Data support the use of enzalutamide instead of bicalutamide in patients with asymptomatic or mildly symptomatic metastatic castration-resistant prostate cancer (mCRPC), researchers concluded.

The findings are from the TERRAIN trial, which showed that enzalutamide use was associated with significantly longer median progression-free survival compared with bicalutamide.

Enzalutamide is an oral androgen-receptor inhibitor approved for patients with mCRPC. Bicalutamide is a non-steroidal oral anti-androgen that is approved for use in conjunction with luteinizing hormone-releasing hormone (LHRH) analogues in men with hormone-treatment naïve prostate cancer. “Despite the widespread use of bicalutamide, evidence of its clinical benefits is scarce in the context of castration-resistant prostate cancer,” Neal D. Shore, MD, of Carolina Urologic Research Center in Myrtle Beach, S.C., and colleagues stated in an online report in Lancet Oncology.

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TERRAIN, which the investigators believe is the first head-to-head trial comparing the drugs, was a double-blind phase 2 study of asymptomatic or minimally symptomatic mCRPC patients with cancer progression on androgen-deprivation therapy (ADT). Investigators randomly assigned 375 patients to receive either enzalutamide (184 patients) or bicalutamide (191 patients). Of these, 126 (68%) and 168 (88%), respectively, discontinued their assigned treatment prior to the end of the study, mainly because of progressive disease.

The median follow-up time was 20 months in the enzalutamide group and 16.7 months in the bicalutamide group. The enzalutamide recipients had significantly improved median progression-free survival (PFS)—the study’s primary endpoint—compared with those in the bicalutamide arm (15.7 vs. 5.8 months). This difference in PFS translated into a significant 56% decreased risk of disease progression in the enzalutamide arm.

Of the most common adverse events, those occurring more frequently in the enzalutamide than bicalutamide group were fatigue, back pain, and hot flushes; those occurring more frequently in the bicalutamide than enzalutamide arm were nausea, constipation, and arthralgia. Serious adverse events were reported by 57 (31%) of 183 patients in the enzalutamide arm and 44 (23%) of 189 patients in the bicalutamide arm.

Data from the TERRAIN trial show that enzalutamide is a more effective treatment than bicalutamide for patients with metastatic prostate cancer who progress on androgen-deprivation therapy, and are consistent with the superior activity of enzalutamide versus bicalutamide in preclinical models of mCRPC, thus continuing to show the importance of inhibiting androgen receptor signaling in mCRPC patients, the researchers stated.

The results from the TERRAIN trial confirm and extend the results of enzalutamide  activity in the phase 3 PREVAIL trial in chemotherapy-naïve patients and in the AFFIRM trial in patients who had received docetaxel.

“These data support the role of enzalutamide use in patients with asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer who had disease progression while on ADT with an LHRH agonist or antagonist, or after bilateral orchiectomy,” the authors wrote.