PSMA Imaging: The Future of Traditional CT Scans

As part of GU Oncology Now’s Round the Wire series on PSMA Imaging, moderator Dr. Neeraj Agarwal and Dr. Scott Tagawa discusses the future use of traditional CT and bone scans in the follow up or surveillance of metastatic prostate cancer in light of PSMA imaging.

Dr. Agarwal:

So let me ask Dr. Tagawa, Scott do you think we may replace the traditional CT scan and bone scan for regular follow up or surveillance of metastatic prostate cancer by PSMA scans in the future?

Dr. Tagawa:

I think that it is highly likely that it’s going to replace to some degree, but I don’t think it’s going to fully replace. So initially what we know is that, CT scan and/or MRI and bone scans, we know exactly what that means in terms of prognosis. We know what that means in terms of not so much in terms of response other than that soft tissue, but at least in terms of radiographic progression, we know what that means and they’re involved in regulatory endpoints. We don’t know yet so much what that means for PSMA PET. So, I’m not sure who is listening to this, but hopefully there are investigators and from pseudo companies that are listening that hopefully add in PSMA PETs to our prospective trials. So, we can really answer that question. I think there’s an opportunity to replace and also to start looking at response, in addition to just radiographic progression-free survival, because we can use a quantitative type of imaging to see some differences.

That being said, so PSMA is highly sensitive, but is particularly in heavily pretreated patients with a lot of AR directed therapy, there is a loss of PSMA. So like we’ve always seen, even if it’s CT and bone scan, we don’t want to follow PSA only because we might miss non-PSA producing tumors. Same thing can happen with PSMA. There can be loss of that target. So, I think we can get stuck missing that patient population, even if it’s only 10%. So, I don’t think we want to 100% tie ourselves to PSMA PET, but I think there is a role for it in all settings, whether it is selection for PSMA directive therapy or selection for other therapies as well as looking at response to other therapies. I think there’s a lot of opportunity, and we’re really just scratching the surface right now.

Dr. Agarwal:

So that’s a great point you made. A lot of PSMA targeting therapeutic agents are coming up. They’re already being tested in phase one and phase two trials. And what you are saying is making perfect sense. Patients may have to undergo PSMA PET scan to assess for their possibility of response to these new therapies. Is that correct?

Dr. Tagawa:

I think that the biggest data set that we have is with small molecule target agents against PSMA radio labeled with beta emitters, such as lutetium-177. So, there’s a small data set that’s treated a population without looking. So we’ve done that, but not a lot of others. The total available data says that in my mind, that we select out for a more optimal patient population by using PSMA PET as a theoretically predictive biomarker for that. I think that when we start going away from using a small molecule for targeting, using drugs or immune targeting of PSMA or maybe alphas that may be different.

So, I think we want to continue to image everyone prior to treatment, so we can look at that as a biomarker, but what is close to primetime lutetium-617, no question in my mind, optimize a patient population. I think that meets the definition as Jeremie mentioned before of his job theranostics, I think that’s there as an individual patient. There may be occasional patients that will respond without having good imaging. So, it’s a little bit different with individual patient sitting in front of me versus drug development.