Scientific Principles of Dosage form Design

Dosage form design and formulation development is essential for the success of a drug delivery. Drugs are rarely administered as pure chemical substances alone and are almost always given as formulated preparations or medicines. These can vary from relatively simple solutions to complex drug delivery systems through the use of appropriate additives or excipients in the formulations. The excipients provide varied and specialized pharmaceutical functions. It is the formulation additives that, amongst other things, solubilize, suspend, thicken, preserve, emulsify, modify dissolution, improve the compactability and flavor drug substances to form various medicines or dosage forms.

The principal objective of dosage form design is to achieve a predictable therapeutic response to a drug included in a formulation which is capable of large-scale manufacture with reproducible product quality. To ensure product quality, numerous features are required: 

• chemical and physical stability with suitable preservation against microbial contamination if appropriate,
• uniformity of dose of drug,
• acceptability to users, including both prescriber and patient, 
• as well as suitable packaging and labelling.

Dosage forms available for different administration routes Administration 

• Oral: Solutions, syrups, suspensions, emulsions, gels, powders, granules, capsules, tablets 
• Rectal: Suppositories, ointments, creams, powders, solutions 
• Topical: Ointments, creams, pastes, lotions, gels, solutions, topical aerosols, foams, transdermal patches 
• Parenteral: Injections: (solution, suspension, emulsion forms), implants, irrigation and dialysis solutions 
• Respiratory: Aerosols (solution, suspension, emulsion, powder forms), inhalations, sprays, gases 
• Nasal: Solutions, inhalations 
• Eye: Solutions, ointments, creams 
• Ear: Solutions, suspensions, ointments, creams

To optimize the bioavailability of drug substances, it is often necessary to carefully select the most appropriate chemical form of the drug. For example, such selection should address: 

• solubility requirements, 
• drug particle size
• physical form 
• appropriate additives 
• manufacturing aids coupled to selecting the most appropriate administration route(s) and dosage form(s)
• Additionally, suitable manufacturing processes, labelling and packaging are required.

Many drugs are formulated into several dosage forms of varying strengths, each having selected pharmaceutical characteristics which are suitable for a specific application. One such drug is the glucocorticoid prednisolone used in the suppression of inflammatory and allergic disorders. Through the use of different chemical forms and formulation additives, a range of effective anti-inflammatory preparations is available, including tablet, enteric coated tablet, injections, eye drops and enema.

It is therefore apparent that before a drug substance can be successfully formulated into a dosage form, many factors must be considered. These can be broadly grouped into three categories:

1. biopharmaceutical considerations, including factors affecting the absorption of the drug substance from different administration routes
2. drug factors, such as the physical and chemical properties of the drug substance
3. therapeutic considerations, including consideration of the clinical indication to be treated and patient factors.

High-quality and efficacious medicines will be formulated and prepared only when all these factors are considered and related to each other. This is the underlying principle of dosage form design.

Source: Aulton's Pharmaceutics