Ultrasound beam triggers 'nanodroplets' to deliver drugs at exactly the right spot

Conventional drug delivery is often like cracking a nut with a sledgehammer. Whether the drug is swallowed, injected, inhaled, or absorbed through the skin, it ultimately diffuses to most parts of the body, including those where it isn't needed—or where it might even cause harm.

But what if the delivery could be targeted at exactly the right spot? This would allow the total dose to be dramatically lower, thus minimizing side-effects.

Now, scientists from the US have found a way to perfect a promising, emerging method that does just that. With their new protocol, the method is made both safe and efficient for the first time, hopefully paving the way for first-in-human trials. The results are published in Frontiers in Molecular Biosciences.

"Here we show a method to deliver drugs to specific areas of the body where they are needed. We do so using ultrasound waves, which trigger drug release from circulating nanocarriers when focused on the target," said Matthew G Wilson, a graduate research assistant at the University of Utah, and the study's first author. "We developed a method to produce stable nanocarriers repeatably, and identified ultrasound parameters that can activate them."

Nanoengineered droplets

The nanocarriers are minuscule droplets, between 470 and 550 nanometers across, with a hollow outer shell composed of polymer molecules. These polymers have two distinct ends: a hydrophilic one, which mixes well with watery solutions like blood and which faces outward, and a hydrophobic one that doesn't mix with water and which faces inwards.

Within the shell is an inner core of hydrophobic perfluorocarbons, molecules that consist mostly of fluorine and carbon, and that are mixed with an equally hydrophobic drug of interest. The shells keep the cores apart, preventing them from coalescing into a single droplet, and form a barrier against the immune system. The effect is much like mayonnaise, where proteins from eggs form droplets of encapsulated oils, where otherwise the oil and water would separate completely.

To release the drug, the researchers played back an ultrasound—a sound wave with a frequency beyond the upper limit of human hearing—of 300 or 900 kilohertz. The beam of ultrasound can be steered across three dimensions, to focus on a desired area within the body that is only a few millimeters across.

The ultrasound is thought to cause the perfluorocarbons to expand, stretching out the droplet's shell and making it more permeable to the drug, which then diffuses out to the organs, tissues, or cells where it is required.

The researchers compared the efficiency of delivery of a representative drug—the anesthetic and sedative propofol—between three different perfluorocarbons, perfluoropentane (PFP), decafluoropentane (DFP), and perfluorooctylbromide (PFOB). The ultrasound was delivered to the nanodroplets in vitro, in 60 pulses of 100 milliseconds over one minute.