Biopharmaceutics is the study of the relation of the physical and chemical properties of a drug to its bioavailability, pharmacokinetics, and pharmacodynamic and toxicologic effects.
- A drug product is the finished dosage form (e.g., tablet, capsule, solution) that contains the active drug ingredient in association with nondrug (usually inactive) ingredients (excipients) that make up the vehicle or formulation matrix.
- Bioavailability is a measurement of the rate and extent (amount) of systemic absorption of the therapeutically active drug.
- Pharmacokinetics is the study of the time course of drug movement in the body during absorption, distribution, and elimination (excretion and biotransformation).
- Pharmacodynamics is the study of the relation of the drug concentration or amount at the site of action (receptor) and its pharmacologic response as a function of time.
Drug absorption requires the drug to be transported across various cell membranes. Drug molecules may enter the bloodstream and be transported to the tissues and organs of the body. Drug molecules may cross additional membranes to enter cells. Drug molecules may also cross an intracellular membrane, such as the nuclear membrane or endoplasmic reticulum, to reach the site of action.
- A cell membrane is a semipermeable structure composed primarily of lipids and proteins.
- Drugs may be transported by passive diffusion, partitioning, carrier-mediated transport, paracellular transport, or vesicular transport.
- Usually, proteins, drugs bound to proteins, and macromolecules do not easily cross cell membranes.
- Nonpolar lipid-soluble drugs traverse cell membranes more easily than do ionic or polar water-soluble drugs.
- Low-molecular-weight drugs diffuse across a cell membrane more easily than do highmolecular-weight drugs.
For most drugs with limited water solubility, the rate at which the solid drug enters into solution (i.e., the rate of dissolution) is often the rate-limiting step in bioavailability. The Noyes–Whitney equation describes the diffusion-controlled rate of drug dissolution.
Physicochemical properties of the API that influences the drug product -
- Solubility
- Particle size
- Partition coefficient and extent of ionization
- Salt formation
- Polymorphism
- Chirality
- Hydrates
- Complex formation