An in vivo/in vitro comparison with a leuprolide osmotic implant for the treatment of prostate cancer

An osmotically driven implantable system was designed and characterized for the delivery of leuprolide over a year-long duration. Leuprolide has been used in the treatment of prostate cancer since the 1980s. The DUROS ® implant consists of a titanium alloy cylinder, measures 4 mm in diameter by 45 mm in length and holds approximately 150 μl of formulation. Stability studies indicated that leuprolide was stable, as a solution formulation in DMSO, for several years at 37°C. In vitro release rate testing, at weekly intervals, showed zero-order delivery for 1 year. DUROS implants demonstrated release rates that were reproducible and similar to one another after storage at 25°C for 18 months prior to testing. In vivo studies, with implants placed subcutaneously, revealed delivery rates comparable to those observed under in vitro conditions. Leuprolide stability was also comparable between in vivo and in vitro conditions. Steady leuprolide serum levels produced by the implant resulted in the desired pharmacodynamic efficacy endpoint of testosterone suppression, both in canines and in humans. The good agreement between in vivo/in vitro delivery rates was as expected for a delivery system based on the principles of osmosis.