The OCT2/MATE1 Interaction Between Trifluridine, Metformin and Cimetidine: A Crossover Pharmacokinetic Study.

BACKGROUND AND OBJECTIVES: Trifluridine/tipiracil, registered for the treatment of patients with metastatic gastric and colorectal cancer, is a substrate and inhibitor for the organic cation transporter 2 (OCT2) and the multidrug and toxin extrusion protein 1 (MATE1), which raises the potential for drug-drug interactions with other OCT2/MATE1 modulators. Therefore, we prospectively examined the effect of an OCT2/MATE1 inhibitor (cimetidine) and substrate (metformin) on the pharmacokinetics of trifluridine. METHODS: In this three-phase crossover study, patients with metastatic colorectal or gastric cancer were sequentially treated with trifluridine/tipiracil alone (phase A), trifluridine/tipiracil concomitant with metformin (phase B) and trifluridine/tipiracil concomitant with cimetidine (phase C). The primary endpoint was the relative difference in exposure of trifluridine assessed by the area under the curve from timepoint zero to infinity. A > 30% change in exposure was considered clinically relevant. A p-value of < 0.025 was considered significant because of a Bonferroni correction. RESULTS: Eighteen patients were included in the analysis. Metformin did not significantly alter the exposure to trifluridine (- 12.6%; 97.5% confidence interval - 25.0, 1.8; p = 0.045). Cimetidine did alter the exposure to trifluridine significantly (+ 18.0%; 97.5% confidence interval 4.5, 33.3; p = 0.004), but this increase did not meet our threshold for clinical relevance. Metformin trough concentrations were not influenced by trifluridine/tipiracil. CONCLUSIONS: Our result suggests that the OCT2/MATE1 modulators cimetidine and metformin can be co-administered with trifluridine/tipiracil without clinically relevant effects on drug exposure. CLINICAL TRIAL REGISTRATION: NL8067 (registered 04-10-2019).

Darolutamide does not interfere with OATP-mediated uptake of docetaxel.

The addition of darolutamide, an androgen receptor signalling inhibitor, to therapy with docetaxel has recently been approved as a strategy to treat metastatic prostate cancer. OATP1B3 is an SLC transporter that is highly expressed in prostate cancer and is responsible for the accumulation of substrates, including docetaxel, into tumours. Given that darolutamide inhibits OATP1B3 in vitro, we sought to characterise the impact of darolutamide on docetaxel pharmacokinetics. We investigated the influence of darolutamide on OATP1B3 transport using in vitro and in vivo models. We assessed the impact of darolutamide on the tumour accumulation of docetaxel in a patient-derived xenograft (PDX) model and on an OATP1B biomarker in patients. Darolutamide inhibited OATP1B3 in vitro at concentrations higher than the reported Cmax. Consistent with these findings, in vivo studies revealed that darolutamide does not influence the pharmacokinetics of Oatp1b substrates, including docetaxel. Docetaxel accumulation in PDX tumours was not decreased in the presence of darolutamide. Metastatic prostate cancer patients had similar levels of OATP1B biomarkers, regardless of treatment with darolutamide. Consistent with a low potential to inhibit OATP1B3-mediated transport in vitro, darolutamide does not significantly impede the transport of Oatp1b substrates in vivo or in patients. Our findings support combined treatment with docetaxel and darolutamide, as no OATP1B3 transporter based drug-drug interaction was identified.

A common germline variant in CYP11B1 is associated with adverse clinical outcome of treatment with abiraterone or enzalutamide.

Extragonadal androgens play a pivotal role in prostate cancer disease progression on androgen receptor signaling inhibitors (ARSi), including abiraterone and enzalutamide. We aimed to investigate if germline variants in genes involved in extragonadal androgen synthesis contribute to resistance to ARSi and may predict clinical outcomes on ARSi. We included ARSi naive metastatic prostate cancer patients treated with abiraterone or enzalutamide and determined 18 germline variants in six genes involved in extragonadal androgen synthesis. Variants were tested in univariate and multivariable analysis for the relation with overall survival (OS) and time to progression (TTP) by Cox regression, and PSA response by logistic regression. A total of 275 patients were included. From the investigated genes CYP17A1, HSD3B1, CYP11B1, AKR1C3, SRD5A1 and SRD5A2, only rs4736349 in CYP11B1 in homozygous form (TT), present in 54 patients (20%), was related with a significantly worse OS (HR = 1.71, 95% CI 1.09 - 2.68, p = 0.019) and TTP (HR = 1.50, 95% CI 1.08 - 2.09, p = 0.016), and was related with a significantly less frequent PSA response (OR = 0.48, 95% CI 0.24 - 0.96, p = 0.038) on abiraterone or enzalutamide in a multivariable analysis. The frequent germline variant rs4736349 in CYP11B1 is, as homozygote, an independent negative prognostic factor for treatment with abiraterone or enzalutamide in ARSi naive metastatic prostate cancer patients. Our findings warrant prospective investigation of this potentially important predictive biomarker.

Early Identification of Patients at Risk of Cabazitaxel-induced Severe Neutropenia.

BACKGROUND: Cabazitaxel frequently causes severe neutropenia. A higher cabazitaxel systemic exposure is related to a lower nadir absolute neutrophil count (ANC). OBJECTIVE: To describe the effect of cabazitaxel systemic exposure on ANC by a population pharmacokinetic/pharmacodynamic (POP-PK/PD) model, and to identify patients at risk of severe neutropenia early in their treatment course using a PK threshold. DESIGN, SETTING, AND PARTICIPANTS: Data from five clinical studies were pooled to develop a POP-PK/PD model using NONMEM, linking both patient characteristics and cabazitaxel systemic exposure directly to ANC. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: A PK threshold, predictive of severe neutropenia (grade ≥3), was determined using a receiver operating characteristic curve. RESULTS AND LIMITATIONS: Ninety-six patients were included with a total of 1726 PK samples and 1081 ANCs. The POP-PK/PD model described both cabazitaxel PK and ANC accurately. A cabazitaxel plasma concentration of >4.96 ng/ml at 6 h after the start of infusion was found to be predictive of severe neutropenia, with a sensitivity of 76% and a specificity of 65%. CONCLUSIONS: Early cabazitaxel plasma levels are predictive of severe neutropenia. Implementation of the proposed PK threshold results in early identification of almost 76% of all severe neutropenias. If prospectively validated, patients at risk could benefit from prophylactic administration of granulocyte colony stimulating factors, preventing severe neutropenia in an early phase of treatment. Implementation of this threshold permits a less restricted use of the 25 mg/m2 dose, potentially increasing the therapeutic benefit. PATIENT SUMMARY: Treatment with cabazitaxel chemotherapy often causes neutropenia, leading to susceptibility to infections, which might be life threatening. We found that a systemic cabazitaxel concentration above 4.96 ng/ml 6 h after the start of infusion is predictive of the occurrence of severe neutropenia. Measurement of systemic cabazitaxel levels provides clinicians with the opportunity to prophylactically stimulate neutrophil growth.

Darolutamide added to docetaxel augments antitumor effect in models of prostate cancer through cell cycle arrest at the G1-S transition.

Resistance to taxane chemotherapy is frequently observed in metastatic prostate cancer. The androgen receptor (AR) is a major driver of prostate cancer and a key regulator of the G1-S cell cycle checkpoint, promoting cancer cell proliferation by irreversible passage to the S-phase. We hypothesized that AR signaling inhibitor (ARSi) darolutamide in combination with docetaxel could augment antitumor effect by impeding the proliferation of taxane-resistant cancer cells. We monitored cell viability in organoids, tumor volume and PSA secretion in patient-derived xenografts (PDXs) and analyzed cell cycle and signaling pathway alterations. Combination treatment increased anti-tumor effect in androgen-sensitive, AR-positive prostate cancer organoids and PDXs. Equally beneficial effects of darolutamide added to docetaxel were observed in a castration-resistant model, progressive on docetaxel, enzalutamide and cabazitaxel. In vitro studies showed that docetaxel treatment with simultaneous darolutamide resulted in a reduction of cells entering the S-phase in contrast to only docetaxel. Molecular analysis in the prostate cancer cell line LNCaP revealed an upregulation of Cyclin Dependent Kinase inhibitor p21, supporting blockade of S-phase entry and cell proliferation. Our results provide a preclinical support for combining taxanes and darolutamide as a multimodal treatment strategy in metastatic prostate cancer patients progressive on ARSi and taxane chemotherapy.

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 ).

Validation of an LC-MS/MS method for simultaneous quantification of abiraterone, enzalutamide and darolutamide in human plasma.

Currently, several oral androgen receptor signalling inhibitors are available for the treatment of advanced prostate cancer. Quantification of plasma concentrations of these drugs is highly relevant for various purposes, such as Therapeutic Drug Monitoring (TDM) in oncology. Here, we report a liquid chromatography/tandem mass spectrometric (LC-MS/MS) method for the simultaneous quantification of abiraterone, enzalutamide, and darolutamide. The validation was performed according to the requirements of the U.S. Food and Drug Administration and European Medicine Agency. We also demonstrate the clinical applicability of the quantification of enzalutamide and darolutamide in patients with metastatic castration-resistant prostate cancer.

Randomized Phase II Cabazitaxel Dose Individualization and Neutropenia Prevention Trial in Patients with Metastatic Castration-Resistant Prostate Cancer.

PURPOSE: There is ongoing controversy about the recommended dose of cabazitaxel in patients with metastatic castration-resistant prostate cancer (mCRPC). PATIENTS AND METHODS: This multicenter phase II open-label, randomized, parallel-group study compared 3-weekly cabazitaxel at 25 mg/m2 (conventional arm A) with cabazitaxel therapeutic drug monitoring (experimental arm B) in mCRPC. The primary objective was to improve the clinical feasibility rate (CFR), defined as the absence of grade 4 neutropenia or thrombocytopenia, any thrombocytopenia with bleeding, febrile neutropenia, severe nonhematologic toxicity, withdrawal for cabazitaxel-related toxicity, or death. A total of 60 patients had to be randomized to detect a difference in CFR of 35% (power 80%, two-sided alpha 10%). RESULTS: A total of 40 patients were randomized to arm A and 33 patients to arm B. CFR was 69.4% in arm A and 64.3% in arm B (P = 0.79). Week-12 PSA response was 38.5% in both arms. A radiological response by RECIST v.1.1 was seen in 3 (9.7%) patients in arm A versus 6 (23.1%) patients in arm B (P = 0.28), disease progression was higher in arm A compared with arm B (61.3% vs. 30.8%, P = 0.05). Median progression-free survival was longer in arm B compared with arm A (9.5 vs. 4.4 months; HR = 0.46; P = 0.005). Median overall survival was higher in arm B compared with arm A (16.2 vs. 7.3 months; HR = 0.33; P < 0.0001). CONCLUSIONS: Pharmacokinetic-guided dosing of cabazitaxel in patients with mCRPC is feasible and improves clinical outcome due to individual dose escalations in 55% of patients.

Cis-mediated interactions of the SARS-CoV-2 frameshift RNA alter its conformations and affect function.

The RNA genome of SARS-CoV-2 contains a frameshift stimulatory element (FSE) that allows access to an alternative reading frame through -1 programmed ribosomal frameshifting (PRF). -1PRF in the 1a/1b gene is essential for efficient viral replication and transcription of the viral genome. -1PRF efficiency relies on the presence of conserved RNA elements within the FSE. One of these elements is a three-stemmed pseudoknot, although alternative folds of the frameshift site might have functional roles as well. Here, by complementing ensemble and single-molecule structural analysis of SARS-CoV-2 frameshift RNA variants with functional data, we reveal a conformational interplay of the 5' and 3' immediate regions with the FSE and show that the extended FSE exists in multiple conformations. Furthermore, limiting the base pairing of the FSE with neighboring nucleotides can favor or impair the formation of the alternative folds, including the pseudoknot. Our results demonstrate that co-existing RNA structures can function together to fine-tune SARS-CoV-2 gene expression, which will aid efforts to design specific inhibitors of viral frameshifting.

CABA-V7: a prospective biomarker selected trial of cabazitaxel treatment in AR-V7 positive prostate cancer patients.

BACKGROUND: Metastatic castration-resistant prostate cancer (mCRPC) patients with positive AR-V7 expression in their circulating tumour cells (CTCs) rarely derive benefit from abiraterone and enzalutamide. DESIGN: We performed a prospective, multicenter, single arm phase II clinical trial (CABA-V7) in mCRPC patients previously treated with docetaxel and androgen deprivation therapy. OBJECTIVE: In this trial, we investigated whether cabazitaxel treatment resulted in clinically meaningful PSA response rates in patients with positive CTC-based AR-V7 expression and collected liquid biopsies for genomic profiling. RESULTS: Cabazitaxel was found to be modestly effective, with only 12% of these patients obtaining a PSA response. Genomic profiling revealed that CTC-based AR-V7 expression was not associated with other known mCRPC-associated alterations. CTC-based AR-V7 status and dichotomised CTC counts were observed as independent prognostic markers at baseline. CONCLUSIONS: AR-V7 positivity predicted poor overall survival (OS). However, cabazitaxel-treated AR-V7 positive patients and those lacking AR-V7 positivity, who received cabazitaxel as standard of care, appeared to have similar OS. Therefore, despite the low response rate, cabazitaxel may still be an effective treatment in this poor prognosis, AR-V7 positive patient population.

A Metabolomics Approach for Predicting OATP1B-Type Transporter-Mediated Drug-Drug Interaction Liabilities.

In recent years, various endogenous compounds have been proposed as putative biomarkers for the hepatic uptake transporters OATP1B1 and OATP1B3 that have the potential to predict transporter-mediated drug-drug interactions (DDIs). However, these compounds have often been identified from top-down strategies and have not been fully utilized as a substitute for traditional DDI studies. In an attempt to eliminate observer bias in biomarker selection, we applied a bottom-up, untargeted metabolomics screening approach in mice and found that plasma levels of the conjugated bile acid chenodeoxycholate-24-glucuronide (CDCA-24G) are particularly sensitive to deletion of the orthologous murine transporter Oatp1b2 (31-fold increase vs. wild type) or the entire Oatp1a/1b(-/-)cluster (83-fold increased), whereas the humanized transgenic overexpression of hepatic OATP1B1 or OATP1B3 resulted in the partial restoration of transport function. Validation studies with the OATP1B1/OATP1B3 inhibitors rifampin and paclitaxel in vitro as well as in mice and human subjects confirmed that CDCA-24G is a sensitive and rapid response biomarker to dose-dependent transporter inhibition. Collectively, our study confirmed the ability of CDCA-24G to serve as a sensitive and selective endogenous biomarker of OATP1B-type transport function and suggests a template for the future development of biomarkers for other clinically important xenobiotic transporters.

POTATO: Automated pipeline for batch analysis of optical tweezers data.

Optical tweezers are a single-molecule technique that allows probing of intra- and intermolecular interactions that govern complex biological processes involving molecular motors, protein-nucleic acid interactions, and protein/RNA folding. Recent developments in instrumentation eased and accelerated optical tweezers data acquisition, but analysis of the data remains challenging. Here, to enable high-throughput data analysis, we developed an automated python-based analysis pipeline called POTATO (practical optical tweezers analysis tool). POTATO automatically processes the high-frequency raw data generated by force-ramp experiments and identifies (un)folding events using predefined parameters. After segmentation of the force-distance trajectories at the identified (un)folding events, sections of the curve can be fitted independently to a worm-like chain and freely jointed chain models, and the work applied on the molecule can be calculated by numerical integration. Furthermore, the tool allows plotting of constant force data and fitting of the Gaussian distance distribution over time. All these features are wrapped in a user-friendly graphical interface, which allows researchers without programming knowledge to perform sophisticated data analysis.

A Pharmacokinetics-Time to Alleviation of Symptoms Model to Support Extrapolation of Baloxavir Marboxil Clinical Efficacy in Different Ethnic Groups with Influenza A or B.

Baloxavir marboxil, the prodrug of baloxavir acid, is an anti-influenza antiviral. Here, a pharmacokinetics-time to alleviation of symptoms (PK-TTAS) model was developed and used to (I) characterize the PK-TTAS relationship, (II) quantify the impact of covariates, and (III) predict TTAS in different ethnic groups. Data from 1781 otherwise-healthy (OwH) or high-risk (HR) patients included in phase II (JapicCTI-153090) and III studies (NCT02954354 and NCT02949011) were used; patients received either placebo or oral baloxavir marboxil. The natural distribution of TTAS in placebo-treated patients was modeled, then TTAS data from the baloxavir marboxil arms were added to model the impact of baloxavir acid concentration on TTAS. PK parameters estimated by a population PK model and informed by phase I data (NCT03959332 and KCT0003535) were included to simulate TTAS in Chinese and South Korean patients. Composite symptom score at baseline (TSS0), ethnicity, sex, and patient type (OwH or HR) significantly impacted the natural TTAS distribution. TTAS reduced with increasing baloxavir acid concentrations. Compared with placebo, high and low baloxavir acid exposures (AUC0-inf 5.13-16.65 and 0.72-5.13 µg.hr/mL, respectively) significantly reduced TTAS; no covariates affected the drug effect on TTAS. Simulated TTAS was similar between OwH or HR Chinese, South Korean, and other Asian patients, with median reductions from placebo between 18.3-18.8 hours and 21.2-22.0 hours in OwH and HR patients, respectively, assuming TSS0 > 10. Ethnicity (Asian vs. non-Asian) did not significantly impact the drug effect on TTAS; predicted TTAS was similar across different Asian populations. This suggests Chinese and South Korean patients may benefit from similar efficacy as other Asian patients.

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 (AUC0-24h) 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.

Optical Tweezers to Study RNA-Protein Interactions in Translation Regulation.

RNA adopts diverse structural folds, which are essential for its functions and thereby can impact diverse processes in the cell. In addition, the structure and function of an RNA can be modulated by various trans-acting factors, such as proteins, metabolites or other RNAs. Frameshifting RNA molecules, for instance, are regulatory RNAs located in coding regions, which direct translating ribosomes into an alternative open reading frame, and thereby act as gene switches. They may also adopt different folds after binding to proteins or other trans-factors. To dissect the role of RNA-binding proteins in translation and how they modulate RNA structure and stability, it is crucial to study the interplay and mechanical features of these RNA-protein complexes simultaneously. This work illustrates how to employ single-molecule-fluorescence-coupled optical tweezers to explore the conformational and thermodynamic landscape of RNA-protein complexes at a high resolution. As an example, the interaction of the SARS-CoV-2 programmed ribosomal frameshifting element with the trans-acting factor short isoform of zinc-finger antiviral protein is elaborated. In addition, fluorescence-labeled ribosomes were monitored using the confocal unit, which would ultimately enable the study of translation elongation. The fluorescence coupled OT assay can be widely applied to explore diverse RNA-protein complexes or trans-acting factors regulating translation and could facilitate studies of RNA-based gene regulation.

Pharmacokinetics, safety, and simulated efficacy of an influenza treatment, baloxavir marboxil, in Chinese individuals.

Baloxavir marboxil is an endonuclease inhibitor indicated for the treatment of influenza in patients ≥12 years. No data exist for Chinese patients in global studies. This randomized, open-label, phase I study evaluated the pharmacokinetics (PK) and safety of baloxavir marboxil in healthy Chinese volunteers and was used to anticipate efficacy in Chinese patients. Patients received a single oral dose of baloxavir marboxil (40 or 80 mg [1:1]). Serial blood samples were collected predose and at various timepoints up to 14 days postdose. Baloxavir marboxil and acid plasma concentrations were determined by liquid chromatography tandem mass spectrometry. PK parameters of baloxavir acid were estimated by noncompartmental analysis. Adverse events (AEs) were recorded. Time to alleviation of symptoms (TTAS) was simulated for otherwise healthy (OwH) and high-risk (HR) Chinese and Asian patients. Thirty-two male patients received baloxavir marboxil. Baloxavir acid plasma concentration peaked 4 h postdose. Mean maximum concentration (Cmax ) was 107.6 and 206.9 ng/ml, and mean area under the plasma concentration-time curve from zero to infinity (AUC0-inf ) was 6955 and 9643 ng·h/ml in the 40 and 80 mg cohorts, respectively. AEs were mild and transient; no new safety signals were identified. Simulated median TTAS for OwH and HR Chinese patients agreed with simulated values in Asian patients. PK parameters were similar to Asian populations in other studies. The globally adopted baloxavir marboxil dosing strategy was consistent with the established safety profile of baloxavir marboxil in this population. Simulated efficacy indicated Chinese patients could benefit from similar efficacy to Asian patients.

The short isoform of the host antiviral protein ZAP acts as an inhibitor of SARS-CoV-2 programmed ribosomal frameshifting.

Programmed ribosomal frameshifting (PRF) is a fundamental gene expression event in many viruses, including SARS-CoV-2. It allows production of essential viral, structural and replicative enzymes that are encoded in an alternative reading frame. Despite the importance of PRF for the viral life cycle, it is still largely unknown how and to what extent cellular factors alter mechanical properties of frameshift elements and thereby impact virulence. This prompted us to comprehensively dissect the interplay between the SARS-CoV-2 frameshift element and the host proteome. We reveal that the short isoform of the zinc-finger antiviral protein (ZAP-S) is a direct regulator of PRF in SARS-CoV-2 infected cells. ZAP-S overexpression strongly impairs frameshifting and inhibits viral replication. Using in vitro ensemble and single-molecule techniques, we further demonstrate that ZAP-S directly interacts with the SARS-CoV-2 RNA and interferes with the folding of the frameshift RNA element. Together, these data identify ZAP-S as a host-encoded inhibitor of SARS-CoV-2 frameshifting and expand our understanding of RNA-based gene regulation.

Continued androgen signalling inhibition improves cabazitaxel efficacy in prostate cancer.

BACKGROUND: The androgen receptor (AR) pathway is a key driver of neoplastic behaviour in the different stages of metastatic prostate cancer (mPCa). Targeting the AR therefore remains the cornerstone for mPCa treatment. We have previously reported that activation of AR signalling affects taxane chemo-sensitivity in preclinical models of castration resistant PCa (CRPC). Here, we explored the anti-tumour efficacy of the AR targeted inhibitor enzalutamide combined with cabazitaxel. METHODS: We used the AR positive CRPC model PC346C-DCC-K to assess the in vitro and in vivo activity of combining enzalutamide with cabazitaxel. Subsequent validation studies were performed using an enzalutamide resistant VCaP model. To investigate the impact of AR signalling on cabazitaxel activity we used quantitative live-cell imaging of tubulin stabilization and apoptosis related nuclear fragmentation. FINDINGS: Enzalutamide strongly amplified cabazitaxel anti-tumour activity in the patient-derived xenograft models PC346C-DCC-K (median time to humane endpoint 77 versus 48 days, P<0.0001) and VCaP-Enza-B (median time to humane endpoint 80 versus 53 days, P<0.001). Although enzalutamide treatment by itself was ineffective in reducing tumour growth, it significantly suppressed AR signalling in PC346C-DCC-K tumours as shown by AR target gene expression. The addition of enzalutamide enhanced cabazitaxel induced apoptosis as shown by live-cell imaging (P<0.001). INTERPRETATION: Our study demonstrates that cabazitaxel efficacy can be improved by simultaneous blocking of AR signalling by enzalutamide, even if AR targeted treatment no longer affects tumour growth. These findings support clinical studies that combine AR targeted inhibitors with cabazitaxel in CRPC.