Drug Excretion

Overview of Drug Excretion

Excretion is the process of eliminating a drug from the body.
Organs involved in excretion are those open to the atmosphere, primarily the kidneys.

Kidneys
- Main organ for drug excretion via urine.
- Kidney function is critical; impaired function (e.g., CKD, AKI) affects drug excretion and can lead to drug accumulation.
Biliary System
- Drugs absorbed across the GI tract can be excreted via bile and anteropatic circulation.
Lungs
- Exhalation of inhaled anesthetics and gases.
GI Tract
- Drugs that aren't absorbed are excreted via feces.

Filtration
- Dependent on Glomerular Filtration Rate (GFR).
- Lower GFR leads to less drug filtration, higher blood concentration, and potential toxicity.
- Protein binding affects filtration; high protein binding reduces filtration.
Secretion
- Involves moving drugs from peritubular capillaries to kidney tubules.
- Dependent on drug solubility, concentration gradient, and ATP-dependent transporters (e.g., OATs, OCTs).
- Drug interactions can inhibit transporters, affecting secretion.
Reabsorption
- Drugs can be reabsorbed from distal convoluted tubule based on solubility and molecular characteristics.
- Liver metabolism can alter drugs to prevent reabsorption.

Concept used to enhance drug elimination, especially in overdoses.
Weak Acid Drugs (e.g., Phenobarbital, Aspirin):
- Alkalinize urine with sodium bicarbonate to trap ionized drug form, preventing reabsorption.
Weak Base Drugs (e.g., Amphetamines):
- Acidify urine with ammonium chloride to trap ionized drug form, preventing reabsorption.

Understanding of how kidneys and other organs excrete drugs is crucial for managing drug dosages and preventing toxicity.
Ability to manipulate drug reabsorption through pH changes in urine is a key therapeutic tool, especially for overdose management.