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Seventeen patients with advanced prostate cancer were studied to evaluate the pharmacokinetics and pharmacodynamics of a hydrogel implant designed to deliver histrelin at a constant rate (50 μg/d) for 1 year. Serum histrelin levels were collected during the 52-week implantation period and after a second implant. Testosterone suppression was the primary pharmacodynamic endpoint, with treatment success defined as serum testosterone less than 50 ng/dL. The histrelin sub-dermal implant delivered constant histrelin levels, with mean serum histrelin of approximately 0.265 ng/mL over 52 weeks. At the end of 52 weeks, mean histrelin concentrations were 0.128 ± 0.0652 ng/mL. Patients achieved chemical castration (testosterone less than 50 ng/mL) by week 4. In patients who had the first implant removed and received a new implant at the end of the first 52 weeks, testosterone suppression was not interrupted. The hydrogel implant provided consistent delivery of histrelin over 1 year and effectively suppressed testosterone in men with prostate cancer.