Lecture 10 Flashcards
Drug Excretion/Elimination
Excretion
- Removes a chemical from the body and prevents drug accumulation
- Liver and kidneys are the primary sites of excretion
- Metabolism, aka biotransformation, is important component of excretion since metabolites are usually charged, hydrophilic, and conjugated (larger)
- Metabolic changes alter the physicochemical properties of the drug and can make it more or less likely to be excreted
Renal v.s. Biliary Secretion
- The one that occurs is a consequence of drug properties (ionization, adducts, etc.)
- Renal - excretion into urine, polar, glomerular filtration, active secretion, passive reabsorption/excretion
- Hepato/Biliary - excretion into bile (stable adducts), diffusion AND active transport
Renal Drug Clearence
- Many can be excreted unchanged
- Clearance is proportional to renal function
- Metabolites MAY be excreted into urine
Key Renal Excretion Mechanisms (3)
- Glomerular Filtration - Bulk flow (remove)
- Tubular Secretion - active transport (remove)
- Tubular Reabsoprtion - passive reabsorption and active reabsoprtion
Glomerular Filtration
- Non-selective, size limited
- Most “free” drugs are small enough to be filtered, MW cut off ~1000-2000
- Specialized pores/fenestrations and the size, shape, and charge affect the filtration
- Pressure gradient regulates filtration, very sensitive to blood pressure
Tubule Secretion
- Diffusion AND active transport
- Passive diffusion utilized for small, uncharged molecules
- Active transport for large, ionized molecules allow for greater urine concentrations
- Depends on specific transport systems for acids or bases: transporters can be saturated, inhibited, require energy, or a specific carrier for a certain drug
Tubule Reabsorption
- Mediated by passive diffusion
- The concentration gradient usually favor reabsorption
- Ionization of solutes, solute size, and solute liquid solubility may modulate passive reabsorption
- Passive reabsorption favors small, neutral, lipophilic molecules
Renal Summary
- Renal elimination is a balance of transport between kidney tubules and plasma
- Multiple transport mechanisms (active and passive)
- Drug characteristics are important for renal elimination
- Slow decline in renal function (~1% per year) throughout adulthood occurs
Hepato-Biliary Excretion
- Many drugs eliminated after hepatic metabolism
- Excretion is into bile which enters the feces
- Liver is optimized for drug metabolism and excretion
- About 5% of administered drug dose will enter bile passively and active secretion can increase this up to 95%
General Pathway of Drug to Plasma to Bile
- Enters hepatocytes by passive diffusion or active transport from the plasma
- Drugs then transported to biliary epithelium by active secretory process
- Enterohepatic cycling then occurs where a drug is secreted into the bile and is reabsorbed from the intestine into the blood.
Biliary Excretion
- Drugs enter hepatocytes actively or passively
- Hepatocytes secrete bile which flow to duaodenum for eleimination
- Conjugation from Phase II aids biliary excretion by increases the molecule’s polarity and molecular size
- Biliary excretion favors drugs with MW > 300
- Drug conjugates are not usually reabsorbed from the bile since they are poorly recognized by reuptake systems, too ionized, and too large
- Most drugs are excreted via the feces
- If reabsobed, enterohepatic cycles occur
Other Routes of Secretion
- Milk - concerning for babies whose Phase I and II activity may not be fully developed until at least 6 years of age, sometimes later
- Lungs
- Intestines
- Sweat
- Saliva
- Other bodily fluids
Drug Clearance
- Clearance is the decrease of drug in the bloodstream
- For most drugs, clearance is constant over the concentration range encountered in the clinical settings (1st order elimination)
- Total Clearance is an additive of the eliminations from all tissues
Excretion Definition
Removal of drugs from the body
Clearance Definition
The volume of plasma cleared of the drug per unit time.
Glomerular Filtration Definition
Removal of drugs by bulk flow
Tubular Secretion Definition
Active transport of drug from the blood into the urine
Tubular Reabsorption Definition
Passive and active reabsorption of drug from tubules back into blood
Enterohepatic Cycling Definition
Biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation