Background: Before the androgen target therapy era, flutamide was widely used for castrationresistant prostate cancer in Japan. Enzalutamide is currently the recommended treatment; however, the efficacy and safety of enzalutamide and flutamide after combined androgen blockade therapy with bicalutamide, has not been compared. Methods: Patients with castration-resistant prostate cancer who received combined androgen blockade therapy with bicalutamide were randomly assigned to receive either enzalutamide or flutamide. The primary endpoint for efficacy was the 3-month prostate-specific antigen response rate. This trial is registered with ClinicalTrials.gov (NCT02346578) and the University hospital Medical Information Network (UMIN000016301) Results: Overall, 103 patients were enrolled. The 3-(80.8% vs. 35.3%; p<0.001) and 6-month (73.1% vs. 31.4%; p<0.001) prostate-specific antigen response rates were higher in the enzalutamide than in the flutamide group. The 3-month disease progression rates (radiographic or prostate-specific antigen progression) were 6.4% and 38.8% in the enzalutamide and flutamide groups, respectively (hazard ratio [HR]: 0.16; 95% confidence interval [CI]: 0.05-0.47; p<0.001); the 6-month rates were 11.4% and 51.1%, respectively (HR: 0.22; 95% CI: 0.09-0.50; p<0.001). Enzalutamide provided superior prostate-specific antigen progression-free survival compared with flutamide (HR: 0.29; 95% CI: 0.15-0.54; p<0.001). Median time to prostate-specific antigen 4 progression-free survival was not reached and was 6.6 months in the enzalutamide and flutamide groups, respectively. Conclusions: As an alternative anti-androgen therapy in patients with castration-resistant prostate cancer who fail bicalutamide-combined androgen blockade therapy, enzalutamide provides superior clinical outcomes compared with flutamide. Enzalutamide should be preferred over flutamide in these patients.

Abstract. Orotate phosphoribosyl transferase (OPRT) is the initial enzyme of 5-fluorouracil (5-FU) activation, in which 5-FU is converted to 5-fluorouridinemonophosphate. Dihydropyrimidine dehydrogenase (DPD) is a degrading enzyme that catabolizes 5-FU. In this study, we investigated the expression of these enzymes in normal prostate gland (NP), hormone-sensitive prostate cancer (HSPC) and hormonerefractory prostate cancer (HRPC). Forty-two prostatic tissue specimens were obtained from patients who had undergone prostate needle biopsies without any treatments or with PSA failure after initial androgen deprivation. The tissue samples derived from formalin-fixed, paraffin-embedded sections were made by laser-captured microdissection and from those RNA was extracted. The levels of OPRT and DPD mRNA expression were examined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). The level of OPRT mRNA expression in the HSPC or the HRPC specimens was significantly higher than that in the NP specimens. Immunohistochemical staining for OPRT revealed strong expression of OPRT in prostate cancer cells. There was a significant correlation between OPRT mRNA expression levels and the tumor pathological grade. Furthermore, the OPRT/DPD expression ratio, a powerful predictive factor to evaluate 5-FU sensitivity, in the HRPC group was significantly higher than that in the low grade HSPC group. Thus, 5-FU may be an effective option for some HRPC patients.

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168 Background: In Asia, including Japan, combined androgen blockade (CAB) therapy with bicalutamide is widely administered for metastatic prostate cancer. Alternative anti-androgen therapy (AAT) with flutamide after CAB therapy with bicalutamide was common before the androgen receptor-targeted therapy era. The objective of the OCUU-CRPC study was to compare the efficacy and safety between second-line hormonal therapy of enzalutamide and flutamide as alternative AAT after CAB therapy that included bicalutamide in patients with CRPC. Methods: A total of 104 patients with CRPC with or without distant metastases after disease progression who received CAB therapy with bicalutamide were randomly assigned at a 1:1 ratio according to distant metastases to the enzalutamide (160 mg/day) and flutamide (375 mg/day) groups. The primary endpoint for the drug efficacy was the response rate of prostate-specific antigen (PSA; i.e., declined by ≥50%) at 3 months. Results: Between January 2015 and February 2018, 103 patients were randomly assigned, 52 to enzalutamide and 51 to flutamide. 25 (48%) and 38 (75%) patients, respectively, discontinued their assigned treatment before study end, mainly due to progressive disease. Median follow-up time was 13.4 months (IQR 9.6–23.8) in the enzalutamide group and 17.0 months (9.7–24.4) in the flutamide group. Enzalutamide resulted in significant improvements in the response rate of PSA at 3 months (84.6% vs 31.4%; p< 0.001). Enzalutamide significantly improved in all secondary endpoints: the PSA progression rate at 3 months (HR, 0.17; 95% CI, 0.05 to 0.47; p< 0.01) and at 6 months (HR, 0.22; 95% CI, 0.09 to 0.50; p< 0.01); and PSA response rate at 6 months (75.0% vs 29.4%; p< 0.01); and time to PSA progression (median: not reached vs 6.6 months; HR, 0.29; 95% CI, 0.15 to 0.54; p< 0.01). The observed adverse event profile was consistent with that from phase III enzalutamide trials. Conclusions: Enzalutamide significantly reduced risk of progression compared with flutamide in patients with CRPC after CAB therapy with bicalutamide. This trial is registered with ClinicalTrials.gov, number. Clinical trial information: NCT02346578.