Phase 1b Study of Abiraterone Acetate Plus Prednisone and Docetaxel in Patients with Metastatic Castration-resistant Prostate Cancer

Neoadjuvant Cabazitaxel plus Abiraterone/Leuprolide Acetate in Patients with High-Risk Prostate Cancer: ACDC-RP Phase II Trial

PURPOSE

Early treatment intensification with neoadjuvant therapy may improve outcomes in patients with high-risk, localized prostate cancer treated with radical prostatectomy. Our objective was to compare pathologic, oncologic, and safety outcomes of neoadjuvant abiraterone acetate plus leuprolide acetate with or without cabazitaxel prior to radical prostatectomy in patients with localized, high-risk prostate cancer.

PATIENTS AND METHODS

This open-label, multicenter, phase II trial randomized men with clinically localized, D'Amico high-risk prostate cancer to neoadjuvant abiraterone acetate (1,000 mg/day) and leuprolide acetate (22.5 mg every 3 months) with or without cabazitaxel (25 mg/m2) prior to radical prostatectomy. The primary outcome was pathologic complete response (pCR) or minimal residual disease (MRD). Secondary outcomes included surgical margins, lymph node involvement, pathologic stage, 12-month biochemical relapse-free survival (BRFS) rates, and safety profile.

RESULTS

The per-protocol population consisted of 70 patients [cabazitaxel arm (Arm A): 37, no cabazitaxel arm (Arm B): 33]. Median patient age and prostate-specific antigen levels were 63.5 years [interquartile range (IQR), 58.0-68.0] and 21.9 ng/mL (IQR, 14.6-42.8), respectively. pCR/MRD occurred in 16 (43.2%) versus 15 patients (45.5%) in arms A and B, respectively (P = 0.85). pCR occurred in two (5.4%) versus three patients (9.1%) in arms A and B, respectively (P = 0.66). Patients with ≤ 25% total biopsy cores positive had increased odds of pCR/MRD (P = 0.04). Patients with pCR/MRD had superior 12-month BRFS rates (96.0% vs. 62.0%, P = 0.03). Grade 3+ adverse events occurred in 42.5% and 23.7% of patients in arms A and B, respectively (P = 0.078).

CONCLUSIONS

Neoadjuvant cabazitaxel addition to abiraterone acetate/leuprolide acetate prior to radical prostatectomy did not improve pCR/MRD in clinically localized, high-risk prostate cancer.

A phase II randomised trial of abiraterone acetate plus prednisone in combination with docetaxel or docetaxel plus prednisone after disease progression to abiraterone acetate plus prednisone in patients with metastatic castration-resistant prostate cancer: The ABIDO-SOGUG trial

BACKGROUND

We aimed to compare the efficacy and safety of maintaining or withdrawing abiraterone acetate plus prednisone (AAP) in patients with metastatic castration-resistant prostate cancer who had experienced cancer progression to this treatment and were beginning a docetaxel-based therapy.

PATIENTS AND METHODS

Phase II, randomised, open-label study conducted in patients with metastatic castration-resistant prostate cancer who were asymptomatic or mildly symptomatic. After open-label treatment with AAP, patients who had experienced cancer progression to AAP were randomised to 75 mg/m² of docetaxel plus AAP or to receive 75 mg/m² of docetaxel plus 10 mg of prednisone orally daily. The primary outcome was the radiographic progression-free survival rate at 12 months as evaluated by the investigators in all randomised patients.

RESULTS

A total of 148 patients were included in open-label treatment with AAP, and of them, 94 patients were randomised to receive either docetaxel plus AAP (intervention group; n = 47) or docetaxel plus prednisone (control group; n = 47). The 12-month radiographic progression-free survival rates did not differ between the intervention group (34.9%; 95% CI 20.7-49.2) and the control group (33.9%; 95% CI 19.5-48.3). There were no significant differences in the time to radiographic progression and the overall survival between the intervention and control groups. Grade 3-5 neutropenia with the combination of docetaxel plus prednisone and AA was more frequent than with docetaxel plus prednisone (59.6% versus 27.7%).

CONCLUSION

Our results indicate that the therapeutic strategy of maintaining AAP added to docetaxel in chemotherapy-naïve patients who have experienced cancer progression to AAP treatment should not be further evaluated and should be avoided in clinical practice.

CLINICAL TRIALS

NCT02036060 https://clinicaltrials.gov/ct2/show/NCT02036060.

Abiraterone plus prednisone added to androgen deprivation therapy and docetaxel in de novo metastatic castration-sensitive prostate cancer (PEACE-1): a multicentre, open-label, randomised, phase 3 study with a 2 × 2 factorial design

BACKGROUND

Current standard of care for metastatic castration-sensitive prostate cancer supplements androgen deprivation therapy with either docetaxel, second-generation hormonal therapy, or radiotherapy. We aimed to evaluate the efficacy and safety of abiraterone plus prednisone, with or without radiotherapy, in addition to standard of care.

METHODS

We conducted an open-label, randomised, phase 3 study with a 2 × 2 factorial design (PEACE-1) at 77 hospitals across Belgium, France, Ireland, Italy, Romania, Spain, and Switzerland. Eligible patients were male, aged 18 years or older, with histologically confirmed or cytologically confirmed de novo metastatic prostate adenocarcinoma, and an Eastern Cooperative Oncology Group performance status of 0-1 (or 2 due to bone pain). Participants were randomly assigned (1:1:1:1) to standard of care (androgen deprivation therapy alone or with intravenous docetaxel 75 mg/m² once every 3 weeks), standard of care plus radiotherapy, standard of care plus abiraterone (oral 1000 mg abiraterone once daily plus oral 5 mg prednisone twice daily), or standard of care plus radiotherapy plus abiraterone. Neither the investigators nor the patients were masked to treatment allocation. The coprimary endpoints were radiographic progression-free survival and overall survival. Abiraterone efficacy was first assessed in the overall population and then in the population who received androgen deprivation therapy with docetaxel as standard of care (population of interest). This study is ongoing and is registered with ClinicalTrials.gov, NCT01957436.

FINDINGS

Between Nov 27, 2013, and Dec 20, 2018, 1173 patients were enrolled (one patient subsequently withdrew consent for analysis of his data) and assigned to receive standard of care (n=296), standard of care plus radiotherapy (n=293), standard of care plus abiraterone (n=292), or standard of care plus radiotherapy plus abiraterone (n=291). Median follow-up was 3·5 years (IQR 2·8-4·6) for radiographic progression-free survival and 4·4 years (3·5-5·4) for overall survival. Adjusted Cox regression modelling revealed no interaction between abiraterone and radiotherapy, enabling the pooled analysis of abiraterone efficacy. In the overall population, patients assigned to receive abiraterone (n=583) had longer radiographic progression-free survival (hazard ratio [HR] 0·54, 99·9% CI 0·41-0·71; p<0·0001) and overall survival (0·82, 95·1% CI 0·69-0·98; p=0·030) than patients who did not receive abiraterone (n=589). In the androgen deprivation therapy with docetaxel population (n=355 in both with abiraterone and without abiraterone groups), the HRs were consistent (radiographic progression-free survival 0·50, 99·9% CI 0·34-0·71; p<0·0001; overall survival 0·75, 95·1% CI 0·59-0·95; p=0·017). In the androgen deprivation therapy with docetaxel population, grade 3 or worse adverse events occurred in 217 (63%) of 347 patients who received abiraterone and 181 (52%) of 350 who did not; hypertension had the largest difference in occurrence (76 [22%] patients and 45 [13%], respectively). Addition of abiraterone to androgen deprivation therapy plus docetaxel did not increase the rates of neutropenia, febrile neutropenia, fatigue, or neuropathy compared with androgen deprivation therapy plus docetaxel alone.

INTERPRETATION

Combining androgen deprivation therapy, docetaxel, and abiraterone in de novo metastatic castration-sensitive prostate cancer improved overall survival and radiographic progression-free survival with a modest increase in toxicity, mostly hypertension. This triplet therapy could become a standard of care for these patients.

FUNDING

Janssen-Cilag, Ipsen, Sanofi, and the French Government.

Docetaxel and prednisone with or without enzalutamide as first-line treatment in patients with metastatic castration-resistant prostate cancer: CHEIRON, a randomised phase II trial

BACKGROUND

Pre-clinical data suggest that docetaxel and enzalutamide interfere with androgen receptor translocation and signalling. The aim of this study is to assess the efficacy of their concurrent administration in the first-line treatment for metastatic castration-resistant prostate cancer (mCRPC).

METHODS

In this open-label, randomised, phase II trial, previously untreated mCRPC patients were randomised 1:1 to receive eight 21-d courses of docetaxel 75 mg/m², oral prednisone 5 mg twice daily and oral enzalutamide 160 mg/d (arm DE), or the same treatment without enzalutamide (arm D). The primary end-point was the percentage of patients without investigator-assessed disease progression 6 months after the first docetaxel administration.

RESULTS

The 246 eligible patients were randomly assigned to receive docetaxel, prednisone and enzalutamide (n = 120) or docetaxel and prednisone (n = 126). The 6-month progression rate was 12.5% (95% confidence interval [CI] 8.1-20.6) in arm DE and 27.8% (95% CI 22.8-39.4) in arm D (chi-squared test 10.01; P = 0.002). The most frequent grade III-IV adverse events were fatigue (12.5% in arm DE versus 5.6% in arm D), febrile neutropenia (9.3% versus 4.0%) and neutropenia (7.6% versus 5.6%).

CONCLUSIONS

The combination of enzalutamide and docetaxel appears to be more clinically beneficial than docetaxel alone in previously untreated mCRPC patients, although serious adverse events were more frequent. Our findings suggest that first-line treatment with this combination could lead to an additional clinical benefit when prompt and prolonged disease control is simultaneously required. Clearly, these results should be considered cautiously because of the study's phase II design and the absence of an overall survival benefit.

TRIAL REGISTRATION NUMBERS

EudraCT 2014-000175-43 - NCT02453009.

Recent Development and Future Prospects of Molecular Targeted Therapy in Prostate Cancer

Prostate cancer (PC) is a rapidly increasing ailment worldwide. The previous decade has observed a rapid advancement in PC therapies that was evident from the number of FDA approvals during this phase. Androgen deprivation therapies (ADT) have traditionally remained a mainstay for the management of PCs, but the past decade has experienced the emergence of newer classes of drugs that can be used with or without the administration of ADT. FDA approved poly (ADP-ribose) polymerase inhibitors (PARPi), such as olaparib and rucaparib, after successful clinical trials against gene-mutated metastatic castration-resistant prostate cancer. Furthermore, drugs like apalutamide, darolutamide, and enzalutamide with an androgen-targeted mechanism of action have manifested superior results in non-metastatic castration-resistant prostate cancer (nmCRPC), metastatic castration-sensitive prostate cancer (mCSPC), and metastatic castration-resistant prostate cancer (mCRPC), respectively, with or without previously administered docetaxel. Relugolix, an oral gonadotropin-releasing hormone antagonist, and a combination of abiraterone acetate plus prednisone were also approved by FDA after a successful trial in advanced PC and mCRPC, respectively. This review aims to analyze the FDA-approved agents in PC during the last decade and provide a summary of their clinical trials. It also presents an overview of the ongoing progress of prospective molecules still under trial.

Prognostic Value of Androgen Receptor Splice Variant 7 in the Treatment of Metastatic Castration-Resistant Prostate Cancer: A Systematic Review and Meta-Analysis

Background

The prognostic value of androgen receptor splice variant 7 (AR-V7) for the treatment response of metastatic castration-resistant prostate cancer (mCRPC) remains unclear. In this study, we aimed to synthesize relevant studies that assessed the prognostic value of AR-V7 status for the treatment response of mCRPC patients treated with androgen receptor signalling inhibitors (ARSis) and chemotherapy.

Methods

We searched the PubMed, Embase, and MEDLINE databases by using the keywords AR-V7 and prostate cancer to identify relevant studies published before 25 September 2019. The main outcomes were prostate-specific antigen (PSA) response, progression-free survival (PFS), and overall survival (OS). Pooled odds ratios (ORs) and hazard ratios (HRs) with 95% confidence intervals (CIs) were calculated using a random effects model. The quality of the included studies was assessed using the Newcastle-Ottawa Quality Assessment Scale.

Results

A total of 1,545 patients from 21 studies were included. For the mCRPC patients treated with ARSis, AR-V7-positive patients had a lower PSA response rate (OR 6.01, 95% CI 2.88–12.51; P < 0.001), shorter PFS (HR 2.56, 95% CI 1.80–3.64; P < 0.001) and shorter OS (HR 4.28, 95% CI 2.92–6.27; P < 0.001) than AR-V7-negative patients. Although AR-V7-positive patients treated with chemotherapy also had a lower PSA response rate (OR 2.23, 95% CI 1.38–3.62; P = 0.001) and shorter OS than AR-V7-negative patients (HR 1.60, 95% CI 1.02–2.53; P = 0.043), there was no significant difference in PFS (HR 1.05, 95% CI 0.74–1.49; P = 0.796) between these groups. Furthermore, AR-V7-positive patients receiving ARSis had a shorter median OS than those receiving chemotherapy (HR 3.50, 95% CI 1.98–6.20; P < 0.001); There was no significant difference among AR-V7-negative patients (HR 1.30, 95% CI 0.64–2.62; P = 0.47).

Conclusions

AR-V7 is a potential biomarker of treatment resistance in mCRPC patients. AR-V7-positive mCRPC patients had poorer treatment outcomes than AR-V7-nagetive patients when treated with ARSis. AR-V7-positive patients have better outcomes when treated with taxane than ARSis. Furthermore, the ability of AR-V7 status to predict treatment outcomes varies from different detection methods. The detection of AR-V7 before treatment is important for the selection of treatment modalities for mCRPC patients.

Neutropenia and docetaxel exposure in metastatic castration-resistant prostate cancer patients: A meta-analysis and evaluation of a clinical cohort

The incidence of neutropenia in metastatic castration‐resistant prostate cancer (mCRPC) patients treated with docetaxel has been reported to be lower compared to patients with other solid tumors treated with a similar dose. It is suggested that this is due to increased clearance of docetaxel in mCRPC patients, resulting in decreased exposure. The aims of this study were to (1) determine if exposure in mCRPC patients is lower vs patients with other solid tumors by conducting a meta‐analysis, (2) evaluate the incidence of neutropenia in patients with mCRPC vs other solid tumors in a clinical cohort, and (3) discuss potential clinical consequences. A meta‐analysis was conducted of studies which reported areas under the plasma concentration‐time curves (AUCs) of docetaxel and variability. In addition, grade 3/4 neutropenia was evaluated using logistic regression in a cohort of patients treated with docetaxel. The meta‐analysis included 36 cohorts from 26 trials (n = 1150 patients), and showed that patients with mCRPC had a significantly lower mean AUC vs patients with other solid tumors (fold change [95% confidence interval (CI)]: 1.8 [1.5‐2.2]), with corresponding AUCs of 1.82 and 3.30 mg∙h/L, respectively. Logistic regression, including 812 patient, demonstrated that patients with mCRPC had a 2.2‐fold lower odds of developing grade 3/4 neutropenia compared to patients with other solid tumors (odds ratio [95%CI]: 0.46 [0.31‐0.90]). These findings indicate that mCRPC patients have a lower risk of experiencing severe neutropenia, possibly attributable to lower systemic exposure to docetaxel.

Prognostic Association of Prostate-specific Antigen Decline with Clinical Outcomes in Men with Metastatic Castration-resistant Prostate Cancer Treated with Enzalutamide in a Randomized Clinical Trial

BACKGROUND

In the PREVAIL study, enzalutamide provided significant improvements versus placebo in clinical outcomes in chemotherapy-naïve men with metastatic castration-resistant prostate cancer (mCRPC). The association of post-treatment prostate-specific antigen (PSA) decline with clinical outcomes may provide important prognostic information.

OBJECTIVE

To evaluate associations between the magnitude of PSA decline from baseline to month 3 and clinical outcomes among enzalutamide recipients.

DESIGN, SETTING, AND PARTICIPANTS

This was a post hoc retrospective analysis of PREVAIL, an international, randomized, double-blind, placebo-controlled phase 3 study. Men with mCRPC and no prior chemotherapy from the enzalutamide arm were included (n=872). Patients were grouped by confirmed maximal PSA decline from baseline to month 3 of treatment (n=795 evaluable).

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Primary outcomes were overall survival and radiographic progression-free survival. Secondary outcomes included PSA progression-free survival, radiographic response, and degradation of Functional Assessment of Cancer Therapy-Prostate score, which were estimated using the Kaplan-Meier method.

RESULTS AND LIMITATIONS

Following 3mo of enzalutamide treatment, 88% (701/795), 80% (639/795), and 39% (307/795) of patients had postbaseline confirmed maximal PSA declines of ≥30%, ≥50%, and ≥90%, respectively, whereas 12% (94/795) had no confirmed maximal PSA decline or a decline of <30%. Greater degrees of PSA decline within the first 3mo of enzalutamide treatment were increasingly associated with longer overall survival, time to PSA and radiographic progression, higher objective soft-tissue responses, and longer time to quality-of-life deterioration than no PSA decline or declines of <30% from baseline. PSA flares (rise followed by fall) after enzalutamide treatment were rare (<1%).

CONCLUSIONS

The magnitude of PSA decline after 3mo of enzalutamide therapy was strongly associated with better clinical and patient-reported outcomes. This updated prognostic information is of clinical value to this patient population and their health care providers.

PATIENT SUMMARY

We report that decreases in PSA levels are closely linked to better health and survival after 3mo of enzalutamide treatment in men with metastatic prostate cancer. The PREVAIL trial is registered at clinicaltrials.gov as NCT01212991.

Collateral resistance to taxanes in enzalutamide-resistant prostate cancer through aberrant androgen receptor and its variants

Currently, the optimal sequential use of androgen receptor (AR) axis-targeted agents and taxane chemotherapies remains undetermined. We aimed to elucidate the resistance status between taxanes and enzalutamide, and the functional role of the AR axis. Enzalutamide-resistant 22Rv1 cells showed collateral resistance to taxanes, including docetaxel and cabazitaxel. However, taxane-resistant cells showed no collateral resistance to enzalutamide; taxane-resistant cells expressed comparable protein levels of full-length AR and AR variants. Knockdown of both full-length AR and AR variants rendered cells sensitive to taxanes, whereas knockdown of AR variants sensitized cells to enzalutamide, but not to taxanes. In contrast, overexpression of full-length AR rendered cells resistant to taxanes. Consistently, the prostate-specific antigen response and progression-free survival in docetaxel chemotherapy were worse in cases with prior use of ARAT agents compared with cases without. Collateral resistance to taxanes was evident after obtaining enzalutamide resistance, and aberrant AR signaling might be involved in taxane resistance.

Abiraterone acetate/androgen deprivation therapy combination versus docetaxel/androgen deprivation therapy combination in advanced hormone-sensitive prostate cancer: a network meta-analysis on safety and efficacy

BACKGROUND

A major, yet precisely studied, shift has occurred in the treatment of advanced hormone-sensitive prostate cancer (HSPC) by the addition of docetaxel to androgen deprivation therapy (ADT) in the first line. Recently, two landmark trials showed that abiraterone acetate (AA) can be an effective alternative along with ADT in the same setting. We implemented a network meta-analysis to compare the safety and efficacy of the two combinations.

METHODS

PubMed database, ASCO and ESMO meeting library databases of all results published until June 2017 were searched using the keywords: "prostate cancer" AND "docetaxel" OR "abiraterone acetate". Efficacy endpoints including progression-free survival (PFS) and overall survival (OS), and safety endpoints (including treatment related deaths and selected adverse events) were assessed.

RESULTS

Twenty relevant studies were retrieved and assessed for eligibility. Of those trials, eight were found potentially eligible. Inconsistent reporting of efficacy outcomes limited our analysis to M1 HSPC. The pooled hazard ratios (HRs) of OS and PFS of the direct comparison of abiraterone acetate plus ADT versus ADT were 0.63 (95% CI: 0.545-0.717) and 0.38 (95% CI: 0.34-0.43), respectively. Meanwhile, in the trials of docetaxel plus ADT the pooled HRs of OS and PFS were 0.75 (95% CI: 0.65-0.86) and 0.634 (95% CI: 0.57-0.70), respectively. The indirect comparison showed that the HRs of OS and PFS in DOC + ADT in comparison to AA + ADT were 1.2 (95% CI: 0.98-1.46) and 1.65 (1.40-1.94), respectively. The pooled RR of treatment-related mortality in docetaxel + ADT versus AA + ADT was 1.438 (95% CI: 0.508-4.075).

CONCLUSION

Patients with metastatic HSPC (mHSPC) who received abiraterone acetate with ADT had better PFS and less toxicity compared to those receiving docetaxel with ADT. A trend towards superior OS and fewer treatment-related deaths was also observed, but was statistically non-significant. In view of lacking clear OS advantage, the choice between docetaxel and AA should include a discussion with the patient about the potential toxicities and impact on quality of life of each regimen.

Androgen receptor reverts dexamethasone‑induced inhibition of prostate cancer cell proliferation and migration

The aim of the present study was to determine the role of androgen receptor in the effect of dexamethasone on cell proliferation and migration of multiple prostate cancer cells. The prostate cancer cell lines LNCaP, 22Rv1, C4-2 and PC3 were cultured in vitro. For glucocorticoid-induced experiments, the cells were transferred and cultured in RPMI-1640 medium with 10% charcoal-stripped serum from RPMI-1640 medium with 10% fetal bovine serum for at least 24 h. The effects of dexamethasone on the proliferation and migration of various cell lines were analyzed by MTT and migration assays. Dexamethasone exhibited no effect on LNCaP, C4-2 and 22Rv1 cell lines, but suppressed proliferation of glucocorticoid receptor (GR)⁺ androgen receptor (AR)⁻ PC3 cell line. Dexamethasone suppressed PC3 cell migration, and did not affect migration of PC3-AR9 cells. Dexamethasone positively or negatively regulated proliferation of various prostate cancer cells based on AR and GR expression profiles. The data presented in the present study indicates that androgen receptor reverts the dexamethasone-induced inhibition of prostate cancer cell proliferation and migration.

Taxane-based Combination Therapies for Metastatic Prostate Cancer

CONTEXT

Multiple single-agent therapies improving survival are approved for the treatment of metastatic castration-resistant prostate cancer (mCRPC), including two chemotherapies, two androgen-signaling axis-targeting agents, an immunotherapeutic vaccine, and a radiopharmaceutical. Combination therapy can target multiple oncogenic pathways simultaneously, while potentially curbing the development of treatment resistance.

OBJECTIVE

To provide a succinct overview of taxane-based combination therapies currently being evaluated for the treatment of metastatic prostate cancer.

EVIDENCE ACQUISITION

We searched MEDLINE/PubMed^® and relevant congress databases for literature focused on taxane-based combination therapies being evaluated for the treatment of metastatic prostate cancer. In addition, a systematic search of www.clinicaltrials.gov was performed to gather information regarding ongoing taxane-based combination trials in prostate cancer. This search included phase II or III trials starting after January 1, 2010, which included the terms "docetaxel" or "cabazitaxel" and "prostate", and was then manually filtered for combination studies.

EVIDENCE SYNTHESIS

Single-agent therapy yields modest increments in survival. The success of combining docetaxel with androgen deprivation to improve overall survival (OS) for metastatic hormone-sensitive disease suggests the potential of similar approaches in mCRPC. Several classes of biological drugs have previously been combined with docetaxel for mCRPC in clinical trials without improvement in OS. However, combining docetaxel or cabazitaxel with newer agents with established single-agent benefit, such as radium-223, second-generation androgen pathway-targeted agents, or other chemotherapies, has the potential to benefit patients when compared with taxane chemotherapy alone. Our search revealed that the majority of trials currently assessing taxanes are focused on combination therapies: a combination approach is being evaluated in 37 of 47 trials assessing docetaxel and in 18 of 34 trials assessing cabazitaxel.

CONCLUSIONS

Despite prior failures, novel taxane-based combination therapies have the potential to improve outcomes in mCRPC. Challenges include the absence of validated predictive biomarkers for the selection of suitable patients and the potential for enhanced toxicity.

PATIENT SUMMARY

Patients with metastatic prostate cancer have access to multiple therapies improving survival. Many advanced epithelial cancers are treated with combinations of drugs; however, prostate cancer has remained an exception. A number of clinical studies have shown that combining chemotherapy with other classes of therapy may improve patient outcomes in prostate cancer. Here, we summarize the various combinations that are tested in the clinic and review the results.

Influence of Enzalutamide on Cabazitaxel Pharmacokinetics: a Drug-Drug Interaction Study in Metastatic Castration-resistant Prostate Cancer (mCRPC) Patients

Purpose: In ongoing clinical research on metastatic castration-resistant prostate cancer (mCRPC) treatment, the potential enhanced efficacy of the combination of taxanes with AR-targeted agents, that is, enzalutamide and abiraterone, is currently being explored. Because enzalutamide induces the CYP3A4 enzyme and taxanes are metabolized by this enzyme, a potential drug-drug interaction needs to be investigated.Experimental Design: Therefore, we performed a pharmacokinetic cross-over study in mCRPC patients who were scheduled for treatment with cabazitaxel Q3W (25 mg/m²). Patients were studied for three consecutive cabazitaxel cycles. Enzalutamide (160 mg once daily) was administered concomitantly after the first cabazitaxel cycle, during 6 weeks. Primary endpoint was the difference in mean area under the curve (AUC) between the first (cabazitaxel monotherapy) and third cabazitaxel cycle, when enzalutamide was added.Results: A potential clinically relevant 22% (95% CI, 9%-34%; P = 0.005) reduction in cabazitaxel exposure was found with concomitant enzalutamide use. The geometric mean AUC_0-24h of cabazitaxel was 181 ng*h/mL (95% CI, 150-219 ng*h/mL) in cycle 3 and 234 ng*h/mL (95% CI, 209-261 ng*h/mL) in cycle 1. This combination did not result in excessive toxicity, whereas PSA response was promising.Conclusions: We found a significant decrease in cabazitaxel exposure when combined with enzalutamide. In an era of clinical trials on combination strategies for mCRPC, it is important to be aware of clinically relevant drug-drug interactions. Because recent study results support the use of a lower standard cabazitaxel dose of 20 mg/m², the clinical relevance of this interaction may be substantial, because the addition of enzalutamide may result in subtherapeutic cabazitaxel exposure. Clin Cancer Res; 24(3); 541-6. ©2017 AACR.

Mechanisms of resistance to systemic therapy in metastatic castration-resistant prostate cancer

Patients with metastatic castration-resistant prostate cancer (mCPRC) now have an unprecedented number of approved treatment options, including chemotherapies (docetaxel, cabazitaxel), androgen receptor (AR)-targeted therapies (enzalutamide, abiraterone), a radioisotope (radium-223) and a cancer vaccine (sipuleucel-T). However, the optimal treatment sequencing pathway is unknown, and this problem is exacerbated by the issues of primary and acquired resistance. This review focuses on mechanisms of resistance to AR-targeted therapies and taxane-based chemotherapy. Patients treated with abiraterone, enzalutamide, docetaxel or cabazitaxel may present with primary resistance, or eventually acquire resistance when on treatment. Multiple resistance mechanisms to AR-targeted agents have been proposed, including: intratumoral androgen production, amplification, mutation, or expression of AR splice variants, increased steroidogenesis, upregulation of signals downstream of the AR, and development of androgen-independent tumor cells. Known mechanisms of resistance to chemotherapy are distinct, and include: tubulin alterations, increased expression of multidrug resistance genes, TMPRSS2-ERG fusion genes, kinesins, cytokines, and components of other signaling pathways, and epithelial-mesenchymal transition. Utilizing this information, biomarkers of resistance/response have the potential to direct treatment decisions. Expression of the AR splice variant AR-V7 may predict resistance to AR-targeted agents, but available biomarker assays are yet to be prospectively validated in the clinic. Ongoing prospective trials are evaluating the sequential use of different drugs, or combination regimens, and the results of these studies, combined with a deeper understanding of mechanisms of primary and acquired resistance to treatment, have the potential to drive future treatment decisions in mCRPC.