Internal & External Suspensions

Question 1

Define what a suspension is and be aware of the medicines in which they are used

Answer 1

• A suspensions is defined as a mixture in which particles are dispersed throughout the bulk of a fluid.
• A coarse suspension is cloudy with particles greater than 1 micrometer in size
• A colloidal suspension is more translucent with particles less than 1 micrometer in size

Question 2

Know different types of suspensions and their applications

Answer 2

Oral suspensions

• antiacids for the stomach e.g. Gaviscon
• digestive treatments for the gut e.g. Kaolin mixture BP
• antifungals for the mouth, throat or gut e.g Nystan suspension

Drops and sprays for the eyes, ears and nose
- some products such as tetracycline hydrochloride can have an oil as the vehicle

External lotions and creams
- e.g calamine lotion BP forming a thin coating of solid on the skin

Intramuscular or subcutaneous injections
- prolongs release of the drug if not fully dissolved in solution

Inhalers

• magnesium carbonate can act as a mineral to absorb volatile substances for inhalation
• e.g. menthol and eucalyptus

Question 3

Understand common excipients of suspensions and their functions

Answer 3

WETTING AGENTS

• reduce interfacial tension between indiffusible solid particles they can also charge up the particles so that when they collide they repel and bounce off each other and do not stick together
• the function of wetting agents is not related to stokes law unlike the other two excipients

Hydrophobic colloids
- e.g. acacia and tragacanth
- care should be taken as these colloids can promote deflocculations
Surfactants - used at concentrations below critical Michelle concentrations
- e.g. polysorbates in internal preparations
- e.g. sodium Laurent sulfate in external preparations
Solvents
- e.g. ethanol and glycerol

SUSPENDING AGENTS
- also known as thickeners, reduce the velocity of particles in the suspension principally by increasing the viscosity of the liquid, which in turn reduces the number of collisions of the particles. Some examples below include long polymer molecules which entangle the components in the liquid and suppress their diffusion. As such, suspending agents exploit the velocity component of stokes law

Natural polysaccharide (gums)
- often used in extemporaneous dispensing
- e.g. acacia tragacanth, xanthan
Semi-synthetic polysaccharide (cellulose derivatives)
- e.g sodium carboxymethylcellulose
Clays
- naturally occurring inorganic minerals
- e.g. bentonite - external use only
Synthetic thickness
- e.g. carbomer
Gelatin and glycerol

DENSITY MODIFIERS 
reduce the difference in density between particles and the liquid, which means that when particle do flocculate they sediment more slowly. As such, density modifiers exploit the gravity component of Stoke’s law.
Sugars
-e.g. sucrose or dextrose 
Sugar alcohols 
- e.g. sorbitol or mannitol

Question 4

Describe the stability and packaging of different formulations

Answer 4

Special storage conditions because they are less stable than solutions
- e.g keep in the fridge

The need for fresh reconstitution because they are too unstable to ship in their final form
- e.g. erythromycin oral suspension (limited to 14 days expiry date after reconstitution)

Communication of specific instructions for the patient
SHAKE WELL BEFORE USE
NOT TO BE TAKEN ORALLY or FOR EXTERNAL USE ONLY - external suspensions only

Degradation process taking place in the liquid that are similar to those in solutions:

• bacterial growth
• hydrolysis of the drug

Limitation of the shelf life to 6months after opening rather than expiry date if there is no preservative

• e.g. menthol and eucalyptus inhalation BP
• safety of the product cannot be guaranteed after this period due to possible microbial contamination

Degradation from sedimentation of particles, bacterial growth and hydrolysis of drug can be minimised by reducing exposure of the medicine to light and heat, which can improve its shelf life . This can be achieved through the appropriate storage conditions and packaging:
- plain amber glass/plastic bottles
These reduce degradation of the product by light
- click-locks
- spoon vs syringe
- may be available as measured doses in sachets
e.g. calpol sachets for potting pain relief

Question 5

Discuss similarities and differences between different types of suspensions

Answer 5

A

Question 6

Discuss advantages and disadvantages of the use of suspensions

Answer 6

Advantages

• applications for insoluble drugs and suppression of hydrolysis
• mask the taste of drug compared with solutions to enhance palatability (internal)
• easier to swallow for children and elderly (internal)
• fast absorption from GIT (“)
• use in controlled release (injections)
• deliver drug in finely divided form with high surface area (inhalers)

Disadvantages

• difficult to manufacture
• not very convenient for the patient
• must shake to redisperse or even reduce the viscosity of the liquid
• can separate, sediment or look unattractive
• less stable than solutions
• less accurate dosing than tablets

Question 7

Suspension properties

Answer 7

• slow sedimentation
• easy re-dispersion
• acceptable viscosity
• stable changes in temperature and pH

Difficult to achieve so necessary to compromise on balance between factors