Rate of elimination of different drugs varies.
Methods of elimination/excretion:
1) Renal elimination (accounts for the majority of drug excretion)
a) Glomerular filtration (removes drugs from blood into urine as part of glomerular filtrate; protein drugs are not removed)
b) Proximal renal tubular excretion (accounts for most of renal excretion)
c) Renal tubular reabsorption (lipid soluble drugs are reabsorbed due to concentration gradient)
2) Metabolism (usually in liver) into an inactive metabolite
3) Faecal elimination
4) Pulmonary elimination in exhaled air
5) Other (breast milk, saliva, tears, sweat)
Renal disease will affect the excretion of some drugs – the level of impairment can be measured by a 24hr creatinine clearance measurement.
Pharmacokinetic parameter that describes elimination is clearance (Cl) – this is the volume of plasma completely emptied of a drug per unit time:
Rate of elimination = CL x Cp
Clearance is an important pharmacokinetic parameter – it determines the maintenance dose rate. For most drugs, the rate of clearance is constant – when the rate of administration is equal to rate of elimination the plasma concentration is constant (CpSS). At the beginning of a dosing regimen, plasma concentration will be low until a steady state is reached.
As plasma concentration falls (eg stop taking drug) rate of elimination falls the plasma concentration-time graph curve follows a characteristic non-linear curve – first order elimination (Cp declines exponentially). This is different from the constant elimination irrespective of plasma concentration – zero order elimination (Cp declines linearly; rare; eg ethanol).
Elimination rate constant (ke) – the fraction of the amount of drug in the body eliminated per unit time
Rate of elimination = ke x A