Pharmaceutical Suspensions

🧪 Disperse Systems Overview

Disperse systems consist of two components:

• Dispersed phase (internal phase): could be molecules, particles, or droplets.

• Continuous phase (external phase): the medium in which the dispersed phase is distributed.

🔬 Classification of Disperse Systems

💊 Pharmaceutical Suspensions

Suspensions are heterogeneous mixtures where insoluble particles are dispersed in a liquid.

🔄 Differences: Solutions vs Suspensions

📦 Classification of Suspensions

1. By Route of Administration

• Oral: Paediatric formulations, antacids (e.g. amoxicillin)

• Topical: Calamine lotion

• Parenteral: Procaine penicillin G, vaccines (must be sterile)

• Inhalation: Asthma treatments (fine particles)

• Ophthalmic: Sterile, small particle size

2. By Solid Concentration

• Dilute suspensions (2–10% w/w)
  E.g. Prednisolone acetate eye drops

• Concentrated suspensions (up to 50% w/w)
  E.g. Zinc oxide topical formulations

3. By Particle Size

📈 Bioavailability (Absorption Rate) Order

From slowest to fastest:

1. Coated tablets

2. Compressed tablets

3. Capsules

4. Suspensions

5. Solutions

💡 Pharmaceutical Uses of Suspensions

• To deliver poorly soluble drugs (e.g. Prednisolone acetate)

• To enhance stability (e.g. Oxytetracycline)

• To mask taste (e.g. Chloramphenicol palmitate)

• For topical use (e.g. Calamine lotion)

• In injections/vaccines (e.g. Cholera vaccine)

• In X-ray contrast agents (e.g. Barium sulphate)

✅ Pros and Cons of Suspensions

✅ Advantages

• Useful for insoluble/stable drugs

• Palatable for paediatric use

• Easy to swallow

• Higher drug loading

• Allow modified/sustained drug release

❌ Disadvantages

• Dosing accuracy may be poor

• Requires shaking before use

• Bulky, risk of breakage/leakage

• Storage (may need refrigeration)

⚗ Properties of an Ideal Suspension

• Slow sedimentation

• Easily redispersible after settling

• Pourable and smooth

• Uniform particle size to avoid grittiness

• Stable appearance

🧱 Components of Suspensions

Types of Dispersed Solids

a) Diffusible powders

• Light and evenly suspended

• No suspending agent required

• Examples: Light Kaolin BP, Magnesium Trisilicate BP

b) Indiffusible powders

• Heavy, settle quickly

• Need a suspending agent

• Examples: Calamine BP, Zinc Oxide BP, Aspirin BP

⚠ Suspension Challenges

1. Sedimentation

• Particles settle due to gravity

• Controlled by suspending agents and viscosity control

2. Caking

• Formation of a non-redispersible layer

• Avoided by creating flocculated systems

🧪 Suspending Agents

Purpose: Maintain particles in suspension by:

• Preventing aggregation

• Increasing viscosity

Examples:

• Natural gums (acacia)

• Cellulose derivatives (HPMC)

• Clays (bentonite)

Note: Must balance between:

• Sedimentation rate

• Pourability

• Drug release rate

⚙ Particle Size Considerations

• < 5 µm: Avoid grittiness

• > 25 µm: May block needles

• Too small: Risk of hard cake formation

🍭 Other Formulation Components

• Flavours: Improve palatability

• Preservatives: Prevent microbial growth (e.g. Benzoic acid)

• Colours

• Sweeteners: e.g. Sucrose, Glycerol

• Packaging: Amber bottles (oral), fluted (topical/eye/ear)

🏷 Labelling & Storage

• Label: "Shake well before use", "For external use only", etc.

• Storage: Refrigeration may be required

• Transport: Avoid breakage/leakage

📊 Quality Control

• Content uniformity (85–115% of label claim)

• Preservative efficacy

• Shelf life: ≥ 90% active after expiry

• Appearance, pH, viscosity