January 12, 2009

Tiny Bubbles
By Dan Harvey
Radiology Today
Vol. 10 No. 1 P. 16

Researchers investigate microbubbles smaller than a red blood cell for drug delivery.

In 2004, Philips Research, a division of Netherlands-based Royal Philips Electronics, embarked on new research that could change chemotherapeutic regimens: using microbubbles for drug delivery.

Specifically, the research involves in vivo microbubble destruction, an event that’s far from a calamity. This ultrasound-enabled internal explosion can be harnessed to deliver a drug payload to a specific region within the body, such as a tumor site.

Technological Adaptations
Discerning the latent drug-delivery potential inherent in microbubbles, Philips researchers are adapting existing technology to enable these cell-sized tools to deliver precise drug dosages to a tightly targeted area within the human body. They have integrated microbubble modifications with new ultrasonic developments.

“When we started, we envisioned a combination of contrast agent and drug-delivery capabilities, where contrast could track the path of the delivery vehicle that would culminate in a drug-releasing trigger,” says Steve Klink, senior communications manager for Philips’ healthcare research programs. “That vision directed us toward microbubbles combined with advanced ultrasound technology.”

Gas-filled microbubbles have been widely applied as contrast agents in the clinical setting. This application proved effective and gained acceptance as microbubbles engender better ultrasonic reflection than what’s provided by blood or soft tissue. Within the envisioned new therapeutic approach, ultrasound would not only provide the necessary, optimal, noninvasive imaging but also serve as the subsequent release trigger.

“We envision that we will be able to inject drug-filled microbubbles into a patient’s bloodstream then follow those bubbles with ultrasound imaging,” says Klink. “When the bubbles reach the target site, we’d then deploy a high-energy, tightly focused ultrasound wave to burst the bubble, releasing the encapsulated drug.”

The researchers believe that this approach could have an enormous impact on drug treatments, specifically related to different forms of cancer, as it could pack chemotherapy drugs within microbubbles directed toward tumor sites. At the same time, the microbubbles’ contrast capabilities make it easier for radiologists and technologists to locate tumors in the first place.

Cellular Size
Microbubbles provide an optimal, internal, minimally invasive drug-delivery vehicle. They measure 2 to 4 microns, which is smaller than the size of a red blood cell, which measures 6 to 8 microns. This dimension enables these small vehicles to easily course within the flow of the smallest blood vessels. Once injected, microbubbles can transport their backpacked drug cache throughout the circulatory system, with their path tracked by ultrasonic imaging. Ultimately, when the microbubbles reach their destination, a highly focused ultrasound pulse creates an explosive force that ruptures the microbubbles, a microscopic cataclysm that not only destroys the bubble but deposits the drug at the targeted internal location.

Philips believes that this microbubble/ultrasound-based drug-delivery system would increase the therapeutic window by increasing the drug concentration at the site of interest. It is hoped that this will increase therapeutic efficiency while reducing deleterious chemotherapeutic side effects. As the drugs would mainly be released at the disease site, drug uptake in certain vital organs would be greatly limited. “Conversely, traditional chemotherapy is comprehensive in that it impacts all parts of the body,” says Klink. Thus, the treatment approach system theoretically might not impact a patient’s quality of life or sacrifice treatment efficacy.

However, as good as this sounds, the systemic benefits reside in the speculative realm. Actual clinical application is a long way off, as the system has only been tested in preclinical trials.

Philips is developing the technology in collaboration with several academic partners, including the University of Virginia and the University of Münster in Germany. Meanwhile, healthcare organizations such as The Methodist Hospital in Houston are actively researching the viability of ultrasound-mediated drug delivery. Philips is also seeking to partner with a pharmaceutical company to foster further development.

“This represents one of our first ventures into the pharmaceutical area,” Klink says. “We’re engaged in other pharmaceutical projects, but this is the first that involves image-guided drug delivery, and we know we can’t accomplish this alone. Thus, we’re looking to work with a pharmaceutical company in a collaboration where we’d provide the technological tools and expertise. Currently, we’re engaged in active discussions with several companies to develop research studies.”

Shell Shock
In developing the technique, Philips researchers confronted several substantial challenges. One involved the actual construction of microbubbles that serve as drug-carrying vehicles. Typically, microbubbles currently utilized as contrast agents are contained within a shell manufactured from albumin or lipids, with an interior filled with gas or air. However, for its purposes, Philips needed to develop a microbubble with a different external and internal composition.

“While current, clinically applied microbubbles possess an albumin or lipid shell, we developed one that has a shell comprised of a biodegradable polymer,” says Marcel Böhmer, PhD, a principle scientist for Philips Research. Based in Eindhoven, the Netherlands, Böhmer is the company’s expert for the contrast agent/drug-delivery vehicle. “That composition makes them more stable within the circulatory system and provides them with a much better defined size distribution,” Böhmer adds. “Also, compared to the other shells, ours have an efficient activation because we can make the microbubbles fit inside a pretty thin shell. A lot of work went into finding the precise polymer. Ultimately, the polymer type, when combined with the delivery system, provides efficient drug release.”

The researchers also developed a unique method to synthesize microbubbles so that they are filled with one part drug and one part gas. According to the company, the microbubble synthesis involved the creation of a solution mixture that contains a carrier solvent—an oil removable by freeze-drying, in which the drug is dissolved—and the polymer. Droplets of this solution are placed in water, which subsequently leads to the formation of the polymer capsules. Finally, the created capsules undergo filtering and freeze-drying. The end result is a microbubble that contains both gas and drug.

Moreover, the microbubbles retain their contrast-enhancing ultrasonic properties, which make them a good vehicle for image-guided drug delivery. “Simply stated, the air or gas enables the bubbles to keep their contrast capabilities while making it easier to burst the bubbles,” says Klink.

In addition, the process provides accurate control of microbubble size and shell thickness, as well as the amount of the loaded drug. Size and thickness need to be precisely controlled so the microbubbles rupture at the correct ultrasound frequencies and energies.

“These imaged microbubbles should rupture when subjected to the appropriate acoustic pressure,” says Böhmer. “The drug releases upon explosion. So think of the bubble as half filled with liquid and half filled with gas, and the gas is the finger on the trigger.”

Ultrasonic Challenge
The challenge for the ultrasound component of microbubble delivery involves accurately focusing an ultrasonic pulse that ruptures the bubbles and releases the drug into the tissue. The researchers developed a computer-controlled ultrasound device that focuses the pulses required to rupture the microbubbles. The pulses are accurately targeted to the disease site due to an innovative, phased-array transducer that focuses the pulse into a small, ellipsoid volume that can reach as deep as 10 centimeters into the target tissue. The prototype phased-array transducer coaligns with a standard clinical imaging transducer, enabling simultaneous imaging and drug delivery.

“When employing ultrasound, you need one device that delivers the therapy and another that monitors therapy progression,” says Ralf Seip, PhD, director of Philips Research’s ultrasound mediated delivery venture. “Using ultrasound as our energy source, we accomplish that with a system that combines both.”

To monitor the microbubbles as they travel through the bloodstream, Philips uses off-the-shelf ultrasound scanners that are widely applied in clinical settings. “Typically, monitoring occurs at high frequencies—between 5 to 10 megahertz—which provides good resolution and enables you to visualize the flow of the bubbles contained within the agents throughout the body,” explains Seip.

“Once the microbubbles reach the area of interest, we use the same kind of ultrasound technology but delivered through another, high-powered transducer that operates at lower frequencies, which can penetrate deep within the body to oscillate the bubbles or, when necessary, to actually rupture them,” he continues.

Seip notes that, with the system, tissue wouldn’t be damaged. “Ultrasound can have a thermal effect and a pressure-mediated effect,” he explains. “We only use pressure to break the bubbles, as differentiated from high-intensity, focused ultrasound, which is a thermal mechanism that could literally cook the tissue. But that’s not what we’re doing. The tissue temperature doesn’t change at all, so it doesn’t get damaged. Rather, the system only bursts the bubbles, so that they can release their payload.”

However, it is possible that the transducer could be optimized to produce thermal heating and other bioeffects, such as thrombolysis, that could present possibilities for other therapy options, according to Philips. Localized payload delivery via microbubbles also could find future applications in gene therapy.

Early Research
Seip reports that Philips is currently developing an ultrasound device that can be used in clinical studies when the research reaches that point, which is currently well down the road. Philips researchers have tested and found promising results using microbubbles to deliver the cancer drug Paciltaxel in mice models. While these preclinical tests suggest that localized drug delivery is feasible, the company is only in the early stages of developing the technology. Böhmer believes it may take as long as five years before clinical trials on humans begin.

“In our preclinical work, we have looked at the effects of drug release and the matching of the proper ultrasound settings with the exact composition of the microbubble agents,” Böhmer says. “This remains a work in progress, and it will take awhile to translate all of the preclinical data into a working clinical system.”

But, as Klink notes, researchers continue to be driven toward establishing an ultrasound-mediated drug delivery technique that assumes control of drug delivery inside the human body. “The end point is to place the right drugs at the right place at the right time, which widens the therapeutic window and increases therapy efficiency and efficacy,” he says. “Ultrasound imaging and therapy will lead us toward this level of control; at least, that is our expectation.”

— Dan Harvey is a freelance writer based in Wilmington, Del., and a frequent contributor to Radiology Today.