Peptide Half-Life: What the Number Actually Means
A precise quantity with conditions attached. Species, route and in vitro versus in vivo are usually missing from a quoted figure.
Half-life is quoted constantly in peptide literature and misread almost as often. It is a precise quantity with specific conditions attached, and knowing what those are changes how comparisons between compounds should be read.
The definition
Half-life, written t½, is the time for the concentration of a substance in a compartment — usually plasma — to fall by half. It describes elimination, and it assumes first-order kinetics: a constant fraction removed per unit time rather than a constant amount.
Because elimination is fractional, the decline is exponential. After one half-life, 50 per cent remains; after two, 25 per cent; after four, about 6 per cent; after five, about 3 per cent. This is where the working rule that a compound is essentially cleared after four to five half-lives comes from.
Why native peptides are so short-lived
Two independent clearance routes, and both are fast for small peptides:
- Enzymatic degradation — proteases in plasma and tissue. DPP-4 alone accounts for the roughly two-minute half-life of native GLP-1.
- Renal filtration — the glomerulus filters molecules below roughly 60 kDa. Almost every peptide is far below that threshold.
A modification that solves one does nothing for the other, which is why long-acting analogues carry two distinct kinds of change: protease resistance and albumin binding.
Reading a quoted figure
Three things are usually missing from a half-life quoted in isolation.
Species
Rodent and human half-lives can differ by an order of magnitude. Smaller animals have higher metabolic rates and faster clearance. A figure from a mouse study does not transfer.
Route
Half-life describes elimination, but a figure measured after subcutaneous administration is often dominated by absorption rather than elimination — the compound is entering circulation more slowly than it leaves it. This is flip-flop kinetics, and it means the number describes the depot, not the molecule. Intravenous data give the true elimination half-life.
In vitro or in vivo
Plasma stability measured in a test tube gives a half-life for enzymatic degradation only, with no renal clearance in the system. It will be longer than the in vivo figure, sometimes far longer.
Half-life is not duration of effect
This is the most common misreading. The two can differ in either direction:
- A compound that binds its receptor tightly can produce effects well after plasma concentration has fallen.
- A compound triggering a downstream cascade can produce effects lasting far longer than the trigger.
- Conversely, a long half-life at concentrations below a receptor’s threshold produces nothing.
Plasma concentration and biological effect are related, not equivalent.
How it is measured
Serial samples are taken and concentration measured — usually by LC-MS/MS, sometimes by immunoassay. Concentration is plotted against time on a logarithmic scale, where first-order elimination appears as a straight line, and t½ is calculated from its slope.
Many peptides show two phases: a rapid initial fall as the compound distributes into tissues (the distribution or alpha phase) followed by a slower decline as it is eliminated (the beta phase). A single quoted half-life may refer to either, which is another reason figures from different sources are not always comparable.
Our article on half-life engineering covers the modifications that extend it, and the oral bioavailability article covers why route matters so much.
All products and information referenced are for in-vitro research and laboratory use only. Nothing here is medical advice, and no therapeutic claim is made or implied.