order of kinetics
FIRST ORDER KINETICS:
 Absorption, distribution, biotransformation and excretion processes are mostly occurring at rates proportional to the conc. of drug in the plasma
 A constant fraction of drug is absorbed, distributed, biotransformed and excreted per unit time
 These processes increase in rate with increase in conc. and decrease with falling conc.
 A constant fraction of the drug in the body is eliminated per unit time
 The rate of elimination is proportional to the amount of drug in the body
 The majority of drugs are eliminated in this way
 For drugs with first order kinetics the time required to achieve steady state levels can be predicted from Half life
Half life
 Elimination
2t1/2
75%
3 t1/2
87.5%
4 t1/2 93.75%
5 t1/2 96.875
FIRST ORDER METABOLISM:
 A drug may be given in doses that produce blood concentrations less than the Km of the enyzme for the drug.
 v = Vmax [C]
 Km + [C]
 When Km >>> [C],
 then v = Vmax [C] & v σ [C]
 Km
 Metabolism of the drug is a first order process.
 Rate of elimination is directly proportional to drug concentration
 A constant fraction of the remaining drug is metabolized per unit time.
 Most drugs are given at concentrations smaller than the Km of the enzymes of their metabolism.
 A constant amount of drug is eliminated per unit time
 Supply of enzymes are limited in the body
 So, with increasing dose a time will come when the supply of enzymes get saturated
 At this point no elimination occur
 Biotransformation or excretion remains constant at this point
 So, rate of processes or reaction is not proportional to the conc. or dose
 Processes showing such kinetics are known as – Rate limited or zero order or saturation kinetics
 Some times clinically called nonlinear kinetics
 All enzyme mediated processes show this type of kinetics
 Passive diffusion like processes do not show this type of kinetics
 e.g. aspirin, ethanol, phenytoin ,propanolol, tolbutamide etc.
ZERO ORDER METABOLISM:
 A drug may be given in doses that produce blood concentrations greater than the Km of the enyzme for the drug.
K m + [C]v
When [C] >>> Km,
then v = Vmax [C] , and v = Vmax
[C]
 Metabolism of the drug is a zero order process.
 A constant amount of the remaining drug is metabolized per unit time.
 Phenytoin undergoes zero order metabolism at the doses given.
 Phenytoin
 Theophylline
 Alcohol
 Tolbutamide
 Warfarin
 Salicylates (Aspirin)
 Propanolol
First Order Kinetics  Zero Order Kinetics (Non linear Kinetics) 



 Zero order kinetics is independent of Plasma concentration
 Zero order kinetics occur in Phenytoin,Tolbutamide, Propranolol and Ethanol at high dose
 In first order kinetics a constant proportion of plasma concentration is eliminated
 In first order kinetics Rate of elimination is directly proportional to drug concentration
 Zero order kinetics is otherwise known as saturation kinetics
 In first order kinetics Clearance is constant
 Elimination after 4 half lives in first order Kinetics is 93%
 Elimination after 3 half lives in first order kinetics is 87.5%
 For drugs with first order kinetics the time required to achieve steady state levels can be predicted from Half life
 First order kinetics is Elimination of the drug is proportional to the serum concentration
 In Zero order kinetics Constant amount of drug eliminated per unit time
Don't Forget to Solve all the previous Year Question asked on order of kinetics