Drug Excretion

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   Renal excretion is the major route of drug elimination for polar drugs, water-
soluble drugs, low molecular weight drugs (below 500) and drugs with slow
biotransformation rates. Renal excretion of drugs through the kidney nephrons
involves three processes, namely glomerular filtration, active tubular secretion and
tubular reabsorption.
Glomerular filtration:
  This is a passive process in which small molecules and drugs are filtered
through the glomerulus of the nephron. Glomerular filtration rate affects drug
elimination rate and is affected by cardiac output. Glomerular filtration rate is
estimated from the elimination rate of creatinine or inulin which are excreted by
glomerular filtration only (without tubular secretion or reabsorption). Drugs bound to
plasma proteins are too large to be excreted by glomerular filtration.
Active tubular secretion:
  This is a carrier-mediated active transport system requiring energy. The kidney
contains two active tubular secretion systems; one for weak acids and another for
weak bases. The active tubular secretion system is competitive. For example, the
weakly acidic drug probenecid competes with penicillin for the same system and
therefore probenecid decreases renal excretion of penicillin resulting in prolongation
of its half life.
Tubular reabsorption:
  This is a passive process that follows Fick's Law of diffusion where lipid
soluble drugs are reabsorbed from the lumen of the nephron back into systemic
circulation. Weakly acidic drugs are mainly reabsorbed from acidic urine and vice
versa as the drug is more lipophilic in the non-inonized form.
  Diuretics decrease the time of drug contact with renal tubules thus decreasing
tubular reabsorption and increasing drug elimination. 

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