By Dr. Shruti Bhat
In these systems, osmotic pressure provides the driving force to generate controlled release of drug. These systems generally appear in 2 different forms. The first contains the drug as a solid core together with electrolyte, which is dissolved by the incoming water. The electrolyte provides the high osmotic pressure difference. The second system contains the drug in solution in an impermeable membrane within the device. The electrolyte surrounds the bag. Both systems have single or multiple holes bored through the membrane to allow drug release.
In systems with solid drug dispersed with electrolyte, the size or membrane of bored hole (s) are the rate limiting factors for release of drug; since any variations in boring of the hole, accomplished with a laser device, can have a substantial effect on release characteristics. Most of the orally administered osmotic systems, are of this variety e.g. OROS (Acutrim) by Alza Corp. Inc. A variation on this theme is an osmotic system of similar design without a hole. The building osmotic pressure causes the tablet to burst, causing the entire drug to be rapidly released. This design is useful for drugs that are difficult to formulate in tablet or capsule form.
The osmotic systems are advantageous in that they can deliver large volumes. Most important, the release of drug is in theory independent of the drug’s properties. This allows one dosage form design to be extended to almost any drug. Disadvantages are that the systems are relatively expensive and are inappropriate for drugs unstable in solution.
V) ION-exchange SYSTEMS:
Ion-exchange systems generally use resins composed of water-insoluble cross-linked polymers. These polymers contain self-forming functional groups in repeating positions on the polymer chain. The drug is bound to the resin and released by exchanging with appropriately charged ions in contact with the ion-exchange groups.
The rate of drug diffusing out of the resin is controlled by the area of diffusion, diffusional path length and rigidity of the resin, which is a function of the amount of cross-linking agent used to prepare the resin. This system is advantageous for drugs that are highly susceptible to degradation by enzymatic processes, since it offers a protective mechanism by temporarily altering the substrate. This approach to sustained release, however, has the limitations that the release rate is proportionate to the concentration of the ions present in the area of administration. Although the ionic concentration of the GI tract remains more or less constant, the release rate of drug can be affected by variability in diet, water intake and individual intestinal content.
An improvement in this system is to coat the ion-exchange resin with a hydrophobic rate-limiting polymer, such as ethyl cellulose or wax. These systems rely on polymer coat to govern the rate of drug availability.
vi) PRODRUG APPROACH:
The applications of the classical pro-drug approach in the design of oral sustained drug delivery forms has been limited due to various toxicological considerations. However, theophylline, a fairly water soluble compound with good bioavailability having short biological half life and narrow therapeutic range (10 - 20 um /ml in plasma) when given orally but makes plasma concentration monitoring essential. In an effort to overcome these shortcomings, several sustained release products of theophylline have been designed.
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