Often, following radiation therapy or surgery, a radiologist may find a spot that looks suspicious by MRI – it could be harmless scar tissue or a concerning tumor recurrence, but the MRI finding is ambiguous. At St. Jude Children’s Research Hospital, members of the Department of Diagnostic Imaging have an innovative way to clarify ambiguous findings using a positron emission tomography (PET) scan with [11C]Methionine (MET).
For the first time in the institution’s history, St. Jude has received Expanded Access approval from the Food and Drug Administration (FDA). Expanded Access use is an FDA designation that allows use of a drug or agent as a bridge on the way to commercialization and full FDA approval. The combined works of the late Zoltan Patay, MD, PhD, Barry Shulkin, MD, and Amy Vavere, PhD, all of the Department of Diagnostic Imaging led to the submission.
The protocol for a PET scan with MET is named EXAMPT. This provides the Expanded Access use of [11C]Methionine, a PET tracer for amino acid uptake and utilization, in the central nervous system and head and neck growths, such as cancer. The awarding of the Expanded Access designation is a result of the department’s research since 2009, where MET PET imaging has been studied in clinical trials for pediatric tumors in over 1,500 patient scans.
The results have shown it is safe and has value in pinpointing brain tumors. For example, in 2022 Shulkin and his colleagues published a study in the Journal of Nuclear Medicine on the use of [11C]Methionine PET for identification of recurrent pediatric high-grade gliomas (a type of brain tumor).
Methionine is a naturally occurring amino acid necessary for protein synthesis. As tumors are rapidly growing, the formation of new proteins and cellular growth is amplified. PET imaging with this agent measures amino acid transport following the route of its naturally occurring, non-radioactive counterpart. While other amino acid PET tracers have been explored over the last few decades, primarily in adults, little has been done to show their benefit in children.
The most commonly used PET tracer, [18F]-2-fluoro-2-deoxy-D-glucose, or [18F]FDG, is taken in by normal brain tissue. Compared to[18F]FDG, [11C]MET is advantageous because it does not accumulate in healthy brain tissue, but it does accumulate in cancerous tissue. This makes it much easier to see which tissue is cancerous and which tissue is healthy when using [11C]MET.
“Imaging a brain with FDG is like looking for stars when the sun is shining,” Shulkin explained. “The stars are there, but we just can’t see them. Imaging with MET takes out the sun, and we can see them.”
“We are convinced this is the best agent for our patients,” Vavere said. “The Expanded Access approval will allow us and our clinicians to use the protocol independently as part of the patient’s medical care and allow the physician to use the results for treatment planning.”
Since the MET tracer is labeled with carbon-11, which has a half-life of only 20 minutes, it can only be made available in hospitals with a nearby or onsite cyclotron and radiochemistry facility for preparation. For the benefit of St. Jude patients, the Department of Diagnostic Imaging installed such a facility in 2007.
Since 2018, to bolster collaboration, the facility has functioned as the Molecular Imaging Core, an institutional resource within the St. Jude Department of Diagnostic Imaging. The Molecular Imaging Core supports clinical trials with four IND-approved tracers to date, as well as basic and preclinical research projects supporting researchers across the institution.
The Expanded Access designation demonstrates the dedication of St. Jude Department of Diagnostic Imaging to innovate in the field.