Overview Of Pharmaceutical Suspension

A Pharmaceutical suspension is a coarse dispersion in which internal phase is dispersed uniformly throughout the external phase. The internal phase consisting of insoluble solid particles having a specific range of size (0.5 to 5 microns) which is maintained uniformly throughout the suspending vehicle with the aid of a single or combination of suspending agents. The external phase (suspending medium) is generally aqueous in some instance, may be an organic or oily liquid for non oral use.

Pharmaceutical suspension

The term "disperse system" refers to a system in which one substance (the dispersed phase) is distributed, in discrete units, throughout a second substance (the continuous phase). Each phase can exist in a solid, liquid, or gaseous state.

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Most suspensions are classified as a coarse suspension which is a dispersion of particles with a mean diameter greater than 1µm (1 to 100 µm). A colloidal suspension is a dispersion of particles with a mean diameter of less than 1µm (0.5µm to 1µm). Suspensions are not optically clear and will appear cloudy unless the size of the particles is within the colloidal range.

Suspending Agents

Suspending agents are substances that are used to keep finely divided insoluble materials suspended in a liquid media by preventing there agglomeration (coming together) and by imparting viscosity to the dispersion media so that the particles settle more slowly. Most suspending agents perform two functions i.e. besides acting as a suspending agent they also impart viscosity to the solution. Suspending agents form film around particles and decrease interparticle attraction.

A good suspension should have well developed thixotropy. At rest the solution is sufficiently viscous to prevent sedimentation and thus aggregation or caking of the particles. When agitation is applied the viscosity is reduced and provide good flow characteristic from the mouth of the bottle.

Types Of Suspending Agents

1. Natural Agents

This class consists of those from;

a. Animal source e.g. gelatine

b. Plant source e.g. acacia, tragacanth, starch, seaweed (alginates)

c. Mineral sources e.g. bentonite, kaolin

2. Semi-synthetic Agents

These consist of substituted cellulose (minerals) e.g. hydroxyethylcellulose , sodium carboxymethylcellulose, methylcellulose, microcrystalline cellulose

3. Synthetic Agents

They are synthetic polymers e.g. carboxypolymethylene (carbopol), polyvinyl alcohol, polyvinylpyrrolidone iodine complex (PVC)

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Route Of Pharmaceutical Suspension Administration

Suspension dosage forms are given by the oral route, injected intramuscularly or subcutaneously, instilled intranasally, inhaled into the lungs, applied to the skin as topical preparations, or used for ophthalmic or otic purposes in the eye or ear, respectively.

Some suspensions are available in a ready-to-use form that is already distributed through a liquid vehicle with or without stabilizers and other additives. Other preparations are available as a dry powder mixture containing the drug and suitable suspending and dispersing agents intended for reconstitution (diluted and agitated) just before use with a specified quantity of vehicle, most often purified water.

Features Desired in Pharmaceutical Suspension

1. The suspended particles should not settle rapidly and sediment produced, must be easily re-suspended by the use of a moderate amount of shaking

2. It should be easy to pour yet not watery and no grittiness

3. It should have pleasing odour, colour and palatability

4. Good syringeability

5. It should be physically, chemically and microbiologically stable

6. Parenteral/Ophthalmic suspension should be sterilizable

7. Suspended particles should settle slowly and the sediment or creaming produced on storage, if any, should readily redisperse upon gentle shaking of the container

8. For creams, it should be watery enough to spread and not leave stain

Applications Of Suspension

1. Suspension is usually applicable for drugs which are insoluble or poorly soluble

2. To prevent degradation of drugs or to improve stability of drugs

3. To mask the bitter taste of an unpleasant drug

4. Suspension of drug can be formulated for topical application

5. Suspension can be formulated for parenteral application in order to control rate of drug absorption

6. Vaccines as an immunizing agent are often formulated as suspension

7. X-ray contrast agents are also formulated as suspension

Various Components Used In Suspension Formulation

1. API: Active drug substances

2. Wetting agents: They are added to disperse solids in a continuous liquid phase

3. Flocculating agents: They are added to floc the drug particles

4. Thickeners: They are added to increase the viscosity of suspension.

5. Buffers and PH adjusting agents: They are added to stabilize the suspension to a desired PH range.

6. Osmotic agents: They are added to aadjust osmotic pressure comparable to biological fluid.

7. Coloring agents: They are added to impart desired color to suspension and improve elegance.

8. Preservatives: They are added to prevent microbial growth

9. External liquid vehicle: They are added to construct structure of the final suspension

Stability Of Suspensions

Factors that contribute to appreciable stability of a suspension include:

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a) Small particle size: reduce the size of the dispersed particle increases the total surface area of the solid. The greater the degree of subdivision of a given solid the larger the surface area. The increase in surface area means also an increase in interface between the solids and liquids leading to an increase in viscosity of a system.

b) Increasing the viscosity: increasing the viscosity of the continuous phase can lead to the stability of suspensions. This is so because the rate of sedimentation can be reduced by increase in viscosity. Viscosity increase is brought about by addition of thickening agents to the external phase. In water these must be either soluble or swell. It is important to note that the rate of release of a drug from a suspension is also dependent on viscosity of a product. The more viscous the preparation, the slower is likely to be the release of a drug. Sometimes this property may be desirable for depot preparations.

c) Temperature: Another factor which negatively affects the stability and usefulness of pharmaceutical suspensions is fluctuation of temperature. Temperature fluctuations can lead to caking and claying.