Pharmaceutical Suspensions Overview

Suspensions

• Suspensions are preparations with finely divided drug particles distributed throughout a liquid, typically water, where the drug has minimal solubility.
• Some suspensions are ready-to-use, while others are dry powders that need reconstitution.
• Suspensions can be for oral, topical, or parenteral administration.

Reasons for Suspension Preparation

• Certain drugs are unstable in solution but stable in suspension form, maintaining chemical stability while being administered as a liquid.
• Liquid dosage forms (suspensions) are often preferred over solid dosage forms due to easier swallowing.
• Suspensions can mask unpleasant tastes of some drugs, using insoluble drug particles instead of a solution.

Desirable Features of Suspensions

• Properly prepared suspensions should remain homogeneous for the period between shaking and use.
• The sediment created during storage should redisperse easily after gentle shaking.
• Particle size of the suspended drug should remain constant over time, avoiding crystal growth (physical stability).
• Suspension viscosity shouldn't be too high for easy pouring from the container.

Sedimentation Rate

• Stokes' law equation describes the factors affecting the rate at which particles settle in a suspension.
• Factors influencing sedimentation rate include particle size (larger particles settle faster).
• The density of the particles and medium also impact sedimentation rates, with a higher density difference leading to faster sedimentation.
• Higher viscosity of the medium slows down sedimentation rate.
• Particle size reduction (e.g., dry milling) and controlled viscosity are important factors in suspension stability.

Particle Size Reduction Techniques

• Micropulverization is used to produce fine drug powders (10-50 µm) for suspensions.
• Jet milling (micronization) creates even finer particles (under 10 µm).
• Spray drying is another technique for producing extremely small particles.

Flocculated and Deflocculated Suspensions

• In deflocculated suspensions, particles remain separate, settling slowly and often appearing cloudy.
• Flocculated suspensions involve aggregation of particles into loose clusters (floccules), leading to faster sedimentation.
• Floccules have a more porous, loose structure, allowing easier redispersion after shaking.
• Flocculation is preferred over deflocculation for improved stability and ease of redispersion in suspensions.

Evaluation of Suspension Stability

• Sedimentation volume is the ratio of the ultimate sediment height to the initial suspension height, reflecting suspendibility.
• Higher sedimentation volume ratios are preferred for better suspendibility.
• Freeze-thaw cycling is a stability testing technique for evaluating particle growth and potential issues with prolonged storage conditions.