Learning Objective

Understand the mechanisms of renal drug elimination, how glomerular filtration, secretion, and reabsorption contribute to clearance, and the impact of plasma protein binding on renal clearance.

Renal Elimination

The rate of renal drug elimination is determined by:

Rate of elimination=GFR+active secretion−reabsorption (active or passive)

$Latex\text{Rate of elimination} = \text{GFR} + \text{active secretion} – \text{reabsorption (active or passive)}$

Key Points

Glomerular filtration

• Linear, nonsaturable process.
• Ionized and nonionized drug molecules are filtered.
• Protein-bound drugs are not filtered.

Clearance (Cl)

• Definition: Volume of blood cleared of a drug per unit time.
• First-order kinetics: Cl is constant.
• When no reabsorption, secretion, or protein binding occurs:

• Protein-bound drugs: Only the free fraction is cleared:

$LatexCl = f \times GFR$ :where $f$ = fraction unbound in plasma

• Other contributors:
  • Active secretion: Enhances clearance.
  • Reabsorption: Can reduce clearance, especially for lipid-soluble drugs.

Activity