How to Mix Peptides for Injection

Reconstitution is the step where most material is wasted or ruined. It is not difficult, but it is unforgiving of guesswork. This guide covers what to mix with, how to work out the concentration, the technique itself, and what happens to the solution afterwards.

What reconstitution actually is

Peptides are supplied lyophilised, which means freeze dried into a solid cake or powder under vacuum. Water is removed because peptides in solution degrade: bonds hydrolyse, chains aggregate, and the material loses activity over weeks rather than years. Dry, cold and dark, the same peptide is stable for a very long time.

Reconstitution is simply adding a sterile liquid back to that powder so it can be measured and drawn. Nothing about the process changes the amount of peptide in the vial. A 10mg vial contains 10mg whether you add 1ml or 5ml. What changes is the concentration, and therefore how much liquid corresponds to a given quantity of peptide.

Choosing a diluent

There are three liquids commonly used, and they are not interchangeable.

The benzyl alcohol in bacteriostatic water is what makes repeat withdrawals possible. It suppresses bacterial growth in the vial between uses. Without it, once the stopper has been punctured and the vial has sat, there is nothing preventing contamination from multiplying.

A small number of peptides are documented as being sensitive to benzyl alcohol. Where a supplier specifies plain sterile water, follow that rather than defaulting to bacteriostatic.

Working out the concentration

This is the part that causes the most confusion, and it is one division.

A 10mg vial with 2ml of bacteriostatic water added gives 10 ÷ 2 = 5mg per ml. A 10mg vial with 1ml added gives 10mg per ml. Same vial, same total peptide, different concentration.

Most insulin syringes are marked in units rather than millilitres, and this is where errors creep in. On a standard U-100 syringe, 100 units equals 1ml, so 10 units equals 0.1ml. That relationship is fixed and has nothing to do with what is in the syringe.

There is no correct volume to add. Adding more diluent makes each unit represent less peptide, which makes small quantities easier to measure accurately. Adding less concentrates it. Choose the volume that puts your intended measurement somewhere in the middle of the syringe rather than at the very bottom of the scale, because a two unit error at the bottom of the scale is proportionally enormous.

The technique

1. Let the vial reach room temperature. Taking a vial straight from the fridge and adding liquid encourages condensation and makes the cake harder to dissolve.
2. Wipe both stoppers with an alcohol swab and let them dry. Wet alcohol carried into the vial on a needle is not helpful.
3. Draw your diluent into the syringe to the volume you calculated.
4. Aim at the glass wall, not the powder. Insert the needle at an angle and let the liquid run down the inside of the vial. A jet fired directly into the cake shears the peptide and is the single most common handling mistake.
5. Let it dissolve on its own. Most peptides clear within a minute or two. If it is slow, swirl gently or roll the vial between your palms.
6. Never shake. Agitation denatures peptides and the foam it produces is a sign of damage, not mixing.
7. Inspect the solution. It should be clear. Cloudiness, floating particles or material that will not dissolve means something is wrong and the vial should not be used.

Storage after mixing

Once liquid is added the clock starts. Reconstituted peptide belongs in the fridge at 2 to 8 degrees Celsius, upright, away from light. Do not freeze a reconstituted vial: ice crystal formation during freezing and thawing damages peptide structure, and repeated cycles compound it.

How long it remains usable depends on the peptide and on what the batch documentation states. Bacteriostatic water vials themselves are commonly specified for 28 days after first puncture, which in practice is often the limiting factor rather than the peptide. Treat the supplier's figure as authoritative over any general rule of thumb.

More detail in storage and stability.

Where pens differ

None of the above applies to a pre-filled pen. The solution is already made up at a fixed concentration, and dose is set by dialling units on the mechanism rather than by drawing to a mark. That removes the arithmetic and the reconstitution error, at the cost of flexibility. See pens compared with vials for how the two presentations differ in practice.

Common mistakes

• Firing diluent directly onto the powder rather than down the vial wall.
• Shaking to speed dissolution.
• Confusing units with millilitres on an insulin syringe.
• Recalculating concentration when changing vial size but forgetting to change the diluent volume.
• Using plain sterile water for a vial that will be drawn from repeatedly.
• Freezing a reconstituted vial to make it last longer.
• Using a vial that did not go fully clear.

Related reading

Current specifications and batch documentation for individual compounds are listed at The PED Shop. Background on the SerenQ pen range is at seren-q.com.