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MDV3100 is an androgen-receptor antagonist that blocks androgens from binding to the androgen receptor and prevents nuclear translocation and co-activator recruitment of the ligand-receptor complex. It also induces tumour cell apoptosis, and has no agonist activity. Because growth of castration-resistant prostate cancer is dependent on continued androgen-receptor signalling, we assessed the antitumour activity and safety of MDV3100 in men with this disease. This phase 1–2 study was undertaken in five US centres in 140 patients. Patients with progressive, metastatic, castration-resistant prostate cancer were enrolled in dose-escalation cohorts of three to six patients and given an oral daily starting dose of MDV3100 30 mg. The final daily doses studied were 30 mg (n=3), 60 mg (27), 150 mg (28), 240 mg (29), 360 mg (28), 480 mg (22), and 600 mg (3). The primary objective was to identify the safety and tolerability profile of MDV3100 and to establish the maximum tolerated dose. The trial is registered with ClinicalTrials.gov, number NCT00510718. We noted antitumour effects at all doses, including decreases in serum prostate-specific antigen of 50% or more in 78 (56%) patients, responses in soft tissue in 13 (22%) of 59 patients, stabilised bone disease in 61 (56%) of 109 patients, and conversion from unfavourable to favourable circulating tumour cell counts in 25 (49%) of the 51 patients. PET imaging of 22 patients to assess androgen-receptor blockade showed decreased 18F-fluoro-5α-dihydrotestosterone binding at doses from 60 mg to 480 mg per day (range 20–100%). The median time to progression was 47 weeks (95% CI 34–not reached) for radiological progression. The maximum tolerated dose for sustained treatment (>28 days) was 240 mg. The most common grade 3–4 adverse event was dose-dependent fatigue (16 [11%] patients), which generally resolved after dose reduction. We recorded encouraging antitumour activity with MDV3100 in patients with castration-resistant prostate cancer. The results of this phase 1–2 trial validate in man preclinical studies implicating sustained androgen-receptor signalling as a driver in this disease. Medivation, the Prostate Cancer Foundation, National Cancer Institute, the Howard Hughes Medical Institute, Doris Duke Charitable Foundation, and Department of Defense Prostate Cancer Clinical Trials Consortium.

Background: Although the use of complementary and alternative medicines is widespread in cancer patients, clinical evidence of their benefits is sparse. Furthermore, while they are often assumed to be safe with regard to concurrent use of anticancer therapies, few studies have been carried out to investigate possible interactions. Fucoidans are a group of sulfated carbohydrates, derived from marine brown algae, which have long been used as dietary supplements due to their reported medicinal properties, including anticancer activity. The aim of this study was to investigate the effect of co-administration of fucoidan, derived from Undaria pinnatifida, on the pharmacokinetics of 2 commonly used hormonal therapies, letrozole and tamoxifen, in patients with breast cancer. Methods: This was an open label non-crossover study in patients with active malignancy taking letrozole or tamoxifen (n = 10 for each group). Patients took oral fucoidan, given in the form of Maritech extract, for a 3-week period (500 mg twice daily). Trough plasma concentrations of letrozole, tamoxifen, 4-hydroxytamoxifen, and endoxifen were measured using HPLC-CAD (high-performance liquid chromatography charged aerosol detector), at baseline and after concomitant administration with fucoidan. Results: No significant changes in steady-state plasma concentrations of letrozole, tamoxifen, or tamoxifen metabolites were detected after co-administration with fucoidan. In addition, no adverse effects of fucoidan were reported, and toxicity monitoring showed no significant differences in all parameters measured over the study period. Conclusions: Administration of Undaria pinnatifida fucoidan had no significant effect on the steady-state trough concentrations of letrozole or tamoxifen and was well tolerated. These results suggest that fucoidan in the studied form and dosage could be taken concomitantly with letrozole and tamoxifen without the risk of clinically significant interactions.

In this trial involving women taking endocrine therapy for HR-positive breast cancer, elinzanetant (a neurokinin-targeted therapy) significantly reduced the frequency of moderate-to-severe vasomotor symptoms.

Purpose Fulvestrant is a selective estrogen receptor downregulator (SERD) that is approved for first- or second-line use as a single agent or in combination with cyclin dependent kinase or phosphatidylinositol 3-kinase inhibitors for the treatment of metastatic breast cancer. Fulvestrant exhibits exceptionally effective antitumor activity in preclinical models of breast cancer, a success that has been attributed to its robust SERD activity despite modest receptor downregulation in patient tumors. By modeling human exposures in animal models we probe the absolute need for SERD activity. Methods Three xenograft models of endocrine therapy-resistant breast cancer were used to evaluate the efficacy of fulvestrant administered in doses historically used in preclinical studies in the field or by using a dose regimen intended to model clinical exposure levels. Pharmacokinetic and pharmacodynamic analyses were conducted to evaluate plasma exposure and intratumoral ER downregulation. Results A clinically relevant 25 mg/kg dose of fulvestrant exhibited antitumor efficacy comparable to the historically used 200 mg/kg dose, but at this lower dose it did not result in robust ER downregulation. Further, the antitumor efficacy of the lower dose of fulvestrant was comparable to that observed for other oral SERDs currently in development. Conclusion The use of clinically unachievable exposure levels of fulvestrant as a benchmark in preclinical development of SERDs may negatively impact the selection of those molecules that are advanced for clinical development. Further, these studies suggest that antagonist efficacy, as opposed to SERD activity, is likely to be the primary driver of clinical response.

Background The plasma concentration of endoxifen, the active metabolite of tamoxifen, might be affected by different CYP2D6 genotypes in patients with breast cancer, but solid evidence is still lacking in Asian patients. This prospective study aimed to investigate the relationship between CYP2D6 genetic polymorphisms and endoxifen plasma concentrations among Chinese patients with breast cancer treated with tamoxifen. Methods From August 2015 to June 2018, 110 patients with breast cancer were enrolled. CYP2D6 variant alleles and endoxifen plasma concentration were determined using Sanger sequencing and high-performance liquid chromatography-tandem mass spectrometry, respectively. Results The most frequent allele of CYP2D6 was *10 (56.4%). The most frequent genotype of CYP2D6 was *10/*10 (33%), *1/*10(28.2%) and *2/*10(14.5%). Sixty-four patients (58.2%) were Normal Metabolizers (NM), while 46 (41.8%) were Intermediate Metabolizers (IM). All patients except two had endoxifen concentrations above the threshold of 5.9ng/ml. The median endoxifen plasma concentrations for patients with CYP2D6 genotypes *1/*2 and *1/10 were higher compared to other genotypes ( p =  0.012). The median endoxifen plasma concentration was higher in NM than in IM (18ng/ml vs. 13ng/ml, p  = 0.0077). Patients with CYP2D6*10(T/T) had lower endoxifen concentrations than those with *10(C/T) and *10(C/C) but the difference was not significant. There were no significant differences in adverse events between patients in the NM and IM groups or between patients with the CYP2D6*10 (T/T) genotype and non-*10 (T/T) genotype. Conclusion Only CYP2D6 IMs and NMs were identified in this study. Almost all patients had the endoxifen concentrations above the threshold. The endoxifen plasma concentration was lower in CYP2D6 IMs than in NMs, but these variants did not compromise the adverse effects of tamoxifen in Asian patients with breast cancer. Trial registration The study protocol was approved by the institutional review boards of Sun YatSen University Cancer Center (Ethics approval number, B201506501,20160115).

One in eight women will develop breast cancer in their lifetime, and tamoxifen is used to suppress breast cancer recurrence, but nearly 50% of patients are not effectively treated with this drug. Given that tamoxifen is orally administered and, thus, reaches the intestine, this variable patient response to the drug is likely related to the gut microbiota composed of trillions of bacteria, which are remarkably different among individuals. This study aims to understand the impact of the gut microbiome on tamoxifen absorption, metabolism, and recycling. The significance of our research is in defining the role that gut microbes play in tamoxifen pharmacokinetics, thus paving the way for more tailored and effective therapeutic interventions in the prevention of breast cancer recurrence.

Key Points The selective oestrogen receptor modulator tamoxifen is the most widely used antioestrogen for the treatment of hormone-dependent breast cancer. Hepatic, drug-metabolizing cytochrome P450s (CYPs) catalyse the oxidation of tamoxifen to several metabolites. The metabolites, endoxifen and 4-hydroxytamoxifen, have greater binding affinities for oestrogen receptors and suppress cell proliferation more effectively than tamoxifen does. Plasma concentrations of endoxifen are considerably higher than those of 4-hydroxytamoxifen, suggesting that endoxifen is the main pharmacologically active species of tamoxifen in vivo . The conversion of tamoxifen to endoxifen is predominantly catalysed by CYP2D6. Many polymorphisms in CYP2D6 have been identified. In Caucasian populations, 6–10% of people inherit two alleles containing polymorphisms and/or a gene deletion, leading to no protein expression or the expression of a protein with no CYP2D6 enzymatic activity. These individuals have impaired metabolism of CYP2D6 substrates and are called poor metabolizers of CYP2D6. Some drugs, such as the antidepressants fluoxetine or paroxetine, are potent inhibitors of CYP2D6 and can confer a poor metabolizer phenotype on individuals with normal CYP2D6 activity. The findings of pharmacokinetic studies indicate that the conversion of endoxifen is reduced in poor metabolizers of CYP2D6, either by genotype or by co-prescribed fluoxetine or paroxetine, which are commonly prescribed to manage hot flashes. Recent data suggest that poor metabolizers of CYP2D6 do not derive as much benefit from tamoxifen therapy as other patients do; however, some studies have yielded conflicting results. The analysis of CYP2D6 genotype might represent an early example of a pharmacogenetic tool for optimizing breast cancer therapy; however, the findings of larger, well-designed studies that support the current data are necessary before a change in clinical practice is advocated. The pharmacological activity of tamoxifen is dependent on metabolic conversion by the enzyme cytochrome P450 2D6 (CYP2D6). This Review discusses the existing data that relate CYP2D6 genotypes to tamoxifen response. Can the analysis of CYP2D6 genotype be used to optimize breast cancer therapy? Tamoxifen is the most widely used anti-oestrogen for the treatment of hormone-dependent breast cancer. The pharmacological activity of tamoxifen is dependent on its conversion by the hepatic drug-metabolizing enzyme cytochrome P450 2D6 (CYP2D6) to its abundant metabolite, endoxifen. Patients with reduced CYP2D6 activity, as a result of either their genotype or induction by the co-administration of drugs that inhibit CYP2D6 function, produce little endoxifen and seem to derive inferior therapeutic benefit from tamoxifen. Here we review the existing data that relate CYP2D6 genotypes to response to tamoxifen and discuss whether the analysis of the CYP2D6 genotype might be an early example of a pharmacogenetic tool for optimizing breast cancer therapy.

Oral anti-hormonal drugs are essential in the treatment of breast and prostate cancer. It is well known that the interpatient variability in pharmacokinetic exposure is high for these agents and exposure–response relationships exist for many oral anti-hormonal drugs. Yet, they are still administered at fixed doses. This could lead to underdosing and thus suboptimal efficacy in some patients, while other patients could be overdosed resulting in unnecessary side effects. Therapeutic drug monitoring (TDM), individualized dosing based on measured blood concentrations of the drug, could therefore be a valid option to further optimize treatment. In this review, we provide an overview of relevant clinical pharmacokinetic and pharmacodynamic characteristics of oral anti-hormonal drugs in oncology and translate these into practical guidelines for TDM. For some agents, TDM targets are not well established yet and as a reference the median pharmacokinetic exposure could be targeted (exemestane: minimum plasma concentration ( C min ) 4.1 ng/mL and enzalutamide: C min 11.4 mg/L). However, for most drugs, exposure–efficacy analyses could be translated into specific targets (abiraterone: C min 8.4 ng/mL, anastrozole: C min 34.2 ng/mL, and letrozole: C min 85.6 ng/mL). Moreover, prospective clinical trials have shown TDM to be feasible for tamoxifen, for which the exposure–efficacy threshold of its active metabolite endoxifen is 5.97 ng/mL. Based on the available data, we therefore conclude that individualized dosing based on drug concentrations is feasible and promising for oral anti-hormonal drugs and should be developed further and implemented into clinical practice.

Tamoxifen is the mainstay treatment for estrogen-positive breast cancer for over half a century. However, a significant proportion of patients experience disease recurrence due to treatment failure attributed to various factors, including disease pathology, genetics, and drug metabolism. Alam et al. introduce gut microbiota as a key factor influencing tamoxifen pharmacokinetics (Y. Alam, S. Hakopian, L. Ortiz de Ora, I. Tamburini, et al., mBio 16:e01679-24, 2024, https://doi.org/10.1128/mbio.01679-24). The authors present compelling evidence that functional differences in the gut microbiota, specifically the bacterial enzyme β-glucuronidase, leads to inter-individual variability in systemic exposure of tamoxifen, affecting drug efficacy. This study provides novel insights into the impact of the gut microbiota on tamoxifen pharmacokinetics, the latest example of how pharmacomicrobiomics, or the study of drug-microbe interactions, can enhance precision medicine for numerous diseases.

Tamoxifen decreases breast cancer recurrence, mortality, and breast cancer risk in high-risk women. Despite these proven benefits, tamoxifen use is often limited due to side effects. We identified predictors of tamoxifen-induced side effects based on clinical variables and serum tamoxifen metabolite biomarkers in a cross-sectional study of patients taking tamoxifen. We enrolled 241 women and collected data on demographics, tamoxifen use and side effects, as well as potential clinical and serum predictors. We used logistic regression models and adjusted for age, body mass index, ethnicity, education, prior post-menopausal hormone therapy (HT), tamoxifen duration, and endoxifen levels to identify factors associated with side effects. Common tamoxifen attributed side effects were hot flashes (64%), vaginal dryness (35%), sleep problems (36%), weight gain (6%), and depression, irritability or mood swings (6%). In multi-variate models, tamoxifen duration, age, prior post-menopausal HT, and endoxifen levels all predicted side effects. Women who had been on tamoxifen for >12 months were less likely to report side effects (OR 0.15, 95% CI 0.04–0.58) or severe side effects (OR 0.05, 95% CI 0.005–0.58) compared to women on tamoxifen for <12 months. Compared to women younger than 50, women who were age 60–70 and older than 70 were less likely to report side effects (OR 0.22, 95% CI 0.03–1.35; OR 0.13, 95% CI 0.01–0.99; respectively). Women who previously took post-menopausal HT were more likely to report severe side effects. Women with higher endoxifen levels were more likely to report side effects (OR 1.67, 95% CI 1.01–2.77 per standard deviation increase in endoxifen). Clinicians should consider closely monitoring adherence in women taking tamoxifen, especially in younger women, and women who previously took HT. The association between endoxifen levels and side effects is consistent with the data that suggest that endoxifen is the most highly active metabolite of tamoxifen.