Osmotic delivery systems

For osmotic drug delivery systems, Eq. (2) is of critical importance. This equation demonstrates that the quantity of water that can pass a semipermeable film is directly proportional to the pressure differential across the film as measured by the difference between the hydrostatic and osmotic pressures. Osmotic delivery systems are generally composed of a solid core formulation coated with a semipermeable film. Included in the core formulation is a quantity of material capable of generating an osmotic pressure differential across the film. When placed in an aqueous environment, water is transported across the film. This transported water in turn builds up a hydrostatic pressure within the device which leads to expulsion of the core material through a suitably placed exit port. 

GM Zentner, GA McClelland, SC Sutton. Controlled porosity solubility- and resin-modulated osmotic delivery systems for release of diltiazem hydrochloride. J Controlled Release 16 237-244, 1991. 

Dong, L., V fong, P, and Espinal S., L-OF S)BARDCAP a new osmotic delivery system for controlled release of liquid formulation, poster presentat28tfi International Symposium of Controlled Release of Bioactive Materials, San Diego, June, 2001. 

Osmotic pressure, a colligative property, depends on the concentration of solute (neutral molecule or ionic species) that contributes to the osmotic pressure. Solutions of different concentrations having the same solute and solvent system exhibit an osmotic pressure proportional to their concentrations. Thus a constant osmotic pressure, and thereby a constant influx of water, can be achieved by an osmotic delivery system that results in a constant release rate of drug. Therefore, zero-order release, which is important for a controlled release delivery system when indicated, is possible to achieve using these platforms. In 1974,... 

The elementary osmotic delivery system consists of an osmotic core containing drug and, as necessary, an osmogen surrounded by a semiper-meable membrane with an aperture (Fig. 7.1). A system with constant internal volume delivers a volume of saturated solution equal to the volume of solvent uptake in any given time interval. Excess solids present inside a system ensure a constant delivery rate of solute. The rate of delivery generally follows zero-order kinetics and declines after the solute concentration falls below saturation. The solute delivery rate from the system is controlled by solvent influx through the semiper-meable membrane. 

Figure 7.1 Schematic diagram of elementary osmotic delivery system.

The major formulation components of a typical osmotic delivery system include drug, osmotic agents, and a semipermeable membrane. 

Based on their design and the state of active ingredient, osmotic delivery systems can be classified as follows ... 

Figure 7.7 Classification of osmotic delivery systems types I and II.

Figure 7.8 Osmotic delivery system for delivery of a liquid active agent.

The invention that positioned osmosis as a major driving force for controlled drug delivery was the elementary osmotic delivery system. ALZA has developed elementary osmotic delivery systems under the name OROS. A successful modification that overcame the disadvantages of the elementary osmotic pump was the Push-Pull osmotic drug delivery system. The following sections are devoted to the principal features of these systems. 

Push-Pull osmotic delivery system (ALZA Corp.) Multilayered tablet for drugs with low to high solubility... 

Theeuwes, F., Swanson, D. R., Guittard, G., et al. Osmotic delivery systems for the beta-adrenoceptor antagonists metoprolol and oxprenolol Design and evaluation of systems for once-daily administration. Br. J. Clin. Pharmacol. 19 (suppl 2) 69S-76S, 1985. 

Such an osmotic delivery system is capable of providing not only a prolonged zero-order release but also a delivery rate much higher than that achievable by the solution-diffusion mechanism. 

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