Suspensions

Definition and Classification of Pharmaceutical Suspensions

• Definition of a Pharmaceutical Suspension: A pharmaceutical suspension is a preparation containing finely divided drug particles distributed throughout a vehicle in which the drug exhibits minimal solubility.

• Classification by Particle Size:

  • Coarse Suspension: Defined as a suspension where the particles have a mean diameter of > 1\,\mu m.

  • Colloidal Suspension: Defined as a suspension where the particles have a mean diameter of < 1\,\mu m.

Objectives and Learning Outcomes

• Definition: Define what constitutes a pharmaceutical suspension.

• Evaluation: Explain the various advantages, disadvantages, and the key properties that define a "good" suspension.

• Excipient Functionality: Explain the specific roles of common excipients in maintaining the stability of a suspension, including:

  • Wetting agents.

  • Suspending agents.

  • Flocculating agents.

• System Comparison: Explain the fundamental differences between flocculated and deflocculated suspension systems.

Advantages and Disadvantages of Suspensions

• Advantages:

  • Low Solubility Solution: They are useful for therapeutic agents that possess low solubility in common vehicles.

  • Taste Masking: They may be formulated specifically to mask the unpleasant taste of certain therapeutic agents.

  • Patient Compliance: They may be administered to patients who have difficulty swallowing solid dosage forms (e.g., pediatric or geriatric patients).

  • Controlled Release: They may be formulated to provide controlled drug delivery over time.

• Disadvantages:

  • Bulkiness: They may be bulky and difficult for a patient to carry.

  • Instability: They are considered fundamentally unstable systems because the drug particles are not dissolved are not dissolved and will tend to settle over time due to gravity.

Properties of a High-Quality Suspension

• Dosing Accuracy: The suspension must remain uniform long enough for accurate dosing after the container has been shaken.

• Redispersibility: Any sediment formed during storage must redisperse easily with gentle shaking.

• Pourability: The preparation must flow easily to allow for pouring or removal from the container.

• Product Elegance: The suspension should contain small, uniform particles to ensure a smooth and elegant final product.

• Particle Size Control: Particle size is a critical parameter as it affects:

  • The rate of sedimentation.

  • The texture of the product.

  • The ease of use for the patient.

  • The bioavailability of the drug.

The Role of Wetting Agents

• Mechanism of Action: To allow particles to disperse easily in a liquid, the solid-liquid interfacial tension must be reduced. This allows the liquid to replace air on the particle surface. Wetting agents specifically reduce this interfacial tension, allowing air trapped in the crevices of particles to be displaced by the liquid vehicle.

• Examples of Wetting Agents:

  • Hygroscopic Liquids: Glycerin and Propylene glycol.

  • Hydrophilic Colloids: Acacia, Bentonite, and Xanthan gum.

  • Surface-Active Agents (Surfactants): These should typically have an HLB (Hydrophile-Lipophile Balance) value between $7-9$.

Suspending and Thickening Agents

• Purpose:

  • To increase the viscosity of the dispersion medium.

  • To prevent the rapid settling of particles.

  • Constraints: They should not render the suspension too viscous to agitate or pour, and they should not interfere with the availability of the drug.

• Types and Examples:

  • Natural Polysaccharides: Tragacanth powder, Acacia gum, Starch, Agar, and Sodium alginate.

  • Water-Soluble Celluloses: Methyl cellulose and Sodium carboxymethyl cellulose.

  • Clays: Bentonite and Aluminium magnesium silicate.

Miscellaneous Excipients

• Preservatives: Used to prevent microbial growth.

• Flavourings: Used to improve the palatability of the preparation.

• Sweeteners: Used to mask bitterness or improve taste.

• Colorants: Used for aesthetic purposes and product identification.

Comparison of Deflocculated and Flocculated Suspensions

• Deflocculated Suspensions:

  • Sedimentation Rate: Features a slow sedimentation rate.

  • Storage Behavior: Over time, the sediment forms a compact "cake" during storage (Stage A: After a few minutes; Stage B: After several hours; Stage C: After prolonged storage).

  • Redispersion: Once particles settle and form a cake, they are very difficult to redisperse.

• Flocculated Suspensions:

  • Sedimentation Rate: Features a fast sedimentation rate.

  • Sediment Structure: Forms a loose sediment (floccules) that is easily redispersed by shaking.

  • Dosing Risk: There is a risk of inaccurate dosing if the container is not shaken well before use.

  • Flocculating Agents: These states are achieved through the use of flocculating agents such as electrolytes, polymers, or surfactants.