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First Order Elimination Kinetic



The Fundamental Concept

  • A drug molecule is eliminated when it comes in contact with its elimination mechanism.

  • The elimination mechanism may be an inactivating enzyme or excretion process.

  • First-order elimination kinetic is seen when the elimination process is not saturated.

At high concentration of drug, more drug molecules come in contact with elimination mechanism

More drugs molecule are eliminated. i.e. inactivated or excreted

Drug concentration decreases

Fewer molecules come in contact with elimination mechanism

Fewer molecules are eliminated

If we give next dose of drug

Concentration of drug increases

Again more drug molecules come in contact with elimination mechanism

Elimination increases

  • Thus, at higher concentration, more molecules are eliminated, and at lower concentration, fewer molecules are eliminated.

  • All the properties of the first-order elimination kinetic are different manifestations of this same fundamental process.


Properties of First-Order Elimination Kinetic.

  • Rate of drug elimination is directly proportional to plasma concentration of the drug.

  • Constant fraction of drug is eliminated at constant interval of time.

  • Clearance remains constant.

  • Plasma half-life remains constant.

  • Plot of plasma concentration against time is curvilinear.

  • Single dose of drug is eliminated almost completely eliminated after 5 half-lives.

 

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