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Targeting the Androgen Receptor Confers In Vivo Cross-resistance Between Enzalutamide and Docetaxel, But Not Cabazitaxel, in Castration-resistant Prostate Cancer
European Urology [Epub ahead of print]
Treatment options for metastatic castration-resistant prostate cancer (CRPC) have evolved with the established benefit of the novel androgen receptor (AR)-targeted agents abiraterone and enzalutamide in the prechemotherapy setting. However, concerns regarding cross-resistance between the taxanes docetaxel and cabazitaxel and these AR-targeted agents have arisen, and the optimal drug treatment sequence is unknown. We investigated the in vivo efficacy of docetaxel and cabazitaxel in enzalutamide-resistant CRPC, and mechanisms of cross-resistance between these agents. Castrated mice harboring enzalutamide-resistant tumors and enzalutamide-naïve tumors were treated with docetaxel and cabazitaxel. Tumor growth kinetics, AR nuclear localization, AR-regulated gene expression, Ki67 expression, and serum levels of prostate-specific antigen, docetaxel, and cabazitaxel were analyzed. Docetaxel inhibited tumor growth, AR nuclear localization, and AR-regulated gene expression in enzalutamide-naïve tumors, but did not in enzalutamide-resistant tumors, demonstrating in vivo cross-resistance. By contrast, cabazitaxel remained highly effective in enzalutamide-resistant tumors and demonstrated superior antitumor activity compared to docetaxel, independent of the AR pathway. These findings demonstrate that the AR pathway is able to confer in vivo cross-resistance between enzalutamide and docetaxel, but not cabazitaxel, in CRPC.
We found reduced efficacy of docetaxel, but not cabazitaxel, in enzalutamide-resistant prostate cancer.
Keywords: Androgen receptor, Cabazitaxel, Castration-resistant prostate cancer, Cross-resistance, Docetaxel, Enzalutamide, Taxanes.
For almost a decade, docetaxel has been the standard first-line chemotherapy for metastatic castration-resistant prostate cancer (mCRPC). In recent years, treatment options for mCRPC have evolved with the introduction of cabazitaxel, abiraterone, and enzalutamide, all of which prolong survival in the post-docetaxel setting . Recently, the treatment paradigm has changed with evidence that novel androgen receptor (AR)-targeted therapies abiraterone and enzalutamide are effective when administered to men with mCRPC also before chemotherapy  and . With these novel AR-targeting therapies also available in the prechemotherapy setting, treatment sequencing has become increasingly challenging, especially since concerns have been raised regarding the efficacy of docetaxel when used after abiraterone  and . Clinical cross-resistance has been suggested in retrospective studies that demonstrated reduced efficacy of docetaxel in men with mCRPC who had previously been treated with abiraterone  and . Moreover, a preclinical study by our group identified inhibition of AR nuclear translocation as an overlapping working mechanism that potentially confers cross-resistance between taxanes and the AR-targeted agents abiraterone and enzalutamide . Interestingly, retrospective clinical data suggested that cabazitaxel, in contrast to docetaxel, remains effective in men with mCRPC after prior abiraterone  and . The efficacy of docetaxel and cabazitaxel after first-line enzalutamide is unknown.
With the availability of novel hormonal agents before chemotherapy, there is an urgent need to investigate the optimal treatment sequence and potential mechanisms of cross-resistance among the current treatment options. Here we investigated the in vivo efficacy of docetaxel and cabazitaxel in CRPC with acquired resistance to enzalutamide, as well as mechanisms of cross-resistance between these agents.
We performed in vivo studies using patient-derived enzalutamide-naïve PC346C  and enzalutamide-resistant PC346Enza tumors as described in the Supplementary Methods. Tumors were analyzed for AR nuclear localization, Ki67 expression, and AR-regulated gene expression. Serum levels of prostate-specific antigen (PSA), docetaxel, and cabazitaxel were measured.
We first confirmed that the PC346Enza xenograft was resistant to enzalutamide in vivo (Fig. 1A,B). Docetaxel showed good tumor responses compared with placebo in castrate male mice bearing enzalutamide-naïve PC346C tumors (mean tumor volume change from baseline [TVC] –78%, standard error of the mean [SEM] ±7%), whereas its efficacy was impaired in mice bearing enzalutamide-resistant PC346Enza tumors (TVC +364%, SEM ±69%) demonstrating cross-resistance between docetaxel and enzalutamide in vivo (p < 0.01; Fig. 1C,D). Progression-free survival and tumor growth curves over time are shown in Figure 1E,F, and Supplementary Figure 1. Concordant with the tumor responses observed, docetaxel reduced serum PSA levels compared to placebo in castrate mice bearing PC346C tumors, but not in mice bearing PC346Enza tumors (Supplementary Fig. 2A,B). Thus, cross-resistance between docetaxel and enzalutamide was observed not only at the level of tumor growth, but also in terms of clinically relevant serum PSA response, which is directly related to tumor volume.
Tumor responses for cabazitaxel were similar for PC346Enza and PC346C tumors, demonstrating that there was no cross-resistance between enzalutamide and cabazitaxel (Fig. 1C,D). Whereas docetaxel efficacy was impaired in mice bearing PC346Enza tumors (TVC +364%, SEM ±69%), cabazitaxel remained very effective (TVC –70%, SEM ±10%) and led to greater antitumor activity (p < 0.01) and serum PSA declines (Supplementary Fig. 2A,B) compared to docetaxel.
Plasma concentrations of docetaxel and cabazitaxel were similar in mice with and without enzalutamide pretreatment, indicating that enzalutamide had no effect on the pharmacokinetics of both taxanes (Supplementary Fig. 3, Supplementary Table 1). Furthermore, plasma concentrations of docetaxel and cabazitaxel in mice were similar to those reported in patients (Supplementary Table 1), indicating that our observed cross-resistance occurs at clinically relevant concentrations.
Whereas AR expression was similar among treatment groups (Supplementary Fig. 4A,B), docetaxel was able to affect the downstream AR pathway by inhibiting intratumoral AR nuclear localization (Fig. 2A,C) and the AR target gene PSA (Supplementary Fig. 4C) in PC346C tumors. By contrast, while expressing lower baseline levels, docetaxel did not inhibit AR nuclear localization and PSA expression compared to placebo in PC346Enza tumors (Fig. 2A,C and Supplementary Fig. 4D), indicating a reduced antitumor activity via the AR pathway in these tumors. This impaired anti-AR effect in PC346Enza tumors was also observed for cabazitaxel (Fig. 2A and Supplementary Fig. 4C,D) and enzalutamide (Supplementary Fig. 4G,H). Although the effects of cabazitaxel via the AR were impaired in enzalutamide-resistant tumors, it exhibited stronger antiproliferative properties compared to docetaxel, as observed from Ki67 staining (Fig. 2B,C), independent of the AR pathway.
In this study, we present the first evidence of in vivo cross-resistance between docetaxel and enzalutamide in CRPC. We showed that docetaxel efficiently impairs AR nuclear localization and consequently AR signaling in enzalutamide-naïve tumors, but not in enzalutamide-resistant tumors. These results indicate that the inhibitory effects of docetaxel on the AR represent part of its antitumor activity, which is impaired by previous AR-targeted therapy such as enzalutamide. These results could also explain the reduced efficacy of docetaxel when used after abiraterone that was observed in retrospective clinical studies  and . Our findings are especially of interest concerning the increasing use of enzalutamide and abiraterone prechemotherapy.
In contrast to docetaxel, cabazitaxel demonstrated robust tumor and PSA responses in enzalutamide-resistant tumors, while the effects on AR signaling were reduced compared to those in enzalutamide-naïve tumors. These observations indicate that cabazitaxel is less dependent on its inhibitory effects on the AR pathway, and exerts greater antitumor activity via AR-independent mechanisms compared to docetaxel. This is concordant with clinical observations  and , and is probably caused by a higher potency of cabazitaxel in suppressing microtubule dynamics compared to docetaxel, with faster drug uptake and better intracellular retention . This is further supported by the lower Ki67 expression observed in our enzalutamide-resistant tumors treated with cabazitaxel compared to docetaxel, indicating stronger antiproliferative properties. The greater potency of cabazitaxel after AR-targeted treatment might have clinical implications, as docetaxel is currently the standard first-line chemotherapy for men with mCRPC. Considering the superior efficacy of cabazitaxel over docetaxel in enzalutamide-resistant tumors, our results provide a rationale for clinical studies comparing cabazitaxel with docetaxel in men with mCRPC with disease progression on first-line enzalutamide or abiraterone.
In summary, we demonstrated that a reduced inhibition of the AR pathway by docetaxel in enzalutamide-resistant CRPC confers cross-resistance between these drugs in vivo. Cabazitaxel remained highly effective in enzalutamide-resistant tumors, demonstrating greater antiproliferative properties independent of the AR pathway. This merits further clinical evaluation of cross-resistance and the optimal treatment sequence for patients with mCRPC.
Author contributions: Robert J. van Soest had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: van Soest, de Morrée, van Weerden, de Wit.
Acquisition of data: van Soest, de Ridder, Kweldam.
Analysis and interpretation of data: van Soest, Kweldam.
Drafting of the manuscript: van Soest.
Critical revision of the manuscript for important intellectual content: van Soest, de Morrée, Kweldam, Wiemer, Mathijssen, de Wit, van Weerden.
Statistical analysis: van Soest.
Obtaining funding: de Wit, van Weerden.
Administrative, technical, or material support: de Ridder.
Supervision: van Weerden, de Wit.
Other (specify): None.
Financial disclosures: Robert J. van Soest certifies that all conflicts of interest, including specific financial interests and relationships and affiliations relevant to the subject matter or materials discussed in the manuscript (eg, employment/affiliation, grants or funding, consultancies, honoraria, stock ownership or options, expert testimony, royalties, or patents filed, received, or pending), are the following: Ronald de Wit has received consultancy and speaker honoraria from Sanofi, Janssen, and Millennium, and research funding from Sanofi. Wytske M. van Weerden has received research funding from Sanofi, Janssen, and Millennium. Robert J. van Soest has received research funding and speakers honoraria from Sanofi.
Funding/Support and role of the sponsor: This work was funded by a research grant from Sanofi. The sponsor approved the manuscript.
Acknowledgments: We thank Agnes Boer, Sander Hoeben, and Debra Stuurman for their work and contributions to the in vivo experiments, Peter de Bruijn for the pharmacokinetic measurements, and Hans Stoop for optimizing the immunohistochemistry.
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a Department of Urology, Erasmus University Medical Center, Erasmus MC Cancer Institute, Rotterdam, The Netherlands
b Department of Pathology, Erasmus University Medical Center, Erasmus MC Cancer Institute, Rotterdam, The Netherlands
c Department of Medical Oncology, Erasmus University Medical Center, Erasmus MC Cancer Institute, Rotterdam, The Netherlands
⁎ Corresponding author. Department of Urology, Erasmus University Medical Center, Dr. Molewaterplein 50, Be-331, 3015 GE Rotterdam, The Netherlands. Tel. +31-107043381; Fax: +31-107044661.
© 2014 European Association of Urology, Published by Elsevier B.V.