When it comes to mucus, bigger is better
Larger nanoparticles are better than smaller ones at penetrating human mucous membranes, according to US researchers who say this holds hope for using such particles to deliver drugs in the body.
"In contrast to the prevailing belief, these results demonstrate that large nanoparticles, if properly coated, can rapidly penetrate physiological human mucus," write Lai and team.
"And they offer the prospect that large nanoparticles can be used for mucosal drug delivery."
Worldwide, medical researchers are investigating the potential for delivering drugs to different parts of the body by encapsulating them in small particles.
Scientists prefer larger particles because they can carry more of the drug.
But until now scientists thought that those larger than 200 nanometres wide would not get through the protective mucus barriers that line surfaces such as the gastrointestinal tract, lungs and vagina.
The researchers tested the rate of transport of tiny test particles through fresh human cervicovaginal mucus, which has similar properties to mucus elsewhere in the body.
They found that large polystyrene particles 200-500 nanometres wide penetrated the mucus faster than 100 nanometre particles.
"Much larger fractions of the 100 nanometre particles were immobilised or otherwise hindered by mucus than the large 100-500 nanometre particles," the research team says.
The particles were coated in polyethylene glycol, a chemical that minimises interactions of the particle with the surface of the mucus.
This enabled the researchers to focus on just the role of the particle size in transport through mucus.
The researchers suggest that the smaller particles were slowed down because they interacted more the mucus surface than the larger particles.
Only the first step
Australian pharmacologist Professor Rod Minchin of the University of Queensland says the findings are "groundbreaking".
"What they've found is the opposite of what people would have thought," says Minchin, who is also studying nanoparticle interaction with the body.
"Most people have been saying the smaller the nanoparticle the better it's likely to move through mucus."
He says the researchers have convincingly showed the ability to penetrate mucous membranes that are a barrier to getting drugs into the body.
But, he says, there are other barriers that also need to be penetrated.
Some, like the cell layer, is penetrated better by smaller nanoparticles, says Minchin.
"So there is going to have to be a balance in size here [depending on the application]," he says.
Safety of nanoparticles?
There are many unanswered questions about the safety of some nanoparticles, including those used in consumer products such as sunscreens and cosmetics.
One of the key questions is whether nanoparticles will penetrate various surfaces in the body.
But Minchin says the current study sheds no light on such questions.