Order Of Kinetics

Order Of Kinetics


FIRST ORDER KINETICS:

  • Absorption, distribution, biotransformation and excretion processes are mostly occurring at rates proportional to plasma drug concentration.
  • 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 drug in body is eliminated per unit time.
  • Rate of elimination is proportional to amount of drug in body.
  • For drugs with first order kinetics time required to achieve steady state levels can be predicted from half life. 
  • The majority of drugs are eliminated in this way.
 

First Order Kinetics???Absorption, distribution, biotransformationand excretion processes are mostlyoccurring at ...

Half life

Elimination

1 t1/2 50%
2 t1/2

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 Km of enyzme for drug.
  • v = Vmax  [C]
  • Km + [C]
  • When     Km >>> [C],
  • then      v = Vmax [C]  &  v σ [C]   
  • Km
  • Rate of elimination is directly proportional to drug concentration.
  • A constant fraction of remaining drug is metabolized per unit time. 
  • Most drugs are given at concentrations smaller than Km of enzymes of their metabolism.
ZERO ORDER KINETICS:
  • A constant amount of drug is eliminated per unit time.
  • Supply of enzymes are limited in body.
  • So, with increasing dose enzymes gets saturated at a particular stage.
  • At this point there is,
  • No elimination.
  • Constant biotransformation.excretion.
  • So, rate of processes or reaction is not proportional to conc. or dose.
  • Processes also referred as “Rate limited/zero order/saturation kinetics/clinically called “Non-linear kinetics”.

Eg:

  • All enzyme mediated processes.
  • Drugs involved: Aspirin, ethanol, phenytoin ,propanolol, tolbutamide etc. 
Zero order kinetics cont..?So, rate of processes or reaction is notproportional to the conc. or dose?Processes sho...
ZERO ORDER METABOLISM:
  • A drug may be given in doses that produce blood concentrations greater than Km of enyzme for drug.
  • v = Vmax  [C]

  • K m + [C]

  • When   [C]  >>> Km,

  • then     v = Vmax [C]  ,   and      v = Vmax

  • [C] 

    A constant amount of remaining drug is metabolized per unit time. 

    Phenytoin undergoes zero order metabolism at doses given.

Drugs showing zero/pseudo-zero order kinetics :

  • Phenytoin
  • Theophylline
  • Alcohol
  • Tolbutamide
  • Warfarin 
  • Salicylates (Aspirin)
  • Propanolol

First Order Kinetics

  • Constant fraction of drug is eliminated per unit time.
  • Rate of elimination is proportional to plasma concentration.
  • Clearance remains constant.
  • Half life remains constant.
  • Most of the drugs follow first order kinetics

Zero Order Kinetics (Non linear Kinetics)

  • Constant amount of drug is eliminated per unit time.
  • Rate of elimination is independent of plasma concentration.
  • Clearance is more at low concentrations and less at high  concentrations.
  • Half life is less at low concentrations and more at high concentrations.
  • Very few drugs follow pure zero order kinetics e.g. alcohol
  • Any drug at high concentration (when metabolic or elimination pathway is saturated) may show zero order kinetics.
Exam Question
 
  • Zero order kinetics is independent of plasma concentration.
  • Zero order kinetics is otherwise known as saturation kinetics.
  • In zero order kinetics constant amount of drug eliminated per unit time.
  • 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.
  • 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.
Don’t Forget to Solve all the previous Year Question asked on Order Of Kinetics

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