Biopharmaceutics is the study of the physicochemical properties of the drug and the drug product, in vitro, on the bioavailability of the drug, in vivo, to produce a desired therapeutic effect. Biopharmaceutics links the physical and chemical properties of the drug and the drug product to their performance, in vivo. A primary concern in biopharmaceutics is the bioavailability of drugs. Bioavailability refers to the measurement of the rate and extent of active drug that becomes available at the site of action. Oral drug absorption involves at least three distinct steps: drug release and dissolution from the drug product, permeation of the drug across the gastrointestinal (GI) linings, and drug disposition during GI transit. Additional drug disposition may occur in the body. Because the systemic blood circulation delivers therapeutically active drug to the tissues and to the site of action of the drug, changes in bioavailability affect changes in the pharmacodynamics and toxicity of a drug. The aim of biopharmaceutics is to adjust the delivery of drug from the drug product in such a manner as to provide optimal therapeutic activity and safety for the patient.
Drugs are not usually given as a pure chemical drug substances, but are formulated into a finished dosage forms (drug products), such as tablets, capsules, ointments, solutions, etc, which are then administered to patients. Drug products are designed to deliver the drug for local or systemic effects. The design of the dosage form, the formulation of the drug product, and the manufacturing process requires a thorough understanding of the biopharmaceutic principles of drug delivery. Considerations in the design of a drug product to deliver the active drug with the desired bioavailability characteristics and therapeutic objectives include (1) the physicochemical properties of the drug molecule, (2) the type of drug product (eg, tablet, capsule, transdermal delivery system, topical ointment, parenteral solution), (3) the nature of the excipients in the drug product, (4) the method of manufacturing, and (5) the route of drug administration.
Drug products include the active drug substance combined with selected additional ingredients (excipients) that make up the dosage form. Common drug products include liquids, tablets, capsules, injectables, suppositories, transdermal systems, and topical products such as creams and ointments. These finished dosage forms or drug products are then given to patients to achieve a specific therapeutic objective. Although excipients are considered inert with respect to pharmacodynamic activity, excipients are important in the manufacture of the drug product and provide functionality to the drug product with respect to drug release and dissolution (see also Chapter 16).
Biopharmaceutics allows for the rational design of drug products and is based on
- The physical and chemical properties of the drug substance
- The route of drug administration, including the anatomic and physiologic nature of the application site (eg, oral, topical, injectable, implant, transdermal patch, etc)
- Desired pharmacodynamic effect (eg, immediate or prolonged activity)
- Toxicologic properties of the drug
- Safety of ...