Chemotherapy-naive Metastatic Castration-resistant Prostate Cancer Research Articles

Efficacy and safety of enzalutamide in patients 75 years or older with chemotherapy-naive metastatic castration-resistant prostate cancer: results from PREVAIL

Prostate cancer disproportionately affects older men. Because age affects treatment decisions, it is important to understand the efficacy and tolerability of therapies for advanced prostate cancer in elderly men. This analysis describes efficacy and safety outcomes in men aged ≥75 years who received enzalutamide, an androgen receptor inhibitor, in the phase III PREVAIL trial. PREVAIL was a randomised, double-blind, multinational study of oral enzalutamide 160 mg/day (N = 872) versus placebo (N = 845) in chemotherapy-naive men with metastatic castration-resistant prostate cancer. Overall survival (OS) and radiographic progression-free survival (rPFS) were coprimary end points. Subgroup analysis of men aged ≥75 years (elderly) and men aged <75 years was pre-specified for the coprimary end points and adverse events (AEs). Among 609 elderly patients (35%) who participated in PREVAIL, median treatment duration was 16.6 and 5.0 months in the enzalutamide and placebo arms, respectively. In the elderly subgroup, OS was greater with enzalutamide than with placebo [32.4 months (95% confidence interval (CI) 27.7-not yet reached] versus 25.1 months (95% CI 22.6-28.0); hazard ratio (HR) = 0.61 (95% CI 0.47-0.79); P = 0.0001], as was rPFS [not yet reached (95% CI 12.3-not yet reached) versus 3.7 months (95% CI 3.6-5.3); HR = 0.17 (95% CI 0.12-0.24); P < 0.0001]. Irrespective of treatment assignment, incidence of AEs was similar between the two age groups, except for an overall higher incidence of falls among elderly patients than younger patients [84/609 (13.8%) versus 62/1106 (5.6%)] and among elderly patients receiving enzalutamide than those receiving placebo [61/317 (19.2%) versus 23/292 (7.9%)]. Elderly men benefited from treatment with enzalutamide in terms of OS and rPFS. Enzalutamide was well tolerated in the elderly subgroup and those aged <75 years. Age and enzalutamide treatment were associated with a higher incidence of falls. NCT01212991, ClinicalTrials.gov.

The New Combination Docetaxel, Prednisone and Curcumin in Patients with Castration-Resistant Prostate Cancer: A Pilot Phase II Study

Objectives: Favorable phase I results justified this pilot phase II study to assess the efficacy of docetaxel/curcumin in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (CRPC). Methods: Thirty patients with progressing CRPC and a rising prostate-specific antigen (PSA) received docetaxel/prednisone in standard conditions for 6 cycles in combination with per os curcumin, 6,000 mg/day (day -4 to day +2 of docetaxel). The co-primary endpoint was the overall response rate determined by PSA and target assessments. An ancillary study assessed the seric values of chromogranin A (CgA) and neuron-specific enolase (NSE). Results: Twenty-six patients received the scheduled treatment, 2 progressed and 2 died before the end of treatment. A PSA response was observed in 59% of patients (14% of PSA normalization) and achieved within the first three cycles for 88% of responders. Partial response was reached for 40% of evaluable patients. The regimen was well tolerated, and no adverse event was attributed to curcumin. Twenty patients were 100% curcumin compliant. The PSA level and objective response rate were not correlated with the serum values of CgA and NSE. Conclusion: This study produced additional data on curcumin as a treatment for cancer, with a high response rate, good tolerability and patient acceptability, justifying the interest to conduct a randomized trial.

Pharmacokinetics, Antitumor Activity, and Safety of ODM-201 in Patients with Chemotherapy-naive Metastatic Castration-resistant Prostate Cancer: An Open-label Phase 1 Study

BackgroundODM-201 is a novel second-generation androgen receptor inhibitor for the treatment of metastatic castration-resistant prostate cancer (mCRPC). ObjectiveTo evaluate the pharmacokinetics of ODM-201 tablet products and preliminary long-term safety, tolerability, and antitumor activity of ODM-201 in chemotherapy-naive men with mCRPC. Design, setting, and participantsThirty patients were enrolled in this open-label phase 1 trial. Patients received a single 600-mg dose of ODM-201 in capsules with food and one 600-mg dose of ODM-201 tablet product (TabA or TabB) with food and in the fasted state in a random order. In the extension, patients received 600mg twice daily ODM-201 taken with food in capsules. Outcome measurements and statistical analysisWe analyzed the pharmacokinetics of ODM-201 tablet formulations. Safety and tolerability were assessed until disease progression or an intolerable adverse event (AE). Antitumor activity was assessed by prostate-specific antigen (PSA) levels and imaging. Results and limitationsThe capsule:TabA ratio of area under the concentration-time curve from time zero to the last sample at 48h was 1.06 (90% confidence interval [CI], 0.91–1.24); the capsule:TabB ratio was 0.97 (90% CI, 0.82–1.14). At week 12, 25 of 30 patients (83%) had a PSA response (≥50% reduction from baseline). Median time to radiographic progression was 66 wk (95% CI, 41–79). Most common AEs were fatigue (n=4 [13%]) and nausea (n=4 [13%]). ConclusionsThe study showed that the tablet formulation of ODM-201 had similar pharmacokinetics compared with the capsule. Treatment with a 600-mg twice daily dose of ODM-201 provided anticancer activity and was well tolerated in men with chemotherapy-naive mCRPC. Patient summaryThe findings of this study showed that ODM-201 is well tolerated and provided antitumor activity in chemotherapy-naive patients with metastatic castration-resistant prostate cancer (mCRPC) and that the 300-mg tablet formulation can be used in further clinical studies. A phase 3 trial with ODM-201 600mg twice daily in patients with non-mCRPC is ongoing.

Development of PROSTVAC immunotherapy in prostate cancer.

PROSTVAC immunotherapy is a heterologous prime-boost regimen of two different recombinant pox-virus vectors; vaccinia as the primary immunotherapy, followed by boosters employing fowlpox, to provoke immune responses against prostate-specific antigen. Both vectors contain transgenes for prostate-specific antigen and a triad of T-cell costimulatory molecules (TRICOM). In a placebo-controlled Phase II trial of men with minimally symptomatic, chemotherapy-naive metastatic castration-resistant prostate cancer, PROSTVAC was well tolerated and associated with a 44% reduction in death. With a novel mechanism of action, and excellent tolerability, PROSTVAC has the potential to dramatically alter the treatment landscape of prostate cancer, not only as a monotherapy, but also in combination with other novel agents, such as immune check point inhibitors and novel androgen receptor blockers. A Phase III trial recently completed accrual.

Moving the goal posts in prostate cancer trials

In The Lancet Oncology, Fred Saad and colleagues 1 Saad F Fizazi K Jinga V et al. for the ELM-PC 4 investigatorsOrteronel plus prednisone in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (ELM-PC 4): a double-blind, multicentre, phase 3, randomised, placebo-controlled trial. Lancet Oncol. 2015; (published online Feb 18.)http://dx.doi.org/10.1016/S1470-2045(15)70027-6 PubMed Google Scholar present final data from ELM-PC 4, a phase 3 trial of orteronel plus prednisone versus placebo plus prednisone in patients with chemotherapy-naive, metastatic castration-resistant prostate cancer. Orteronel was developed to inhibit CYP17-mediated secondary androgen production via selective blocking of 17,20-lyase. Although abiraterone acetate has similar biological effects and is already approved for metastatic castration-resistant prostate cancer, orteronel represented another candidate androgen biosynthesis inhibitor. This study 1 Saad F Fizazi K Jinga V et al. for the ELM-PC 4 investigatorsOrteronel plus prednisone in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (ELM-PC 4): a double-blind, multicentre, phase 3, randomised, placebo-controlled trial. Lancet Oncol. 2015; (published online Feb 18.)http://dx.doi.org/10.1016/S1470-2045(15)70027-6 PubMed Google Scholar enrolled 1560 patients, randomly assigning 781 patients to receive orteronel plus prednisone and 779 to receive placebo plus prednisone. The co-primary endpoints were radiographic progression-free survival and overall survival. Despite a benefit in radiographic progression-free survival (median 13·8 months with orteronel plus prednisone vs 8·7 months with placebo plus prednisone; hazard ratio [HR] 0·71; 95% CI 0·63–0·80; p<0·0001) as well as improvements in PSA response and circulating tumour cells, orteronel plus prednisone did not improve overall survival (median 31·4 months with orteronel plus prednisone vs 29·5 months with placebo plus prednisone; HR 0·92; 95% CI 0·79–1·08; p=0·31). Of note, EML-PC 5, a study in docetaxel-treated metastatic castration-resistant prostate cancer, also failed to show an overall survival advantage for oteronel and prednisone versus placebo and prednisone, despite a benefit in radiographic progression-free survival. 2 Fizazi K Jones R Oudard S et al. Phase III, randomized, double-blind, multicenter trial comparing orteronel (TAK-700) plus prednisone with placebo plus prednisone in patients with metastatic castration-resistant prostate cancer that has progressed during or after docetaxel-based therapy: ELM-PC 5. J Clin Oncol. 2015; (published online Jan 26.)https://doi.org/10.1200/JCO.2014.56.5119 Crossref Scopus (108) Google Scholar Orteronel plus prednisone in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (ELM-PC 4): a double-blind, multicentre, phase 3, randomised, placebo-controlled trialIn chemotherapy-naive patients with metastatic castration-resistant prostate cancer, radiographic progression-free survival was prolonged with orteronel plus prednisone versus placebo plus prednisone. However, no improvement was noted in the other primary endpoint, overall survival. Orteronel plus prednisone was associated with increased toxic effects compared with placebo plus prednisone. On the basis of these and other data, orteronel is not undergoing further development in metastatic castration-resistant prostate cancer. Full-Text PDF

Orteronel plus prednisone in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (ELM-PC 4): a double-blind, multicentre, phase 3, randomised, placebo-controlled trial

Orteronel is an investigational, partially selective inhibitor of CYP 17,20-lyase in the androgen signalling pathway, a validated therapeutic target for metastatic castration-resistant prostate cancer. We assessed orteronel in chemotherapy-naive patients with metastatic castration-resistant prostate cancer. In this phase 3, double-blind, placebo-controlled trial, we recruited patients with progressive metastatic castration-resistant prostate cancer and no previous chemotherapy from 324 study centres (ie, hospitals or large urologic or group outpatient offices) in 43 countries. Eligible patients were randomly assigned in a 1:1 ratio to receive either 400 mg orteronel plus 5 mg prednisone twice daily or placebo plus 5 mg prednisone twice daily. Randomisation was done centrally with an interactive voice response system and patients were stratified by region (Europe, North America, and not Europe or North America) and the presence or absence of radiographic disease progression at baseline. The two primary endpoints were radiographic progression-free survival and overall survival, determined in the intention-to-treat population. This trial is registered with ClinicalTrials.gov, number NCT01193244. From Oct 31, 2010, to June 29, 2012, 2353 patients were assessed for eligibility. Of those, 1560 were randomly assigned to receive either orteronel plus prednisone (n=781) or placebo plus prednisone (n=779). The clinical cutoff date for the final analysis was Jan 15, 2014 (with 611 deaths). Median follow-up for radiographic progression-free survival was 8·4 months (IQR 3·7-16·6). Median radiographic progression-free survival was 13·8 months (95% CI 13·1-14·9) with orteronel plus prednisone and 8·7 months (8·3-10·9) with placebo plus prednisone (hazard ratio [HR] 0·71, 95% CI 0·63-0·80; p<0·0001). After a median follow-up of 20·7 months (IQR 14·2-25·4), median overall survival was 31·4 months (95% CI 28·6-not estimable) with orteronel plus prednisone and 29·5 months (27·0-not estimable) with placebo plus prednisone (HR 0·92, 95% CI 0·79-1·08; p=0·31). The most common grade 3 or worse adverse events were increased lipase (137 [17%] of 784 patients in the orteronel plus prednisone group vs 14 [2%] of 770 patients in the placebo plus prednisone group), increased amylase (77 [10%] vs nine [1%]), fatigue (50 [6%] vs 14 [2%]), and pulmonary embolism (40 [5%] vs 27 [4%]). Serious adverse events were reported in 358 [46%] patients receiving orteronel plus prednisone and in 292 [38%] patients receiving placebo plus prednisone. In chemotherapy-naive patients with metastatic castration-resistant prostate cancer, radiographic progression-free survival was prolonged with orteronel plus prednisone versus placebo plus prednisone. However, no improvement was noted in the other primary endpoint, overall survival. Orteronel plus prednisone was associated with increased toxic effects compared with placebo plus prednisone. On the basis of these and other data, orteronel is not undergoing further development in metastatic castration-resistant prostate cancer. Millennium Pharmaceuticals, Inc, a wholly owned subsidiary of Takeda Pharmaceutical Company Limited.

Abiraterone acetate plus prednisone versus placebo plus prednisone in chemotherapy-naive men with metastatic castration-resistant prostate cancer (COU-AA-302): final overall survival analysis of a randomised, double-blind, placebo-controlled phase 3 study

Abiraterone acetate plus prednisone significantly improved radiographic progression-free survival compared with placebo plus prednisone in men with chemotherapy-naive castration-resistant prostate cancer at the interim analyses of the COU-AA-302 trial. Here, we present the prespecified final analysis of the trial, assessing the effect of abiraterone acetate plus prednisone on overall survival, time to opiate use, and use of other subsequent therapies. In this placebo-controlled, double-blind, randomised phase 3 study, 1088 asymptomatic or mildly symptomatic patients with chemotherapy-naive prostate cancer stratified by Eastern Cooperative Oncology performance status (0 vs 1) were randomly assigned with a permuted block allocation scheme via a web response system in a 1:1 ratio to receive either abiraterone acetate (1000 mg once daily) plus prednisone (5 mg twice daily; abiraterone acetate group) or placebo plus prednisone (placebo group). Coprimary endpoints were radiographic progression-free survival and overall survival analysed in the intention-to-treat population. The study is registered with ClinicalTrials.gov, number NCT00887198. At a median follow-up of 49.2 months (IQR 47.0-51.8), 741 (96%) of the prespecified 773 death events for the final analysis had been observed: 354 (65%) of 546 patients in the abiraterone acetate group and 387 (71%) of 542 in the placebo group. 238 (44%) patients initially receiving prednisone alone subsequently received abiraterone acetate plus prednisone as crossover per protocol (93 patients) or as subsequent therapy (145 patients). Overall, 365 (67%) patients in the abiraterone acetate group and 435 (80%) in the placebo group received subsequent treatment with one or more approved agents. Median overall survival was significantly longer in the abiraterone acetate group than in the placebo group (34.7 months [95% CI 32.7-36.8] vs 30.3 months [28.7-33.3]; hazard ratio 0.81 [95% CI 0.70-0.93]; p=0.0033). The most common grade 3-4 adverse events of special interest were cardiac disorders (41 [8%] of 542 patients in the abiraterone acetate group vs 20 [4%] of 540 patients in the placebo group), increased alanine aminotransferase (32 [6%] vs four [<1%]), and hypertension (25 [5%] vs 17 [3%]). In this randomised phase 3 trial with a median follow-up of more than 4 years, treatment with abiraterone acetate prolonged overall survival compared with prednisone alone by a margin that was both clinically and statistically significant. These results further support the favourable safety profile of abiraterone acetate in patients with chemotherapy-naive metastatic castration-resistant prostate cancer. Janssen Research & Development.

Efficacy of Enzalutamide Following Abiraterone Acetate in Chemotherapy-naive Metastatic Castration-resistant Prostate Cancer Patients

BackgroundThe activity of enzalutamide after prior treatment with both abiraterone acetate (abiraterone) and docetaxel has been examined in several retrospective studies. However, limited data are available on the efficacy of enzalutamide following abiraterone in chemotherapy-naive patients with metastatic castration-resistant prostate cancer (mCRPC). ObjectiveTo compare the activity of enzalutamide after abiraterone in docetaxel-experienced and docetaxel-naive mCRPC patients. Design, setting, and participantsThe British Columbia Cancer Agency Cancer Registry was searched for mCRPC patients who received enzalutamide after prior abiraterone. Clinicopathologic characteristics, confirmed prostate-specific antigen (PSA) response rates (PSA decline ≥50% confirmed ≥3 wk later), and survival data were collected. Outcome measurements and statistical analysisOutcomes on enzalutamide were compared between docetaxel-experienced and docetaxel-naive patients using chi-square for PSA response and log-rank test for time to PSA progression and overall survival (OS). Univariate analysis was performed to identify variables associated with confirmed PSA response on enzalutamide, using either chi-square for categorical variables or logistic regression for continuous variables. Results and limitationsA total of 115 patients received enzalutamide after abiraterone: 68 had received prior docetaxel and 47 were docetaxel naive. Median time on enzalutamide was 4.1 mo. Confirmed PSA response rates (22% vs 26%; p=0.8), median time to radiologic/clinical progression (4.6 mo vs 6.6 mo; p=0.6), and median OS (10.6 mo vs 8.6 mo; p=0.2) did not differ significantly between docetaxel-experienced and docetaxel-naive patients. No clinical variables (including prior response to abiraterone) were found to associate significantly with confirmed PSA response to enzalutamide. ConclusionsAntitumour activity of enzalutamide following abiraterone was limited in mCRPC patients irrespective of prior docetaxel use. Identifying clinical and molecular factors predictive of response to enzalutamide remains a high priority for future research. Patient summaryWe looked at the effectiveness of enzalutamide after abiraterone acetate for treatment of advanced prostate cancer. We found that patients who had received docetaxel chemotherapy before abiraterone gained similar benefit from enzalutamide compared with patients who had not received docetaxel. These results suggest that earlier treatment with docetaxel does not have a large impact on the activity of enzalutamide after abiraterone.

Impact of prior endocrine therapy on radiographic progression-free survival (rPFS) in patients (pts) with chemotherapy-naive metastatic castration-resistant prostate cancer (mCRPC): Results from COU-AA-302.

14 Background: Abiraterone acetate (AA), a prodrug of abiraterone, which is a selective androgen biosynthesis inhibitor, prolongs overall survival (OS) in mCRPC pts and is approved for use in this population. An exploratory analysis was conducted to determine the impact of prior endocrine therapy on clinical benefit of AA in COU-AA-302, a randomized phase 3 study of pts with chemotherapy-naive mCRPC. Methods: 1,088 pts (testosterone &lt; 50 ng/dL; receiving continuous gonadotropin-releasing hormone [GnRH] agonist if not orchiectomized) were randomized 1:1 to AA (1 g) + prednisone (P) (5 mg BID) or placebo + P alone. rPFS and OS were co-primary end points. rPFS was defined as time to first occurrence of bone scan progression by PCWG2 criteria, progression by CT/MRI by modified RECIST 1.0 criteria, or death, whichever came first. Comparison of the treatment effect on rPFS (investigator review at 55% of OS events) was made for the subgroups above or below the median duration of prior endocrine therapy in 2 categories: either prior GnRH agonist or prior antiandrogens. Cox model was used to obtain the HR and associated 95% CI with statistical inference by log rank statistic. Results: Most pts received prior GnRH agonists and antiandrogens (Table). rPFS was significantly more favorable with AA than P irrespective of the median duration of prior GnRH agonist or antiandrogen therapy. Increased median rPFS was observed in pts with longer prior exposure to GnRH agonists or antiandrogens comparing within both AA and P treatment groups. Conclusions: As shown in this COU-AA-302 exploratory analysis, duration of prior endocrine therapy, as analyzed after the onset of mCRPC, does not appear to impact clinical benefit of AA in pts with chemotherapy-naive mCRPC. Clinical trial information: NCT00887198. [Table: see text]

Relationship of baseline PSA and degree of PSA decline to radiographic progression-free survival (rPFS) in patients with chemotherapy-naive metastatic castration-resistant prostate cancer (mCRPC): Results from COU-AA-302.

5010 Background: Abiraterone acetate (AA), an androgen biosynthesis inhibitor, prolongs overall survival (OS) in mCRPC patients and is approved for use in this population. The relationship of PSA kinetics to rPFS was evaluated in an exploratory analysis of patients from COU-AA-302, a randomized phase III study of AA in chemotherapy-naive mCRPC patients. Methods: 1,088 patients were randomized 1:1 to AA (1 g) + prednisone (P) (5 mg BID) or P alone. rPFS and OS were co-primary endpoints. rPFS was defined as time to first occurrence of bone scan progression by PCWG2 criteria, progression by CT/MRI by modified RECIST 1.0 criteria, or death from any cause; PSA changes were not a factor in determining rPFS. Quartiles of baseline PSA and % PSA decrease from baseline to nadir were analyzed. Stratified Cox regression models were used with factors for treatment, PSA outcomes, and baseline covariates performed at 55% of OS events. Results: 54/546 patients (10%) in AA + P arm achieved undetectable PSA vs 14/542 patients (3%) in the P arm; at median follow-up of 27.1 mos, radiographic progression was observed in 28% (AA + P) vs 50% of patients (P). There was a consistent trend of decreasing hazard of progression with decreasing baseline PSA and increasing % PSA decline (Table). Treatment effect of AA + P vs P with decreasing baseline PSA or % PSA decline remained significant (p=0.001) after adjusting for other factors (PSA, LDH, alk phos, hemoglobin, bone metastasis) in the model. Conclusions: rPFS was positively associated with the magnitude of PSA decline and inversely associated with baseline PSA. These effects remained after correcting for covariates. In all analyses, treatment with AA led to rPFS outcomes superior to P. Clinical trial information: NCT00887198. [Table: see text]

Combining active immunotherapy with immune checkpoint blockade for the treatment of advanced prostate cancer

Combination immunotherapy is emerging as a promising approach for the treatment of advanced prostate cancer. This research highlight discusses the combination of a PSA-directed poxviral vaccine and a monoclonal antibody blocking an important immune checkpoint molecule to treat men with metastatic castration-resistant prostate cancer. The results demonstrate feasibility and safety, as well as intriguing clinical responses to this combination therapy.

A randomized phase II study of OGX-427 plus prednisone versus prednisone alone in patients with chemotherapy-naive metastatic castration-resistant prostate cancer.

121 Background: Heat Shock Protein 27 (Hsp27) is a stress-activated, multi-functional chaperone protein highly expressed in cancer that regulates cell signaling and survival pathways implicated in cancer progression. In prostate cancer models, Hsp27 complexes with androgen receptor (AR) and enhances transactivation of AR-regulated genes. OGX-427 is a 2nd generation antisense that inhibits Hsp27 expression with in vitro and in vivo efficacy. Phase I studies have demonstrated tolerability and single agent activity. Methods: Patients (pts) with CRPC, no/minimal symptoms and any prior treatment other than chemotherapy were randomized 1:1 to receive prednisone (P) 5 mg PO BID or P with OGX-427 600 mg IV x 3 loading doses followed by 1000 mg IV weekly. Primary endpoint was the proportion of pts progression free (PSAWG 2 criteria) at 12 weeks. A 2-stage MinMax design (H0 = 5%, HA &gt;20%, α=0.1, β=0.1) will enrol 32 pts total per arm and provide 70% power to detect the difference at a 0.10 1-sided significance. Secondary endpoints include PSA decline, measurable disease response, and circulating tumour cell (CTC) enumeration. Results: In the first 22 pts randomized (11 to OGX-427+P, 11 to P), baseline median age was 71 years (53-86), ECOG PS 0 or 1 in 64% and 36% of pts, median PSA 89 (6-606), metastases in bone/lymph nodes/liver or lung in 77%/64%/10%, 23% had prior treatment with P, and 91% had ≥5 CTC/7.5 ml (median 18/7.5 ml). Thus far, 82% of pts randomized to OGX-427+P have had a PSA decline (55% with ≥30% decline) and 18% a PSA increase; 40% of pts treated with P have had a PSA decline (20% with ≥30% decline), 10% no change and 50% with PSA increase. CTC conversion from ≥5 to &lt;5/7.5 ml has occurred in 60% of pts randomized to OGX-427+P and 20% of pts treated with P alone. Grade 1-2 infusion reactions (e.g., chills, diarrhea, flushing) have occurred in 45% of pts receiving OGX-427+P and 1 pt developed hemolytic uremic syndrome after week 7 probably related to OGX. Conclusions: Preliminary data provide clinical support for the role of Hsp27 in AR signalling and as a therapeutic target for prostate cancer. Enrolment on this study continues. Funded by a grant from the Terry Fox Research Institute.

Phase II Study of Abiraterone Acetate in Chemotherapy-Naive Metastatic Castration-Resistant Prostate Cancer Displaying Bone Flare Discordant with Serologic Response

Abiraterone is an oral inhibitor of CYP17, which is essential for androgen biosynthesis. This multicenter study assessed its efficacy in patients with castration-resistant prostate cancer (CRPC), without prior chemotherapy or CYP17-targeted therapy, and frequency of bone scans discordant with prostate-specific antigen (PSA) and clinical response. Thirty-three patients received abiraterone acetate 1,000 mg daily with prednisone 5 mg twice daily in continuous 28-day cycles. Patients were evaluated monthly for efficacy and safety. Bone scan flare was defined as the combination, after 3 months of therapy, of an interpreting radiologist's report indicating "disease progression" in context of a 50% or more decline in PSA level, with scan improvement or stability 3 months later. A 50% or more decline in PSA level at week 12 was confirmed in 22 of 33 (67%) patients. Declines in PSA level of 50% or more were seen in 26 of 33 (79%) patients. Undetectable PSA levels (≤0.1 ng/mL) occurred in 2 patients. Median time on therapy and time to PSA progression were 63 weeks and 16.3 months, respectively. Twenty-three patients were evaluable for bone scan flare. Progression was indicated in radiologist's report in 12 of 23 (52%), and 11 of 12 subsequently showed improvement or stability. As prospectively defined, bone scan flare was observed in 11 of 23 (48%) evaluable patients or 11 of 33 (33%) enrolled patients. Adverse events were typically grade 1/2 and consistent with prior published abiraterone reports. Clinical responses to abiraterone plus prednisone were frequent and durable in men with metastatic CRPC. Further investigation is needed to clarify the confounding effect of bone scan flare on patient management and interpretation of results. Clin Cancer Res; 17(14); 4854-61. ©2011 AACR.