XTANDI® (enzalutamide) for the treatment of prostate cancer

On August 31, 2012, the U. S. Food and Drug Administration approved enzalutamide (XTANDI® Capsules, Medivation, Inc. and Astellas Pharma US, Inc.), for the treatment of patients with metastatic castration-resistant prostate cancer who have previously received docetaxel. 

The approval was based on a single randomized, placebo-controlled, multicenter trial enrolling 1199 patients with metastatic castration-resistant prostate cancer who had received prior docetaxel.  Patients were randomly allocated to receive enzalutamide 160 mg orally once daily (N = 800) or placebo (N = 399).  Study treatment continued until disease progression, initiation of new systemic antineoplastic treatment, unacceptable toxicity, or withdrawal.  Patients were required to continue androgen deprivation therapy and were allowed, but not required, to continue or initiate glucocorticoids during the study period.  Forty-eight percent (48%) of patients on enzalutamide and 46% on placebo received glucocorticoids. 

The primary efficacy endpoint was overall survival (OS).  At the pre-specified interim analysis after 520 events, a statistically significant improvement in OS [HR 0.63 (95% CI: 0.53, 0.75), p < 0.0001, log rank test] was observed.  The median OS was 18.4 and 13.6 months in the enzalutamide and placebo arms, respectively.

The most common (≥5%) grade 1-4 adverse reactions included asthenia or fatigue, back pain, diarrhea, arthralgia, hot flush, peripheral edema, musculoskeletal pain, headache, upper respiratory infection, muscular weakness, dizziness, insomnia, lower respiratory infection, spinal cord compression and cauda equina syndrome, hematuria, paresthesia, anxiety, and hypertension.  Grade 3-4 adverse reactions were reported in 47% of patients treated with enzalutamide and in 53% of those on placebo.

Seizures occurred in 0.9% of patients on enzalutamide.  No patients on the placebo arm experienced seizures.  In the clinical trial, patients experiencing a seizure were permanently discontinued from therapy.  All seizures resolved.  Patients with a history of seizure, taking medications known to decrease the seizure threshold, or with other risk factors for seizures were excluded from the clinical trial.  The safety of enzalutamide in patients with predisposing factors for seizures is unknown. 

The recommended dose and schedule for enzalutamide is 160 mg orally once daily.


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