The kidneys are the most important excretion organ for hydrophylic drugs or polar metabolites. The biotransformation in the liver has resulted in increased water solubility of drugs and their metabolites (glucuronides and sulphate conjugates).
Excretion of drugs by the kidney is dependent on 3 main mechanisms:
- Glomerular filtration of unbound drug.
- Active tubular secretion of drug by transporters.
- Tubular reabsorption by passive diffusion following the concentration gradient.
The rate of drug elimination depends on the balance between filtration, secretion and reabsorption.