Zero-Order Half-Life

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Chapter: Biopharmaceutics and Pharmacokinetics : Pharmacokinetics Basic Considerations

Half-life (t½) or half-time is defined as the time period required for the concentration of drug to decrease by one-half.


Zero-Order Half-Life

Half-life (t½) or half-time is defined as the time period required for the concentration of drug to decrease by one-half. When t = t½, C = Co/2 and the equation 8.7 becomes:


Equation 8.9 shows that the t½ of a zero-order process is not constant but proportional to the initial concentration of drug Co and inversely proportional to the zero-order rate constant Ko. Since the zero-order t ½ changes with the decline in drug concentration, it is of little practical importance. Zero-order equations do not require logarithmic transformations.

Examples of zero-order processes are –

1. Metabolism/protein-drug binding/enzyme or carrier-mediated transport under saturated conditions. The rate of metabolism, binding or transport of drug remains constant as long as its concentration is in excess of saturating concentration.

2. Administration of a drug as a constant rate i.v. infusion.

3. Controlled drug delivery such as that from i.m. implants or osmotic pumps.

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