CHICAGO—Enzalutamide alone is an effective treatment for men with hormone-naïve prostate cancer (PCa), researchers reported at the American Society of Clinical Oncology annual meeting.

The drug, which is a novel oral androgen receptor inhibitor, received FDA approval in August 2012 as a treatment for metastatic castration-resistant PCa in men previously treated with docetaxel. The approval was based on the results of the phase 3 AFFIRM trial, which showed that enzalutamide increased median overall survival by 4.8 months compared with placebo.

Matthew R. Smith, MD, PhD, of Massachusetts General Hospital Cancer Center in Boston and colleagues conducted a phase 2 open-label single-arm trial that enrolled 67 men with histologically confirmed PCa for which androgen deprivation therapy is indicated. All men had non-castration testosterone levels (230 ng/dL or higher), a PSA level of 2 ng/mL or higher, and a life expectancy of at least 12 months.

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The patients were treated with enzalutamide at a dosage of 160 mg daily for 24 weeks. The primary endpoint was PSA response, defined as an 80% of greater decline in PSA.

Of the 67 patients, 62 (92.5%) had a PSA response. The median PSA decrease was 99.6%. The decline in PSA was generally similar between patients with and without metastases. Sixteen patients were evaluable for objective response based on RECIST (Response Evaluation Criteria for Solid Tumors) Of the 16 patients, three (19%) had a complete response, five (31%) had a partial response. Three patients (19%) had stable disease and three (19%) had progressive disease.

Meanwhile, serum levels of sex hormones increased from baseline to week 25. Luteinizing hormone levels increased by 184.7%, testosterone levels increased by 114.3%, and estradiol levels increased by 71.7%.

Bone mineral density (BMD) remained stable and serum cholesterol and triglycerides changed only slightly during the study period, which is in contrast to castration. From baseline to week 25, BMD in the femoral neck and forearm increased by 0.4% and 0.6%, respectively. It decreased by 0.02% and 0.3% in the trochanter and spine (L1-L4), respectively. Total cholesterol and triglycerides rose by 4.6% and 6.5%, respectively.

Adverse events (AEs) occurred in 60 patients (89.6%). Fifty-three patients (79.1%) experienced drug-related AEs. Serious AEs (including one death) occurred in five patients (7.5%). There were no drug-related serious AEs. The most common treatment-emergent AEs were gynecomastia and fatigue, which developed in 24 and 23 patients (35.8% and 34.3%), respectively.