Andrew J. Armstrong, MD, MSc: I think when you ask most physicians in the clinic if they are even ordering circulating tumor cells, your answer is no. Many patients and physicians are not aware of this test. Largely, that’s because the publications for this test have come out in single institutional studies. There has not been an external validation study. The test is not FDA-approved at this point largely because of this. And so, once there is an FDA approval or a clearance for the test, once there is external validation, and once we can say for certainty when this test should be ordered and used, I think that’s when there will be increasing awareness. Many physicians are aware of this cross-resistance and would probably like to have a test that could spare a patient an ineffective therapy that would be otherwise costly.

So, right now the test results, when they come back to a patient or provider, would be essentially a positive or negative test. You’re either AR-V7—positive or AR-V7–negative. There are also patients who just don’t have circulating tumor cells where the test is neither positive or negative, it’s uninformative. Those are really the 3 possible outcomes of the test. If there are no circulating tumor cells, that’s really good news for the patient. It’s not an evaluable biomarker in those patients, and the patients tend to really respond quite well to abiraterone and enzalutamide and have a longer progression-free and overall survival.

The patients who test negative for the biomarker have a predictive probability that’s somewhat higher of a response and a longer progression. However, I would point out that patients who are negative for AR-V7 can still have resistance to these drugs. Patients who test positive for AR-V7 have a very low chance of responding to abiraterone or enzalutamide, particularly in the second- or third-line settings. In the frontline setting, it may have less predictive value. That’s why the recommendation, at least as of this point, would be in the second- or third-line setting, when you’re trying to triage a patient for either a taxane, a clinical trial, radium-223, and immunotherapy, something other than abiraterone or enzalutamide.

The cost of AR-V7 testing has not been set yet. It’s not FDA-approved, and it hasn’t been set. Right now, there are 2 tests that are being proposed: the protein test, which is by Epic, and the RT-PCR test, which is the QIAGEN assay. The first one likely that will come to market is Epic. That has been really overseen by Genomic Health as a standalone test where a clinician can order this test and have a result come back within about 3 to 4 days.

Just as a reference, the cost of a PSA test—which is pretty useful in the clinic—is about $50, whereas the cost of the circulating tumor cell, cell search enumeration, is several hundred dollars. So, I expect it to be somewhere in that range, although you could argue that the clinical utility of this test may help to drive those decisions on the cost of the drug based on the external validation studies that we’re doing.

Transcript Edited for Clarity