The FDA recently approved gallium (Ga) 68 PSMA-11, a radioactive diagnostic agent, for prostate-specific membrane antigen (PSMA)-targeted positron emission tomography (PET) imaging of patients with recurrent prostate cancer. Ga 68 PSMA-11, the approval of which was granted to the University of California San Francisco (UCSF) and the University of California Los Angeles (UCLA), is the first agent to be approved specifically for the detection of PSMA-positive lesions. In this interview with i3 Health, Thomas Hope, MD, Director of Molecular Therapy at UCSF and principal investigator of one of the studies which led to the approval, discusses the benfits of Ga 68 PSMA-11 for PET imaging of patients with prostate cancer and shares advice for the optimal use of this agent.
Can you comment on the significance of the approval of Ga 68 PSMA-11 for PET imaging of PSMA-positive lesions in patients with prostate cancer?
Thomas Hope, MD: There are two sides to that coin. One is the significance of the agent itself. PSMA-targeted PET imaging is a marked improvement in terms of detection and localization of metastatic disease in patients with prostate cancer. This improved ability to detect and localize disease allows treating physicians, such as radiation oncologists, urologists, and medical oncologists to tailor their treatments more specifically to the patient in the hopes of producing better outcomes with more appropriate treatment. The improved imaging is what's important.
The second aspect of significance has to do with the fact that this approval is a nonproprietary approval, as UCSF and UCLA are academic institutions; we're not companies and we don't intend to distribute or sell the agent. This approval allows other companies to submit abbreviated new drug applications (ANDAs) to the FDA, to be able to sell this agent and help make it more widely available. This will actually also increase its availability around the world. With an approval from the FDA, people from Japan, for example, now have an easier way to get Ga 68 PSMA-11 approved in their country. This approval will help to increase the availability of the drug both within and outside the United States.
How do you recommend that clinicians minimize the risk of image interpretation error and misdiagnosis when using Ga 68 PSMA-11 PET imaging?
Dr. Hope: In terms of interpretation, thankfully, PSMA-targeted PET imaging is easier to interpret than previously existing imaging modalities, the main comparator of which would be fluciclovine. There is a lot of inter-reader variability with fluciclovine, and that occurs much less frequently with PSMA PET. That's primarily because the uptake in the tumor is much higher relative to the background than it is with previously existing modalities, so it's easier to interpret. Nonetheless, there are definitely known false positives that can mimic tumor on PSMA scans. Examples of those would be benign bone regions in the ribs and sacral ganglia. Hopefully, with the appropriate training, clinicians will be able to effectively interpret these imaging studies. There are training modules currently being developed by different organizations, and there's also literature and multiple review papers focused on how to interpret PSMA PET scans. There's definitely a level of training that's needed for nuclear medicine physicians and radiologists in order to appropriately interpret the studies, but nonetheless, Ga 86 PSMA-11 scans are actually easier to interpret than those done with agents such as fluciclovine.
The other issue is not just knowing how to interpret the scans but knowing how to use them. If you're a radiation oncologist and you get the results for a PSMA PET, what's the right way to plan your treatment based on the results? That is where the significant work remains. We need to run clinical trials and define the role of PSMA PET in the management of patients with prostate cancer. There's so much literature out there right now in terms of bone scans, computed tomography (CT) scans, and magnetic resonance imaging (MRI) about how to use that information to stage patients and determine whether or not they should receive systemic therapy. That remains yet to be done with PSMA PET.
How do you foresee the use of PET imaging for prostate cancer evolving in the coming years?
Dr. Hope: Over the next five or more years, the way that imaging is going to evolve will have to do with a number of novel PSMA-targeted PET radiotracers coming to the market. Ga 68 PSMA-11 is the first, and oddly, it's also the first as an academic one that's not proprietary, but there are a number of other agents currently in clinical trials. The next one will be fluorine-18 DCFPyL, probably followed by fluorine-18 rhPSMA-7. You get the drift that there are going to be quite a lot of these agents approved in the future, and it will be interesting to see how this market plays out.Overall, I view these as a class of drugs, with little difference between each one in terms of detection sensitivity.
Do you have any words of advice for members of the cancer care team using Ga 68 PSMA-11 PET for patients with prostate cancer?
Dr. Hope: If you're new to PSMA PET, I think it's important to note that there's a significant stage migration with this type of imaging. If you have a patient who has no evidence of disease on CT or bone scan—standard, conventional imaging—PSMA-targeted PET may detect metastatic disease that was not previously known. That does not mean that your patient has progressed or gotten worse; it's just a reclassification of patients. It's important to realize that we don't really know how to handle that stage migration, so we have to be prudent and careful in the way that we treat those patients. Think about them in the setting of conventional imaging, in terms of which treatments are available to them and which ones should be used, and then just be cognizant of that in terms of implementing your new treatment modality.
About Dr. Hope
Thomas Hope, MD, is Director of Molecular Therapy in the Department of Radiology and Biomedical Imaging at the University of California San Francisco (UCSF). He is also Chief of Nuclear Medicine at the San Francisco Veterans Affairs (VA) Medical Center and Chair of the Molecular Imaging and Radionuclide Therapy Site Committee at the UCSF Cancer Center. Dr. Hope's research focuses on the development of molecular imaging agents, novel techniques for PET and MRI imaging, and the use of targeted radiotherapy for patients with prostate cancer. He has authored or coauthored numerous publications in peer-reviewed journals.
For More Information
Fendler WP, Calais J, Eiber M, et al (2019). Assessment of 68Ga-PSMA-11 PET accuracy in localizing recurrent prostate cancer: a prospective single-arm clinical trial. JAMA Oncol, 5(6):856-863. DOI:10.1001/jamaoncol.2019.0096
Eiber M, Kroenke M, Wurzer A, et al (2020). 18F-rhPSMA-7 PET for the detection of biochemical recurrence of prostate cancer after radical prostatectomy. J Nucl Med, 61(5):696-701. DOI:10.2967/jnumed.119.234914
Transcript edited for clarity. Any views expressed above are the speaker's own and do not necessarily reflect those of i3 Health.