Chronic (60-Week) Toxicity Study of DUROS Leuprolide Implants in Dogs

Abstract

The toxicity and pharmacodynamics of leuprolide acetate delivered from subcutaneously implanted DUROS leuprolide implants were examined in sexually mature male beagle dogs. The DUROS leuprolide implant is a sterile, nonpyrogenic, nonerodible, single-use, implantable, osmotically driven, drug delivery system for the palliative treatment of advanced prostate cancer. It contains 65 mg of leuprolide and is designed to deliver leuprolide continuously at a nominal rate of 120 μg per day for at least 12 months. Serum drug and testosterone concentrations were compared to values from dogs receiving monthly intramuscular injections of Lupron Depot 3.75 mg or no treatment (sham-operated). The local tissue response induced by subcutaneously implanted DUROS leuprolide implants was also evaluated. Beagles were implanted with a DUROS leuprolide implant for 52 weeks followed by removal and implantation of a new DUROS leuprolide implant for an additional 8 weeks. No mortality or morbidity occurred in this study. No treatment-related changes occurred in mean body weights, blood chemistry, or hematology during the study. Treatment-related atrophy of the testes, epididymides, and prostate gland, consistent with the known pharmacological effects of the drug, was observed in all dogs receiving the DUROS leuprolide implant or the Lupron Depot. Histological examination of these organs showed no distinguishable difference between dogs treated with the DUROS leuprolide implant or Lupron Depot. Weekly serum samples from dogs with DUROS leuprolide implants indicated continuous leuprolide delivery over 60 weeks, while some samples from the Lupron Depot group fell below measurable concentrations. Analysis of serum samples collected every 28 days just before Lupron Depot injection showed that 80 % of these samples had leuprolide concentrations below the limit of quantitation (0.1 ng/ml). Serum testosterone concentrations were below castrate levels (< 50 ng/dl) by 4 weeks after implantation of DUROS leuprolide implant and remained so for the duration of the study. Lupron Depot 3.75 mg also effectively lowered serum testosterone concentrations, but required reinjection every 28 days. All local tissue reactions to the DUROS leuprolide implant at implant sites were classified as mild following macroscopic examination. Microscopic site scores were mild to moderate. The DUROS leuprolide implant was shown to be safe, to provide continuous leuprolide delivery, and to effectively lower serum testosterone concentrations below castrate levels.