Pharmaceutical excipients are substances other than the active pharmaceutical ingredient that is added during the manufacturing process or is contained in the final dosage. In the formulation of pharmaceutical suspensions, excipients are added with the active pharmaceutical ingredients in order to:
- a) Protect, support, or enhance the stability of the formulation
- b) Bulk up the formulation in case of a potent chemical for assisting in the formulation of an accurate dosage form.
- c) Improve acceptance
- d) Help improve the bioavailability of active chemical
- e) Enhance the overall safety and effectiveness of the formulation during its storage and use.
Excipients used when formulating a pharmaceutical suspension are sub-divided into various functional classifications, depending on what role they will play in the resulting formulation. Some excipients can function differently depending on the type of formulation additionally, individual excipients can have different grades, types, and sources depending on those different functional roles. The common types of excipients used when formulating pharmaceutical suspensions include:
Solvents (or vehicles) are used as a base in which chemicals and other excipients are dissolved or dispersed. Purified water is the most commonly used vehicle for the formulation of suspensions. This is because of its lack of toxicity, physiological compatibility, and good solubilizing power (high dielectric constant).
In certain cases, viscous nonaqueous solvents, such as propylene glycol and polyethylene glycols, are used in order to impart stability to suspended chemical particles. The choice of solvent used depends on the nature and physicochemical properties of the chemical substance and the intended use of the formulation.
Other examples of solvents used in suspension formulation include alcohol, acetic acid, acetone, ethyl acetates, etc.
The purpose of a Co-solvent is to promote the solubility of solute in solvents and they act by decreasing the interfacial tension between predominantly aqueous solutions and hydrophobic solutes. Sorbitol, dextrose, etc. are often added as solubilizers, as well as base sweeteners. Other examples of co-solvents include Ethanol, Sorbitol, Glycerin, Propylene glycol, etc.
- Buffers are a mixture of a weak acid or base and one of its salts which when dissolved in a solvent will resist any change in pH when an acid or base is added. Buffering agents/ pH modifiers are added to suspensions to
- a) Ensure physiological compatibility
- b) Maintain/ optimize chemical stability
- c) Maintain/ optimize anti-microbial effectiveness
- d) Optimize solubility (or insolubility if the taste is an issue).
Normally, the pH of pharmaceutical suspension should be kept between 7.0 – 9.5 preferably between 7.4 – 8.4. Substances commonly used buffering agents in suspensions include salts of week acids such as carbonates, citrates, gluconates, phosphate, and tartrates.
Amongst these, citrates and phosphates are commonly used in suspensions. Citrate buffers are used to stabilize suspensions in the pH range of 3 – 5, while phosphate buffers are used in the pH range of 7 – 8.
Excipients are important ingredients that are used to create a stable and effective pharmaceutical suspension. Much consideration is required when choosing which excipients to use, as a bad choice can lead to intoxication.