ABSTRACT
In this chapter we review the theory, design, and performance of asymmetric membrane filmcoated systems that function by an osmotic drug release mechanism.
Osmotic drug delivery devices are composed of an osmotically active, drug-containing core dosage form surrounded by a rate-controlling, semipermeable membrane. The literature con tains over 240 patented osmotic drug delivery systems, including variations in hydrogel driving layers (1). Osmotic drug delivery differs from diffusional-based systems in that the delivery of the active agent is driven by an osmotic gradient rather than by the concentration of drug in the device per se. Through the use of a semipermeable membrane and osmotic excipients, the rate of release of the drug can be made independent of such variables as pH and agitation rate. In addition, when properly designed, osmotic systems exhibit zero-order release of the drug be cause the drug content in the system (which is continuously depleting) does not affect the os motic gradient. More sophisticated designs use a separate osmotic driving element (“push-pull” designs) or additional features, and can deliver the drug at increasing rates, introduce a delay before release begins, or achieve other release profiles (2). The design-performance character istics of osmotic systems (pH and agitation-independence, reliable delivery profile) contribute to the robust clinical performance that is often observed with osmotic systems, including achievement of highly constant drug plasma concentrations, lack of a food effect, and, fre quently, an in vitro!in vivo correlation (3-6).
