Bypassing the Mucus Barrier: A “Slick” Answer

Summer 2007

Nanoparticles coated with a protein called PEG can penetrate mucus for targeted drug delivery.
Nanoparticles coated with a protein called PEG can penetrate mucus for targeted drug delivery.

Though no one who’s ever been diagnosed with cystic fibrosis has ever been cured of it, new research by Justin Hanes and his colleagues offers a potential solution to one of the biggest obstacles impeding treatment of this devastating and chronic illness.

“The gene that could cure cystic fibrosis has been known since 1989. However, the disease hasn’t been cured because no one knows how to deliver the curative gene to cells lining the airways of the lungs,” says Hanes, an associate professor in the Department of Chemical and Biomolecular Engineering. He adds, “A major change in the next decade will be an increased focus on delivery technologies.”

The way most drugs are delivered now is by flooding the body’s bloodstream with them—an approach that can lead to unwanted side effects or drugs that are too weak for effective treatment. Hanes advocates a more targeted approach. He is at work on an aerosol spray that will deliver drug or therapeutic genecarrying particles to the lungs, gastrointestinal (GI) tract, female reproductive tract, and more.

The challenge? Getting past the mucus. The sticky and highly viscous substance— which lines the lungs, eyes, GI tract, and female reproductive tract—is proficient at blocking particles from penetrating the body. This is a good thing when those particles are bacteria or viruses, but bad when they are vehicles for life-saving therapies.

So, in his lab, Hanes and fellow researchers are finding ways to get drug-delivering particles past those sticky mucus linings. Most importantly, they’re discovering how to get higher density nanoparticles (those with more “bang for the buck” in terms of drug concentration) through mucus at a faster pace, thereby beating the body’s speedy attempts to flush its contaminated mucus away.

In a paper published this past January in Proceedings of the National Academy of Sciences, Hanes’ team reported that they had found a material coating called PEG that keeps particles from sticking to mucus. PEG had previously been reported as highly adhesive to mucus, but the team showed that PEG molecules with low enough molecular weight (i.e., smaller versions of the molecule) were not.

In one of the most surprising and important findings, the researchers, including first author Samuel K. Lai (PhD candidate in Chemical and Biomolecular Engineering) demonstrated that particles coated with PEG moved through human mucus almost as fast as they move through water; particles without the coating had previously been shown to be completely immobile in human mucus.

The team also reported that openings in the mucus mesh lining are much larger than previously thought. This, in turn, means that much larger particles than previously believed possible have the potential to pass through the protective mucus barrier, Hanes says. Larger particles are desired for commercial products since they are easier to efficiently load with drugs and are capable of sustaining the release of drug molecules for longer periods of time.

“These findings set the stage for a new generation of nanomedicines that can be delivered directly to affected areas to treat a host of important diseases, such as lung, colon, and cervical cancer, asthma, COPD, inflammatory bowel disease, cystic fibrosis, and more,” Hanes notes.