What is Half-life?
Also called: t½, Elimination half-life, Plasma half-life
The time taken for the concentration of a compound to fall to half its starting value; for peptides it can describe clearance from the body, breakdown in plasma or loss in an assay medium.
By the APL Research Team · Updated
A half-life (t½) is the time over which a concentration falls by half. When loss follows first-order kinetics, a fair approximation for many peptide degradation and clearance processes, the half-life is constant and the fraction remaining after time t is (½)^(t / t½), whatever the starting concentration.
Which half-life?
The same word covers measurements that are not interchangeable:
| Type | What is measured | Published example |
|---|---|---|
| In-vitro stability | Loss of intact peptide in plasma, serum or medium at 37 °C | GLP-1(7-36)amide in human plasma in vitro: 20.4 min, through N-terminal cleavage by dipeptidyl peptidase-IV [1] |
| Plasma elimination, long-acting analogue | Fall in blood concentration after administration to animals | Semaglutide after intravenous administration in minipigs: 46.1 h [2] |
| Plasma elimination, unprotected peptide | As above | BPC-157 in rats and beagle dogs: under 30 min [3] |
Species, route and matrix all change the value. A rodent plasma half-life says little about a human one, and neither predicts how long a peptide survives in a culture well. Shelf life is a separate idea again: degradation of stored material is normally reported as percentage loss over a period, not as a half-life.
How half-lives are engineered
The GLP-1 example shows both problem and solution. In human plasma, dipeptidyl peptidase-IV removes the hormone's first two residues, and inhibiting that enzyme prevented the cleavage entirely [1]. Semaglutide's design combined two substitutions relative to human GLP-1 (Aib at position 8, Arg at position 34) with a fatty-acid side chain on lysine 26; the aims were full stability against metabolic degradation and higher albumin affinity, and the fatty acid and its linker were the key features behind prolonged exposure [2]. Other common strategies include D-amino acids, terminal acetylation or amidation, and cyclisation. Its pharmacology at the GLP-1 receptor is covered in the semaglutide research guide.
Worked example: a peptide in a culture well
The arithmetic is unforgiving. With the 20.4 min in-vitro half-life above, the fraction of intact peptide left after a 2 h incubation is (½)^(120/20.4) ≈ 1.7%.
| Half-lives elapsed | Fraction remaining |
|---|---|
| 1 | 50% |
| 3 | 12.5% |
| 5 | 3.1% |
| 10 | 0.1% |
An experiment that adds a labile peptide once and reads out 24 hours later may be measuring the response to a brief pulse, or to breakdown products, rather than steady exposure. BPC-157 illustrates the point: in its rat and dog pharmacokinetic study, radiolabelled peptide was rapidly broken down in vivo into small fragments and then single amino acids [3]. Measuring the intact peptide in the actual medium, for example by HPLC at the start and end of an incubation, turns an assumption into data.
Common misunderstandings
- Half-life is not duration of effect. Downstream signalling can outlast the peptide, or end long before it is cleared.
- Ten half-lives is not zero. About 0.1% remains, which can still matter for a picomolar agonist.
- Albumin-dependent half-lives need albumin. A long half-life built on albumin binding does not carry over to serum-free medium, and in serum-containing medium the same binding lowers the free concentration.
- Chemical and enzymatic loss add together. Oxidation and deamidation continue alongside proteolysis.
The peptide half-life guide goes further, and in vitro covers how stability is tested outside a living system.
References
- 1.Deacon CF, Johnsen AH, Holst JJ. Degradation of glucagon-like peptide-1 by human plasma in vitro yields an N-terminally truncated peptide that is a major endogenous metabolite in vivo. J Clin Endocrinol Metab. 1995. PubMed 7883856
- 2.Lau J, Bloch P, Schäffer L, et al. Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide. J Med Chem. 2015. PubMed 26308095
- 3.He L, Feng D, Guo H, et al. Pharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol. 2022. PubMed 36588717