New capabilities for a cutting-edge in vivo imaging method: activatable photoacoustic contrast agent

The recent invention of an imaging method that promises to bridge the gap between microscopy and current low-resolution imaging methods used in the clinic, such as X-ray tomography, has spurred the introduction of a host of new contrast agents to enable detection of specific molecules. This imaging method, called photoacoustic tomography (PAT), was established roughly ten years ago and detects the ultrasonic waves produced when colored molecules absorb light. All photoacoustic imaging agents so far introduced detect disease by binding to proteins present at higher-than-normal concentrations in disease sites. As even unbound contrast agents produce signal, however, considerable background signal may remain; to more clearly distinguish disease sites from healthy tissue, activatable contrast agents that only produce signal upon encountering disease characteristics could be adapted from those already in use in optical imaging.

A research group at Stanford led by Jianghong Rao recently saw an opportunity for the materials they have developed for use in solar cells to serve as photoacoustic imaging agents. Because these semiconducting pi-conjugated polymers are designed to absorb near infrared light efficiently, the investigators hypothesized that they would produce high photoacoustic signal. Further, by incorporating a dye that’s inactivated by certain reactive oxygen species (ROS) into nanoparticles composed of one of these polymers, they created an “activatable” agent. ROS are present at higher-than-normal levels in atherosclerotic plaques, tumors, and degenerating brain tissue, so many ROS-activated imaging agents have been developed towards the goals of early detection and examining the role of ROS in the progression of these diseases. In the present design, the ratio of the signal of the semiconducting polymer (which remains constant) to that of the ROS-responsive dye correlates with the concentration of ROS with which it reacts.

ROS sensing by polymeric photoacoustic contrast agent. ROS inactivate the dye (IR775S, blue) that produces photoacoustic (PA) signal in response to irradiation at 820 nm, while signal upon 700 nm irradiation remains constant. SP1, superconducting polymer 1; DPPC, surfactant. Reprinted by permission from Nature Publishing Group.

The team, led by first author Kanyi Pu, demonstrated in a Nature Nanotechnology paper that nanoparticles of one type of semiconducting polymer provides much greater photoacoustic signal on a per mass basis than gold nanorods or carbon nanotubes in vitro and in vivo. (Not all of these polymers work equally well for this purpose; the study began by comparing two, and the polymer used in later experiments has four-fold greater signal.) They also showed that the activatable version detects inflammation caused by treatment with an immune-activating agent (zymosan).

While this is a clever demonstration of the potential relevance of this class of materials to photoacoustic imaging, the usefulness of this advance remains far from clear. As this is the first use of these materials in animals and involved only one acute exposure, their safety must be examined much more thoroughly. Another caveat is that while induced inflammation is detectable with this approach, ROS levels characteristic of chronic diseases may be less easily distinguished. More broadly, while PAT is a hot topic, the method is only now being introduced to the clinic (so far in non-contrast-enhanced applications, such as monitoring breast cancer response to therapy by measuring blood volume and oxygenation).