Pharmaceutical Suspension

A Pharmaceutical suspension is a dispersion in which the Active Pharmaceutical Ingredient (internal phase)is dispersed in the vehicle (external phase).

As per this definition, the solubility of the therapeutic agent in the vehicle is low.

The internal phase consists of insoluble or poorly soluble solid particles. These particles lie in the size range from 0.5-5 µthat is maintained consistently throughout the suspending medium with the aid of a single/combination of the suspending agent. The external phase (suspending medium) is generally aqueous in some instances. It may be an organic or oily liquid for non-oral use.

The reasons to formulate a pharmaceutical suspension:

If the drug is insoluble or poorly soluble in the delivery vehicle.
To mask the bitter or unpleasant taste of the drug.
To increase the stability of the drug.
To achieve a sustained or controlled drug release.

Examples of Pharmaceutical Suspensions

Antacid oral suspensions
Antibacterial oral suspension
Dry powders (antibiotic) for oral suspension
Analgesic oral suspension
Anthelmentic oral suspension
Anticonvulsant oral suspension
Antifungal oral suspension

Classification

Based on General Classes

· Oral suspension – e.g.Antacids, Paracetamol suspension, and Tetracycline hydrochloride.

· Externally applied suspension - e.g. Calamine lotion.

· Parenteral suspension - e.g. Procaine penicillin G, Insulin Zinc Suspension.

Based on the Proportion of Solid Particles

· Dilute suspension (2-10%w/v solid) - e.g.Cortisone acetate, Predinisolone acetate.

· Concentrated suspension (50%w/v solid) - e.g. Zinc oxide suspension.

Based on Electro-kinetic Nature of Solid Particles

· Flocculated suspension

· Deflocculated suspension

Based on Size of Solid Particles

Colloidal suspensions -Particle sizes of suspended solid less than about 1 micron in size.

Coarse suspensions -Particle sizes of greater than about 1micron in diameter.

Nano-suspensions -Biphasic colloidal dispersions of Nano-sized drug particles stabilized by surfactants.

Advantages of Pharmaceutical Suspension

It enhances the chemical stability of some drugs like Procaine penicillin G.
Therapeutic agents present in the suspension gives a higher bioavailability rate as compared to other dosage forms. Solution > Suspension > Capsule > Compressed Tablet > Coated tablet.
Duration and the onset of action can be controlled like Protamine Zinc-Insulin suspension.
It can mask the unpleasant or bitter taste of therapeutic agents like Chloramphenicol.
Pharmaceutical suspensions are a useful drug delivery system for drugs that have a low solubility.
Pharmaceutical suspensions may be employed to administer drugs to patients who have difficulty in swallowing of solid-dosage forms.
Pharmaceutical suspensions may be formulated to provide controlled drug delivery, e.g. as intramuscular injections.

Disadvantages of Pharmaceutical Suspension

Physical stability, sedimentation, and compaction can cause some troubles.
It is bulky sufficient care must be taken during handling and transport and therefore difficult for a patient to carry.
It is difficult to formulate.
Uniform and accurate dose of the therapeutic agent cannot be obtained unless it packed in the unit dosage form.
Pharmaceutical suspensions are essentially unstable. Hence, it requires formulation skill to ensure the physical stability of the formulation is retained over the period of the shelf-life.
The formulation of aesthetic suspension is difficult.

Applications

A suspension is usually suitable for poorly soluble or insoluble drugs E.g. Prednisolone suspension.
To preventdrug degradation or to improve the drug stability. E.g. Oxytetracycline suspension.
To maskthe bitter or unpleasant taste of the drug. e.g. Chloramphenicol palmitate suspension.
It can be formulated for topical application e.g. Calamine lotion.
Vaccines are often formulated as a suspension. e.g. Cholera vaccine.

Features expected in Pharmaceutical suspensions

It should have a low rate of sedimentation it means suspended particles should not settle rapidly.
It should be easy to pour.
It should have good syringeability.
It should be physically, chemically, and microbiologically stable.
In case of parenteral or ophthalmic, it should be sterilizable.
The disperse phase must be easily redispersed with gentle shaking.
The flow properties of the suspension should be such that the formulation to be easily removed from the container.
It should be aesthetically pleasing.