Pharmaceutical Suspensions: A Guide to Formulation and Stability 💊

A pharmaceutical suspension is an essential liquid dosage form, primarily designed to deliver water-insoluble or hydrophobic drugs. It is a biphasic system where finely divided solid drug particles are homogeneously dispersed in a liquid (usually aqueous) or semi-solid medium.

Understanding the formulation and stability of these systems is crucial for ensuring the drug's therapeutic success and bioavailability.

What is a Pharmaceutical Suspension?

A suspension is a two-phase system consisting of:

Dispersed Phase (Internal Phase): The solid, insoluble drug particles (typically ranging from 0.5–5 µm in size).
Dispersion Medium (Vehicle/External Phase): The liquid or semi-solid in which the particles are suspended.

To maintain the uniform distribution of the dispersed phase, suitable suspending agents are incorporated into the formulation.

Essential Features of an Acceptable Suspension

For a pharmaceutical suspension to be successful, it must exhibit several key characteristics:

Redispersibility: Particles that settle must not form a hard cake and should be easily and homogeneously redispersed upon mild shaking.
Viscosity: Must not be too viscous, ensuring it is easy to pour and administer.
Aesthetics: Should have a smooth, elegant appearance, along with acceptable color, odor, and taste.
Stability: Must be physically and chemically stable throughout its shelf life.
Particle Integrity: The particle size must remain fairly constant over the storage period.
Application-Specific: External suspensions (like lotions) must spread easily without running off the skin.

📈 Advantages of Using Suspensions

Suspensions offer significant benefits over other dosage forms, particularly for certain types of drugs:

Stability Solution: They are the preferred choice for drugs that are unstable or degradable when dissolved in a solution form.
Insoluble Drug Delivery: They provide a liquid dosage option for water-insoluble drugs (e.g., Corticosteroid suspensions) when non-aqueous vehicles are not suitable.
Taste Masking: Suspensions effectively mask the unpleasant or bitter taste and odor of certain drugs (e.g., Chloramphenicol palmitate) by keeping the insoluble drug particles out of contact with the taste receptors.
Sustained Release: Suspensions can act as a drug reservoir, allowing the medicine to be absorbed slowly over a prolonged period in the systemic circulation (e.g., protamine zinc-Insulin).
Improved Bioavailability: In some cases, suspensions offer better bioavailability compared to the equivalent dose in tablet or capsule form.

👨‍⚕️ Classification by Route of Administration: Oral Suspensions

Oral suspensions are biphasic liquid dosage forms where one or more active ingredients are suspended in a suitable vehicle.

They are easily administered, especially to pediatric and geriatric patients, who may have difficulty swallowing tablets or capsules.
The concentration of the suspended drug can be relatively high (e.g., 125–500 mg/5 ml for antibiotics).
They are used for drugs with activities spanning antibiotics, antacids, analgesics, anticonvulsants, and more (e.g., Azithromycin and Ofloxacin pediatric suspensions).

⚖️ The Electrokinetic Nature: Flocculated vs. Deflocculated Suspensions

Suspensions are classified based on the nature of particle-to-particle interactions, which dictates their stability and redispersibility. This distinction depends on the balance between electrostatic forces of repulsion and van der Waals forces of attraction between the particles.

1. Flocculated Suspensions (Desired System)

Flocculation is the formation of a loose, network-like structure known as floccules.

Particle Interaction: Particles experience attraction at moderate distances, forming an open aggregate structure.
Sedimentation: The sedimentation rate is high (they settle quickly).
Redispersibility: The sediment formed is loosely packed and does not form a hard cake. It is easily redispersed with mild agitation, ensuring dose uniformity.
Aesthetics: They may lack elegance because the solid phase tends to separate quickly from the medium.

2. Deflocculated Suspensions (Undesired System)

In a deflocculated or peptized suspension, the electrical repulsive force between particles predominates.

Particle Interaction: Individual particles remain separate entities, resulting in a smooth-looking, elegant suspension.
Sedimentation: The sedimentation rate is low (they settle slowly).
Caking: Because interparticle distances are small, the sediment formed is tightly packed, leading to the unfavorable phenomenon of caking. This hard cake is extremely difficult to resuspend, which compromises accurate dosing and bioavailability.

From a practical and patient compliance perspective, partial aggregation (flocculation) is the desired objective as it resists caking and ensures the suspension can be easily administered uniformly.