Pharmacokinetics: Drug Absorption and Routes

Study Notes

Absorption is the movement of a drug from its administration site to the systemic circulation.
The systemic circulation is the blood circulation throughout the body.
The site of administration can be oral, intravenous, intramuscular, subcutaneous, etc.
The bioavailability of a drug refers to the fraction of the administered dose that reaches the systemic circulation.
Biological membranes are composed of phospholipids, proteins, and carbohydrates.
Drugs cross biological membranes by various mechanisms, including passive diffusion (driven by the concentration gradient), active transport (requires energy), and facilitated diffusion (requires a carrier protein).

Enteral administration: involves the gastrointestinal tract.
Oral: Most common, convenient, and economical. Examples include tablets, capsules, and syrups.
Rectal: Useful for patients who cannot swallow due to nausea, vomiting, or unconsciousness.
Sublingual: Medication is placed under the tongue for absorption.
Parenteral administration: administration bypasses the GI tract.
Intravenous (IV): Directly into the bloodstream, bypassing absorption.
Intramuscular (IM): Injected into a muscle.
Subcutaneous (SC): Injected beneath the skin.
Intradermal: Injected into the skin.
Intrathecal: Injected into the cerebrospinal fluid.

AUC (Area Under the Curve): Represents the total drug exposure over time. An important parameter for both pharmacokinetic and pharmacodynamic analyses.
MEC (Minimum Effective Concentration): The lowest concentration of a drug in the bloodstream required for a therapeutic effect.
Onset: The time taken from drug administration to the first appearance of its pharmacological effect.
Duration: The time between the appearance of the drug's pharmacological effect and its disappearance, also known as the time between two MECs.
Cmax: The maximum concentration of the drug in plasma.
Tmax: The time taken to reach Cmax.
MTC (Minimum Toxic Concentration): The lowest concentration of a drug that produces toxic effects.

Gastrointestinal Motility and Emptying: Increased GI motility can lead to reduced absorption.
Food: Can decrease the absorption of many drugs. For example, tetracycline's absorption is reduced in the presence of calcium.
Intestinal Blood Flow (Splanchnic Blood Flow): Reduced blood flow can decrease drug absorption.
Particle Size: Smaller particle size allows for faster and increased absorption.
Chemical Interactions: Drugs can interact with food or other drugs, affecting pH and dissociation, thereby impacting absorption.

After absorption from the upper gastrointestinal tract, drugs enter the portal circulation.
The portal vein carries the drug-containing blood to the liver, where it may undergo metabolism before reaching systemic circulation.
This first-pass metabolism can reduce the bioavailability of orally administered drugs.