Objectives. To determine the efficacy of the gonadotropin-releasing hormone antagonist abarelix in patients with androgen-independent prostate cancer progressing after orchiectomy and to measure its effect on serum follicle-stimulating hormone (FSH). Methods. Sixteen patients with prostate cancer progressing after orchiectomy received abarelix-depot 100 mg by intramuscular injection on days 1, 15, and 29 and then every 28 days for up to 24 weeks (52 weeks in patients who met the criteria for a prostate-specific antigen [PSA] response after 24 weeks). PSA response was the primary endpoint and was defined as a 50% reduction confirmed 4 weeks later. The time to progression and effect of therapy on serum FSH were secondary endpoints. Results. No patient met the criteria for a PSA response. Five patients (31%, 95% confidence interval 11% to 58%) experienced confirmed reductions in the PSA level ranging from 9.3% to 31.8%. At the end of the six cycles of therapy, 6 patients remained stable without PSA progression or other signs of disease progression. The median time to progression was 12 weeks (95% confidence interval 6 to 18). The mean serum FSH concentration declined after 4 weeks of study treatment by nearly 90% from a baseline of 45.1 IU/L (95% confidence interval 34.0 to 56.2) and remained suppressed throughout the observation period. Treatment was well tolerated, with one grade 3 allergic reaction. Conclusions. Treatment with abarelix in patients with androgen-independent prostate cancer after orchiectomy results in marked reduction in circulating FSH. None of the patients met the PSA response criteria; nonetheless, minor reductions in serum PSA were observed in 5 of 16 patients.
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