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An injectable substance consisting of microscopic porous polymer spheres less than a third the size of red blood cells and containing perfluorocarbon gas can show blood flow into the heart muscle as accurately as a nuclear stress test, but faster, cheaper, and without radiation. Early results from phase III clinical trials have shown that the product, Acusphere Inc.’s AI-700, is just as accurate as nuclear stress tests at identifying patients with coronary artery disease, reports Frank Roylance in the Baltimore Sun. The microspheres reflect ultrasound, enabling perfusion — blood flow through the heart muscle – to be imaged with echocardiography rather than the expensive gamma cameras used to detect radiation from nuclear tracers. But because ultrasound loses energy as it penetrates tissue, the product may not work for very large patients or smokers with emphysema. After about five minutes — plenty of time to capture the echocardiograms — the spheres begin to break down. The gas is expelled through the lungs, and the polymer is broken down by the liver and spleen and eliminated from the body.

Nuclear stress tests not only expose patients to much more radioactivity than they would receive from x rays, but also potentially expose clinical staff. They can cost US$1,500 to $2,000 each and take five hours to complete. By contrast, a stress echo test using AI-700 can be performed in 15 to 30 minutes, on short notice, at about half the cost. Many hospitals are already equipped for echocariography, but fewer for nuclear stress testing. Microspheres with different structures and porosities are also being developed to deliver toxic anti-cancer drugs more quickly and with fewer side effects, to transport drugs that don’t dissolve well in the bloodstream, and (in inhalable form) to deliver an asthma drug directly to the lungs, in a once-a-day sustained release form.

Combining Modalities

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Combined SPECT-CT scanners help determine whether a patient needs a cardiac catheterization, but they are expensive and rare, especially in commuity hospitals in the US. Yet many hospitals have separate CT and SPECT scanners, and researchers at the University of Pittsburgh Medical Center have developed a system of software and sensors that enable images taken separately by both scanners to be combined. The leads researcher said his “whole approach is to develop something that can be applied in the community.” A study of the system’s effectiveness is just getting underway.