Definition
The time required for a compound's concentration in a given system to fall by half.
Half-life is the time taken for the concentration of a compound to fall by 50%. In pharmacokinetic studies it describes elimination from circulation; in stability studies it can describe degradation in solution or plasma. Native peptides such as GLP-1 and GHRH have half-lives of minutes because enzymes such as DPP-4 cleave them and the kidneys filter them rapidly. Research analogues extend half-life through amino acid substitutions that resist cleavage, fatty-acid chains that bind serum albumin, or covalent albumin-binding groups such as the DAC on CJC-1295. Reported values depend on species, model and analytical method.