Excretion Of Drugs

a. The kidneys are the most important organs for elimination of drugs and their metabolites.

b. Substances excreted in feces are either unabsorbed orally ingested drugs or metabolites excreted in bile.

c. Excretion of drugs in milk is important because of potential unwanted pharmacological effects in nursing infants.

d Pulmonary excretion is important mainly for elimination of anesthetic gases & vapors.

Renal excretion

a. Excretion of drugs & metabolites in the urine involves three processes: glomerular filtration, active tubular secretion, and passive tubular reabsorption.

b. The amount of drug entering the tubular lumen by filtration is dependent on its fractional plasma protein binding and glomerular filtration rate (GFR).

c. In the proximal renal tubules, certain organic anions and cations are added to glomerular filtrate by active, carrier-mediated tubular secretion (e.g., penicillin)

d. In the proximal & distal tubules the nonionized forms of weak acids & bases undergo net passive reabsorption (following a concentration gradient between the renal tubular fluid and the blood

e. P-glycoprotein is an "efflux drug transporter" that may be important in the transport of certain drugs (e.g., renal elimination of digoxin; movement of quinidine across the blood brain barrier; increased first pass effect with felodipine).

Biliary & fecal excretion

Many metabolites formed in the liver are excreted into the intestinal tract via the bile. They may be excreted in feces or may be enzymatically converted to the active drug and be reabsorbed (termed enterohepatic circulation).

Category: Pharmacology Notes