part 25: CHEMICAL KINETICS

Chemical Kinetics

Study of the rates of reactions and the mechanism by which these reactions occur

Degradation Rate

Rate of Reaction is also known as?

Rate of Reaction

• The velocity with which the reactions occur

• Depends on:

  • Reactant concentration

  • Temperature

  • pH

  • Presence of solvents or additives

Order of Reaction

The way in which the concentration of the drug or reactant in a chemical reaction affects the rate

Constant amount

Drug loss of Zero order

Constant fraction

Drug loss of First order

Independent

Concentration of Zero order

Dependent

Concentration of First order

conc/time

Unit of Zero order

per time

Unit of First order

Variable

Half-life of Zero order

Constant

Half-life of First order

1. Theophylline

2. Aspirin

3. Phenytoin

4. Ethanol

remember, “TAPE”

Example of drugs under Zero order

Most drugs!

Example of drugs under First order

Zero order elimination

• The same amount of drug is eliminated per unit of time

• Regardless of the plasma conc of the drug, the rate of elimination is constant

First order elimination

• The same proportion of the drug is eliminated per unit of time

• This leads to a variable amount eliminated based on the plasma conc

• As the con drops, the elimination rate drops as well

Half-life

The period of time required for the concentration of a drug to decrease by one half

Shelf life

The period of time where 90 % of the original concentration is left

c = -kt +Co

General formula of Zero order reaction

Inc = -kt +InCo

General formula of First order reaction

t1/2 = 0.5 Co/K

Half-life of Zero order reaction

t1/2 = 0.693/K

Half-life of First order reaction

t90 = 0.1 Co/K

Shelf life of Zero order reaction

t90 = 0.105/K

Shelf life of First order reaction

conc/time

Units of k of Zero order reaction

per time

Units of k of First order reaction

89 mg/mL

Sample problem: CHEMICAL KINETICS

A suspension (125 mg/ml) decays by zero order kinetics with a reaction rate constant of 0.5 mg/ml/hr. What is the concentration of the active drug remaining after 3 days?

will remain after 120 days = 4.71 mg/mL

half-life = 1386 days

shelf life = 210 days

Sample problem: CHEMICAL KINETICS

An ophthalmic solution of a mydriatic drug at 5 mg/mL exhibits 1st order degradation with a k=0.0005/day. How much will remain after 120 days? Compute also for the half life and shelf life

3 mg/L

Sample problem: CHEMICAL KINETICS

If the plasma concentration just after a gentamicin dose is 10 mg/L and the patient's elimination rate constant is 0.15 hr-1, predict what the plasma concentration will be 8 hours later.

3 mg/L

Sample problem: CHEMICAL KINETICS

Using the equation C = C0e -Kt, determine the plasma concentration of a drug 24 hours after a peak level of 10 mg/L is observed if the elimination rate constant is 0.05 hr-1 .

7.5 mg/L

Sample problem: CHEMICAL KINETICS

For a drug that has an initial plasma concentration of 120 mg/L and a half-life of 3 hours, what would the plasma concentration be 12 hours after the initial concentration?