The emerging advantages of PET myocardial perfusion imaging (MPI) for coronary artery disease (CAD) diagnosis and assessment of cardiovascular event risk has prompted growing use of this technology as an alternative to the more commonly used single photon–emission CT (SPECT) MPI.
The advantages of PET MPI include better diagnostic performance and shorter acquisition times. , including consistent, high-quality images and low radiation exposure. It also allows quantification of myocardial blood flow, and it has “strong prognostic power.”
Tracer availability
Despite these advantages, that position paper and subsequent studies note that PET MPI has been underutilized in the United States, largely owing to issues with the available tracers, which have characteristics that limit widespread use in the clinic.
Rubidium, arguably the most commonly used tracer for PET MPI, is not available in unit dosing and so can be expensive for low-volume centers, plus it also requires an on-site generator, Michael Salerno, MD, PhD, a member of the American College of Cardiology’s Imaging Council and section chief of cardiovascular imaging, Stanford (Calif.) University, told this news organization.
N-ammonia, the other U.S. Food and Drug Administration–approved tracer, is available in unit dosing, but its short half-life means that centers need an onsite cyclotron, Dr. Salerno said.
For cardiac perfusion imaging and myocardial blood flow (MBF) quantification, 15O-water is considered the gold standard, although it’s not approved by the FDA. This tracer also requires an on-site cyclotron and “is challenging to use,” Dr. Salerno said. Use has been largely restricted to research purposes, though efforts are underway to widen its availability.
Enter flurpiridaz F-18 (GE Healthcare), a novel PET MPI tracer labeled with fluorine-18. Its longer half-life – similar to that of fluorodeoxyglucose, a tracer used to detect various cancers – could broaden the number of sites that could perform perfusion PET studies, Dr. Salerno said.
“Flurpiridaz also is supposed to have a more linear relationship between flow and tracer uptake, which could improve the ability to perform quantification of perfusion,” he noted. “It also offers the ability to do exercise PET, which is impossible for rubidium and challenging for ammonia, given its 11-minute half-life.”
Flurpiridaz status
The FDA requires two phase-3 studies that show safety and sufficient diagnostic performance before it will approve a new tracer. The first required study, published in the Journal of the American College of Cardiology, showed that the tracer’s sensitivity for detection of greater than or equal to 50% stenosis by ICA was significantly higher than SPECT; however, the specificity did not meet the prespecified noninferiority criterion.
The second FDA-required study, published online recently, also in the Journal of the American College of Cardiology, was designed differently from the first in that only patients with suspected – not known – CAD were enrolled. The primary efficacy endpoint was sensitivity and specificity of flurpiridaz PET for overall detection of CAD, rather than comparing it to SPECT MPI (which became a secondary endpoint). PET and SPECT studies were both performed before invasive coronary angiography to minimize referral bias; SPECT studies included cadmium zinc telluride cameras.
In that study, which included 578 patients (mean age, 64; 32.5% women) from 48 centers in the United States, Canada, and Europe, flurpiridaz met the efficacy endpoints: Its sensitivity and specificity were significantly higher than the prespecified threshold value by two of the three readers; its sensitivity was higher than SPECT (80.3% vs. 68.7%); and its specificity was noninferior (63.8% vs. 61.7%).
PET areas under the receiver-operating characteristic curves were higher than SPECT in the overall population and in women and obese patients, at half the radiation dose of SPECT.
“Cardiac PET MPI is positioned to serve as the leading modality for the functional evaluation of suspected and known CAD,” Jamieson M. Bourque, MD, MHS, medical director of nuclear cardiology, echocardiography, and the Stress Laboratory, University of Virginia, Charlottesville, wrote in an editorial accompanying the second study . “18F-flurpiridaz will facilitate this upward progression with beneficial tracer characteristics that will increase access and availability, enable exercise stress, and optimize MBF quantification.”
At this point, FDA approval of flurpiridaz is expected sometime in 2024, said James E. Udelson, MD, principal investigator of the recent study, chief of the division of cardiology, and director of the Nuclear Cardiology Laboratory at Tufts University School of Medicine, Boston.