Two experts in the management of prostate cancer provide key insight into the value of blood and urine liquid biopsy tests when deciding whether to perform a prostate tissue biopsy.
David Albala, MD: I want to focus on the markers we use to determine whether patients should have a biopsy. Over the last couple of minutes, I’ve tried to give an overview of where we are, but I want to drill down a little so we can talk about the blood and urine tests. Why don’t we start with the urine tests, because you gave a great description of the PCA3 [prostate cancer antigen 3] test. Maybe you could tell us about exosomes, what they are, and how is the ExoDx test useful?
Judd W. Moul, MD: We started using the exosome ExoDx test close to 2 years ago. I wanted a urinary replacement for PCA3 because I needed a secondary blood test. But it was important to have a urine test as well. I’ll be honest, at first I was thinking more practically, that I needed a secondary blood test and a urine test, and not necessarily looking at the sensitivity and specificity. Maybe I should have looked at that first. And when you do an exosome ExoDx test, you made the key point that you don’t need to do the rectal exam. With PCA3, which we had used before, you had to do an attentive digital rectal exam prior to the urine test. It wasn’t a big deal, but it was a bit of a hassle. With the exosome ExoDx test the patients would give a first voided urine through a special device. We initially did it in house, meaning the patients would give the urine sample in the clinic, or in 1 of the exam rooms.
More recently—I know you’re involved in this as well—the mail-order tests became available, which is very similar to the Cologuard. For anybody who may have done colon cancer screening with the Cologuard test, the exosome ExoDx urine test is very similar. You put the order in the website, the company sends the kit by FedEx with instructions, the patient gives the urine sample, puts the specimen in a return FedEx, and sends it back. Then we get the results in about a week. It’s important to point out that, just to compare some of the secondary tests—when we talk about the blood, for example, I have more experience with the PHI test, the Prostate Health Index. That predicts the presence of cancer—or the likelihood of cancer, period—in 4 risk groups: low, low-intermediate, intermediate, and high risk. The way that breaks down is that if the patient falls into the best group, they have a 9% risk of cancer. The next is 16%, the next is 33%, and the last group—the high-risk group—is 50%.
It’s important for the doctors listening today who haven’t used the exosome ExoDx test before to know that the test is not predicting cancer or no cancer. It’s predicting the probability of Gleason 7 or higher. You get a dichotomous answer. The cut point is 15.6, so patients who have an exosome ExoDx test less than 15.6 have a 90% to 92% probability that they don’t have Gleason 7 or higher, whereas if they have a score greater than 15.6, it means they have a probability of having Gleason 7. In my mind, the PHI and the exosome ExoDx are somewhat complementary, meaning that secondary blood test is measuring just cancer—yes or no—in those 4 risk groups, whereas the exosome is a little more nuanced; it’s predicting Gleason 7 or higher. Basically, a guy who has an exosome ExoDx less than 15.6 could have Gleason 6 disease. It’s important for folks who haven’t used those tests to be aware of that.
David Albala, MD: That’s a great point.
Transcript edited for clarity.