical setting.

Bioavailability and Pharmacokinetics of Endoxifen in Female Rats and Dogs: Evidence to Support the Use of Endoxifen to Overcome the Limitations of CYP2D6-Mediated Tamoxifen Metabolism.

Endoxifen (ENDX) is an active metabolite of tamoxifen (TAM), a drug commonly used for the treatment of estrogen receptor-positive breast cancer and metabolized by CYP2D6. Genetic or drug-induced reductions in CYP2D6 activity decrease plasma ENDX concentrations and TAM efficacy. It was proposed that direct oral administration of ENDX would circumvent the issues related to metabolic activation of TAM by CYP2D6 and increase patient response. Here, we characterized the pharmacokinetics and oral bioavailability of ENDX in female rats and dogs. Additionally, ENDX exposure was compared following equivalent doses of ENDX and TAM. ENDX exposure was 100-fold and 10-fold greater in rats and dogs, respectively, with ENDX administration compared with an equivalent dose of TAM. In single-dose administration studies, the terminal elimination half-life and plasma clearance values were 6.3 hours and 2.4 L/h per kg in rats given 2 mg/kg i.v. ENDX and 9.2 hours and 0.4 L/h/kg in dogs given 0.5 mg/kg i.v. ENDX, respectively. Plasma concentrations above 0.1 µM and 1 µM ENDX were achieved with 20-mg/kg and 200-mg/kg doses in rats, and concentrations above 1 µM and 10 µM were achieved with 15-mg/kg and 100-mg/kg doses in dogs. Oral absorption of ENDX was linear in rats and dogs, with bioavailability greater than 67% in rats and greater than 50% in dogs. In repeated-dose administration studies, ENDX peak plasma concentrations reached 9 µM in rats and 20 µM in dogs following four daily doses of 200 mg/kg or 30 mg/kg ENDX, respectively. The results indicate that ENDX has high oral bioavailability, and therapeutic concentrations were maintained after repeated dosing. Oral dosing of ENDX resulted in substantially higher ENDX concentrations than a similar dose of TAM. These data support the ongoing development of ENDX to overcome the limitations associated with CYP2D6-mediated metabolism of TAM in humans. SIGNIFICANCE STATEMENT: This study presents for the first time the pharmacokinetics and bioavailability of endoxifen and three key tamoxifen metabolites following repeated oral dosing in female rats and dogs. This study reports that endoxifen has high oral bioavailability, and therapeutic concentrations were maintained after repeated dosing. On the basis of these data, Z-endoxifen (Z-ENDX) was developed as a drug based upon the hypothesis that oral administration of Z-ENDX would overcome the limitations of CYP2D6 metabolism required for full metabolic activation of tamoxifen.

Action of Estrogen In Breast Cancer: A review Study

Estrogen hormone is one of the steroid hormones has a critical role in breast cancer etiology. It has been implicated in proliferation and differentiation of cells through its action promoting binding of its receptor to DNA, changing transcriptional expression of target genes. In addition to the classical DNA binding mechanism, estrogen can also regulate gene expression through a nongenomic mechanism associated with activation a variety of signal transduction pathways (e.g. PI3`K/AKT, ERK/MAPK, PLC/PKCs , p38/MAPK). Dysregulation of the balance between pro-apoptotic and anti-apoptotic members of the Bcl-2 family, would result in apoptosis inhibition and tumorigenesis. Also, poor responses towards hormonal therapy, radiotherapy, and chemotherapy and treatment resistance are likely caused by dysregulation of apoptosis. Importantly, E2 has been shown to prevent apoptosis, first through its action in activation of anti-apoptotic proteins like Bcl-xl and Bcl-2 in breast cells like MCF-7, T47D and ZR-75-1 or due to its metabolites independently of ER. Estrogen metabolism includes several very important pathways that can possibly induce de novo DNA mutation. These pathways include:2, &amp;4-hydroxylation, 16 α hydroxylation and 4 hydroxyestradiol-quinone-adenine/guanine adduct depurination, which subsequently participate in DNA damage that can lead to breast cancer. Endocrine resistance is considered one of the most permanent problems that can reduce the benefits of breast cancer treatment. Alteration of microRNAs expression, polymorphisms occuring in tamoxifen metabolism, and using reduntant alternative signaling pathways, resulting in poor treatment responses of breast cancer patients and induction of endocrine resistance.Thus, most proposed therapeutic strategies will be based on a combination of drugs targeting various pathways alongside endocrine therapy that may improve the outcomes of endocrine responses in resistant breast cancer cells.

Influence of CYP2D6, CYP2C19 and CYP3A5 polymorphisms on plasma levels of tamoxifen metabolites in Algerian women with ER+ breast cancer

BackgroundTamoxifen, a selective estrogen receptor modulator, is indicated for breast cancer developed in response to estrogen.FindingsIn the current study we explored the relationship between the different variants of CYP2D6, CYP2C19, CYP3A5 and plasma Endoxifen levels in Algerian patients with ER + breast cancer. We further conducted the relationship between the candidate genes and the recurrences rate. Endoxifen levels differed significantly (p < .005) between carriers of two functional alleles and patients genotyped as CYP2D6*10, CYP2D6*17, CYP2D6*41 or CYP2D6*5/*5. Patients with elevated Endoxifen concentrations were significantly more likely to not report recurrences than patients with reduced or nul alleles. Such nul/nul, red/red, and red/nul diplotypes have been associated with a higher rate of recurrences than other genotypes during treatment.ConclusionOur findings suggest that the CYP2D6 genotype should be considered in tamoxifen-treated women. While quantitatively, CYP2D6 represents only a minor fraction of the total drug metabolizing capacity of the liver, it is polymorphic and, therefore, may alter the balance of metabolism of tamoxifen toward the activation pathways. Breast cancer patients with the CYP2D6 nul/nul or red/nul diplotype may benefit less from Tamoxifen treatment and are more likely to develop recurrences. Comprehensive CYP2D6 genotyping has a good predictive value for CYP2D6 activity. Common variants in CYP2C19 and CYP3A5 did not have a significant impact on the recurrences in this cohort of patients with ER + breast cancer.

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&lt;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.

Polysaccharide Peptide Extract From Coriolus versicolor Increased Tmax of Tamoxifen and Maintained Biochemical Serum Parameters, With No Change in the Metabolism of Tamoxifen in the Rat.

Background: Polysaccharide peptide (PSP) extract of Coriolus versicolor (L.) Quél. (1886) (Trametes; Polyporaceae) is increasingly used in cancer to support the immune system. However, its interaction with tamoxifen is unknown. Aim of the study: To investigate the effect of a PSP extract on the pharmacokinetics, biochemical parameters, and depletion of tamoxifen. Methods: The pharmacokinetic and biochemical parameters of tamoxifen (20 mg/mL oral single dose and repeated dosing for 12 days) was investigated in female Sprague Dawley rats with or without PSP (340 mg/kg orally for 7 days) (n = 5 per group). Tamoxifen (5 µM) depletion rate with PSP (10–100 μg/mL) was measured in female rat hepatic microsomes in vitro. Results: Compared to tamoxifen alone, the time to reach maximum concentration (Tmax) significantly increased by 228% (4.15 ± 1.15 versus 13.6 ± 2.71 h) in the single tamoxifen dose with PSP and 93% (6 ± 2.17 versus 11.6 ± 0.4 h) in the repeated tamoxifen dosing with PSP (p < 0.05). No significant changes in the area-under-curve and maximum concentration were observed in the single dose and repeated tamoxifen dosing plus PSP compared to tamoxifen alone. Pharmacodynamically, the repeated tamoxifen dosing with PSP maintained 19 out of 23 hepatic, renal and cardiac biochemical serum parameters in rats compared to untreated rats (p > 0.05). PSP extract did not significantly alter in vitro intrinsic clearance of tamoxifen compared to tamoxifen control. Conclusion: With the increased use of PSP as an adjunct therapy, this study highlights the importance of clinician’s knowledge of its interaction with tamoxifen to avoid compromising clinical actions and enhancing clinical therapy.

(Z)-Endoxifen and Early Recurrence of Breast Cancer: An Explorative Analysis in a Prospective Brazilian Study.

Adherence to treatment and use of co-medication, but also molecular factors such as CYP2D6 genotype, affect tamoxifen metabolism, with consequences for early breast cancer prognosis. In a prospective study of 149 tamoxifen-treated early-stage breast cancer patients from Brazil followed up for 5 years, we investigated the association between the active tamoxifen metabolite (Z)-endoxifen at 3 months and event-free survival (EFS) adjusted for clinico-pathological factors. Twenty-five patients (16.8%) had recurred or died at a median follow-up of 52.3 months. When we applied a putative 15 nM threshold used in previous independent studies, (Z)-endoxifen levels below the threshold showed an association with shorter EFS in univariate analysis (p = 0.045) and after adjustment for stage (HR 2.52; 95% CI 1.13–5.65; p = 0.024). However, modeling of plasma concentrations with splines instead of dichotomization did not verify a significant association with EFS (univariate analysis: p = 0.158; adjusted for stage: p = 0.117). Hence, in our small exploratory study, the link between impaired tamoxifen metabolism and early breast cancer recurrence could not be unanimously demonstrated. This inconsistency justifies larger modeling studies backed up by mechanistic pharmacodynamic analyses to shed new light on this suspected association and the stipulation of an appropriate predictive (Z)-endoxifen threshold.

Impact of Irradiation on the Pharmacokinetics and Biotransformation of Tamoxifen.

BackgroundThe optimal procedure for combining radiotherapy (RT) with tamoxifen treatment is controversial as RT may alter the pharmacokinetics and biotransformation of tamoxifen. The present study investigated this potential interaction by assessing the pharmacokinetics of tamoxifen during concurrent and sequential RT.MethodPlasma tamoxifen concentration was measured in rats with or without RT 2.0 Gy (RT2.0Gy) or 0.5 Gy (RT0.5Gy) with ultra-high-performance liquid chromatography-tandem mass spectrometry after tamoxifen administration (10 mg/kg, p.o., n = 6). Tamoxifen was either administered 1 h after RT (concurrent condition) or 24 h after RT (sequential condition).ResultsPharmacokinetic data analysis demonstrated that the area under the curve (AUC) and half-life of tamoxifen were 2,004 ± 241 h ng/ml and 6.23 ± 1.21 h, respectively, after tamoxifen administration (10 mg/kg, p.o.). The respective conversion rate of 4-hydroxytamoxifen, N-desmethytamoxifen, and endoxifen for tamoxifen metabolism was 20%, 16%, and 5%. The AUC value of tamoxifen in the RT0.5Gy group was 1.5- to 1.7-fold higher than in the sham and RT2.0Gy groups. The relative bioavailability of tamoxifen at concurrent RT0.5Gy and RT2.0Gy groups ranged from 127% to 202% and from 71% to 152%, respectively. The magnitude of endoxifen, which converted from 4-hydroxytamoxifen and N-desmethyltamoxifen, increased 3- to 5-fold in the concurrent RT groups. By contrast, the AUC of tamoxifen decreased by roughly 24% in the sequential RT2.0Gy group. The conversion ratio of endoxifen was four times higher than that in the sequential RT2.0Gy group compared with rats not exposed to RT.ConclusionThe current study provides advanced pharmacokinetic data to confirm the interaction between RT and hormone therapy. Our findings indicate that RT facilitates the metabolism of tamoxifen to active metabolites and thus imply that combination RT-tamoxifen has potential benefits for the treatment of hormone-dependent breast cancer.

Inhibition of Cytochrome P450s by Strobilanthes crispus Sub-Fraction (F3): Implication for Herb-Drug Interaction.

Strobilanthes crispus Blume sub-fraction (F3) has been reported to be cytotoxic against cancer cells and to cause murine mammary tumor regression. Potential utilization of F3 as an adjuvant in breast cancer treatment to alleviate chemotherapeutic drug resistance is currently hampered by potential cytochrome P450 (CYP)-mediated herb-drug interactions (HDIs). The current study assessed the inhibitory potency of F3 towards five CYP enzymes involved in tamoxifen metabolism. Potential CYP inhibition by F3 was first determined using fluorescence assays, using known CYP inhibitors as reference. To further ascertain the inhibitory potency and mode of inhibition, high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis of specific metabolites of a CYP probe substrate was conducted. The half-maximal inhibitory concentration (IC50) values indicate that F3 exhibited relatively weak inhibition on CYP2B6, CYP2C19, CYP2D6, and CYP3A4. Highest susceptibility to inhibition by F3 was observed for CYP2C9, where the IC50 value from fluorescence-based assay was 35-fold higher than control. Further analysis by HPLC-MS/MS revealed relatively weak mixed-type inhibition of F3 on CYP2C9, as indicated by IC50 and inhibition constant (KI) values. The risk of clinically significant CYP2C9 inhibition by F3 was then predicted based on the attained KI value and the presumed amount of F3 absorbed from S. crispus leaves following consumption. The calculated maximum plasma concentration to inhibition constant Cmax/KI) ratio suggests that F3 consumption could potentially result in clinically significant drug interactions with medications metabolized by CYP2C9. Taken together, the results revealed a low probability of inhibition by F3 on CYP enzymes involved in tamoxifen metabolism. However, further in vivo investigation is necessary for potential F3 interaction with CYP2C9. The utility of a preliminary in vitro approach in the assessment of potential HDI was demonstrated in this study.

Toward predicting CYP2D6-mediated variable drug response from CYP2D6 gene sequencing data.

Pharmacogenomics is a key component of personalized medicine that promises safer and more effective drug treatment by individualizing drug choice and dose based on genetic profiles. In clinical practice, genetic biomarkers are used to categorize patients into *-alleles to predict CYP450 enzyme activity and adjust drug dosages accordingly. However, this approach leaves a large part of variability in drug response unexplained. Here, we present a proof-of-concept approach that uses continuous-scale (instead of categorical) assignments to predict enzyme activity. We used full CYP2D6 gene sequences obtained with long-read amplicon-based sequencing and cytochrome P450 (CYP) 2D6-mediated tamoxifen metabolism data from a prospective study of 561 patients with breast cancer to train a neural network. The model explained 79% of interindividual variability in CYP2D6 activity compared to 54% with the conventional *-allele approach, assigned enzyme activities to known alleles with previously reported effects, and predicted the activity of previously uncharacterized combinations of variants. The results were replicated in an independent cohort of tamoxifen-treated patients (model R 2 adjusted = 0.66 versus *-allele R 2 adjusted = 0.35) and a cohort of patients treated with the CYP2D6 substrate venlafaxine (model R 2 adjusted = 0.64 versus *-allele R 2 adjusted = 0.55). Human embryonic kidney cells were used to confirm the effect of five genetic variants on metabolism of the CYP2D6 substrate bufuralol in vitro. These results demonstrate the advantage of a continuous scale and a completely phased genotype for prediction of CYP2D6 enzyme activity and could potentially enable more accurate prediction of individual drug response.

Effect of Genetic Variability in 20 Pharmacogenes on Concentrations of Tamoxifen and Its Metabolites.

Background: Tamoxifen, as a treatment of estrogen receptor positive (ER+) breast cancer, is a weak anti-estrogen that requires metabolic activation to form metabolites with higher anti-estrogenic activity. Endoxifen is the most-studied active tamoxifen metabolite, and endoxifen concentrations are highly associated with CYP2D6 activity. Associations of tamoxifen efficacy with measured or CYP2D6-predicted endoxifen concentrations have been inconclusive. Another active metabolite, 4-OHtam, and other, less active metabolites, Z-4′-endoxifen and Z-4′-OHtam, have also been reported to be associated with tamoxifen efficacy. Method: Genotype for 20 pharmacogenes was determined by VeriDose® Core Panel and VeriDose® CYP2D6 CNV Panel, followed by translation to metabolic activity phenotype following standard activity scoring. Concentrations of tamoxifen and seven metabolites were measured by UPLC-MS/MS in serum samples collected from patients receiving 20 mg tamoxifen per day. Metabolic activity was tested for association with tamoxifen and its metabolites using linear regression with adjustment for upstream metabolites to identify genes associated with each step in the tamoxifen metabolism pathway. Results: A total of 187 patients with genetic and tamoxifen concentration data were included in the analysis. CYP2D6 was the primary gene associated with the tamoxifen metabolism pathway, especially the conversion of tamoxifen to endoxifen. CYP3A4 and CYP2C9 were also responsible for the metabolism of tamoxifen. CYP2C9 especially impacted the hydroxylation to 4-OHtam, and this involved the OATP1B1 (SLCO1B1) transporter. Conclusion: Multiple genes are involved in tamoxifen metabolism and multi-gene panels could be useful to predict active metabolite concentrations and guide tamoxifen dosing.

Modeling shows the NK3R antagonist, ACER-801, reduces treatment-induced vasomotor symptoms.

e13013 Background: Hot flashes or vasomotor symptoms (VMS) are a common side effect of hormone deprivation (HD) therapy. Up to 80 % of cancer patients treated with tamoxifen (antiestrogen treatment) or leuprolide (androgen deprivation) have VMS. In some cases, patients discontinue HD therapy due to VMS severity and lower quality of life; therefore, reducing VMS is critical for patient compliance. NK3 receptor (NK3R) antagonists have previously been shown to reduce VMS in postmenopausal women. ACER-801 is a candidate NK3R antagonist drug intended to alleviate VMS severity when used with HD therapy. We used Quantitative Systems Pharmacology (QSP) modeling to predict the likely efficacy of ACER-801 in patients on HD therapy and evaluate the potential of hepatic drug-drug interactions between ACER-801 and tamoxifen or leuprolide. Methods: The model is composed of KNDy neurons in the arcuate nucleus with NKB, dynorphin, and estradiol effects on KNDy neurons, HPG axis, sex hormones, and neuroendocrine feedback. ACER-801, tamoxifen, and leuprolide PK, PD, and hepatic metabolism are included in the model. The model was developed, qualified, and tested using literature data. A tamoxifen-treated postmenopausal female virtual patient (VP) and leuprolide-treated male VP were created based on typical patients included in clinical trials. VMS severity and frequency were estimated based on the level of NKB binding to NK3R. Results: In the male VP, simulated leuprolide administration induced hypertrophy of KNDy neurons and VMS over six months. Coadministration of ACER-801 with leuprolide reduced VMS frequency and severity to near 0 in short-term (5-week) simulations. Simulations predict ACER-801 will not alter leuprolide metabolism nor increase plasma testosterone concentrations. Tamoxifen treatment increased VMS by 15% in simulations with the postmenopausal VP. Coadministration of ACER-801 with tamoxifen in this VP reduced VMS by 75% compared to tamoxifen monotherapy. ACER-801 had minimal effects on plasma estradiol concentrations in the postmenopausal VP. Drug-drug interactions between ACER-801 and tamoxifen were dependent on the simulated bioavailability of ACER-801. Using current estimates of ACER-801 bioavailability in the model, the hepatic concentration of ACER-801 had limited effects on tamoxifen metabolism, which are not expected to necessitate dose adjustments. Conclusions: Using a QSP neurobiology model as a research tool enabled us to evaluate the efficacy of the NK3R antagonist, ACER-801, to treat HD therapy-induced VMS. Simulations show ACER-801 may be highly efficacious for the treatment of induced-VMS. The research provided estimates of DDI with ACER-801 and tamoxifen and what clinical experiments would be needed to confirm those estimates.