The in vitro and in vivo performance of an osmotically controlled delivery system — IVOMEC SR®1®Registered Trademark of Merck and Co.1 bolus

Osmotic micropumps for drug delivery

This paper reviews miniaturized drug delivery systems applying osmotic principles for pumping. Osmotic micropumps require no electrical energy and consequently enable drug delivery systems of smallest size for a broad field of new applications. In contrast to common tablets, these pumps provide constant (zero-order) drug release rates. This facilitates systems for long term use not limited by gastrointestinal transit time and first-pass metabolism. The review focuses on parenteral routes of administration targeting drug delivery either in a site-specific or systemic way. Osmotic pumps consist of three building blocks: osmotic agent, solvent, and drug. This is used to categorize pumps into (i) single compartment systems using water from body fluids as solvent and the drug itself as the osmotic agent, (ii) two compartment systems employing a separate osmotic agent, and (iii) multi-compartment architectures employing solvent, drug and osmotic agent separately. In parallel to the micropumps, relevant applications and therapies are discussed.

Critical variables associated with nonbiodegradable osmotically controlled implants

Osmotically controlled implants yield precise zero-order drug delivery kinetics and are utilized in a number of applications. The implants deliver drugs for extended periods (weeks to years) and exhibit good in vivo/in vitro correlation. This paper reviews critical variables associated with these implants, with a focus on release rate testing. The extended-duration kinetics can be problematic when attempting to test for >70% cumulative delivery. An innovative scheme based on the scientific principles of operation of the system is described to ensure > 70% delivery at the target rate and duration for the DUROS Viadur (leuprolide acetate) implant.

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 microl of formulation. Stability studies indicated that leuprolide was stable, as a solution formulation in DMSO, for several years at 37 degrees 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 degrees 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.

Intraruminal devices

Intraruminal boluses are drug delivery devices designed to provide long term delivery of drug to the reticulo-rumen compartments of ruminant animals. This cavity is the site of the bacterial fermentation and breakdown of foodstuffs and represents an ideal site for the long term delivery of drugs and nutrients to these grazing animals. Often such animals are 'turned out' in the spring to spend much of the spring, summer and autumn seasons grazing open range. As such, the administration of drugs during the grazing season is complicated by the need to first retrieve the animals prior to drug administration. Long term delivery of drug circumvents this problem providing for decreased costs and improved efficiencies for the farmer. Numerous types of delivery devices have been developed to meet this need. This review summarizes the various approaches that have been developed, provides information on the critical design features and reviews the relevant anatomy and physiology of these animals. Some comments are provided regarding needed future developments for this class of drug delivery system.