Question

Three patients in your ward are due to receive a single oral dose of the same drug. The first patient is getting a 60mg dose, the second patient is getting a 120mg dose, and the third patient is getting a 180mg dose.

Assuming that all else is equal in these patients and that there is no unusual pharmacokinetics going on:

Predict what will happen (increase/decrease/no change) to half-life, Cmax and Tmax with these increasing oral doses if all other factors remain constant.

Answer 1

Pharmacokinetics means that the study of the time course of drug absorption, distribution, metabolism, and excretion. Pharmacokinetics is the study of what the body does to the drug. But pharmacodynamics is the study of what the drug does to the body.

The duration of action of a drug is known as its half life. This is the period of time required for the concentration or amount of drug in the body to be reduced by one-half.

• The half-life of a drug is the time taken for the plasma concentration of a drug to reduce to half its original value.
• Drugs which follow the first order kinetics have a constant half life. Drugs following zero order kinetics do not have a constant half life. Their increased dose increases plasma concentration which increases half life.
• In toxic doses, drugs follow the zero order kinetics. Certain drugs follow zero order kinetics in therapeutics after a small increase in dose.

Here it is evident that there is no unusual pharmacokinetics going on. Hence the half life is constant, no change in the half life.

• Cmax is the maximum or peak serum concentration that a drug achieves in a specified compartment or test area of the body after the drug has been administered and before the administration of a second dose.
• Tmax is the time to peak drug and/or metabolites concentration.
• After the oral administration, Cmax and Tmax are dependent on the extent, and the rate of drug absorption and the disposition profile of the drug.
• Drug absorption is the movement of a drug into the bloodstream after administration. Drug absorption is determined by the drug's physicochemical properties, formulation, and route of administration.

Here all the doses are administered via oral routes. Hence the Cmax and Tmax remains same.