Half-life

Half-life is the time required for the measured concentration of a compound to fall to half of its starting value. In peptide research the term is used in two related but distinct senses. Chemical half-life refers to the time over which a peptide degrades in a given buffer, matrix, or storage condition - a stability property of the molecule itself. Biological half-life (more precisely, elimination half-life, often written t1/2) refers to the time over which the compound is cleared from a biological compartment once administered to an organism or added to a living system. Both are usually reported from first-order kinetic fits, in which concentration declines exponentially and the half-life is the natural logarithm of two divided by the elimination rate constant.

Why half-life is a design parameter for peptides

Native peptide hormones are generally cleared from circulation on timescales of seconds to minutes. GLP-1(7-36) amide, for example, has a reported intact-peptide circulating half-life of approximately one to two minutes because the protease DPP-4 cleaves the N-terminal dipeptide and the resulting metabolite is itself cleared renally (Deacon et al., Diabetes 1995 (NLM 7789648)). This short duration is a feature of the hormone's signaling role but a serious obstacle when the goal is a research probe with a predictable exposure window. Synthetic analogs extend half-life through several mechanisms: resistance to proteolytic cleavage (e.g., substitution of the DPP-4-susceptible residue), reversible binding to serum albumin (fatty-acid conjugation), engineered PEGylation, and Fc fusion. Each of these changes extends the measured half-life by shifting the dominant clearance pathway from rapid proteolysis or renal filtration to slower hepatic or receptor-mediated clearance.

How half-life is measured

In vitro chemical half-life is measured by incubating the peptide under controlled conditions (buffer, pH, temperature, matrix) and quantifying intact peptide over time by HPLC or LC-MS. In vivo half-life is measured by collecting serial samples from a dosed organism and quantifying intact peptide by a validated bioanalytical method, typically LC-MS/MS or an immunoassay with specificity for the intact form. Immunoassays that cannot distinguish intact peptide from cleaved metabolites tend to overestimate the apparent half-life and are an important source of discrepancy across published values for the same peptide. Methodological considerations are reviewed in Werle & Bernkop-Schnürch, Amino Acids 2006 (NLM 16622600).

How half-life shows up on Open Assay

Half-life values that appear on Open Assay pages are drawn from published literature on the peptide in question and are cited to their source, not extrapolated from related molecules. Where multiple values exist from different methods (for example, intact-peptide LC-MS vs. total-immunoreactivity ELISA), the source method is identified so a reader can compare values on a like-for-like basis. Values are presented as research findings about the molecule, not as instructions for any kind of administration.