Suspension Definition in Pharmacy

What is suspension in pharmacy?

Suspension refers to the insoluble drug particles dispersed in the liquid medium to form a non-uniform dispersion system of liquid preparations.

The drug particles in the suspension should be fine and evenly distributed to ensure the uniformity of the drug; the settling speed is moderate and does not affect the accurate dosage. It is not easy to agglomerate after sinking, and can be evenly dispersed by gently shaking; maintain stable particle size during long-term storage. Suspension should have a moderate viscosity, easy to pour and use; attractive appearance and good taste to improve patient compliance. At the same time, the suspension should have certain anti-corrosion ability to ensure the safety of the preparation. For topical suspension, it should be easy to apply, not quickly disperse, and can dry quickly on the skin to form a long-lasting protective film.

Suspension particles are usually larger than 1 micron in size and have a wide range of applications in the medical field, including oral, topical, injectable, eye drops, aerosol and long-acting formulations. Commonly used suspensions include ibuprofen suspension, budesonide suspension for inhalation, calamine lotion, etc.

Advantages of suspension

• Rapid gastrointestinal absorption
• It is beneficial to improve bioavailability and large divergence
• Suitable for children
• Insoluble drugs can be made into suspension for clinical application
• Suspension can be considered to make the drug produce a slow-release effect

Formulation of suspension

The formulation of suspension is a complex process that requires careful consideration of various components and their interactions. Key ingredients include:

Active pharmaceutical ingredients (APIs): Pharmaceutical substances that are usually insoluble in continuous phases.

Suspending agent: By increasing the viscosity of the dispersion medium to slow down the settlement rate of insoluble solid particles, and improve the hydrophilicity of particles, prevent crystallization transition. Commonly used suspension AIDS are natural polymers (e.g., xanthan gum, Arabic gum), synthetic polymers (e.g., carboxymethyl cellulose), and clays (e.g., bentonite).

Humectants: For many hydrophobic drugs (such as sulfur, sterols, etc.), because the surface of the particles is easy to absorb air, it is not easy to be wet by water, which brings challenges to the preparation of suspension. Adding humectants, such as polysorbate and sorbitan ester, can be adsorbed on the surface of the particle to improve its hydrophilicity.

Flocculant and deflocculant: In order to achieve good dispersion and stability, deflocculant is usually added to the suspension that requires fine particles and good dispersion; For the suspension that needs to be stored for a long time, more flocculants are selected to form a loose settling system and facilitate dispersion. The same electrolyte may appear as flocculant or deflocculant for different drugs. The same electrolyte may also play a role of flocculation or deflocculation in different drugs because of the different dosage. Common flocculants include electrolytes such as sodium chloride and some surfactants.

Preservatives: To prevent microbial contamination, preservatives such as methyl parabens and propyl parabens are often added.

Buffers and pH regulators: Help maintain API stability and proper solubility by controlling the pH of the suspension.

Various components used in suspension formulation. (Kumar, R. S., 2016)

Suspension related services at BOC Sciences

Preparation of suspension

Dispersion method

Hydrophilic drugs: After the drug powder is pulverized to a certain fineness, add an appropriate amount of liquid, and then grind to a suitable dispersion, and finally add the remaining liquid in the prescription.

Hydrophobic drugs: Not easy to be moistened by water, must first add a certain amount of wetting agent and drug grinding and then add liquid grinding and mixing.

Drugs with high quality and hardness: Use the levigation method. Levigation is a method that utilizes the impact of water flow to grind medications.

Condensation method

Physical condensation method: The method of adding a drug solution dispersed in a molecular or ionic dispersion state to another dispersion medium to condense into a suspension.

Chemical condensation method: The method of producing insoluble drug particles by chemical reaction of two drugs and then suspending them in a dispersing medium to prepare a suspension agent.

Evaluation of suspension

Particle size: The size of the particles in the suspension is directly related to the quality, stability and bioavailability of the preparation. Therefore, measuring the size and distribution of the particles in the suspension is one of the key indexes to evaluate its quality. The available methods include microscopy, Coulter counting and light scattering.

Sedimentation volume ratio: Sedimentation volume ratio refers to the ratio of the volume of the sedimentation to the volume of the pre-sedimentation suspension. Place the suspension in the measuring cylinder, shake for 1min, record the total volume V₀ of the suspension (or the original degree is H₀), stand for 3h, observe that the settling surface does not change, is the volume V (or height H) of the fallout, and the settling volume ratio is F:

The larger the F number, the more stable the suspension is. The value of F is between 1 and 0. Sedimentation ratio, the larger the better. For oral suspensions, the sedimentation volume ratio should not be less than 0.90.

Degree of flocculation: The degree of flocculation (β) reflects the influence of flocculation on the increase of sedimentation volume of suspension. For example, if the de-flocculating suspension has an F number of 0.15 and the de-flocculating suspension has an F number of 0.75, then β = 5.0, which means that the sedimentation volume of the de-flocculating suspension is 5 times that of the de-flocculating suspension. The larger the beta value, the better the flocculation effect. The degree of flocculation can be used to evaluate the effect of the flocculant and to predict the stability of the suspension.

Redispersability: High-quality suspension after a period of storage, after light shaking should be able to quickly re-disperse, to ensure uniformity and dose accuracy when taking. The experimental method is usually to rotate the suspension in the measuring cylinder at a specific speed to observe the rate at which the bottom fallout redisperses. The fewer the number of rotations required for redispersion, the better the redispersion of the suspension.

The main factor of suspension stability

• Properties of drugs
• Repulsion and attraction between particles.
• Sedimentation of suspended particles
• Growth of particles
• Drug crystal transformation

Types of suspensions in pharmacy

Drug suspensions can be roughly classified according to their properties and uses as follows:

Oral suspensions: Designed for oral use, these suspensions usually contain flavorings and sweeteners to improve palatability.

Topical suspensions: For skin or mucous membrane applications, these suspensions require careful formulation to ensure that they spread easily and adhere firmly to the application site.

Parenteral suspensions: These suspensions are designed for injection and require strict aseptic treatment and stability.

Ophthalmic suspension: Specifically designed for ocular administration, isotonic and sterility need to be carefully considered.

In addition, suspensions can also be divided into flocculated and deflocculated suspension based on the interaction between particles:

Flocculated suspension: This type of suspension is characterized by rapid settling and easy redispersion. The flocculated particles form a loose network structure to avoid the formation of hard blocks that are difficult to disperse. Electrolytes and surfactants are often used to induce flocculation, such as the antacid alumina hydroxide gel.

Deflocculated suspension: This type of suspension has a low settling rate and forms a dense precipitate that may be difficult to redisperse. Suspension aids such as carboxymethyl cellulose can be used to stabilize deflocculating suspensions, such as sulfamethoxazol-trimethoprene suspensions.

Suspension example

Antibiotic suspensions: Amoxicillin suspensions, for example, provide a tasty and effective form of medication for pediatric use.

Acid resistant suspension: Such as magnesium hydroxide suspension provides immediate relief from excess stomach acid and is easy to take.

Antifungal suspension: Such as nystatin suspension, used to treat oral thrush and other fungal infections.

Antipsychotic suspension: Risperidone suspension, for example, provides a convenient dosage form for patients who have difficulty swallowing pills.