Tamoxifen metabolism predicts drug concentrations and outcome in premenopausal patients with early breast cancer

Tamoxifen Dose De-Escalation: An Effective Strategy for Reducing Adverse Effects?

Tamoxifen, a cornerstone in the adjuvant treatment of estrogen receptor-positive breast cancer, significantly reduces breast cancer recurrence and breast cancer mortality; however, its standard adjuvant dose of 20 mg daily presents challenges due to a broad spectrum of adverse effects, contributing to high discontinuation rates. Dose reductions of tamoxifen might be an option to reduce treatment-related toxicity, but large randomized controlled trials investigating the tolerability and, more importantly, efficacy of low-dose tamoxifen in the adjuvant setting are lacking. We conducted an extensive literature search to explore evidence on the tolerability and clinical efficacy of reduced doses of tamoxifen. In this review, we discuss two important topics regarding low-dose tamoxifen: (1) the incidence of adverse effects and quality of life among women using low-dose tamoxifen; and (2) the clinical efficacy of low-dose tamoxifen examined in the preventive setting and evaluated through the measurement of several efficacy derivatives. Moreover, practical tools for tamoxifen dose reductions in the adjuvant setting are provided and further research to establish optimal dosing strategies for individual patients are discussed.

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.

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.

Pharmacogenetics of tamoxifen in breast cancer patients of African descent: Lack of data

Tamoxifen, a selective estrogen receptor modulator, is used to treat hormone receptor-positive breast cancer. Tamoxifen acts as a prodrug, with its primary therapeutic effect mediated by its principal metabolite, endoxifen. However, tamoxifen has complex pharmacokinetics involving several drug-metabolizing enzymes and transporters influencing its disposition. Genes encoding enzymes involved in tamoxifen disposition exhibit genetic polymorphisms which vary widely across world populations. This review highlights the lack of data on tamoxifen pharmacogenetics among African populations. Gaps in data are described in this study with the purpose that future research can address this dearth of research on the pharmacogenetics of tamoxifen among African breast cancer patients. Initiatives such as the African Pharmacogenomics Network (APN) are crucial in promoting comprehensive pharmacogenetics studies to pinpoint important variants in pharmacogenes that could be used to reduce toxicity and improve efficacy.

Circulating Biomarkers Instead of Genotyping to Establish Metabolizer Phenotypes

Pharmacogenomics (PGx) enables personalized treatment for the prediction of drug response and to avoid adverse drug reactions. Currently, PGx mainly relies on the genetic information of absorption, distribution, metabolism, and excretion (ADME) targets such as drug-metabolizing enzymes or transporters to predict differences in the patient's phenotype. However, there is evidence that the phenotype-genotype concordance is limited. Thus, we discuss different phenotyping strategies using exogenous xenobiotics (e.g., drug cocktails) or endogenous compounds for phenotype prediction. In particular, minimally invasive approaches focusing on liquid biopsies offer great potential to preemptively determine metabolic and transport capacities. Early studies indicate that ADME phenotyping using exosomes released from the liver is reliable. In addition, pharmacometric modeling and artificial intelligence improve phenotype prediction. However, further prospective studies are needed to demonstrate the clinical utility of individualized treatment based on phenotyping strategies, not only relying on genetics. The present review summarizes current knowledge and limitations.

Predictive modeling of adverse drug reactions to tamoxifen therapy for breast cancer on base of pharmacogenomic testing

OBJECTIVES

The present study investigated the analysis of adverse drug reactions (ADRs) to tamoxifen (TAM) in breast cancer patients in relation to the carriage of genetic polymorphisms of genes encoding enzymes of CYP system and transporters of P-glycoprotein (Pg) and predictive models based on it.

METHODS

A total of 120 women with breast cancer taking adjuvant TAM were examined for the gene polymorphisms such as CYP2D6*4, CYP3A5*3, CYP2C9*2, CYP2C9*3, CYP2C19*2, CYP2C19*3 and ABCB1 (C3435T). Allelic variants were determined using the real-time polymerase chain reaction method. The research material was double sampling of buccal epithelium. Medical history data and extracts from case histories were used as sources of medical information, on the basis of which questionnaires specially created by us were filled out.

RESULTS

An associative analysis showed association with the development of ADRs to TAM indicating their clinical significance from different genetic polymorphisms of CYP2D6, CYP3A5, CYP2C9 and ABCB1. The complex associative analysis performed using mathematical modeling made it possible to build predictive risk models for the development of ADRs such as hot flashes, dyspepsia, bone pain, and asthenia.

CONCLUSIONS

Models that include both genetic and non-genetic determinants of ADRs of TAM may further improve the prediction of individual response to TAM.

Value of therapeutic drug monitoring of endoxifen in Egyptian premenopausal patients with breast cancer given tamoxifen adjuvant therapy: A pilot study

BACKGROUND

The complex metabolic profile of tamoxifen anticancer drug and polymorphism in its metabolizing enzymes particularly CYP2D6 contribute to the high-observed inter-individual variability in its main active metabolite endoxifen. Therapeutic drug monitoring of endoxifen may play a key role in optimizing tamoxifen therapy, and control of both adverse effects and cancer recurrence. This pilot study aims to assess the clinical benefits of applying endoxifen measurement during tamoxifen therapy in patients with breast cancer.

METHODS

Adult premenopausal breast cancer patients ≥ 18 years who received tamoxifen at a fixed dose of 20 mg daily were included. The primary endpoint was to identify the inter-subject variability in serum concentration of the drug and its metabolites especially endoxifen, through fixation of the tamoxifen dose. The secondary endpoint was to check the correlation between endoxifen metabolite concentration and the development of tamoxifen's adverse effects and cancer recurrence.

RESULTS

Sixty patients were included in the study with a mean age of 38.4  ±  0.6 years (range: 26-50). The mean concentration of tamoxifen and endoxifen was 181  ±  9.6 ng/mL and 31.49 ng/mL, respectively. The inter-individual variability in concentrations for the drug and its active metabolite as estimated by the coefficient of variation percentage was in 41% and 31%, respectively. Cancer recurrence was observed in a group of patients (n  =  16) with an average endoxifen level of 24.48 ng/mL. Another group of patients (n  =  25) developed different tamoxifen adverse effects including hot flashes, vaginal bleeding, endometrial thickness, and ovarian cysts with the average endoxifen level of 38.61 ng/mL. The rest of the patients (n  =  19) who responded smoothly to the drug with no complications had an average endoxifen level of 31.37 ng/mL. Analysis of variance test showed a significant difference in endoxifen levels between the three groups (p  =  0.002).

CONCLUSION

The measurement of the endoxifen active metabolite of tamoxifen in breast cancer patients can help dose optimization in light of the observed wide inter-individual variability in drug fixed-dose related concentration of the metabolite. Monitoring of serum concentration of endoxifen can help to reveal, reduce and control tamoxifen's adverse effects and cancer recurrence.

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).

CBD-oil as a potential solution in case of severe tamoxifen-related side effects

Tamoxifen may lead to bothersome side effects contributing to non-compliance and decreased quality of life. Patients searching for relief are increasingly turning to cannabinoids such as CBD-oil. However, CBD-oil might affect tamoxifen pharmacokinetics (PK) through CYP2D6 inhibition. The aims of this open-label, single-arm study were (1) to determine the PK profile of tamoxifen when using CBD-oil, and (2) to subsequently investigate whether CBD-oil has a beneficial influence on side effects. Study patients had to have steady-state endoxifen concentrations ≥16 nM (conservative threshold). PK sampling and side effect assessment was done at initiation of CBD-oil and 28 days thereafter. Bio-equivalence could be concluded if the 90% confidence interval (CI) for the difference in endoxifen AUC fell within the [-20%; +25%] interval. The effect of CBD-oil on side effects was evaluated using the FACT-ES questionnaire. Endoxifen AUC decreased after CBD-oil by 12.6% (n = 15, 90% CI -18.7%, -6.1%) but remained within bio-equivalence boundaries. The endocrine sub-scale of the FACT-ES improved clinically relevant with 6.7 points (n = 26, p < 0.001) and health-related quality of life improved with 4.7 points after using CBD (95% CI + 1.8, +7.6). We conclude that CBD-oil, if of good quality and with a dosage below 50 mg, does not have to be discouraged in patients using it for tamoxifen-related side effects. Clinical trial registration: International Clinical Trial Registry Platform (NL8786; https://www.who.int/clinical-trials-registry-platform ).

The beneficial effects of tamoxifen on arteries: a key player for cardiovascular health of breast cancer patient

Breast cancer is the most common cancer in women. Over the past few decades, advances in cancer detection and treatment have significantly improved survival rate of breast cancer patients. However, due to the cardiovascular toxicity of cancer treatments (chemotherapy, anti-HER2 antibodies and radiotherapy), cardiovascular diseases (CVD) have become an increasingly important cause of long-term morbidity and mortality in breast cancer survivors. Endocrine therapies are prescribed to reduce the risk of recurrence and specific death in estrogen receptor-positive (ER+) early breast cancer patients, but their impact on CVD is a matter of debate. Whereas aromatase inhibitors and luteinizing hormone-releasing hormone (LHRH) analogs inhibit estrogen synthesis, tamoxifen acts as a selective estrogen receptor modulator (SERM), opposing estrogen action in the breast but mimicking their actions in other tissues, including arteries. This review aims to summarize the main clinical and experimental studies reporting the effects of tamoxifen on CVD. In addition, we will discuss how recent findings on the mechanisms of action of these therapies may contribute to a better understanding and anticipation of CVD risk in breast cancer patients.

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.

Early increase in tamoxifen dose in CYP2D6 poor metaboliser breast cancer patients and survival: A propensity score matching analysis

PURPOSE

Tamoxifen is a drug used for hormone receptor-positive breast cancers, primarily metabolised by the CYP2D6 enzyme into active metabolites such as endoxifen. CYP2D6 displays varying degrees of activity depending on its genotype. This study aims to analyse the effect of an early increase in tamoxifen dose in poor metabolisers (PM) on survival.

METHODS

We enrolled 220 patients diagnosed with breast cancer who were treated with tamoxifen. CYP2D6 polymorphisms were determined, and the phenotype was estimated according to the Clinical Pharmacogenetics Implementation Consortium. Disease-free survival (DFS) and overall survival (OS) were analysed considering the entire patient group, and a subgroup of 110 patients selected by Propensity Score Matching (PSM). All women were treated with 20 mg/day of tamoxifen for 5 years, except PM, who initially received 20 mg/day for 4 months, followed by 40 mg/day for 4 months and 60 mg/day for 4 months before returning to the standard dose of 20 mg/day until completing 5 years of treatment.

RESULTS

The analysis of the influence of CYP2D6 polymorphisms in the complete group and in the PSM subgroup revealed no significant differences for DFS or OS. Furthermore, DFS and OS were analysed in relation to various covariates such as age, histological grade, nodal status, tumour size, HER-2, Ki-67, chemotherapy, and radiotherapy. Only age, histological grade, nodal status, and chemotherapy treatment demonstrated statistical significance.

CONCLUSION

An early increase in tamoxifen dose in PM patients is not associated with survival differences among CYP2D6 phenotypes.

Tamoxifen pharmacokinetics and pharmacodynamics in older patients with non-metastatic breast cancer

BACKGROUND

We aimed to study the pharmacokinetics and -dynamics of tamoxifen in older women with non-metastatic breast cancer.

METHODS

Data for this analysis were derived from the CYPTAM study (NTR1509) database. Patients were stratified by age (age groups < 65 and 65 and older). Steady-state trough concentrations were measured of tamoxifen, N-desmethyltamoxifen, 4-hydroxy-tamoxifen, and endoxifen. CYP2D6 and CYP3A4 phenotypes were assessed for all patients by genotyping. Multiple linear regression models were used to analyze tamoxifen and endoxifen variability. Outcome data included recurrence-free survival at time of tamoxifen discontinuation (RFSt) and overall survival (OS).

RESULTS

668 patients were included, 141 (21%) were 65 and older. Demographics and treatment duration were similar across age groups. Older patients had significantly higher concentrations of tamoxifen 129.4 ng/ml (SD 53.7) versus 112.2 ng/ml (SD 42.0) and endoxifen 12.1 ng/ml (SD 6.6) versus 10.7 ng/ml (SD 5.7, p all < 0.05), independently of CYP2D6 and CYP3A4 gene polymorphisms. Age independently explained 5% of the variability of tamoxifen (b = 0.95, p < 0.001, R² = 0.051) and 0.1% of the variability in endoxifen concentrations (b = 0.45, p = 0.12, R² = 0.007). Older patients had worse RFSt (5.8 versus 7.3 years, p = 0.01) and worse OS (7.8 years versus 8.7 years, p = 0.01). This was not related to differences in endoxifen concentration (HR 1.0, 95% CI 0.96-1.04, p = 0.84) or CYP polymorphisms.

CONCLUSION

Serum concentrations of tamoxifen and its demethylated metabolites are higher in older patients, independent of CYP2D6 or CYP3A4 gene polymorphisms. A higher bioavailability of tamoxifen in older patients may explain the observed differences. However, clinical relevance of these findings is limited and should not lead to a different tamoxifen dose in older patients.

Toward model-informed precision dosing for tamoxifen: A population-pharmacokinetic model with a continuous CYP2D6 activity scale

BACKGROUND

Tamoxifen is important in the adjuvant treatment of breast cancer. A plasma concentration of the active metabolite endoxifen of > 16 nM is associated with a lower risk of breast cancer-recurrence. Since inter-individual variability is high and > 20 % of patients do not reach endoxifen levels > 16 nM with the standard dose tamoxifen, therapeutic drug monitoring is advised. However, ideally, the correct tamoxifen dose should be known prior to start of therapy. Our aim is to develop a population pharmacokinetic (POP-PK) model incorporating a continuous CYP2D6 activity scale to support model informed precision dosing (MIPD) of tamoxifen to determine the optimal tamoxifen starting dose.

METHODS

Data from eight different clinical studies were pooled (539 patients, 3661 samples) and used to develop a POP-PK model. In this model, CYP2D6 activity per allele was estimated on a continuous scale. After inclusion of covariates, the model was subsequently validated using an independent external dataset (378 patients). Thereafter, dosing cut-off values for MIPD were determined.

RESULTS

A joint tamoxifen/endoxifen POP-PK model was developed describing the endoxifen formation rate. Using a continuous CYP2D6 activity scale, variability in predicting endoxifen levels was decreased by 37 % compared to using standard CYP2D6 genotype predicted phenotyping. After external validation and determination of dosing cut-off points, MIPD could reduce the proportion of patients with subtherapeutic endoxifen levels at from 22.1 % toward 4.8 %.

CONCLUSION

Implementing MIPD from the start of tamoxifen treatment with this POP-PK model can reduce the proportion of patients with subtherapeutic endoxifen levels at steady-state to less than 5 %.

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.

The impact of HIV on non-adherence for tamoxifen among women with breast cancer in South Africa

PURPOSE

Women living with HIV (WLWH) and breast cancer (BC) have worse overall survival than HIV-negative women with BC, and poor adherence to prescribed tamoxifen is known to contribute to poor survival. We therefore investigated the association of HIV infection with adherence to adjuvant tamoxifen among women with localized hormone receptor (HR)-positive breast cancer in South Africa.

METHODS

Among 4,097 women diagnosed with breast cancer at six hospitals in the prospective South African Breast Cancer and HIV Outcomes (SABCHO) cohort study between July 2015 and December 2020, we focused on black women with stages I-III HR-positive breast cancer who were prescribed 20 mg of adjuvant tamoxifen daily. We collected venous blood once from each participant during a routine clinic visit, and analyzed concentrations of tamoxifen and its metabolites using a triple quadruple mass spectrometer. We defined non-adherence as a tamoxifen level < 60 ng/mL after 3 months of daily tamoxifen use. We compared tamoxifen-related side effects, and concurrent medication use among women with and without HIV and developed multivariable logistic regression models of tamoxifen non-adherence.

RESULTS

Among 369 subjects, 78 (21.1%) were WLWH and 291 (78.9%) were HIV-negative. After a median (interquartile range) time of 13.0 (6.2-25.2) months since tamoxifen initiation, the tamoxifen serum concentration ranged between 1.54 and 943.0 ng/mL and 208 (56.4%) women were non-adherent to tamoxifen. Women < 40 years of age were more likely to be non-adherent than women > 60 years (73.4% vs 52.6%, odds ratio (OR) = 2.49, 95% confidence interval (CI) = 1.26-4.94); likewise, WLWH (70.5% vs 52.6%, OR = 2.16, 95% CI = 1.26-3.70) than HIV-negative women. In an adjusted model WLWH had twice the odds of non-adherence to tamoxifen, compared to HIV-negative women (OR = 2.40, 95% CI = 1.11-5.20).

CONCLUSION

High rates of non-adherence to adjuvant tamoxifen may limit the overall survival of black South African women with HR-positive breast cancer, especially among WLWH.

The impact of endoxifen-guided tamoxifen dose reductions on endocrine side-effects in patients with primary breast cancer

BACKGROUND

Tamoxifen is important in the adjuvant treatment of hormone-sensitive breast cancer and substantially reduces recurrence; however, almost 50% of patients are non-compliant mainly due to side-effects. The aim of this study was to investigate whether endoxifen-guided tamoxifen dose reduction could lead to fewer side-effects.

MATERIALS AND METHODS

Effects of tamoxifen dose reduction were investigated in patients with bothersome side-effects and endoxifen levels ≥32 nM and compared to patients with side-effects who remained on tamoxifen 20 mg. Endocrine symptoms and health-related quality of life (HR-QOL) were assessed after 3 months with the Functional Assessment of Cancer Therapy-Endocrine Symptoms (FACT-ES) questionnaire.

RESULTS

Tamoxifen dose was reduced in 20 patients, 17 of whom were assessable for side-effect analyses. A clinically relevant improvement of >6 points was observed in endocrine symptoms and HR-QOL in 41% and 65% of the patients, respectively. In total, there was a significant and clinically relevant improvement in endocrine symptoms [5.7, 95% confidence interval (CI) -0.5-11.5] and HR-QOL (8.2, 95% CI 0.9-15.4) after dose reduction. This was not seen in patients whose doses were not reduced (n = 60). In 21% of patients, endoxifen dropped slightly below the 16-nM threshold (12.8, 15.5, 15.8, 15.9 nM).

CONCLUSIONS

Endoxifen-guided dose reduction of tamoxifen significantly improved tamoxifen-related side-effects and HR-QOL. Nearly 80% of patients remained above the most conservative endoxifen threshold.

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

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Implementation of pharmacogenomics: Where are we now?

Pharmacogenomics (PGx), examining the effect of genetic variation on interpatient variation in drug disposition and response, has been widely studied for several decades. However, as cost, as well as turnaround time associated with PGx testing, has significantly improved, the use of PGx in the clinical setting has been gaining momentum. Nevertheless, challenges have emerged in the broader clinical implementation of PGx. In this review, we will outline current models of PGx delivery and methodologies of evaluation, and discuss clinically relevant PGx tests and associated medications. Additionally, we will describe our approach for the broad implementation of pre-emptive DPYD genotyping in patients taking fluoropyrimidines in Ontario, Canada, as an example of clinically actionable PGx testing with sufficient clinical evidence of patient benefit that can become a new standard of patient care. We will highlight challenges associated with PGx testing, including a lack of diversity in PGx studies as well as general limitations that impact the broad adoption of PGx testing. Lastly, we examine the future of PGx, discussing new clinical targets, methodologies and analysis approaches.

CYP2D6 pharmacogenetics and phenoconversion in personalized medicine

INTRODUCTION

CYP2D6 contributes to the metabolism of approximately 20-25% of drugs. However, CYP2D6 is highly polymorphic and different alleles can lead to impacts ranging from null to increase in activity. Moreover, there are commonly used drugs that potently inhibit the CYP2D6, thus causing 'phenoconversion' which can convert the genotypic normal metabolizer into phenotypic poor metabolizer. Despite growing literature on the clinical implications of non-normal CYP2D6 genotype and phenoconversion on patient-related outcomes, implementation of CYP2D6 pharmacogenetics and phenoconversion to guide prescribing is rare. This review focuses on providing the clinical importance of CYP2D6 pharmacogenetics and phenoconversion in precision medicine and summarizes the challenges and approaches to implement these into clinical practice.

AREAS COVERED

A literature search was performed using PubMed and clinical studies documenting the effects of CYP2D6 genotypes and/or CYP2D6 inhibitors on pharmacokinetics, pharmacodynamics or treatment outcomes of CYP2D6-metabolized drugs, and studies on implementation challenges and approaches.

EXPERT OPINION

Considering the extent and impact of genetic polymorphisms of CYP2D6, phenoconversion by the comedications, and contribution of CYP2D6 in drug metabolism, CYP2D6 pharmacogenetics is essential to ensure drug safety and efficacy. Utilization of proper guidelines incorporating both CYP2D6 pharmacogenetics and phenoconversion in clinical care assists in optimizing drug therapy.

Factors affecting inter-individual variability in endoxifen concentrations in patients with breast cancer: results from the prospective TOTAM trial

INTRODUCTION

Endoxifen-the principal metabolite of tamoxifen-is subject to a high inter-individual variability in serum concentration. Numerous attempts have been made to explain this, but thus far only with limited success. By applying predictive modeling, we aimed to identify factors that determine the inter-individual variability. Our purpose was to develop a prediction model for endoxifen concentrations, as a strategy to individualize tamoxifen treatment by model-informed dosing in order to prevent subtherapeutic exposure (endoxifen < 16 nmol/L) and thus potential failure of therapy.

METHODS

Tamoxifen pharmacokinetics with demographic and pharmacogenetic data of 303 participants of the prospective TOTAM study were used. The inter-individual variability in endoxifen was analyzed according to multiple regression techniques in combination with multiple imputations to adjust for missing data and bootstrapping to adjust for the over-optimism of parameter estimates used for internal model validation.

RESULTS

Key predictors of endoxifen concentration were CYP2D6 genotype, age and weight, explaining altogether an average-based optimism corrected 57% (95% CI 0.49-0.64) of the inter-individual variability. CYP2D6 genotype explained 54% of the variability. The remaining 3% could be explained by age and weight. Predictors of risk for subtherapeutic endoxifen (< 16 nmol/L) were CYP2D6 genotype and age. The model showed an optimism-corrected discrimination of 90% (95% CI 0.86-0.95) and sensitivity and specificity of 66% and 98%, respectively. Consecutively, there is a high probability of misclassifying patients with subtherapeutic endoxifen concentrations based on the prediction rule.

CONCLUSION

The inter-individual variability of endoxifen concentration could largely be explained by CYP2D6 genotype and for a small proportion by age and weight. The model showed a sensitivity and specificity of 66 and 98%, respectively, indicating a high probability of (misclassification) error for the patients with subtherapeutic endoxifen concentrations (< 16 nmol/L). The remaining unexplained inter-individual variability is still high and therefore model-informed tamoxifen dosing should be accompanied by therapeutic drug monitoring.

(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.

Influence of probenecid on endoxifen systemic exposure in breast cancer patients on adjuvant tamoxifen treatment

Introduction:

In breast cancer patients treated with the anti-estrogen tamoxifen, low concentrations of the active metabolite endoxifen are associated with more disease recurrence. We hypothesized that we could increase endoxifen concentrations by induction of its formation and inhibition of its metabolism by co-administration of probenecid.

Methods:

We conducted a crossover study and measured endoxifen concentrations in patients on steady-state tamoxifen monotherapy and after 14 days of combination treatment with probenecid. Eleven evaluable patients were included.

Results:

Treatment with tamoxifen and probenecid resulted in a 26% increase of endoxifen area under the plasma concentration–time curve from 0 to 24 h (AUC_0–24h) compared to tamoxifen monotherapy (95% confidence interval [CI]: 8–46%; p < 0.01), while the maximum observed endoxifen concentration increased with 24% (95% CI: 7–44%; p < 0.01). The metabolic ratio of endoxifen to tamoxifen increased with 110% (95% CI: 82–143%; p < 0.001) after the addition of probenecid.

Conclusion:

Probenecid resulted in a clinically relevant increase of endoxifen concentrations in breast cancer patients treated with adjuvant tamoxifen. This combination therapy could provide a solution for patients with a CYP2D6-poor metabolizer phenotype or endoxifen concentrations below the threshold despite earlier tamoxifen dose.

Therapeutic Drug Monitoring-Guided Adjuvant Tamoxifen Dosing in Patients with Early Breast Cancer: A Cost-Effectiveness Analysis from the Prospective TOTAM Trial

Background and Objectives

Endoxifen is the active metabolite of tamoxifen, and a minimal plasma concentration of 16 nM has been suggested as a threshold above which it is effective in reducing the risk of breast cancer recurrence. The aim of the current analysis was to investigate the cost-effectiveness of therapeutic drug monitoring (TDM)-guided tamoxifen dosing.

Methods

A cost-effectiveness analysis was performed from a Dutch healthcare perspective, using a partitioned survival model and a lifetime horizon. The reduction in subtherapeutic treatment following TDM is modelled as improved rates of recurrence-free survival (RFS) and overall survival (OS) in comparison to standard tamoxifen treatment. A probabilistic sensitivity analysis (PSA) and a series of scenario analyses were performed to assess the robustness of the results.

Results

Base-case results estimated a total increase in life years and quality-adjusted life years (QALYs) for TDM of 0.40 and 0.53, respectively. Total costs for TDM and standard tamoxifen treatment are €32,893 and €39,524, respectively. The TDM intervention results in both more QALYs and less healthcare costs, indicating a dominating effect for TDM. The PSA results indicate that the probability of TDM being cost-effective is 92% when using a willingness-to-pay threshold of €20,000.

Conclusions

TDM-guided dose optimization of tamoxifen is estimated to save costs and increase QALYs for early breast cancer patients.

Association between genetic polymorphisms in cytochrome P450 enzymes and survivals in women with breast cancer receiving adjuvant endocrine therapy: a systematic review and meta-analysis

Tamoxifen is commonly prescribed for preventing recurrence in patients with breast cancer. However, the responses of the patients on tamoxifen treatment are variable. Cytochrome P450 genetic variants have been reported to have a significant impact on the clinical outcomes of tamoxifen treatment but no tangible conclusion can be made up till now. The present review attempts to provide a comprehensive review on the associative relationship between genetic polymorphisms in cytochrome P450 enzymes and survival in breast cancer patients on adjuvant tamoxifen therapy. The literature search was conducted using five databases, resulting in the inclusion of 58 studies in the review. An appraisal of the reporting quality of the included studies was conducted using the assessment tool from the Effective Public Health Practice Project (EPHPP). Meta-analyses were performed on CYP2D6 studies using Review Manager 5.3 software. For other studies, descriptive analyses were performed. The results of meta-analyses demonstrated that shorter overall survival, disease-free survival and relapse-free survival were found in the patients with decreased metabolisers when compared to normal metabolisers. The findings also showed that varying and conflicting results were reported by the included studies. The possible explanations for the variable results are discussed in this review.

Endoxifen, an Estrogen Receptor Targeted Therapy: From Bench to Bedside

The selective estrogen receptor (ER) modulator, tamoxifen, is the only endocrine agent with approvals for both the prevention and treatment of premenopausal and postmenopausal estrogen-receptor positive breast cancer as well as for the treatment of male breast cancer. Endoxifen, a secondary metabolite resulting from CYP2D6-dependent biotransformation of the primary tamoxifen metabolite, N-desmethyltamoxifen (NDT), is a more potent antiestrogen than either NDT or the parent drug, tamoxifen. However, endoxifen's antitumor effects may be related to additional molecular mechanisms of action, apart from its effects on ER. In phase 1/2 clinical studies, the efficacy of Z-endoxifen, the active isomer of endoxifen, was evaluated in patients with endocrine-refractory metastatic breast cancer as well as in patients with gynecologic, desmoid, and hormone-receptor positive solid tumors, and demonstrated substantial oral bioavailability and promising antitumor activity. Apart from its potent anticancer effects, Z-endoxifen appears to result in similar or even greater bone agonistic effects while resulting in little or no endometrial proliferative effects compared with tamoxifen. In this review, we summarize the preclinical and clinical studies evaluating endoxifen in the context of breast and other solid tumors, the potential benefits of endoxifen in bone, as well as its emerging role as an antimanic agent in bipolar disorder. In total, the summarized body of literature provides compelling arguments for the ongoing development of Z-endoxifen as a novel drug for multiple indications.

Evaluating the Risk of Breast Cancer Recurrence and Metastasis After Adjuvant Tamoxifen Therapy by Integrating Polymorphisms in Cytochrome P450 Genes and Clinicopathological Characteristics

Tamoxifen (TAM) is the most commonly used adjuvant endocrine drug for hormone receptor-positive (HR+) breast cancer patients. However, how to accurately evaluate the risk of breast cancer recurrence and metastasis after adjuvant TAM therapy is still a major concern. In recent years, many studies have shown that the clinical outcomes of TAM-treated breast cancer patients are influenced by the activity of some cytochrome P450 (CYP) enzymes that catalyze the formation of active TAM metabolites like endoxifen and 4-hydroxytamoxifen. In this study, we aimed to first develop and validate an algorithm combining polymorphisms in CYP genes and clinicopathological signatures to identify a subpopulation of breast cancer patients who might benefit most from TAM adjuvant therapy and meanwhile evaluate major risk factors related to TAM resistance. Specifically, a total of 256 patients with invasive breast cancer who received adjuvant endocrine therapy were selected. The genotypes at 10 loci from three TAM metabolism-related CYP genes were detected by time-of-flight mass spectrometry and multiplex long PCR. Combining the 10 loci with nine clinicopathological characteristics, we obtained 19 important features whose association with cancer recurrence was assessed by importance score via random forests. After that, a logistic regression model was trained to calculate TAM risk-of-recurrence score (TAM RORs), which is adopted to assess a patient's risk of recurrence after TAM treatment. The sensitivity and specificity of the model in an independent test cohort were 86.67% and 64.56%, respectively. This study showed that breast cancer patients with high TAM RORs were less sensitive to TAM treatment and manifested more invasive characteristics, whereas those with low TAM RORs were highly sensitive to TAM treatment, and their conditions were stable during the follow-up period. There were some risk factors that had a significant effect on the efficacy of TAM. They were tissue classification (tumor Grade < 2 vs. Grade ≥ 2, p = 2.2e-16), the number of lymph node metastases (Node-Negative vs. Node < 4, p = 5.3e-07; Node < 4 vs. Node ≥ 4, p = 0.003; Node-Negative vs. Node ≥ 4, p = 7.2e-15), and the expression levels of estrogen receptor (ER) and progesterone receptor (PR) (ER < 50% vs. ER ≥ 50%, p = 1.3e-12; PR < 50% vs. PR ≥ 50%, p = 2.6e-08). The really remarkable thing is that different genotypes of CYP2D6*10(C188T) show significant differences in prediction function (CYP2D6*10 CC vs. TT, p < 0.019; CYP2D6*10 CT vs. TT, p < 0.037). There are more than 50% Chinese who have CYP2D6*10 mutation. So the genotype of CYP2D6*10(C188T) should be tested before TAM therapy.

Therapeutic Drug Monitoring of Endoxifen for Tamoxifen Precision Dosing: Feasible in Patients with Hormone-Sensitive Breast Cancer

Background

Endoxifen is the most important active metabolite of tamoxifen. Several retrospective studies have suggested a minimal or threshold endoxifen systemic concentration of 14–16 nM is required for a lower recurrence rate. The aim of this study was to investigate the feasibility of reaching a predefined endoxifen level of ≥ 16 nM (5.97 ng/mL) over time using therapeutic drug monitoring (TDM).

Methods

This prospective open-label intervention study enrolled patients who started treatment with a standard dose of tamoxifen 20 mg once daily for early breast cancer. An outpatient visit was combined with a TDM sample at 3, 4.5, and 6 months after initiation of the tamoxifen treatment. The tamoxifen dose was escalated to a maximum of 40 mg if patients had an endoxifen concentration < 16 nM. The primary endpoint of the study was the percentage of patients with an endoxifen level ≥ 16 nM at 6 months after the start of therapy compared with historical data, in other words, 80% of patients with endoxifen levels ≥ 16 nM with standard therapy.

Results

In total, 145 patients were included. After 6 months, 89% of the patients had endoxifen levels ≥ 16 nM, compared with a literature-based 80% of patients with endoxifen levels ≥ 16 nM at baseline (95% confidence interval 82–94; P = 0.007). In patients with an affected CYP2D6 allele, it was not always feasible to reach the predefined endoxifen level of ≥ 16 nM. No increase in tamoxifen-related adverse events was reported after dose escalation.

Conclusion

This study demonstrated that it is feasible to increase the percentage of patients with endoxifen levels ≥ 16 nM using TDM. TDM is a safe strategy that offers the possibility of nearly halving the number of patients with endoxifen levels < 16 nM.

Endoxifen: A new, protein kinase C inhibitor to treat acute and mixed mania associated with bipolar I disorder

OBJECTIVES

Endoxifen is a protein kinase C inhibitor. The objective of the present phase III study was to demonstrate the safety and efficacy of endoxifen in treating bipolar I disorder (BPD I) patients.

METHODS

A multicenter, double-blind, active-controlled study was conducted using a daily dose of 8 mg endoxifen compared to 1000 mg divalproex, the current standard treatment, in patients with BPD I acute manic episodes with/without mixed features. The primary endpoint of our study was the mean change in total Young Mania Rating Scale (YMRS) score at day 21.

RESULTS

Endoxifen (n = 116) significantly (p < 0.0001) reduced total YMRS score (from 33.1 to 17.8. A significant (p < 0.001) improvement in Montgomery-Åsberg Depression Rating Scale (MADRS) score was observed for endoxifen (4.8 to 2.5). Early time to remission of the disease was observed with endoxifen compared to divalproex. None of the patients required rescue medication and there was no drug-associated withdrawals. Changes in Clinical Global Impressions-Bipolar Disorder and Clinical Global Impression-Severity of Illness scores showed that treatment with endoxifen was well-tolerated.

CONCLUSIONS

Endoxifen at a low daily dose of 8 mg was as efficacious and safe in patients with BPD I acute manic episodes with/without mixed features.

Exosomal miRNA-205 promotes breast cancer chemoresistance and tumorigenesis through E2F1

Breast cancer (BC) is a common malignant tumor in females. The challenge in treating BC is overcoming chemoresistance. Exosome-mediated transfer of miRNAs is a molecule-shuttle in intercellular communication. Thus, we aimed to investigate whether exosomal miRNA-205 could affect chemoresistance and tumorigenesis in recipient tumor cells and to elucidate the underlying mechanism in vivo and in vitro. Microarray and qRT-PCR assays demonstrated that miRNA-205 was upregulated in tamoxifen resistance MCF-7/TAMR-1 (M/T) cells and M/T cell-derived exosomes (M/T-Exo). The M/T-Exo was internalized by human BC cells (BCCs), causing increased expression of miRNA-205 in BCCs. Coculturing with M/T-Exo promoted tamoxifen resistance, proliferation, migration, and invasion while suppressed apoptosis in recipient BCCs, which were associated with activating the caspase pathway and phosphorylating Akt. Luciferase reporter assays showed that miRNA-205 directly targeted E2F Transcription Factor 1 (E2F1) in BCCs. Furthermore, knockdown of miRNA-205 or overexpression of E2F1 reversed the roles of M/T-Exo in BCCs. In vivo experiments showed that the intratumoral injection of M/T-Exo caused greater tamoxifen resistance and larger tumor size relative to mice treated with miRNA-205-knockdown or E2F1-overexpressing BCCs. Together, the results suggest that exosomal miRNA-205 may promote tamoxifen resistance and tumorigenesis in BC through targeting E2F1 in vivo and in vitro.

Genetic polymorphisms on the effectiveness or safety of breast cancer treatment: Clinical relevance and future perspectives

Breast cancer (BC) is the most frequent neoplasm and one of the main causes of death in women. The pharmacological treatment of BC consists of hormonal therapy, chemotherapeutic agents and targeted therapy. The response to BC therapy is highly variable in clinical practice. This variability can be explained by the presence of genetic polymorphisms in genes involved in the pharmacokinetics, pharmacodynamics or immune response of patients. The abundant evidence of associations between low-activity alleles CYP2D6*3, *4, *5, *6, *10 and *41 and poor results with tamoxifen therapy, and between DPYD gene polymorphisms rs3918290, rs55886062, rs67376798 and rs75017182 and increased risk of toxicity to fluoropyrimidine therapy, justify the existence of clinical pharmacogenetic guidelines. The NQO1 rs1800566 polymorphism is related to poorer results in BC therapy with chemotherapy agents. The polymorphism rs1695 of the GSTP1 gene has been associated with the effectiveness and toxicity of fluorouracil, cyclophosphamide and epirubicin therapy. Finally, the HLA-DQA1*02:01 allele is significantly associated with the occurrence of liver toxicity events in patients receiving lapatinib. There is moderate evidence to support the aforementioned associations and, therefore, a high probability of these being considered as future predictive genetic biomarkers of response. However, further studies are required to reinforce or clarify their clinical relevance.

Volumetric Absorptive Microsampling as a New Biosampling Tool for Monitoring of Tamoxifen, Endoxifen, 4-OH Tamoxifen and N-Desmethyltamoxifen in Breast Cancer Patients

INTRODUCTION

In this research, we used a volumetric absorptive microsampling (VAMS) technique to collect blood samples from the patients. A rapid and simple sample preparation method and LC-MS.MS assay was then developed and validated for the simultaneous analysis of tamoxifen and its three active metabolites.

METHODS

VAMS extraction was performed in methanol by sonication-assisted extraction method for 25 min after 2 hof VAMS drying. Separation was carried out using Acquity UPLC BEH C18 column (2.1 x 100 mm; 1.7 µm), with a flow rate of 0.2 mL/min, and the mobile phase gradient of formic acid 0.1% and formic acid 0.1% in acetonitrile for 5 min. The multiple reaction monitoring (MRM) values were set at m/z 358.31>58.27 for N-desmethyltamoxifen, m/z 372.33>72.28 for tamoxifen, m/z 388.22>72.28 for 4-hydroxytamoxifen, m/z 374.25>58.25 for endoxifen, and m/z 260.26>116.12 for propranolol.

RESULTS AND DISCUSSION

The lower limit of quantification value (LLOQ) was 2.50 ng/mL for tamoxifen, 2.50 ng/mL for endoxifen, 1.50 ng/mL for 4-hydroxitamoxifen, and 2.00 ng/mL for N-desmethyltamoxifen. Accuracy (%bias) and precision (%CV) were within 20% for LLOQ and 15% for other concentrations. There were no interference responses >20% of the LLOQ and 5% of the internal standard. The level of ion suppression in all analytes was less than 7%. The preparation system developed in this study successfully extracted more than 90% of analytes from the matrix with precision below 15%. Carryover was shown to be below 6% in all analytes. Stability of analytes in VAMS was demonstrated for up to 30 days, under room temperature storage in a sealed plastic bag with desiccant. This method was successfully applied to analyze tamoxifen and the metabolites level in 30 ER+ breast cancer patients.

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.

Computational Treatment Simulations to Assess the Need for Personalized Tamoxifen Dosing in Breast Cancer Patients of Different Biogeographical Groups

Simple Summary

Tamoxifen is a drug often used to treat the most common type of breast cancer. Its metabolite endoxifen is formed by the liver enzyme CYP2D6, whose activity is variable and depends on a patient’s genetic profile. The frequency of CYP2D6 variants with different functional enzymatic activity varies largely between populations. To ensure sufficient efficacy of tamoxifen, a certain target concentration of endoxifen is needed, and 20% of White breast cancer patients have been shown not to reach this target concentration. However, little is known about the risk of not attaining the endoxifen target amongst other ethnic populations. This study investigated the risk for suboptimal endoxifen concentration in nine different biogeographical populations based on their distinct CYP2D6 genetic profile. The variability between the populations was high (up to three-fold), and East Asian breast cancer patients were identified as the population with the highest need for personalized tamoxifen dosing.

Abstract

Tamoxifen is used worldwide to treat estrogen receptor-positive breast cancer. It is extensively metabolized, and minimum steady-state concentrations of its metabolite endoxifen (C_{SS,min ENDX}) >5.97 ng/mL have been associated with favorable outcome. Endoxifen formation is mediated by the enzyme CYP2D6, and impaired CYP2D6 function has been associated with lower C_SS,min ENDX. In the Women’s Healthy Eating and Living (WHEL) study proposing the target concentration, 20% of patients showed subtarget C_{SS,min ENDX} at tamoxifen standard dosing. CYP2D6 allele frequencies vary largely between populations, and as 87% of the patients in the WHEL study were White, little is known about the risk for subtarget C_{SS,min ENDX} in other populations. Applying pharmacokinetic simulations, this study investigated the risk for subtarget C_{SS,min ENDX} at tamoxifen standard dosing and the need for dose individualization in nine different biogeographical groups with distinct CYP2D6 allele frequencies. The high variability in CYP2D6 allele frequencies amongst the biogeographical groups resulted in an up to three-fold difference in the percentages of patients with subtarget C_{SS,min ENDX}. Based on their CYP2D6 allele frequencies, East Asian breast cancer patients were identified as the population for which personalized, model-informed precision dosing would be most beneficial (28% of patients with subtarget C_{SS,min ENDX}).

Therapeutic drug monitoring of oral targeted antineoplastic drugs

PURPOSE

This review provides an overview of the current challenges in oral targeted antineoplastic drug (OAD) dosing and outlines the unexploited value of therapeutic drug monitoring (TDM). Factors influencing the pharmacokinetic exposure in OAD therapy are depicted together with an overview of different TDM approaches. Finally, current evidence for TDM for all approved OADs is reviewed.

METHODS

A comprehensive literature search (covering literature published until April 2020), including primary and secondary scientific literature on pharmacokinetics and dose individualisation strategies for OADs, together with US FDA Clinical Pharmacology and Biopharmaceutics Reviews and the Committee for Medicinal Products for Human Use European Public Assessment Reports was conducted.

RESULTS

OADs are highly potent drugs, which have substantially changed treatment options for cancer patients. Nevertheless, high pharmacokinetic variability and low treatment adherence are risk factors for treatment failure. TDM is a powerful tool to individualise drug dosing, ensure drug concentrations within the therapeutic window and increase treatment success rates. After reviewing the literature for 71 approved OADs, we show that exposure-response and/or exposure-toxicity relationships have been established for the majority. Moreover, TDM has been proven to be feasible for individualised dosing of abiraterone, everolimus, imatinib, pazopanib, sunitinib and tamoxifen in prospective studies. There is a lack of experience in how to best implement TDM as part of clinical routine in OAD cancer therapy.

CONCLUSION

Sub-therapeutic concentrations and severe adverse events are current challenges in OAD treatment, which can both be addressed by the application of TDM-guided dosing, ensuring concentrations within the therapeutic window.

Generating a Precision Endoxifen Prediction Algorithm to Advance Personalized Tamoxifen Treatment in Patients with Breast Cancer

Tamoxifen is an endocrine treatment for hormone receptor positive breast cancer. The effectiveness of tamoxifen may be compromised in patients with metabolic resistance, who have insufficient metabolic generation of the active metabolites endoxifen and 4-hydroxy-tamoxifen. This has been challenging to validate due to the lack of measured metabolite concentrations in tamoxifen clinical trials. CYP2D6 activity is the primary determinant of endoxifen concentration. Inconclusive results from studies investigating whether CYP2D6 genotype is associated with tamoxifen efficacy may be due to the imprecision in using CYP2D6 genotype as a surrogate of endoxifen concentration without incorporating the influence of other genetic and clinical variables. This review summarizes the evidence that active metabolite concentrations determine tamoxifen efficacy. We then introduce a novel approach to validate this relationship by generating a precision endoxifen prediction algorithm and comprehensively review the factors that must be incorporated into the algorithm, including genetics of CYP2D6 and other pharmacogenes. A precision endoxifen algorithm could be used to validate metabolic resistance in existing tamoxifen clinical trial cohorts and could then be used to select personalized tamoxifen doses to ensure all patients achieve adequate endoxifen concentrations and maximum benefit from tamoxifen treatment.

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

Simple Summary

Tamoxifen 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.

Abstract

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.

Simulation-Based Assessment of the Impact of Non-Adherence on Endoxifen Target Attainment in Different Tamoxifen Dosing Strategies

Tamoxifen is widely used in breast cancer treatment and minimum steady-state concentrations of its active metabolite endoxifen (C_{SS,min ENDX}) above 5.97 ng/mL have been associated with favourable disease outcome. Yet, about 20% of patients do not reach target C_{SS,min ENDX} applying conventional tamoxifen dosing. Moreover, 4-75% of patients are non-adherent, resulting in worse disease outcomes. Assuming complete adherence, we previously showed model-informed precision dosing (MIPD) to be superior to conventional and CYP2D6-guided dosing in minimising the proportion of patients with subtarget C_{SS,min ENDX}. Given the high non-adherence rate in long-term tamoxifen therapy, this study investigated the impact of non-adherence on C_{SS,min ENDX} target attainment in different dosing strategies. We show that MIPD allows to account for the expected level of non-adherence (here: up to 2 missed doses/week): increasing the MIPD target threshold from 5.97 ng/mL to 9 ng/mL (the lowest reported C_{SS,min ENDX} in CYP2D6 normal metabolisers) as a safeguard resulted in the lowest interindividual variability and proportion of patients with subtarget C_{SS,min ENDX} even in non-adherent patients. This is a significant improvement to conventional and CYP2D6-guided dosing. Adding a fixed increment to the originally selected dose is not recommended, since it inflates interindividual variability.

Pharmacogenetics of tamoxifen therapy in Asian populations: from genetic polymorphism to clinical outcomes

BACKGROUND

Compared with western countries, Asian breast cancer patients have unique pathological and biological characteristics. Most of them are premenopausal women with HR positive. Tamoxifen as the first-line drug for premenopausal women with HR+ is involved in multiple enzymes and transporters during metabolizing and transporting process. Variants that cause decreased or inactive gene products leading to abnormal responses in tamoxifen therapy have well been studied in western countries, whereas such information is much less reported in Asian populations.

OBJECTIVE

In order to elucidate the relationship between genetic variants and tamoxifen-induced individual drug reactions in different Asian populations and further identify genotypes/phenotypes with potential therapeutic significance.

METHODS

We reviewed the frequencies of genetic variants in major enzymes and transporter genes involved in the metabolism and transport of tamoxifen across Asian populations as well as significant correlations between genotypes/metabolic phenotypes and metabolites concentrations or BC clinical outcomes.

RESULTS

Significant inter-ethnic differences in allele frequencies was found among Asian populations, such as CYP2D6*4, *10, *41, CYP2C9*2, ABCB1 C3435T and SLCO1B1*5, and CYP2D6*10/*10 is the most common genotype correlated with adverse clinical outcomes. Moreover, we summarized the barriers and controversies of implementing pharmacogenetics in tamoxifen therapy and concluded that more population-specific pharmacogenetic studies are needed in the future.

CONCLUSION

This review revealed more systematic pharmacogenomics of genes involved in the metabolism and transport besides CYP2D6, are required to optimize the genotyping strategies and guide the personalized tamoxifen therapy in Asian populations.

The effect of IL-1β on MRP2 expression and tamoxifen toxicity in MCF-7 breast cancer cells

BACKGROUND

Chronic inflammation is considered to be a risk factor for carcinogenesis, tumor development and metastasis by providing tumor-related factors.

OBJECTIVES

We aimed to evaluate the effect of cytokine interleukin-1β (IL-1β) as a key mediator of inflammation on multidrug resistance associated protein 2 (MRP2) expression and tamoxifen toxicity in estrogen receptor positive (ER+) MCF-7 breast cancer cells.

METHODS

The effects of IL-1β on tamoxifen toxicity following 20-day treatment of MCF-7 cells with IL-1β and/or 17β-estradiol (E2) were measured by MTT assay. Furthermore, the effects of IL-1β and/or E2 on the mRNA expression and protein levels of MRP2 and NF-κB (p65) in breast cancer cells were evaluated by QRT-PCR and Western blot analysis, respectively.

RESULTS

Treatment of breast cancer cells with IL-1β+ E2 decreased the sensitivity to 4-OH tamoxifen compared to both E2-treated and untreated cells. The mRNA expression levels of MRP2 and NF-κB (p65) were significantly increased following treatment with IL-1β+ E2, compared to control. In addition, breast cancer cells treatment with IL-1β+ E2 increased protein expression of MRP2 and it had no significant effect on NF-κB/p65 protein expression in these cells.

CONCLUSION

Increased expression of mRNA and protein level of MRP2 following 20-day treatment of MCF-7 cells with IL-1β + E2 might be a possible elucidation for the increased tamoxifen resistance which was observed in these cells. More researches are essential to clarify the molecular mechanisms of inflammation on drug-resistance in the tumor environment in order to reducing or eliminating chemotherapy resistance and developing more effective treatment strategies.

Recent advances in the personalized treatment of estrogen receptor-positive breast cancer with tamoxifen: a focus on pharmacogenomics

Introduction: Tamoxifen is still an important drug in hormone-dependent breast cancer therapy. Personalization of its clinical use beyond hormone receptor positivity could improve the substantial variability of the treatment response.Areas covered: The overview of the current evidence for the treatment personalization using therapeutic drug monitoring, or using genetic biomarkers including CYP2D6 is provided. Although many studies focused on the PK aspects or the impact of CYP2D6 variability the translation into clinical routine is not clearly defined due to the inconsistent clinical outcome data.Expert opinion: We believe that at least the main candidate factors, i.e. CYP2D6 polymorphism, CYP2D6 inhibition, endoxifen serum levels may become important predictors of clinical relevance for tamoxifen treatment personalization in the future. To achieve this aim, however, further research should take into consideration more precise characterization of the disease, epigenetic factors and also utilize an appropriately powered multifactorial approach instead of a single gene evaluating studies.

Low Z-4OHtam concentrations are associated with adverse clinical outcome among early stage premenopausal breast cancer patients treated with adjuvant tamoxifen

Low steady-state levels of active tamoxifen metabolites have been associated with inferior treatment outcomes. In this retrospective analysis of 406 estrogen receptor-positive breast cancer (BC) patients receiving adjuvant tamoxifen as initial treatment, we have associated our previously reported thresholds for the two active metabolites, Z-endoxifen and Z-4-hydroxy-tamoxifen (Z-4OHtam), with treatment outcomes in an independent cohort of BC patients. Among all patients, metabolite levels did not affect survival. However, in the premenopausal subgroup receiving tamoxifen alone (n = 191) we confirmed an inferior BC -specific survival in patients with the previously described serum concentration threshold of Z-4OHtam ≤ 3.26 nm (HR = 2.37, 95% CI = 1.02-5.48, P = 0.039). The 'dose-response' survival trend in patients categorized to ordinal concentration cut-points of Z-4OHtamoxifen (≤ 3.26, 3.27-8.13, > 8.13 nm) was also replicated (P-trend log-rank = 0.048). Z-endoxifen was not associated with outcome. This is the first study to confirm the association between a published active tamoxifen metabolite threshold and BC outcome in an independent patient cohort. Premenopausal patients receiving 5-year of tamoxifen alone may benefit from therapeutic drug monitoring to ensure tamoxifen effectiveness.

Model-Based Quantification of Impact of Genetic Polymorphisms and Co-Medications on Pharmacokinetics of Tamoxifen and Six Metabolites in Breast Cancer

Variations in clinical response to tamoxifen (TAM) may be related to polymorphic cytochromes P450 (CYPs) involved in forming its active metabolite endoxifen (ENDO). We developed a population pharmacokinetic (PopPK) model for tamoxifen and six metabolites to determine clinically relevant factors of ENDO exposure. Concentration-time data for TAM and 6 metabolites come from a prospective, multicenter, 3-year follow-up study of adjuvant TAM (20 mg/day) in patients with breast cancer, with plasma samples drawn every 6 months, and genotypes for 63 genetic polymorphisms (PHACS study, NCT01127295). Concentration data for TAM and 6 metabolites from 928 patients (n = 27,433 concentrations) were analyzed simultaneously with a 7-compartment PopPK model. CYP2D6 phenotype (poor metabolizer (PM), intermediate metabolizer (IM), normal metabolizer (NM), and ultra-rapid metabolizer (UM)), CYP3A4*22, CYP2C19*2, and CYP2B6*6 genotypes, concomitant CYP2D6 inhibitors, age, and body weight had a significant impact on TAM metabolism. Formation of ENDO from N-desmethyltamoxifen was decreased by 84% (relative standard error (RSE) = 14%) in PM patients and by 47% (RSE = 9%) in IM patients and increased in UM patients by 27% (RSE = 12%) compared with NM patients. Dose-adjustment simulations support an increase from 20 mg/day to 40 and 80 mg/day in IM patients and PM patients, respectively, to reach ENDO levels similar to those in NM patients. However, when considering Antiestrogenic Activity Score (AAS), a dose increase to 60 mg/day in PM patients seems sufficient. This PopPK model can be used as a tool to predict ENDO levels or AAS according to the patient's CYP2D6 phenotype for TAM dose adaptation.

Simplified phenotyping of CYP2D6 for tamoxifen treatment using the N-desmethyl-tamoxifen/ endoxifen ratio

Introduction

CYP2D6 protein activity can be inferred from the ratio of N-desmethyl-tamoxifen (NDMT) to endoxifen (E). CYP2D6 polymorphisms are common and can affect CYP2D6 protein activity and E level. Some retrospective studies indicate that E < 16 nM may relate to worse outcome.

Materials and methods

A target NDMT/E ratio was defined as associated with an E level of 15 nM in the 161 patient Test cohort of tamoxifen-treated patients, dichotomizing them into ‘Normal’ (NM) and ‘Slow’ (SM) CYP2D6 metabolizer groups. This ratio was then tested on a validation cohort of 52 patients. Patients were phenotyped based on the standard method (ultrarapid/extensive, intermediate or poor metabolizers; UM/EM, IM, PM) or a simplified system based on whether any variant allele (V) vs wildtype (wt) was present (wt/wt, wt/V, V/V). Comprehensive CYP2D6 genotyping was undertaken on germline DNA.

Results

A target NDMT/E ratio of 35 correlated with the 15 nM E level, dichotomizing patients into NM (<35; N = 117) and SM (>35; N = 44) groups. The ratio was independently validated by a validation cohort. The simplified system was better in predicting patients without slow metabolism, with specificity and sensitivity of 96% and 44% respectively, compared with the standard method - sensitivity 81% and specificity 83%.

Conclusions

The simplified classification system based on whether any variant was present better identified patients who were truly not CYP2D6 slow metabolizers more accurately than the current system. However, as CYP2D6 genotype is not the only determinant of endoxifen level, we recommend that direct measurement of endoxifen should also be considered.

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We used a ratio of two tamoxifen metabolites to categorize CYP2D6 metabolizer groups.

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We developed a simplified system to identify slow metabolizers based on genotype.

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The simplified system was more accurate than the standard complex system.

Predicting steady-state endoxifen plasma concentrations in breast cancer patients by CYP2D6 genotyping or phenotyping. Which approach is more reliable?

In previous studies, steady-state Z-endoxifen plasma concentrations (ENDOss) correlated with relapse-free survival in women on tamoxifen (TAM) treatment for breast cancer. ENDOss also correlated significantly with CYP2D6 genotype (activity score) and CYP2D6 phenotype (dextromethorphan test). Our aim was to ascertain which method for assessing CYP2D6 activity is more reliable in predicting ENDOss. The study concerned 203 Caucasian women on tamoxifen-adjuvant therapy (20 mg q.d.). Before starting treatment, CYP2D6 was genotyped (and activity scores computed), and the urinary log(dextromethorphan/dextrorphan) ratio [log(DM/DX)] was calculated after 15 mg of oral dextromethorphan. Plasma concentrations of TAM, N-desmethyl-tamoxifen (ND-TAM), Z-4OH-tamoxifen (4OH-TAM) and ENDO were assayed 1, 4, and 8 months after first administering TAM. Multivariable regression analysis was used to identify the clinical and laboratory variables predicting log-transformed ENDOss (log-ENDOss). Genotype-derived CYP2D6 phenotypes (PM, IM, NM, EM) and log(DM/DX) correlated independently with log-ENDOss. Genotype-phenotype concordance was almost complete only for poor metabolizers, whereas it emerged that 34% of intermediate, normal, and ultrarapid metabolizers were classified differently based on log(DM/DX). Multivariable regression analysis selected log(DM/DX) as the best predictor, with patients' age, weak inhibitor use, and CYP2D6 phenotype decreasingly important: log-ENDOss = 0.162 - log(DM/DX) × 0.170 + age × 0.0063 - weak inhibitor use × 0.250 + IM × 0.105 + (NM + UM) × 0.210; (R2  = 0.51). In conclusion, log(DM/DX) seems superior to genotype-derived CYP2D6 phenotype in predicting ENDOss.

Obesity Alters Endoxifen Plasma Levels in Young Breast Cancer Patients: A Pharmacometric Simulation Approach

Endoxifen is one of the most important metabolites of the prodrug tamoxifen. High interindividual variability in endoxifen steady-state concentrations (CSS,min ENDX ) is observed under tamoxifen standard dosing and patients with breast cancer who do not reach endoxifen concentrations above a proposed therapeutic threshold of 5.97 ng/mL may be at a 26% higher recurrence risk compared with patients with endoxifen concentrations exceeding this value. In this investigation, 10 clinical tamoxifen studies were pooled (1,388 patients) to investigate influential factors on CSS,min ENDX using nonlinear mixed-effects modeling. Age and body weight were found to significantly impact CSS,min ENDX in addition to CYP2D6 phenotype. Compared with postmenopausal patients, premenopausal patients had a 30% higher risk for subtarget CSS,min ENDX at tamoxifen 20 mg per day. In treatment simulations for distinct patient subpopulations, young overweight patients had a 3.1-13.8-fold higher risk for subtarget CSS,min ENDX compared with elderly low-weight patients. Considering ever-rising obesity rates and the clinical importance of tamoxifen for premenopausal patients, this subpopulation may benefit most from individualized tamoxifen dosing.

Impairment of endoxifen formation in tamoxifen-treated premenopausal breast cancer patients carrying reduced-function CYP2D6 alleles

Aims

Tamoxifen is bioactivated to endoxifen by polymorphic CYP2D6‐dependent metabolism. Here, endoxifen levels were compared to CYP2D6 diplotypes, tentative target concentrations and side effects.

Methods

In total, 118 Swedish premenopausal breast cancer patients diagnosed 2006–2014, with on‐going postoperative tamoxifen treatment January 2017, were included. Biobanked DNA from peripheral blood was used for CYP2D6 genotyping by TaqMan real‐time polymerase chain reaction (CYP2D6*1, *3, *4, *5, *6, *9, *10, *41, *1xN). Plasma levels of tamoxifen and 3 major metabolites were quantified by liquid chromatography–tandem mass spectrometry. Clinical information on treatment and side effects was retrospectively obtained from medical records.

Results

In the final analysis of 114 patients, a clear relationship between CYP2D6 genotype and plasma endoxifen levels was evident. Low endoxifen (1.6–5.2 ng/mL), i.e. below the suggested threshold for clinical efficacy, was found in all patients with 2 reduced‐function alleles, 2 null‐alleles, or a null/reduced‐function combination. CYP2D6*41 was the most common reduced‐function allele (82%) and 17 of 21 CYP2D6*41‐carriers exhibited a lower CYP2D6 activity than predicted from published guidelines. No difference in endoxifen levels was observed between carriers of 2 null‐alleles vs patients homozygous for CYP2D6*41 or the corresponding heterozygous combination (P = .338). In patients with endoxifen levels <5.9 ng/mL (36/114), side effects were either mild or absent. At higher endoxifen levels moderate‐to‐severe side effects were reported in a concentration‐dependent manner.

Conclusion

Significantly reduced endoxifen levels were observed not only in all homozygous carriers of CYP2D6 null‐alleles, but also in carriers of 2 reduced‐function alleles. This finding may be highly relevant for future, genotype‐based dose considerations.

Serum Detection of Nonadherence to Adjuvant Tamoxifen and Breast Cancer Recurrence Risk

PURPOSE

Nonadherence to long-term treatments is often under-recognized by physicians and there is no gold standard for its assessment. In breast cancer, nonadherence to tamoxifen therapy after surgery constitutes a major obstacle to optimal outcomes. We sought to evaluate the rate of biochemical nonadherence to adjuvant tamoxifen using serum assessment and to examine its effects on short-term, distant disease-free survival (DDFS).

PATIENTS AND METHODS

We studied 1,177 premenopausal women enrolled in a large prospective study (CANTO/NCT01993498). Definition of biochemical nonadherence was based on a tamoxifen serum level < 60 ng/mL, assessed 1 year after prescription. Self-reported nonadherence to tamoxifen therapy was collected at the same time through semistructured interviews. Survival analyses were conducted using an inverse probability weighted Cox proportional hazards model, using a propensity score based on age, staging, surgery, chemotherapy, and center size.

RESULTS

Serum assessment of tamoxifen identified 16.0% of patients (n = 188) below the set adherence threshold. Patient-reported rate of nonadherence was lower (12.3%). Of 188 patients who did not adhere to the tamoxifen prescription, 55% self-reported adherence to tamoxifen. After a median follow-up of 24.2 months since tamoxifen serum assessment, patients who were biochemically nonadherent had significantly shorter DDFS (for distant recurrence or death, adjusted hazard ratio, 2.31; 95% CI, 1.05 to 5.06; P = .036), with 89.5% of patients alive without distant recurrence at 3 years in the nonadherent cohort versus 95.4% in the adherent cohort.

CONCLUSION

Therapeutic drug monitoring may be a useful method to promptly identify patients who do not take adjuvant tamoxifen as prescribed and are at risk for poorer outcomes. Targeted interventions facilitating patient adherence are needed and have the potential to improve short-term breast cancer outcomes.