Peptide aggregation (revision 11)

Peptide aggregation is the association of peptide molecules with one another. It ranges from reversible self-association into defined oligomers, which may be a stabilising feature of a formulation, to irreversible formation of amyloid-like fibrils or amorphous particles, which is a degradation route.^[1]

The distinction between the two is central and is frequently lost. Liraglutide self-associates into heptamers in its formulation, and that association contributes to its duration of action; human amylin forms fibrils, which is why it could not be developed as a medicine without engineering. Both are "aggregation" in a loose sense and they are not the same phenomenon.^[2]

Aggregation is driven by concentration, by pH near the isoelectric point, by hydrophobic surface exposure, by agitation at air–liquid interfaces, and by temperature. It is the degradation route most likely to be visible: a solution that has gone faintly hazy after reconstitution has changed.^[1]

Aggregation proceeds through partially unfolded or conformationally exposed states rather than from the native state directly. Anything that increases the population of such states — heat, interfaces, shear, extremes of pH — increases the rate.^[1]

Nucleation-dependent aggregation, characteristic of amyloid formation, shows a lag phase during which nuclei form, followed by rapid growth. The lag is why a solution can appear stable for a period and then change quickly, and why seeding with pre-formed aggregate abolishes the lag entirely — a relevant consideration when material is drawn repeatedly from one container.

Interfacial aggregation is the mechanism most often encountered in handling. The air–liquid interface presents a hydrophobic surface at which peptides adsorb and unfold, so shaking a vial is substantially worse than swirling it. The same applies to repeated passage through a narrow needle.^[3]