Mark Kidd: Thank you, Leslie. I appreciate the opportunity.
Leslie Ballas: Tell me first of all a little bit about PROSTest. Okay.
Mark Kidd: PROSTest is a mRNA-based blood assay which focuses on genes involved in prostate cancer tumor biology. So what we do is we collect a blood sample, we extract RNA, and we measure genes that reflect tumor biology. So it's a molecular tool that captures how the tumor's growing and how aggressive it is.
Leslie Ballas: Tell us a little bit about the study. Give us the design.
Mark Kidd: It is a prospective multicenter involving men who are undergoing Lu-PSMA and men with prostate cancer in the metastatic castration-resistant setting, so late-stage disease. The idea for the study really was to evaluate whether changes in this molecular marker, particularly early changes could predict which men responded to therapy.
Leslie Ballas: Before use of PROSTest, how would we know if someone was going to respond to therapy?
Mark Kidd: That's based on PSA, but PSA accuracy is quite low in this setting. PSA is more of a prognostic than a predictor. And then of course, there are a number of clinical factors that clinicians have used to try and identify who could respond best to therapy. More recently, there's been a focus on, for example, circulating tumor DNA and looking very carefully at Lu-PSMA imaging. Because PSMA imaging, as you know, has value if you're looking at SUV max and some of those parameters in trying to identify who could best respond. Clinical factors have value, but I would say accuracy is probably in about the 70% range. So there's still this very large unmet need for identifying those men who will not respond to therapy.
Leslie Ballas: And what made you think that PROSTest was going to be able to discern who would respond and not?
Mark Kidd: Well, because the assay measures the molecular fingerprint of the tumor, the biology of the tumor, we anticipated that an increase in score, so the PROSTest scored zero to a hundred. So if, for example, before a treatment your score was say 70, we anticipated that men who would not respond to therapy, the score would increase and that score would reflect tumor growth. In other words, aggressive biology, not responding to being targeted. Conversely, if the score was to decrease, that would tell us that the tumor's been appropriately targeted. So it was I think quite a simple but powerful hypothesis, changes in score early on could predict who would respond best.
Leslie Ballas: And what did you find?
Mark Kidd: We found exactly that. And we looked both at PFS and at overall survival. And over 90%, in fact, 90.4% of men who responded to therapy and lived had a decrease in PROSTest score after one cycle. Those that did not respond and typically men who ended up dying within about eight months of starting therapy, 8.6 months to be specific, over 80% of them had an increase in the score. So we've got something very early on that can be used to identify who should continue with therapy perhaps and whether there should be a change of therapy or not.
Leslie Ballas: And is PROSTest just limited to Lu-PSMA? It seems like it could be used to tell who would respond to any kind of medication.
Mark Kidd: You're absolutely correct. So we do have one study looking at chemotherapy and we found exactly the same kind of response. Men who responded well earlier on in the landscape of prostate cancer, the responders had a decrease, the non-responders had an increase in the PROSTest score. So yes, that approach appears to be agnostic. It can be used for multiple different therapeutic approaches.
Leslie Ballas: Well, this is very exciting. Congratulations on your abstract and on PROSTest. We will look forward to seeing more excitement in the future.
Mark Kidd: We do too and we look forward to seeing how we could help clinicians in their decision-making.