Half-life

Definition

Half-life (often denoted as t½) is a pharmacokinetic parameter that describes how quickly a compound is eliminated from the body. Specifically, it is the time required for the concentration of the compound to fall to half its initial level through metabolism, excretion, or degradation. For example, if a peptide has a half-life of 30 minutes, then 30 minutes after administration, 50% of the dose remains in the body; 60 minutes after administration (two half-lives), 25% remains; at 90 minutes (three half-lives), 12.5% remains. Half-life is an important pharmacokinetic parameter for determining dosing frequency: compounds with short half-lives must be administered frequently to maintain therapeutic concentrations, while compounds with long half-lives require less frequent dosing. Different peptide compounds have markedly different half-lives depending on their structure, their susceptibility to proteolytic enzymes, and their rate of renal clearance. Understanding the half-life of a compound is essential for designing appropriate dosing schedules in both research protocols and clinical applications.

Half-life is related to the rate constant of elimination; compounds with faster elimination rates have shorter half-lives. The relationship between half-life and clearance is described by the formula: t½ = 0.693 / k, where k is the elimination rate constant. Compounds with longer half-lives accumulate in the body when given repeatedly; after approximately five half-lives, a compound reaches a steady-state concentration where the amount administered equals the amount eliminated per dose. For research applications, understanding steady-state is important because effects may not be apparent until steady-state is achieved, and it may take a considerable time for a compound to be completely eliminated after discontinuation.

Natural peptides are often rapidly degraded by proteolytic enzymes in the body, resulting in short half-lives (minutes to hours). To extend half-life, researchers have developed modified peptides that resist enzymatic degradation—such as peptides with D-amino acids, peptides conjugated to polyethylene glycol (PEGylated peptides), or peptides with altered sequences that reduce recognition by proteases. These modifications allow for less frequent dosing, improved convenience, and potentially better patient compliance. The half-life of research compounds is typically determined through pharmacokinetic studies in animal models or human volunteers.