Biopharmaceutics and Bioavailability

Absorption: Uptake of the drug into the bloodstream.
Distribution: Movement of drugs from the site of absorption to the target site and to sites of metabolism or excretion.
Metabolism: Chemical modification of the drug, primarily occurs in the liver.
Excretion: Elimination of the drug from the body via kidneys, lungs, or faeces.
Liberation: Passage of the drug out of its dosage form into the solution for absorption.

Definition: Bioavailability is the rate and extent to which an active drug is absorbed and becomes available at the site of action.
Intravenous (IV): Represents 100% bioavailability due to direct entry into systematic circulation.
Oral (PO): Typically has less than 100% bioavailability due to factors like first-pass metabolism.
Absolute Bioavailability (F): Calculated as $F = \frac{AUC<em>{test}}{AUC</em>{iv}}$ where AUC is the area under the concentration-time curve.

Various routes affect the absorption rates and bioavailability.
- Oral: passes through GI tract then to hepatic portal system and liver.
- IV Injection: Directly enters the bloodstream, bypassing most barriers.
- Subcutaneous (s.c.) and Intramuscular (i.m.) injections: quicker absorption than oral, slower than IV.
- Topical: Absorption can be variable based on the formulation.

Definition: The therapeutic window is the range of drug concentrations in plasma that achieves the desired effect without causing toxicity.
Narrow Window: Small margin between effective and toxic doses. Requires careful monitoring.
Wide Window: Larger difference between effective and toxic doses, allowing for more variability in absorption.

Dosage Form: Tablets, capsules, liquids can influence the rate of drug dissolution and absorption.
Co-Administration: Interaction with other drugs, food, or existing health conditions can alter absorption.
Physical/Chemical Properties: Solubility, stability, pH, and molecular size can affect how well a drug is absorbed.
Patient Factors: Age, sex, genetic factors, and disease can modify absorption efficiency.

Example calculations demonstrate how to use AUC values from different forms to find bioavailability.
- For a drug administered IV and orally, compare AUC of oral to the AUC of IV: $F = \frac{AUC<em>{oral}}{AUC</em>{iv}}$
- Utilize given AUCs to perform related calculations for different cases.

Gain knowledge about the importance of biopharmaceutics in drug design and delivery.
Understand the differential effects of administration routes on drug exposure.
Ability to define and utilize key terms related to bioavailability and bioequivalence.
Proficiency in calculating bioavailability using AUC values from pharmacokinetic studies and understanding how these calculations impact pharmacotherapy decisions.