Medications such as Ozempic and Mounjaro are composed of molecules called acylated peptides that are designed to circulate in the body and regulate insulin production. This enables adults with Type 2 diabetes to take a weekly injection instead of monitoring their insulin levels every few hours. With slight modification, this class of therapeutics is also approved for weight-loss use in the treatment of obesity.

But these molecules sometimes become unstable when in contact with certain container surfaces, making formulation and manufacturing of these types of medications challenging.

University of Delaware Professor Norman Wagner and a team of researchers worked with partners at the pharmaceutical company Eli Lilly to study why this class of materials experiences this instability, a phenomenon known as ouzo formation, which can make the solution cloudy when in contact with certain surfaces designed to repel water, rendering the medication unusable. 

The research team studied how the peptides behaved when in contact with surfaces such as glass, thermoplastics and synthetic polymers, to understand the fundamental mechanics behind what was happening in order to provide solutions for reducing drug product manufacturing failure. 

The researchers reported their findings in a paper in the Proceedings of the National Academy of Sciences (PNAS), offering valuable insights to guide peptide synthesis, formulation, manufacturing and storage of this class of molecules. It’s work that could help across this class of medications.

In addition to Wagner, co-authors on the paper from Eli Lilly and Company include Ken Qian, director, and Kevin Seibert, vice president. Other co-authors from UD include the paper’s lead author Qi Li, a former postdoctoral researcher in UD’s chemical and biomolecular engineering department and Center for Neutron Science, and Vasudev Tangry, a former undergraduate student on the project.

Stability is key

The commonality, Wagner explained, is that drug formulations need to remain stable in solution, so the molecules don’t form large aggregates, which can be harmful if injected into the body. 

“It's a fairly tricky pathway to go from a stable molecule in solution and formulation to an unstable one,” said Wagner, Unidel Robert L. Pigford Chair, chemical and biomolecular engineering at UD. “It involves the molecule adsorbing, or adhering, onto a surface like a container and creating small aggregates that become a nuclei for the growth of visible aggregates, leading to a solution that looks like the classic drink ouzo.” 

Ouzo is a Greek liquor that is stable on its own but becomes cloudy when mixed with water. The cloudiness occurs because the anise contained in the liquor is not water soluble, so the droplets stay in solution, creating the drink’s murky appearance.

This might be okay for a refreshing libation, but when it happens to molecules in the pharmaceutical industry, it's problematic. It also can be costly.

“So, I might have a vessel where I am making this molecule and, in final stages, this vessel might contain something like the equivalent of over a million dollars’ worth of drug substance,” said Wagner. “If that solution forms an ouzo, you've got a real problem. You've now lost that production.”

It’s not just a production problem either. If a formulation of drug product formed an ouzo while being transported to or stored at a doctor's office, the result would be the same—it would have to be thrown out. So, on both the production side and on the storage and delivery side of the formulation, preventing this from occurring is critical. 

How they approached the problem

The Wagner group’s work focused on predicting what surfaces cause problems and how rapidly this ouzo effect might occur. The research team used light and X-ray scattering along with other techniques to investigate how the molecules interacted with each other and solution. They also looked at how the molecules interacted with numerous different types of surfaces, ranging from glass to polystyrene and polytetrafluoroethylene, all common materials used in the industry. 

The researchers measured the spherical shape, size, structure and the internal composition of the droplets in the solution, too. Their analysis revealed that the droplet formation was sparked by the water-repellent nature of the surfaces and depended on the rate at which the particles were stirred and mixed. The size of the particles was affected by the solution’s salt concentration, independent of the surface material. Interestingly, it seems that the particles remained in solution (rather than sinking to the bottom) due to interaction between the surface tension of the solution and the particles’ electric charge. 

While explaining, Wagner gave the example of oil and water — a classic example of two types of molecules that separate when left alone. In salad dressing, two solutions are emulsified, shaken and mixed up, while products called surfactants sit at the interface between the solutions to prevent the molecules from aggregating and separating. This allows salad dressings to remain mixed over long time periods.