New PET & SPECT Tracers Portend to Radically Change Practice

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Results from Phase III of the ADMIRE-HF study presented at the 2009 American College of Cardiology meeting showed that GE Healthcare’s AdreView (Iobenguane I-123 [MIBG]) SPECT tracer could help determine if heart failure (HF) patients with low ejection fractions will actually benefit from an ICD. The radiotracer images myocardial nerve activity at the cellular level: the more damage to the nerves, the less AdreView uptake. Researchers discovered that patients with low uptake had more cardiac events, a higher risk of sudden death and a better likelihood that their defibrillator—if they had one—would fire.

“Not everyone with a low ejection fraction will benefit from an ICD,” says Gary Heller, MD, director of the nuclear cardiology laboratory at Hartford Hospital in Connecticut and an investigator in the trial. “If you’re not sure if a patient should have an ICD, this test may help in that regard.”

The advantage right now of a tracer like AdreView is that it is not affected by the technetium shortage, says Manuel Cerquiera, MD, chairman of molecular and functional imaging at the Cleveland Clinic Foundation.

“This agent could be a major breakthrough, not only for its clinical relevance, but because it’s used with conventional gamma cameras, which have a huge installed base,” says Cerquiera, who is also an AdreView investigator. The Phase III data have been submitted to the FDA for approval, he says.

SPECT vs. PET

Jamshid Maddahi, MD, professor of molecular and medical pharmacology and medicine at David Geffen School of Medicine at UCLA

The technetium shortage might make some cardiologists take a second look at PET myocardial perfusion imaging (MPI). While SPECT MPI is well established, it has some limitations, which PET MPI does not, according to Dennis A. Calnon, MD, head of the division of nuclear cardiology at McConnell Heart Hospital at Riverside Methodist Hospital, Columbus, Ohio.

The technetium-labeled tracers used for SPECT MPI underestimate the extent and severity of coronary artery disease (CAD), whereas rubidium-82 and ammonia-13 tracers do not. Soft tissue attenuation artifacts are common in SPECT, but not with PET. A SPECT study can last up to four hours, whereas PET MPI can be performed in 35 minutes. Finally, reimbursement is more favorable for PET, Calnon says.

CardioGen 82 (Bracco Diagnostics) is the only FDA-approved generator-based PET agent to evaluate CAD. But there is ongoing research into developing fluorine 18-labeled perfusion tracers for PET. The advantage of F18 is that it is produced widely in regional cyclotrons (for FDG), has a two-hour half-life (compared to minutes for oxygen-15 and ammonia-13) and can be used during exercise stress testing, says Jamshid Maddahi, MD, a professor of molecular and medical pharmacology (nuclear medicine), and medicine (cardiology) at David Geffen School of Medicine at UCLA.

Lantheus Medical Imaging is the farthest along in developing such an F18-labeled agent. Phase I results of BMS747158 indicated a good safety and biodistribution profile. Phase II studies have begun and preliminary results show that “short protocols are feasible and an imaging time of approximately 10 minutes will be adequate with the agent,” Maddahi says.