The plasma level time curve is plotted after measured drug concentration in plasma of an animal at different time intervals. The concentrations of drug in each plasma sample are plotted on an ordinary graph paper against the corresponding time at which the plasma samples were collected.
As the drug reaches in systemic circulation, plasma drug concentration will rise to a maximum. The absorption of a drug is more rapid than its elimination. As the drug is being absorbed into systemic circulation, the drug is distributed to all the tissues in the body and is also simultaneously being eliminated.
Elimination of a drug can proceed by excretion or metabolism or combination of both. The relationship of the drug level time-curve and pharmacological parameters for a drug is given in figure 2.2. Minimum effective concentration (MEC) of a drug is the concentration of drug in plasma that is required for producing an effect.
For antibiotics, it is minimum inhibitory concentration (MIC) for preventing the growth of microorganism. The MIC of an antibiotic differs for different microorganism.
MTC is the minimum toxic concentration of a drug. The onset time is the time required for the drug to reach the MEC or MIC. The intensity of the pharmacological effect is proportional to the number of drug receptors occupied which is reflected in observation that high plasma drug concentrations produce a greater pharmacological response, up to a maximum.
The duration of a drug action is the difference between the onset time and the time for the drug to decline back to the MEC or MIC.
Sometimes, the term peak plasma level or maximum plasma concentration (Cp max), time for peak plasma level (tCp, max) and area under plasma drug concentration (AUC) also come into pharmacokinetic consideration (Fig. 2.3) The peak plasma level is the term often used for the attainment of the highest plasma level of a drug when given by other than the i.v. route.
The same is known as Cp max when the drug is given through i.v. route. The tCp max is the time of maximum drug concentration in the plasma and is roughly proportional to the average rate of drug absorption.
The peak plasma level or Cp max is usually related to the dose and rate constant for absorption (Ka, after oral or i.m.) and distribution (α, after i.v.) as well as elimination (β) of a drug. The AUC is related to the amount of drug absorbed systematically.
