Long-term efficacy and tolerability of once-yearly histrelin acetate subcutaneous implant in patients with advanced prostate cancer

Objective: To evaluate the efficacy and tolerability of a once-yearly histrelin implant during an open-label extension of a pivotal study.

Patients and methods: Men with advanced prostate cancer and a clinical response to 52 weeks of treatment with the histrelin implant. Implants were placed annually. The primary efficacy variable was achievement of serum testosterone levels of ≤50 ng/dL. Secondary efficacy variables were disease progression, analgesia use, performance status and tolerability of therapy.

Results: Of 104 patients enrolled, over 70% received three consecutive histrelin implants, and the longest, single treatment period was greater than 4 years. Serum testosterone was consistently suppressed below 50 ng/dL in all patients and mean testosterone levels were 13.1, 14.8 and 10.8 ng/dL after 104 weeks (year 2), 156 weeks (year 3) and 208 weeks (year 4) of treatment, respectively. Most patients showed no clinical worsening of their disease, were able to continue normal daily activities, and did not require analgesic medication during the extension period. Mean (SD) time to place the histrelin implant was 4.5 (6.2) min, with only three patients having insertions that were considered difficult. Adverse events were reported in 100 (96.2%) patients. The eight deaths and 28 (26.9%) serious adverse events were judged as unrelated to the study drug. The most commonly reported drug-related adverse events was hot flashes in 67 (64.4%) patients. Most of these cases was judged as mild or moderate.

Conclusions: The once-yearly histrelin implant maintained testosterone suppression for repeated treatment cycles and was generally well tolerated. The histrelin implant provides a clinically attractive option for long-term androgen deprivation therapy in patients with advanced prostate cancer seeking fewer office visits and repeated injections.