What Are Surfactants, and Why Do They Show Up in Almost Every Biologic Formulation?
If you work with protein therapeutics, you have seen at lest polysorbate 80 on an excipient list. It can be one of the most consequential ingredients in the formulation.
What do surfactants actually do and why do formulators rely on them so heavily? What do they mean for anyone working with protein solutions?
The basic idea: molecules with two personalities
A surfactant is amphiphilic, meaning one part of the molecule is water-loving (hydrophilic) and another part is water-avoiding (hydrophobic). That split personality is the whole story. Because no single environment suits both parts, the molecule migrates to boundaries: air and water, oil and water, liquid and container wall. It settles there with the hydrophilic portion in the water and the hydrophobic portion in whatever the water is up against.
The main families
Surfactants are grouped by the charge on the hydrophilic head.
Nonionic surfactants carry no charge. Polysorbates 20 and 80 and poloxamer 188 belong here, and they are the ones you will see in biologic formulations. Without charge, there is no electrostatic interaction with the protein surface, so they stabilize without denaturing.
Anionic surfactants carry a negative charge. Sodium dodecyl sulfate is the familiar example.
Cationic surfactants carry a positive charge and are common in disinfectants.
Zwitterionic surfactants carry both charges. CHAPS and phospholipids fall into this group, often used in membrane protein work.
For therapeutic proteins, the practical answer is nearly always nonionic, and nearly always a polysorbate.
Why protein formulations need them
Proteins are surface active themselves. A monoclonal antibody has hydrophobic patches that are normally buried in the folded interior. Bring that molecule to an air-water interface and there is a strong thermodynamic incentive to unfold, turn those patches outward into the air, and spread. An unfolded protein at an interface is a nucleation site. It recruits neighbors, and you get aggregates.
Protein solutions meet a lot of interfaces on the way to a patient:
- Air-liquid interfaces during mixing, filling, and shipping agitation
- Container walls, glass and polymer both
- Silicone oil in prefilled syringes
- Interfacial stress of passing through a needle
A surfactant beats the protein to the interface. It is smaller, so it gets there faster, and more surface active, so it holds the position. The protein largely stays in bulk solution, folded.
This is why surfactant use is close to universal in marketed products. Recent tabulations of marketed monoclonal antibody formulations put surfactant inclusion in the low-to-mid 90 percent range, with polysorbate 80 the most common by a wide margin, followed by polysorbate 20 and, occasionally, poloxamer 188.**
The complication: polysorbates degrade
Polysorbates are not single compounds. They are heterogeneous mixtures of fatty acid esters of polyoxyethylene sorbitan, which means lot-to-lot composition varies. They are also chemically fragile, for reasons built into that structure. Two degradation routes matter:
Oxidation of the polyoxyethylene chains generates hydroperoxides, which can then oxidize susceptible residues on the protein, methionine in particular.
Hydrolysis cleaves the ester bonds and releases free fatty acids. Those fatty acids have low solubility and can precipitate as visible or subvisible particles, which is a serious problem for a parenteral product.
Polysorbate degradation is now one of the major challenges in biologics formulation.
TLDR;
A surfactant is a molecule that lives at interfaces because it cannot be fully comfortable anywhere else. In a biologic formulation, that property is used to keep proteins away from the interfaces where they would unfold and aggregate. Nonionic surfactants, above all polysorbate 80, do this job in the overwhelming majority of marketed antibody products. They are not free of problems, they are not present in every protein sample you will encounter, and knowing which situation you are in changes how the material behaves in your hands.
Here are some examples of surfactants:
| Product | Protein conc. | Surfactant | Conc. |
|---|---|---|---|
| Humira (adalimumab), citrate-free | 100 mg/mL | Polysorbate 80 | 1 mg/mL (0.1%) |
| Dupixent (dupilumab) | 150 mg/mL | Polysorbate 80 | 2 mg/mL (0.2%) |
| Rituxan (rituximab) | 10 mg/mL | Polysorbate 80 | 0.7 mg/mL |
| Actemra (tocilizumab) IV | 20 mg/mL | Polysorbate 80 | 0.5 mg/mL |
| Keytruda (pembrolizumab) | 25 mg/mL | Polysorbate 80 | 0.2 mg/mL |
| Stelara (ustekinumab) SC | 90 mg/mL | Polysorbate 80 | 0.04 mg/mL |
| Xolair (omalizumab) PFS | 150 mg/mL | Polysorbate 20 | 0.4 mg/mL |
| Skyrizi (risankizumab) | 150 mg/mL | Polysorbate 20 | 0.2 mg/mL |
| Darzalex Faspro (daratumumab) | 120 mg/mL | Polysorbate 20 | 0.4 mg/mL |
| Eylea (aflibercept) | 40 mg/mL | Polysorbate 20 | 0.3 mg/mL |
| Herceptin (trastuzumab), lyo | 21 mg/mL recon. | Polysorbate 20 | 0.6 mg/vial (150 mg) |
| Hemlibra (emicizumab) | 150 mg/mL | Poloxamer 188 | 0.5 mg/mL |
| Enspryng (satralizumab) | 120 mg/mL | Poloxamer 188 | 0.5 mg/mL |
| Norditropin (somatropin) | 3.3–10 mg/mL | Poloxamer 188 | 3 mg/mL |
Sources:
- **Strickley & Lambert 2021, J Pharm Sci 110(7):2590-2608, DOI 10.1016/j.xphs.2021.03.017 — the paper you sent. It covers 126 commercially available antibodies approved globally between 1986 and February 2021, totaling 136 products, 36 lyophilized and 100 solutions. On surfactants: mostly polysorbate 20 or 80, four containing poloxamer 188, and one that contains no surfactant but uses PEG 3350 instead.