Radiologist Gary Ulaner, MD, PhD, FACNM provides a comprehensive discussion on the FDA-approved PSMA radiotracer options that are available for use as part of PSMA-PET imaging.
David Albala, MD: Gary, talk about what the FDA has approved for the tracer options in the United States.
Gary Ulaner, MD, PhD, FACNM: We have 4 PSMA [prostate-specific membrane antigen]-targeted agents in the United States. The FDA has approved a gallium-68 PSMA-11 molecule, which is available through 2 providers.
David Albala, MD: Which is where you have your largest experience, with USC [University of Southern California] and UCLA [University of California, Los Angeles]?
Gary Ulaner, MD, PhD, FACNM: UCLA definitely has the greatest experience with gallium-68 PSMA-11. My greatest experience is with the fluorinated DCFPyL compound, which is available through Lantheus. There are 2 molecules and 3 producers. Lantheus makes the fluorinated molecule. Telix and Novartis each has its formulation of the gallium-68 molecule. All 3 are collectively referred to as PSMA PET [positron emission tomography] under NCCN [National Comprehensive Cancer Network] Guidelines.
David Albala, MD: Could you delineate the rationale for using each agent? Are they interchangeable? For a urologist in practice in a larger city, what’s the rationale for using them? How are they prepared? Do they need to be administered quickly or not so quickly? Review that for us briefly.
Gary Ulaner, MD, PhD, FACNM: I’m happy to. Gallium-68 is produced by generator, whereas fluorine-18 is produced by cyclotron. Gallium-68 has a half-life of about 68 minutes, whereas fluorine-18 is about 110 minutes. For simple math, I just call it 1 hour and 2 hours. If you want to use your product within 3 half-lives, gallium-68 gives you a time frame of about 3 hours, whereas the fluorine-18 gives you a time frame of 6 hours, or a little more, which is advantageous. There have been no head-to-head published studies comparing the gallium-68 and fluorine-18 PSMA PET imaging agents. Therefore, I don’t think there are any conclusive data suggesting that 1 is superior to the other in terms of sensitivity or positive predictive value.
There are some hypothetical advantages to the fluorine-18 molecule, which comes from the physics. Fluorine-18 has a higher positron yield than gallium-68. That means you get more positrons per dose that you’re administering to the patient. More positrons gives us more signal to background noise. Theoretically, you can see a higher signal to noise on fluorine-18 PSMA studies compared with gallium-68.
The other theoretical advantage comes from positron energy. Fluorine-18 has a lower positron energy than gallium-68. That means the positrons travel a shorter distance in tissue or hit an electron and create the protons, which are then detected by the PET camera. The shorter travel distances provide us with a slightly better resolution on fluorine-18 scans compared with gallium-68. Theoretically, from the physics, fluorine-18 has advantages over the gallium-68. Does this translate into a true clinical benefit? There are no head-to-head studies to definitively tell.
David Albala, MD: That’s a great summary. That really puts it in perspective.
Transcript edited for clarity.