18F-rhPSMA-7.3 Lays Groundwork for Better PSMA-PET Imaging Techniques in Prostate Cancer

Mark T. Fleming, MD, medical director for the US Oncology Genitourinary Research Committee

At the American Urological Association 2022 Annual Meeting, results of the phase 3 SPOTLIGHT trial (NCT04186854) were presented on the use of 18F-rhPSMA-7.3 for PET/CT imaging in patients with recurrent prostate cancer. It’s use successfully led to greater baseline post-scan disease upstaging compared with standard imaging.

Mark T. Fleming, MD, medical director for the US Oncology Genitourinary Research Committee, and lead investigator of the study, discussed development of 18F-rhPSMA-7.3 and the need for a prostate-specific membrane antigen (PSMA)–PET imaging agent like this.

“Based upon my experience in this trial as well as other [PSMA-PET] options that are available, I don’t get conventional imaging anymore. To me, [PSMA-PET] has changed the field in that we are detecting localized diseases,” said Fleming.

In an interview with CancerNetwork®, Fleming spoke about results of SPOTLIGHT, how this imaging agent can impact the standard of care, and where he hopes to see future research headed.

What was the rationale for developing the 18F-rhPSMA-7.3 imaging agent?

rhPSMA-PET takes advantage of the importance of prostate-specific membrane antigen in prostate cancer. It’s evolving as both a diagnostic as well as potentially therapeutic modality. Conventional imaging, bone scans, and CT scans are just not adequate to determine early biochemical recurrence in patients with prostate cancer. This is a high-affinity PET radiopharmaceutical, which has the advantage of having a low bladder activity as well as a longer half-life. From a production standpoint, [it also has] a larger batch potential.

What were the results of the use of 18F-rhPSMA-7.3 in the SPOTLIGHT trial?

The SPOTLIGHT trial looked at men with prostate cancer who were treated with curative intent for localized disease. They showed evidence of biochemical recurrence and were eligible for curative intent salvage therapy. The imaging was performed with the rh-PSMA-PET scan. It’s important to highlight that confirmation of PET findings showed a standard of truth—which was either histopathology ie a biopsy or confirmatory conventional imaging—to confirm if there was evidence of disease. There was an exploratory efficacy analysis of true positives scan that led to upstaging of patients with negative baseline imaging and there were about 250 patients in the group.

A detection rate looks at the percent positivity of a study. More importantly in this study, we looked at the correct detection rate which is a percentage of all the patients who were scanned with at least one true positive PET finding as confirmed by that standard of truth. That standard of truth was either the biopsy or the confirmation imaging. When we look at this study, the correct detection rate was 45% to 47%, based upon the 3 readers who were upstaging patients. It’s important to point out that these patients had a rise in their [prostate-specific antigen levels] and they had negative conventional imaging when we did the study. We were able to find a corrected detection rate of 45% to 47%, and that varies based on prior treatment as well as the anatomical region.

What are the next steps in the development of the agent, and when do you anticipate it becoming available for daily practice?

The data are convincing that rh-PSMA PET is a worthwhile modality to detect biochemical recurrence and treat patients earlier. That’s what it’s all about, being able to create options for patients. How would you use it? If I see a site of disease, with the rh-PSMA-PET I could consider either surgical resection or targeted radiation therapy to the oligometastatic sites of disease.

The other added piece of rh-PSMA-PET is it has the ability to potentially label α and β emitting radio models for systemic radiation therapy. Based upon that, it has the advantage of potentially being able to combine this rh-PSMA-PET modality with future treatment options.

Is there anything else you would like to discuss?

The SPOTLIGHT study highlights what I believe is a paradigm shift with its multidisciplinary use in prostate cancer. We’re going to need radiologists who have the ability to read PSMA-PET [scans] and have experience with PET scans. You need to work with a urologist, radiation oncologist, and medical oncologist. To me, when we collaborate and work together on behalf of our patients, we should be utilizing multidisciplinary care. Where we are in prostate cancer treatment in 2022 is a multidisciplinary field of both diagnostics and therapeutics.

Reference

Fleming MT. Impact of 18F-rhPSMA-7.3 PET on upstaging of patients with prostate cancer recurrence: results from the prospective, phase 3, multicenter, SPOTLIGHT study. Presented at: 2022 AUA Annual Meeting; May 13-16, 2022; New Orleans, LA. Abstract PLLBA-02.