Sustained release dosage forms are designed to release a drug at a predetermined rate by maintaining a constant drug level for a specific period of time with minimum side effects. This can be achieved through a variety of formulations, including liposomes and drug-polymer conjugates (commonly referred to as hydrogels). It is the definition for the controlled release dosage form rather than sustain release.
INTRODUCTION TO SUSTAINED-RELEASE DOSAGE FORMS
A sustained-release dosage form is designed to maintain constant levels of a drug in the patient's bloodstream by releasing the drug over an extended period. Maintaining constant blood levels of the drug in the bloodstream increases the therapeutic effectiveness of the drug
ADVANTAGES
One advantage of sustained-release dosage forms is that medication must be administered less often than other dosage forms. Another advantage is that it reduces fluctuations of drug concentration in the blood. (See figures 6-2 and 6-3 for a graphic illustration of this principle.) Thus, the patient is not repeatedly subjected to amounts of the drug which are less than adequate or more than adequate. Nor does the blood chemistry undergo repeated chemical imbalances, which might be detrimental to the patient's health.
One advantage of sustained-release dosage forms is that medication must be administered less often than other dosage forms. Another advantage is that it reduces fluctuations of drug concentration in the blood. (See figures 6-2 and 6-3 for a graphic illustration of this principle.) Thus, the patient is not repeatedly subjected to amounts of the drug which are less than adequate or more than adequate. Nor does the blood chemistry undergo repeated chemical imbalances, which might be detrimental to the patient's health.
DISADVANTAGES
There are several disadvantages and limitations of this form of dosage.
There are several disadvantages and limitations of this form of dosage.
Sustained-release dosage forms are more expensive to produce. Variation of the size
of the dose for a particular individual is difficult or impractical. Once the medication has
been administered, it cannot be discontinued until its effects have finally ceased. In
addition, some drugs are not suitable for preparation in this form
MODES OF OPERATION
The initial dose of sustained-release medication provides sufficient drug at the time it is administered to produce the desired therapeutic effect. Thereafter, as the drug is removed from the body, more drugs, the sustained-release, must be supplied to keep a constant level in the blood. There are various means of achieving this.
The initial dose of sustained-release medication provides sufficient drug at the time it is administered to produce the desired therapeutic effect. Thereafter, as the drug is removed from the body, more drugs, the sustained-release, must be supplied to keep a constant level in the blood. There are various means of achieving this.
a. Capsules. The original medication in this category is a capsule containing a number of small pellets. A full and immediate dose is provided to the bloodstream from these capsules by uncoated pellets that dissolve quickly. Other pellets, coated with varying thicknesses of a slowly soluble aterial, supply the bloodstream with enough drug to keep the concentration at the desired level. In addition, ion exchange resins have been used in some sustained-release capsules to release drugs at the proper intervals.
b. Tablets. In addition to capsules, some tablets provide sustained release.
(1) Granules. Some sustained-release tablets are made by compressing granules with varying coats into a tablet. The coats dissolve at different times, much like those in the capsules discussed above.
(2) Slowly soluble core. Some tablets are manufactured with an outer coating that dissolves quickly to give the initial dose. The core of this tablet, consisting of a slowly soluble base containing additional drug provides the sustained release. It is possible to achieve the same effect with layered tablets, which are often flat and cylindrical to give a constant sustained dose.
(3) Insoluble matrix. This tablet is an insoluble network of channels containing the drug. Enough drug is immediately extracted from the outer channels to provide the initial dose. Thereafter, the drug is extracted from the inner channels of the tablet less rapidly, producing the sustained release.
(4) Ion exchange resins. Ion exchange resins are sometimes used in tablets to provide sustained release.
(5) Repeat-action tablets. Repeat-action tablets are not really sustained-release tablets. They consist of an outer layer that dissolves quickly and provides the initial dose. Within is another tablet with a special coating that dissolves in about four hours and releases another full dosage. It is useful in avoiding the necessity of waking in the middle of the night to take a second tablet
Benefits
- for short half-life drugs, sustained release can mean less frequent dosing and thus better compliance.
- reduce variations in plasma/blood levels for more consistent result.
Problems
- More complicated formulation, may be more erratic in result. A sustained release product may contain a larger dose, i.e. the dose for two or three (or more) 'normal' dosing intervals. A failure of the controlled release mechanism may result in release of a large toxic dose.
- more expensive technology
Types of products
- erosion tablets
- waxy matrix
- matrix erodes or drug leaches from matrix
- coated pellets
- different pellets (colors) have different release properties
- coated ion exchange
- osmotic pump
- insoluble coat with small hole. Osmotic pressure pushes the drug out at a controlled rate.
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