Alicia Morgans: Hi. I'm so excited to be here with Dr. Joaquin Mateo of Vall d'Hebron Cancer Institute in Barcelona, Spain. Thank you so much for being here with me today.

Joaquin Mateo: Thank you for having me.

Alicia Morgans: Wonderful. Well, I wanted to speak with you a little bit about one of the things that vex us in our clinical practices when we're trying to get somatic mutation information from our patients with advanced prostate cancer. One of the things that can be challenging is trying to make sure that we have adequate tissue to really do this testing, at least by the gold standard. We have multiple alternatives to use, but lots of questions about the concordance between the different testing mechanisms. You, as an expert, of course, can comment on this. What are your thoughts on this really clinically relevant problem?

Joaquin Mateo: It is a big issue, and I think we are only starting to see the clinical value of this genomic profiling. I also see that the implementation in clinical practice is challenged by the lack of... I wouldn't say lack of expertise, but lack of experience in using this test and understanding the limitations that they have because they are not perfect. I think that is very important that we as clinicians understand the indications and the limitations of each of these testing modalities so we can use them in the best way possible for our patients.

Alicia Morgans: I think that's completely fair. Now, one of the things that I do in practice is try to use a stepwise approach where I have an ideal test that I might do, then a backup plan, a backup plan, and a backup plan. Somewhere in there, I'm hoping to make sure that I get that somatic testing for my patient. What is the order in which you would sort of prioritize approaches to somatic tumor testing for your patient?

Joaquin Mateo: So probably, you need to find a balance between the ideal plan and what is feasible in your clinical setting with the resources you may have. For example, in our institution, we have relatively easy access to tumor tissue samples, both for archival or fresh biopsies we can order targeted sequencing for any metastatic prostate cancer patient. But it's true that in some patients, this is not going to be a good approach, or the sample is not going to be good enough, the DNA is going to be degraded and you need to look for other approaches. There may be other institutions where the situation may be the contrary, that they may not have the capacity of retrieving samples from other hospitals and they may have access to liquid biopsy assays.

To me, it's still the tissue that comes first. I think that in the long term, we are going to see tissue testing getting integrated into the diagnostic process, and then we can probably use liquid biopsy for testing follow-up. But clearly, liquid biopsy also has a role upfront for those patients where you cannot get a good tissue sample on.

Alicia Morgans: Well, yes. I totally agree. One of the things though I think we should be aware of is something that you mentioned earlier, the limitations of those tests. What are the questions you have or the things that you think about when you're ordering these liquid biopsies? Would you do this in a patient, for example, who you've just started treatment on who's having an incredible response and PSA has gone down to very low? Is that the ideal patient? Is there another setting that it might be more effective or you may have a higher chance of getting your testing done without false negatives?

Joaquin Mateo: So when it comes to liquid biopsy, I mean first of all, with the name of liquid biopsy, we are actually including many different things that would normally refer to ctDNA testing. So getting tumor DNA that is circulating in the bloodstream and analyzing through Next Generation Sequencing. We know that the amount of tumor DNA that you can get from a blood sample is almost proportional to the burden of the disease for that patient, and also to the fact of whether the disease is actively progressing or not. So definitely, patients with very low burden of disease like localized or locally advanced prostate cancer patients, but also patients who are responding to systemic therapy would not be good candidates for liquid biopsy testing, meaning that there is a high chance that you don't get enough DNA in the blood to actually do the testing properly.

The problem is that with many of the commercially available tests, you don't actually get this quality check first, so you just get a result back that says mutation or no mutations. But if there are no mutations, you don't get a proper assessment of whether it may be just a problem that there was not enough tumor DNA in the blood. So physicians, we need to be aware of the difference between a negative result and a lack of ctDNA result when we interpret their results from this test. But definitely, I would prioritize getting liquid biopsy testing in those patients where the disease is actively progressing and that have a lot of metastatic burden.

Alicia Morgans: Great. Yes, I agree. I just wanted to make sure that we highlight that because the timing can be so important, and that's for false negatives as you mentioned. Now false positives can also be an issue, specifically with some of our certain genes. ATM for example, seems to have a higher rate of this. Can you tell us a little bit about this?

Joaquin Mateo: Yeah, they're having a few reports in the last few years about the potential confounding factor of what we call clonal hematopoiesis mutations. Our white blood cells also get mutations as we get older, and these mutations can be picked up by ctDNA analysis. These are normally mutations in a limited number of genes, and are mutations that are usually present in a very small proportion of the DNA fragments. What we call the mutational frequency, so how many reads of the DNA actually get the mutation is shown as a percentage usually is very low for this mutation. The problem is when we start working with samples, as we were saying from patients where the disease may not be progressing and they have little amounts of DNA, is when it gets difficult to differentiate whether that mutation may come from one or another source.

Again, it is important that when we find one of these mutations that may come from a white blood cell rather than from the tumor, we also do in parallel, germline testing from PBMCs to see if the mutation is present there or not. Again, the problem is that many times when you get a result from a targeted panel on ctDNA, you don't get the information on how likely it is to come from a white cell. It's up to the physician to realize that may be a problem, so we need a lot of education on that field for physicians who are using these tests so they are aware of the potential for false positives and false negatives.

Alicia Morgans: Wonderful. What are your final thoughts then? Should we believe our tissue testing most, ctDNA testing most? Can we actually use all of these if we're using them strategically and in the right patients at the right time? What are your thoughts?

Joaquin Mateo: I think these testing approaches are complementary to each other. Of course, there is a limitation of resources that we are not going to do every single test to every single patient, and we need still to find the right path for using them. I think that we are most likely going to a scenario of tumor tissue testing up front, and then a potential for liquid biopsy later on. I think that the most important thing we need to realize is that we all need to be aware of how to interpret the results of this test, and that the value of this test is not in the data, but in the clinical interpretation of this data. Hence, we need to take into account all these other technical issues that may be relevant to interpreting these results.

Alicia Morgans: Well, thank you so much for talking that through with us. I always appreciate your time and your expertise, and this is one area where you are one of the most concise and informative people that we ever talk to, so I always appreciate your guidance. Thank you.

Joaquin Mateo: Thank you very much. It's always great to come here.