Kinetic ‘order’ is a term which simply means that the various processes are ranked in order of increasing complexity. The important kinetic processes which are concerned are those governing the entry of the drug into the blood, its distribution in body tissues, and its elimination from the blood by metabolism and excretion. Such processes are generally of ‘zero-order’ or ‘first-order’.
1. Zero-Order Kinetics:
When the kinetic processes (absorption, distribution and elimination of a drug) is independent of the amount of drug undergoing the process is known as ‘zero-order’ kinetics.
For such a process the plot of drug plasma concentration against time is linear.
Example of Zero-order Processes:
(i) Entry of a drug into the circulation during i.v. infusion.
(ii) The absorption of many depot forms of administration.
(iii) Saturable metabolism.
2. First-Order Kinetics:
In first-order kinetics, the rate of change of drug concentration is proportional to the amount of drug remaining. The rate of disappearance of a drug is expressed as;
Where β is first-order constant for elimination and is expressed in units of time (e.g./hr), x = amount of drug.
Most Kinetic processes affecting drug disposition in therapeutic practice are first-order. For such a process the plot of plasma drug concentration against time, is curvilinear however, the plot of the logarithm of the plasma concentration against time is linear.