Lyophilisation

Lyophilisation, or freeze-drying, removes water from a frozen solution without ever passing through the liquid state. The solution is frozen, chamber pressure is dropped below the triple point of water, and ice sublimes directly to vapour during primary drying; secondary drying then desorbs the water still bound to the solid at a raised shelf temperature. What remains is a porous cake of peptide plus whatever excipients were dissolved with it.

Formulation does most of the work. Bulking and stabilising agents such as mannitol, sucrose or trehalose set the collapse temperature, and primary drying must stay below it or the cake slumps and retains water. Residual moisture is typically specified in the low single-digit percentages and verified by Karl Fischer titration. Many marketed peptide and protein presentations, including glucagon rescue kits and several somatropin products, ship as lyophilised powder for exactly this reason.

The point of the exercise is shelf life. Hydrolysis, deamidation and aggregation all need water and molecular mobility, so removing the solvent converts a product with a refrigerated shelf life measured in weeks into one measured in years. It also fixes composition at the moment of fill: what the vial holds is decided by the solution that went into it, not by anything the powder does afterwards.

Cake appearance gets badly over-read. A shrunken, cracked or partly collapsed cake indicates a process deviation and correlates loosely with higher residual moisture, but a handsome cake is not evidence of identity, purity or content, and much of the visible bulk is often excipient and salt rather than peptide. Weighing a cake tells you even less, since counterion and water together can account for a fifth or more of its mass.