Can a drug follow both zero order and first order kinetics?

 

Can a drug follow both zero order and first order kinetics?

·         Any exogenous substance including drugs when administered by any route to the human body undergoes metabolism and elimination.

·         Lipid soluble drugs must first undergo metabolism, get converted into water soluble compounds and are then eliminated.

·         Water soluble drugs  get directly eliminated but sometimes, a part of them also undergo metabolism.

·         The process of metabolism and elimination are carried out by enzymes and transporters.

·         Most enzymes and transporters  in the body obey saturation kinetics.

·         So theoretically, all drugs will show first order kinetics at low concentration in plasma and zero order kinetics at high concentration in plasma.

·         However, in clinical pharmacology, we are concerned only about the therapeutic range because the dose and dose interval are planned to keep the drug concentration in the therapeutic range.

·         So, drugs for which the enzymes and transporters are yet not saturated in therapeutic range are said to obey first order kinetics. And drugs for which the enzymes are saturated in therapeutic range are said to obey zero order kinetics.