Gallium-68 PSMA Targeted PET Imaging in Prostate Cancer: Staging and Outcomes - Episode 2

History and Benefits of Targeting PET PSMA

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The history and resultant benefits of PET (positron emission tomography) along with the utilization of PSMA (prostate-specific membrane antigen) PET imaging in treating prostate cancer is discussed.

Phillip Kuo, MD, PhD: The next question I’m going to address is comparing PET [positron emission tomography] imaging to conventional imaging. I will take this question since it is a little more imaging orientated. I am going to start by explaining a little of the history of PET imaging. For those of us who have been at this long enough, it used to be easy because when we started with PET scans, the only tracer we had to worry about was fluorodeoxyglucose [FDG]. You could just say “PET scan” and everyone knew you meant FDG PET scans, after you got over the jokes of scanning your cat or dog. Now we have Axumin, also known as fluciclovine. That was a big breakthrough in prostate cancer imaging. It is approved by the FDA for biochemical recurrence, and its mechanism is amino acid imaging. Fluciclovine is a synthetic amino acid and so, as you can imagine, prostate cancer, like all cancers, wants to grow faster and hence needs more proteins. To make more proteins, the cancer needs more amino acids.

This upregulation of the uptake of amino acids is not the mechanism of Axumin, the synthetic amino acid. It is actually not specific for prostate cancer, as you can imagine, since it is an amino acid. That was a really big first step. I make that analogous to penicillin when it comes to antibiotics. It was great. It was a big milestone, but eventually we are going to come up with something better. PSMA [prostate-specific membrane antigen] is right on the cusp, and that is why we’re going to focus on it today. You are now going to have to know yet another—in a good way—another PET tracer with PSMA PET. There will likely be multiple PSMA PET tracers coming out. FDG though, I do want to make a special mention, still needs to be used in certain circumstances when it comes to prostate cancer, particularly the more aggressive ones. For example, it would be best used when it comes to metastatic castrate-resistant prostate cancer, particularly if it’s neuroendocrine differentiation. Then, FDG plays a big role toward that late stage, but not so much early on. Dr Abreu, please comment.

Andre Abreu, MD: I agree with this. This is specifically true for the PET PSMA in terms of patients with high-risk disease. With the PET PSMA, we will probably be able to better stratify these patients. We will likely be able to separate the patients who are indeed high risk but with localized disease, and those patients who are high risk but with oligometastatic disease—whatever definition of oligometastatic disease you pick—or patients who have nodal disease. What we expect is, in the future, both of these patient populations will have better treatment because we have a better understanding of the disease from the baseline and better staging. For example, in the case of some patients who have high-risk disease, where we thought they were high-risk disease by conventional imaging, they actually have nodal disease. Now we know that they have nodal disease. They will then be classified as having oligometastatic disease and will be appropriately treated for this.

The other way around is the patients we thought only had oligometastatic disease, but when we do the imaging like PET PSMA, we realize we have more disease to treat, and then we change the treatment of this patient. This is something that the PET PSMA is providing; we see up to 50% of our cases changing the treatment, most likely for patients with high-risk disease. This is something that PET PSMA is bringing us that we did not have before.

Phillip Kuo, MD, PhD: Well, thank you very much. Those comments really touch upon, when you’re reading the studies, to speak generally, it of course depends upon your patient population. As Dr Abreu’s already mentioned, the high-risk, newly diagnosed patients with prostate cancer are the ones, not surprisingly, we feel that these more advanced imaging modalities are knocking on the door for, and for whom PSMA PET is going to have the highest impact.

We do not recommend it, for example, for patients with low-risk disease, and intermediate-risk disease is that gray area where there’s a lot of research still to be done. Those high-risk patients are for whom we really feel this imaging will work. That is familiar to us, if you think about it. Not every patient with newly diagnosed prostate cancer needs a bone scan to evaluate for just the metastatic bone disease. This is nothing unfamiliar to us if you think about it.

Transcript edited for clarity.