Reconstitution 101

Lyophilised peptide arrives as a dry cake of unknown volume. Reconstitution is simply choosing how much bacteriostatic water to add so that a convenient amount lands on a readable mark of a U-100 syringe.

The one equation

Concentration is milligrams times one thousand, divided by millilitres of water. Everything else follows from it.

Once concentration is known, the volume for any amount is that amount divided by concentration, and the syringe units are the volume times one hundred — because a U-100 syringe reads 100 units to the millilitre.

Choosing the diluent volume

There is no single correct volume. More water gives a lower concentration and more syringe units per amount, which is easier to read accurately but uses more of the vial per draw. Less water concentrates the solution, which means fewer units and a larger relative error from any inaccuracy in the draw.

A practical target is a volume that puts a typical amount somewhere between 10 and 50 units, where a U-100 barrel is easy to read.

What quietly ruins a batch

• Injecting the water directly onto the cake rather than down the vial wall — the force can shear the peptide.
• Shaking instead of swirling. Agitation denatures peptides; gentle swirling until dissolved is enough.
• Reconstituting into a vial that has warmed to room temperature and then refreezing it, which cycles the solution through phase changes.
• Assuming the label mass is exact. Vials are filled to a nominal mass, and net peptide content is lower than gross weight once counterion and residual water are accounted for.

After it is in solution

Reconstituted peptide is far less stable than the dry cake. Bacteriostatic water contains benzyl alcohol, which suppresses microbial growth and is what makes multi-draw use possible at all; sterile water without it does not. Refrigerate, keep it out of light, and treat published stability windows as estimates that depend on the specific peptide.