polysorbate

A polysorbate is a gentle, soap-like ingredient added in tiny amounts to protein medicines to stop the protein from sticking to surfaces and clumping. Proteins are uncomfortable at boundaries — the surface of an air bubble, the inner wall of a vial or syringe — and tend to unfold and aggregate there. A polysorbate, being part water-loving and part oil-loving, rushes to coat those surfaces first, leaving no room for the protein to unfold against them.

Chemically, polysorbates are nonionic surfactants: they carry no charge and have a water-friendly head with fatty tails. The two most used in injectables are polysorbate 20 and polysorbate 80, differing in their fatty-acid tail. Because they lower surface and interfacial tension and form micelles above a certain concentration, only a very small amount (often a fraction of a percent) is needed to protect the protein during filling, shipping (with its shaking), freezing, and storage.

Their role is specifically protection against surface-induced and agitation-induced aggregation, which is why they appear in so many antibody and protein formulations. They are well tolerated and have a long history of safe use, but they are not inert bystanders: they slowly degrade by oxidation and hydrolysis, releasing fatty acids and peroxides that can themselves harm the protein or form their own particles.

An honest caveat: the very surfactant meant to protect the protein can become a liability if it breaks down. Modern formulation work pays close attention to polysorbate quality, the right concentration (too little fails to protect, too much can cause its own issues), and monitoring its degradation over shelf life.