04. Therapeutic Concentrations

• MEC = minimum effective concentration of a drug
• acidic drugs are un-ionised in the stomach so more is absorbed here
• basic drugs are un-ionised in the intestines so more is absorbed here
• when drugs are absorbed through the intestines the hepatic portal system takes drugs to livers they are metabolised here before entering the systemic circulation

  • recirculation may occur through bile
• volume of distribution = volume of fluid required to contain the total amount of drug in the body at the same concentration as that in the plasma

  • amount of drug / plasma concentration of drug
• first pass effect occurs when a drug passes through liver before reaching its target site, this reduces bioavailability
• drug clearance = rate of elimination from blood divided by plasma concentration of drug
• one compartment model = drug stays in plasma, so volume of distribution is amount of plasma
• two compartment model = drug leaves the plasma, so there is lower concentration in plasma meaning volume of distribution is increased
• clearance = how fast a drug is removed from plasma
• zero order kinetics is where the enzymes are saturated
• drug metabolism is split into two phases:

  1. biotransformation - creates highly reactive intermediates using cytochrome P450 enzymes found mostly in the liver
  2. conjugation - creates charged molecule which is easy to get rid of

  some drugs go straight into phase 2
• loading dose = volume of distribution x concentration

  • initial dose given