Fates of drug absorption in the body

• Absorption: drug is absorbed from site of administration, entry into the plasma
• Distribution: drug leaves bloodstream and is distributed into interstitial and intracellular fluids
• Metabolism: drug transformation by metabolism - liver and other tissues
• Excretion: drug and/drug metabolites excreted in urine, faeces or bile

Movement of drugs across cell barriers

Passive diffusion

• Directly through lipid/aquaporins
• Drugs with high lipid solubility - high concentration gradient

Facilitated diffusion

• Via specialised carrier proteins
• Does not require energy
• Movement is down a concentration gradient
• Water-soluble drugs - otherwise would not be able to cross membrane = carrier required
• Can show saturation kinetics - limited amount of carriers

Active transport

• Via specialised carrier proteins
• Requires energy
• Can move molecules against concentration gradient
• Water-soluble drugs